Category: Newsletter articles

REPORT: GeoPRISMS-EarthScope Planning Workshop for Alaska – an SCD Primary Site

Portland, OR, September 22-24, 2011

Jeff Freymueller1, Peter Haeussler2, John Jaeger3, Donna Shillington4, Cliff Thurber5, Gene Yogodzinski6

1University of Alaska-Fairbanks; 2USGS, Anchorage; 3University of Florida; 4Lamont-Doherty Earth Observatory; 5University of Wisconsin-Madison; 6University of South Carolina

A jointly-sponsored GeoPRISMS-EarthScope Planning Workshop for the GeoPRISMS Alaska Primary Site was held in Portland, OR from September 22-24, with some additional support from the U.S. Geological Survey. There were approximately 140 participants, representing more than 60 U.S. academic institutions, as well as key geoscience stakeholders in Alaska, including the USGS, Alaska Volcano Observatory (AVO), Alaska Earthquake Information Center (AEIC, the regional seismic network), and other potential GeoPRISMS partners. International organizations in Germany, Russia, Japan and Canada were also represented. The group included 22 graduate students and post-docs who took part in a one-day pre-workshop Student Symposium. Lively and substantive discussions took place both in breakout and plenary sessions over the 2.5 day workshop, leading to a clear consensus plan for GeoPRISMS science in Alaska.

Figure1. GeoPRISMS-EarthScope Alaska Planning Workshop group photo.

Figure1. GeoPRISMS-EarthScope Alaska Planning Workshop group photo.

Objectives and Process

The objective of the workshop was to solicit community input about research opportunities and priorities that would form the basis for the GeoPRISMS science plan for the Alaska Primary Site. The starting point for the workshop was the Implementation Plan produced during the January 2011 meeting in Bastrop, Texas, where Alaska was identified as the lead primary site for the Subduction Cycles and Deformation (SCD) initiative of GeoPRISMS.

The workshop began with a series of plenary talks that provided an overview and then more focused examination of various aspects of the Alaska-Aleutian subduction system. These talks offered up-to-date summaries of Alaska-Aleutian geology, geophysics and geochemistry, to inform participants and to stimulate participants to think about key opportunities for GeoPRISMS research in the Alaska-Aleutian system. Talks focused on Alaska Margin Tectonics and History (Terry Pavlis and Dave Scholl), Surface Processes and Tectonics (Don Fisher and Sean Gulick), Magma Processes from Deep to Shallow (Peter Kelemen and Stephanie Prejean), and Mantle Processes and Geodynamics (Ikuko Wada and Peter van Keken). Bobby Reece, Rob Harris, Phaedra Upton, Susanne Straub, and Steve Holbrook presented several short talks on subjects proposed in white papers.

Breakout sessions began on the afternoon of the first day of the workshop. The objective of the first breakout was to identify key onshore and offshore research targets and data gaps, and to discuss the concept of “discovery corridors“ as an approach to identifying geographic focus areas within the Alaska-Aleutian system. Participants were encouraged to identify specific locations where GeoPRISMS resources might be most effectively focused on high-impact, shoreline crossing and interdisciplinary research efforts – the hallmarks of the GeoPRISMS program. Participants were encouraged to keep in mind that some important research objectives may be best suited to a thematic research approach, undertaken anywhere in the Alaska-Aleutian system or at any arc on Earth.

Participants were assigned to breakout groups based on their top two research interests chosen prior to the workshop from the SCD key topics. These breakout themes were (1) controls on size, frequency and slip behavior of subduction plate boundaries, (2) spatial and temporal patterns of deformation through the seismic cycle, (3) storage, transfer, and release of volatiles through subduction systems, (4) geochemical products of subduction and creation of continental crust, (5) subduction zone initiation and arc system formation, (6) feedbacks between surface processes and subduction zone dynamics.

Day one of the workshop ended with a series of short presentations on logistical considerations for fieldwork in Alaska. The major points of emphasis were the challenges of Alaskan weather and long distances, and the importance of long-range planning to allow for permitting along the Alaska-Aleutian margin, which is a patchwork of lands mostly under the control of various public agencies.

The second day of the workshop began with reports and discussion of the previous day’s breakout sessions. Next was a series of short presentations by a panel of potential GeoPRISMS partners. National organizations represented on this panel were the USGS and AVO (John Power), USGS Volcano Hazards program (John Eichelberger), USGS Extended Continental Shelf Project (Ginger Barth), the Cascadia Initiative (Richard Allen), and IRIS and USArray (Bob Woodward). International panel representation was from the German-Russian KALMAR Project (Christel van den Bogaard), Japan, IODP and JAMSTEC (Yoshiyuki Tatsumi), and Canada (Kelin Wang).

The second breakout session focused on implementation strategies. Participants considered possible “discovery corridor” locations, and identified overlaps and opportunities for synergistic GeoPRISMS and EarthScope activities. Breakout group leaders and participant attendance was the same as on day one to maintain continuity. Reports from breakout session leaders were given immediately after lunch. The third breakout session commenced late in the afternoon on day two. This time participants were mixed with respect to research interest but grouped with respect to their first and second geographic priorities for discovery corridor selection. The geographic sites were Cook Inlet, Alaska Peninsula, eastern Aleutians, Adak-Amlia area and westernmost Aleutians. A sixth breakout group called the Arc Line was also convened to characterize the “back-bone” of the Aleutian (oceanic) part of the Alaska-Aleutian margin, including geophysical imaging and along-strike changes in geophysical, geochemical, and geologic properties and processes.

The second day of the workshop ended after breakout three discussions, allowing the conveners to synthesize the plenary and breakout discussions so far. Their summary reports were presented in the morning of the third day of the workshop, leading into a productive plenary Q&A and discussion, during which broad consensus about GeoPRISMS science implementation in Alaska was reached.

Figure 2. Jeff Freymueller summarizes the outcomes of the Alaska planning workshop break-out discussions.

Figure 2. Jeff Freymueller summarizes the outcomes of the Alaska planning workshop break-out discussions.

An Implementation Plan for Alaska

A key objective of breakout three discussions was to establish a prioritization of the six geographic areas under consideration for more focused research, measured here by break-out attendance. The cumulative attendances at each of the geographic areas were: the Alaska Peninsula (55); the Adak-Amlia area (48); Cook Inlet (37); the along-arc transect (32); followed by the eastern Aleutians (25) and the western Aleutians (13). An important outcome of breakout three was the similar scientific and geographic focus of the three groups interested in the Aleutian/oceanic part of the margin. Based on this, the convener group presented a proposed science implementation plan, emphasizing a geophysical transect along the oceanic part of the arc in combination with complementary focused studies of the Alaska Peninsula and Cook Inlet areas.

Workshop participants expressed broad support for a large geophysical deployment along the oceanic part of the arc. This geophysical transect is envisioned as the back-bone that provides a framework for focused studies at point locations encompassing varied aspects of the arc, fore-arc, trench and incoming plate. The Aleutian islands provide many advantages for testing ideas about crustal genesis in a subduction setting. The arc has never been rifted, thus the products of ~45 million years of island arc crustal growth are intact and available for study. Additionally, strong contrasts in trench sediment thickness and subducting plate age at the Amlia Fracture Zone area are linked to distinctive magma chemistries in the arc and a change in seismogenic character.

One or more trench/arc-perpendicular transects would intersect the along-arc transect. The highest priority transects are the intersection with the Amlia Fracture zone and focal points in the Adak and Unalaska areas, providing unique opportunities to characterize the birth and evolution of the arc. Volcanoes of the eastern Aleutian area (e.g., Okmok, Akutan, Shishaldin) also provide ideal targets, located on the backbone transect, for slab-to-surface geophysical imaging of the largest and most active volcanic centers in the Alaska-Aleutian subduction system.

The Alaska Peninsula features dramatic along-strike changes in the seismogenic zone, spanning megathrust rupture areas in different parts of their cycles and with a range of locking behaviors. It is the best location for combining onshore and offshore studies to investigate the causes of these changes. It offers the best opportunity to examine links between seismicity and forearc surface process and variable subduction inputs. This area also includes the most productive volcanoes of the continental part of the arc, with both large dominantly basaltic centers and smaller dominantly andesitic centers, including Katmai, which produced the largest eruption of the 20th century. The group also supported the idea of a future deployment of Cascadia Initiative ocean bottom seismometers in this region.

The Cook Inlet area is the continental end-member of the subduction zone, which experienced a watershed megathrust event in 1964, and is dominated in Quaternary time by glacial and other surface processes that direct sediment into the subduction zone and forearc. This region also shows the clearest evidence in Alaska for large slow-slip events and transient changes in seismogenic zone behavior. This region also features a transition to flat slab subduction due to the buoyant thick crust of the subducted Yakutat terrane, intense microseismicity in the downgoing plate, abrupt variations in shear wave splitting orientations, the SE end of a gap in the volcanic arc, and active faulting and folding of a broad region of the overriding plate.

Both Cook Inlet and the Alaska Peninsula are also areas with substantial opportunities for synergy with EarthScope due to the EarthScope instrumentation that will be in place there, and coordinated research opportunities with AVO (described below), AEIC, and other researchers actively studying processes there.

Alaska was chosen as the highest priority GeoPRISMS Primary Site because of the distinct along-arc changes in volcanism, seismicity, forearc structure, and subducting sediment thickness. Participants recognized that specific synoptic studies were needed that address these spatial changes along the entire arc as opposed to specific target areas. These studies could include geodesy, paleoseismology, surface processes and along-arc sediment transfer, arc geochemistry and geochronology, and passive seismic monitoring.

Impact, Influence and Benefits from Partner Organizations

There are clear opportunities for synergy between the GeoPRISMS and EarthScope Programs in Alaska, especially for the Cook Inlet area and also for the Alaska Peninsula. The two programs share many common scientific targets, including the seismogenic zone, fluid cycling, and arc development, The recent report from the May 2011 EarthScope workshop on science opportunities in Alaska discusses many scientific issues and goals that are directly in line with those of GeoPRISMS. EarthScope has supported the installation and operation of ~150 Plate Boundary Observatory (PBO) continuous GPS stations across Alaska, and will support a comprehensive seismic deployment across Alaska in the form of the USArray Transportable Array (TA).

Present and future EarthScope instrumentation in the Cook Inlet area, in particular, offers great opportunities for synergy between the programs on the many shared scientific targets. For example, the TA stations, augmented by EarthScope FlexArray or GeoPRISMS seismic deployments and existing seismic stations on volcanoes, offer the chance for detailed imaging of the mantle wedge and tracking magmas from slab to surface. PBO stations in the area have documented large slow slip events and other transient changes in the behavior of the seismogenic zone, highlighting a great opportunity for research on a topic of great importance for both programs. Other targeted GeoPRISMS investigations would form part of an overall, amphibious, GeoPRISMS and EarthScope research program.

The Alaska Volcano Observatory monitors active volcanoes, assesses the volcanic hazards along the Aleutian arc, and operates seismic networks on 31 of the active volcanoes. John Power, AVO scientist-in-charge, voiced strong support for GeoPRISMS studies. Existing seismic monitoring, geologic mapping, and geodetic monitoring will provide a wealth of background data for focused volcano research. Moreover, AVO is familiar with on-land access and logistical issues in the Aleutians, and they are willing to help provide guidance for involved researchers.

The far western Aleutian area (including the Komandorsky Islands and adjacent Kamchatka Peninsula) is the focus of ongoing work under the German-Russian KALMAR project, which will complement work in GeoPRISMS focus areas further east. Work completed under the first four years of KALMAR focused on several key GeoPRISM themes, including quantifying the volatile flux from active arc volcanoes in the Central Kamchatka Depression, and geochemical and geochronological studies aimed at an improved understanding of the magmatic history and evolution of island arc crust beneath the Komandorsky Islands. KALMAR dredging efforts sampled the incoming plate and fore-arc areas in front of the Komandorsky Islands, and large relict structures in back-arc areas. The prospect for a second four-year phase of the KALMAR project creates a strong international synergy between KALMAR and GeoPRISMS.

Possible international collaboration on the geophysical transect was also discussed, with JAMSTEC indicating strong support.

Broader Impacts

Unquestionably, GeoPRISMS and related studies in Alaska-Aleutian subduction zone have vital societal relevance, in a setting in which geohazards are very visible. The largest US subduction earthquake on record, the M 9.2 1964 Prince William Sound event, ruptured the eastern portion of the subduction megathrust, an area that continues to pose significant seismic hazard for local populations. Tsunamis spawned by large earthquakes and landslides along the Alaska-Aleutian subduction zone can affect the entire Pacific basin. The Aleutian arc is among the most active volcanic regions on the planet, with the potential to disrupt a critical air transport pathway between Asia, North America, and Europe.

The high visibility of geohazards in this setting also offers critical educational and outreach opportunities to GeoPRISMS. Established pathways exist through GeoPRISMS and EarthScope to convey important GeoPRISMS research results in Alaska into college classrooms around the country. Involving nearby schools and communities in instrument deployment and data collection has also proven effective. Efforts to develop a GeoPRISMS REU program would enable new training opportunities for future scientists interested in Alaskan studies. Cooperation with existing statewide programs will provide further outreach as research ramps up in the Alaska Primary Site.

Concluding Thoughts

The conveners thank the meeting attendees for their participation in the process of reaching consensus on the GeoPRISMS science plan for Alaska, and give special thanks to all of the speakers, breakout group leaders, and white paper authors for their contributions in making the workshop such a success. Finally, they want to recognize the enthusiastic participation of the graduate students and post-docs – their input is greatly appreciated.

A number of important tasks lie ahead. The conveners and breakout leaders will prepare a comprehensive workshop report for distribution by November 2011, and an updated draft of the GeoPRISMS Alaska science implementation plan by January 2012. The implementation plan will be made available for public comment prior to final release. It will serve as a guide for proposals submitted for the next NSF GeoPRISMS solicitation, July 1, 2012

 Reference information
REPORT: GeoPRISMS-EarthScope Planning Workshop for Alaska – an SCD Primary Site, Freymueller J. et al;

GeoPRISMS Newsletter, Issue No. 27, Fall 2011. Retrieved from http://geoprisms.nineplanetsllc.com

Bathymetric Surveys of the Cascadia Subduction Zone in Support of the Cascadia Initiative OBS Array Deployment

Chris Goldfinger, Oregon State University

 

As part of the 2009 Stimulus or ARRA (American Recovery and Reinvestment Act) spending, NSF’s Earth Sciences (EAR) and Ocean Sciences (OCE) divisions each received $5M in facility-related investment. The funds were targeted toward the creation of an Amphibious Array Facility to support EarthScope and MARGINS (now GeoPRISMS) science objectives. The initial emphasis and deployment site was onshore/offshore studies of the Cascadia margin, with an expectation that the facility would later move to other locations.

At the October 2010 CEIT OBS workshop held in Portland OR, a number or practical issues were raised in conjunction with this ambitious OBS deployment. One of these was the issue of good bathymetric data needed for good siting and safety of the instruments along the Cascadia margin. In addition, Cascadia is also now the site of the main components of the Ocean Observing Initiative (OOI) and Neptune Canada, the world’s premier cabled observatory systems, and these will be in operation collecting real-time data from a wide spectrum of sensors for decades to come. Cascadia has also now been chosen and a Focus Site for the NSF GeoPRISMS program, which will focus attention on Cascadia earthquake tectonics for the next decade.

Figure 1. “Before” Bathymetric coverage of the Washington-Vancouver Island margin. Translucent overlay shows existing multibeam coverage from most known cruises mostly collected by OSU and restricted until recently. Remaining areas are only covered by sparse NOS soundings. Yellow dots are approximate OBS deployment locations for year one. Some of these were re-located to better sites in terms of trawl protection and topography after the 2011 cruise data were processed.

Figure 1. “Before” Bathymetric coverage of the Washington-Vancouver Island margin. Translucent overlay shows existing multibeam coverage from most known cruises mostly collected by OSU and restricted until recently. Remaining areas are only covered by sparse NOS soundings. Yellow dots are approximate OBS deployment locations for year one. Some of these were re-located to better sites in terms of trawl protection and topography after the 2011 cruise data were processed.

Bathymetric data for the OBS deployments

Good bathymetric data are essential for siting OBS station locations and ensuring the best chance of the instruments settling to the bottom in relatively smooth flat areas with a good chance of good recording fidelity. Equally important is to deploy instruments in areas where the topographic, structural, and hydrologic context is reasonably well understood, so that a maximum number of instruments will be recovered from each deployment. Some of the CI deployed instruments will be located on the abyssal plain of the Juan de Fuca plate, and are relatively safe from local geohazards. However, this initiative specifically addresses the Cascadia subduction boundary, and thus most of the instrument deployments are on the continental margin of this very dynamic plate boundary (Figure 1). Because many of the deployments span a number of active canyon systems and seismically active areas, good bathymetric data are also required to prevent a number of instruments from being swept into channels and canyons and lost or damaged during the deployment.

Current state of Cascadia bathymetric coverage

In the NE pacific region, the Cascadia margin and Cascadia basin has spotty coverage of modern multibeam data. 1980’s vintage EEZ survey data cover Oregon and Northern California from ≈600 m to the abyssal plain out to 1260W. This includes a major survey of the Gorda Plate done during an extended sea trial of the then new AGOR Ronald H. Brown. Much of the ridge system at The Gorda, Juan de Fuca and Explorer ridges and Blanco, Mendocino and the Nootka faults have multibeam data collected over the years during NSF and NOAA sponsored work. The EEZ data were collected in the 1980’s with the original 90 degree Seabeam Classic, now quite antiquated, but adequate for regional context. There are also very large gaps in even these data. The EEZ data collected in the 1980s off Washington was first “classified” by the Navy due to their submarine activity there, and then subsequently “lost” while in storage. Consequently, comprehensive coverage of the Washington margin has not been available. Through the 90’s and early 2000’s, OSU collected data on the WA margin in support of paleoseismic and other cruises, and there were also several cruises of the German vessel Sonne. These data were collected in mission specific areas, and with a variety of systems, and therefore not ideal for regional coverage. Releases of these data were for a time also restricted by the Navy, but since 2008 the data are no longer restricted, nor is collection of new data by academic and Government agencies. These older data were collected with now antiquated systems, including the original Krupp Atlas Hydrosweep and SeaBeam classic and SeaBeam 2000 systems. Shallow data, less than ≈600 m, are even more sparse. Some of the shallow banks in Oregon have been mapped with high resolution systems, and several other NOAA, NSF, MMS and state supported projects have mapped small portions of the shelf, but an estimated 75% of waters shallower than 600m remain unmapped.

Figure 2. “After” Bathymetric coverage of the Washington-Vancouver Island margin following the June 2011 CEIT cruise with modified OBS locations. New and old data have been assembled in CARIS bathymetric database software.

Figure 2. “After” Bathymetric coverage of the Washington-Vancouver Island margin following the June 2011 CEIT cruise with modified OBS locations. New and old data have been assembled in CARIS bathymetric database software.

The need for new bathymetric data

At a minimum, small patches of bathymetric data are desirable for site location of Cascadia margin OBS sites, where even the location of major canyon systems is only approximately known (Washington and Vancouver Island). Minimal patch size should be at least a few km2 in order to ensure safe and effective deployments. A better approach however would be to survey contiguous larger areas that serve to aid in deployments, but also to establish the structural context of the deployments and of the Cascadia margin, which was the approach supported for this project.
Beyond the CIET OBS deployment, this project took the opportunity to, for the first time, obtain a nearly complete image of the Cascadia Subduction Zone that will make significant steps toward filling gaps in our knowledge of the regional tectonics of the Cascadia margin. The project is supportive of the goals of the CIET OBS array, the OOI and GeoPRISMS objectives, and also directly addresses the issue of regional earthquake hazards. The ideal vessel for this purpose was the R/V Thomas Thompson, with its newly upgraded Kongsberg EM-302 multibeam system, with the best resolution for the ≈1000-2500 m target depth range for much of the survey. This vessel, working at survey speeds can also collect concurrent 3.5 kHz Chirp sub-bottom data during the survey work, which is also useful for deployment assessment and many other purposes.

In planning the cruise, we prioritized the most hazardous sites on the margin, and also prioritized completion of margin coverage that is useful for OOI, GeoPRISMS, and regional context. Most mid-plate sites are relatively near existing coverage, but will be surveyed on future cruises if possible before deployment (Figure 1).

The priority survey area on the Washington margin is shown in Figure 1 with existing multibeam coverage along with approximate OBS site locations. Final site choices have and will take advantage of less hazardous sites than shown in Figure 1, as well as trawl closures and the existing Olympic Coast Marine Sanctuary to avoid natural and anthropogenic hazards.

Figure 2 shows the “after” picture of the Washington-Vancouver Island margin. Older bathymetric data have been combined with new data in CARIS in this image. Ongoing work will attempt to reconcile the several generations of sonars used, tide, velocity and other corrections as well as to edit and improve the older data where possible to produce a final surface integrated with existing soundings where multibeam data are still lacking. This will also be done for the Oregon-Northern California parts of the margin where less extensive new data were also collected to fill smaller gaps. A final compilation of the Cascadia margin will be made available to the community when complete in 2012

 Reference information
Bathymetric Surveys of the Cascadia Subduction Zone in Support of the Cascadia Initiative OBS Array Deployment, Goldfinger C.;

GeoPRISMS Newsletter, Issue No. 27, Fall 2011. Retrieved from http://geoprisms.nineplanetsllc.com

Status of the Ocean Bottom Seismology Component of the Cascadia Initiative

By the Cascadia Initiative Expedition Team (CIET)

Figure 1. Oblique shaded relief map showing the Cascadia Array.::The four-year deployments plan of the OBS array, cabled networks associated with NEPTUNE Canada and OOI, earthquake distributions, oceanic spreading centers, and transform faults are all shown.

Figure 1. Oblique shaded relief map showing the Cascadia Array.::The four-year deployments plan of the OBS array, cabled networks associated with NEPTUNE Canada and OOI, earthquake distributions, oceanic spreading centers, and transform faults are all shown.

The Cascadia Initiative (CI) is an onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes to volcanic arc structure to the formation, deformation and hydration of the Juan de Fuca and Gorda plates. This diverse set of objectives are all components of understanding the overall subduction zone system and require an array that provides high quality data that crosses the shoreline and encompasses relevant plate boundaries. An article in the previous GeoPRISMS Newsletter (Spring 2011, issue No. 26) described CI scientific objectives, the outcome of an open community workshop held in October 2010 to develop deployment plans for the offshore component of the experiment, and formation of the Cascadia Initiative Expedition Team (CIET). This article provides an update of CIET activities including the first year of CI OBS deployments (summer 2011) and related Education and Outreach (E&O) efforts.

Over its planned 4-year data acquisition period, the offshore portion of the Cascadia Initiative will involve the deployment and recovery of ~280 OBSs at ~160 different sites and a total of about 14 cruises. Each OBS deployment site requires careful evaluation to ensure that the notional deployment plans developed at the 2010 CI workshop take into consideration local bathymetry, trawling hazards and the presence of fragile ecosystems. The CIET incorporates this information into a detailed deployment plan that includes a prioritized deployment schedule. It is anticipated that the adjustments to most deployment sites will be minor (e.g., small changes in drop coordinates to avoid geological hazards or take advantage of preexisting multibeam bathymetry). However, practical considerations may require some larger changes to the notional plans developed at the 2010 CI workshop in which case the CIET has developed a procedure for revisions, as described on the CIET web site. Scientific oversight is required at sea to ensure that operation decisions driven by instrument failures, bad weather or other factors are guided by the scientific objectives of the experiment. A detailed cruise report will be produced for each cruise to fully document the experiment.

The CIET has been actively discussing and planning the 2011 deployments for several months. Since we are geographically distributed and our schedules are often conflicting we use a variety of communication tools. These include regular emails, a CIET web site that provides wiki capabilities, and bi-weekly conference calls with minutes and action items posted to the CIET site. The CIET website is currently used by the group for communication, discussion and limited data exchange. Much of the website content is viewable by the community. Looking toward the future, the CIET site will also be used for education and outreach, communication with the scientific community, and development and delivery of metadata pertaining to OBS deployments and recoveries (e.g., cruise reports). The CIET will hold its first face-to-face meeting this fall in Seattle on September 13 and 14.

Figure 2: Year 1 Deployment Plan.::Red circles denote the reference array. Yellow circles denote the Regional Array. Yellow squares denote the Focused Array. Yellow diamonds denote the densified coverage of the forearc enabled by requesting 10 additional instruments from the OBSIP pool. Black circles denote on land broadband seismometers. Red squares denote the NEPTUNE Canada seismometers. Blue lines denote slap depth contours (every 10 km). The 1000 m bathymetry contour is shown in bold. See 2010 CI Workshop Report for further descriptions.

Figure 2: Year 1 Deployment Plan.::Red circles denote the reference array. Yellow circles denote the Regional Array. Yellow squares denote the Focused Array. Yellow diamonds denote the densified coverage of the forearc enabled by requesting 10 additional instruments from the OBSIP pool. Black circles denote on land broadband seismometers. Red squares denote the NEPTUNE Canada seismometers. Blue lines denote slap depth contours (every 10 km). The 1000 m bathymetry contour is shown in bold. See 2010 CI Workshop Report for further descriptions.

2011 FIELD SEASON

In accord with the deployment plan developed at the CI Workshop, the CIET proposed to NSF to deploy 70 OBSs during the 2011 field season according to the Year 1 plan (Fig 1). All of the OBS deployments will be done from the R/V Wecoma. Given the limited deck space on this ship, 3 cruises will be required to deploy all 70 OBS. The cruise schedule and chief scientists for 2011 operations are as follows:

  • Leg 1, July 23 – August 2, 2011. Chief Scientists: Maya Tolstoy (LDEO), Anne Trehu (OSU)
  • Leg 2, October 15-29, 2011. Chief Scientists: Robert Dziak (OSU), Del Bohnenstiehl (NCSU)
  • Leg 3, October 30 – November 12, 2011. Chief Scientists: John Collins (WHOI), Emilie Hooft (UO)

CI Leg 1 – W1107A: The first OBS deployment cruise for the Cascadia Initiative took place between July 23rd and August 2nd 2011 aboard the Research Vessel Wecoma. The cruise deployed 15 of the newly designed LDEO-OBSIP Trawl Resistant Mounted OBSs or TRM-OBSs; the original goal was to deploy 20 TRM-OBSs, however, 5 were not fully built. These instruments are designed to provide a shield around the seismometer that should both reduce current noise and provide some protection from the bottom trawl fishing that occurs along the Cascadia margin. The instruments were therefore targeted for deployment at shallow sites (<1000 m) where trawling and currents are most likely to be an issue. The TRM-OBSs contain a Trillium compact seismometer as well as a Paros Instruments Absolute Pressure Gauge (APG), which should be useful in reducing long period noise and in measuring seafloor deformation. The TRM-OBSs will record continuously at 125 samples/sec until they are recovered in early summer 2012 using either an attached pop-up buoy system (instruments < 200 m water depth) or a Remotely Operated Vehicle (ROV).

The TRM-OBSs were largely used to fill the sites of the northern focused array that were < 1000 m depth because of the advantage of having a tight array of APGs. The CIET and the Amphibious Array Steering Committee (AASC) provided a prioritized list of sites prior to the cruise to enable the co-chief scientists to adapt the deployment pattern as necessary while at sea. Individual site locations were adjusted based on feedback from the chair of the Oregon Fisherman’s Cable Committee (OFCC), and different iterations of site locations were identified with the letters B, C and D appended to the site name and number. It is important for both the instruments and the safety of the fishing community and their equipment that regularly fished sites be avoided.
One site was inadvertently deployed at a heavily fished site, and attempts to recover it using the attached pop-up buoy failed. The instrument ended up upside down, which was a serious hazard to the fishing community. This instrument was recovered on August 14th by John Delaney and Deborah Kelley (UW), who generously took time out of their Visions11 cruise to pick up the TRM-OBS using ROV Ropos aboard the R/V Thompson. A video of the recovery can be seen online.

Two community college students were able to join the cruise as part of a summer enrichment program designed by Dean Livelybrooks (Univ. of Oregon) to introduce community college students to science. A PhD graduate student and an undergraduate IRIS summer intern also participated. Having a full complement of watchstanders and 2 co-PIs enabled us to operate research around the clock. While the OBS team slept, 4 & 12 kHz surveys were conducted to image bubbles in the water column from cold seeps in the region.

This cruise was the first full deployment of this new OBS design, and as such, much was learned. You can read a detailed account of the cruise activities in the cruise report which can be found at the CIET website. We are grateful to the Captain and Crew of the R/V Wecoma and the science party, who all worked extremely hard to make the cruise as successful as it was.

Figure 3. An LDEO-TRM OBS being deployed aboard the R/V Wecoma in July 2011.::The white octagonal frame is designed to sink a few inches into the sediment and shield the seismometer from current noise and trawl fishing.

Figure 3. An LDEO-TRM OBS being deployed aboard the R/V Wecoma in July 2011.::The white octagonal frame is designed to sink a few inches into the sediment and shield the seismometer from current noise and trawl fishing.

CI Leg 2: The second CI leg will take place from October 15-29, 2011 aboard the R/V Wecoma, leaving from and returning to Newport, OR. This expedition will deploy 25 OBSs, 15 from SIO and 10 from LDEO. The fifteen SIO OBSs will also be installed in trawl-resistant enclosures and are equipped with Differential Pressure Gauges (DPGs); these instruments are deployable at depths extending from the shelf down to 6,000 m. The remaining ten LDEO instruments are not in trawl resistant enclosures and so must be deployed below 1,000 m; they do carry APGs.

CI Leg 3: The third CI leg will take place from October 30 – November 12, 2011 aboard the R/V Wecoma, leaving from and returning to Newport, OR. This expedition will deploy 25 OBSs, 15 from WHOI and 10 from the OBSIP pool. All these OBSs are not trawl resistant and will be deployed at depths deeper than 1000 m; they carry DPGs.

CI EDUCATION AND OUTREACH

The Cascadia Initiative Education and Outreach (E&O) program is developing two opportunities during its first year, led by Dean Livelybrooks (UO):

1. The ‘CC@sea’ project supports community college (‘CC’) student participation in OBS deployment, retrieval and pre-cruise and follow-up outreach activities in CCs, high schools and the community. CC@sea> leverages another NSF (STEP) program, Undergraduate Catalytic Outreach and Research Experiences (UCORE) that has built strong ties with six Oregon community colleges. Two community college (‘CC’) students participated in the 23-July to 2-August OBS deployment cruise of the R/V Wecoma. The Fellows stood watch, helped with instrument deployment and made movies of all aspects of sea-going research. Dean Livelybrooks also participated in this first sea-going leg to initiate and supervise these activities. The goal of the CC@sea> program is to attract students from diverse, non-traditional backgrounds to a four-year degree in physical sciences and that these students transfer their experiences to the community and their peers. CC@sea> personnel made a very entertaining and informative video suitable for other community college and high school students during the first deployment leg, which will be shown in science classes at participating UCORE campuses and elsewhere.

2. A fall planning, teacher professional development workshop for a seismometers @ schools (S@S) program, where teachers and students, with assistance, install, monitor, and interpret seismograms and characterize shaking at school sites to advocate for seismic retrofit upgrades in older schools in the Pacific NW.

Figure 4. Final deployment locations for W1107A cruise

Figure 4. Final deployment locations for W1107A cruise

CIET Membership:

Doug Toomey (Team Leader, University of Oregon), Richard Allen (University of California, Berkeley), John Collins (Woods Hole Oceanographic Institution), Bob Dziak (OSU/NOAA, Newport, OR) Emilie Hooft (University of Oregon), Dean Livelybrooks (University of Oregon) Jeff McGuire (Woods Hole Oceanographic Institution), Susan Schwartz (University of California, Santa Cruz), Maya Tolstoy (Lamont Doherty Earth Observatory), Anne Trehu (Oregon State University), William Wilcock (University of Washington)

 Reference information
Status of the Ocean Bottom Seismology Component of the Cascadia Initiative, the CIET Team (Toomey et al);

GeoPRISMS Newsletter, Issue No. 27, Fall 2011. Retrieved from http://geoprisms.nineplanetsllc.com

IODP Workshop on Using Ocean Drilling to Unlock the Secrets of Slow Slip Events

Laura Wallace, Eli Silver, Nathan Bangs, Rebecca Bell, Stuart Henrys, Joshu Mountjoy, and Ingo Pecher

Figure 1. Workshop group photo.

Figure 1. Workshop group photo.

From August 1 to 5, 2011, 70 geoscientists and student researchers from a dozen countries gathered in Gisborne, New Zealand, to discuss how scientific ocean drilling can help to elucidate the processes behind slow slip event (SSE) occurrence. Gisborne was chosen as a venue for this workshop due to its close proximity above the source area of shallow slow slip (<5-15 km depth) that occurs at the northern Hikurangi subduction margin in New Zealand.SSEs are a new class of shear slip found at subduction margins around the globe that have revealed the broad spectrum of fault slip behaviour. SSEs are widely acknowledged as one of the most exciting discoveries of the last decade in the Earth Sciences, and have implications for plate boundary processes and the seismic hazard posed by subduction megathrusts. The relatively shallow depths of subduction thrusts exhibiting SSEs in New Zealand (north Hikurangi), central Japan (Boso Peninsula), and Costa Rica (Nicoya Peninsula) (5 – 10 km below seafloor) potentially puts them within reach of IODP drilling. The possibility for direct access to these faults suggests that scientific drilling could play an important role in revealing the physical processes behind SSEs. The main goals of the workshop were to summarize critical requirements of a drilling program to discern the physical mechanisms responsible for SSE behaviour, develop strategies to achieve the scientific goals, determine what types of data are needed to develop an effective drilling program, and identifying the expertise and technologies needed to drill a SSE source area successfully. Additional geophysical experiments in support of any IODP drilling were also addressed.

Oral presentations at the Gisborne workshop were organized into thematic sessions centered around (1) observations of and theories for slow slip event occurrence, (2) lessons learned from previous IODP drilling at subduction zones, and (3) focused talks on potential slow slip drilling targets in New Zealand, Costa Rica, and central Japan. The talks were interspersed each day with breakout discussion sessions and broader group discussions. Breakout sessions over the first 2 days focused on the measurements and experiments needed to understand the origins of SSE and how these plans might be applied to potential IODP drilling projects in New Zealand, Japan, and Costa Rica. On the final day, breakout groups sat down and developed implementation plans for each location.

A number of fundamental conclusions came from the workshop: (1) further development and site characterization is needed at each of the sites to be able to effectively examine slow-slip processes along the plate interface with drilling. At each of the sites, additional data is needed to refine the locations, magnitudes, timing, slip regions, relationship to earthquakes, and cyclicity of SSEs. We concluded that we can achieve these goals with a combination of onshore geodetic and seismic experiments combined with offshore long-term deployment of ocean bottom seismographs equipped with pressure sensors to monitor vertical seafloor deformation. (2) Also critical are complementary, collocated studies for developing regional-scale characterization and development of site locations. These include (but are not limited to): structure and tectonics, physical properties, stratigraphy and lithologies, and thermal structure, using active source 2D and 3D seismic imaging and wide-angle refraction, passive source studies, heat flow surveys, and multibeam seafloor mapping. Auxiliary data are required to both help identify drilling targets and compliment borehole data and monitoring; (3) Shallow level borehole monitoring is key to address questions related to the spatial distribution of slow slip beneath offshore subduction margins, and to reveal the possible relationship between SSEs and normal seismicity, as well as discerning changes in fluid flow and geochemistry within the upper plate during the SSE cycle. Monitoring will be supported with coring and logging for ground truth and detailed characterization of lithology, stratigraphy, structure, fluids, and physical properties above the SSE source regions; (4) Drilling, logging and sampling of the SSE source area will provide the most direct information on the physical conditions (frictional properties, mineralogical composition, fluid pressure conditions, temperature, among others) that lead to and control slow slip event behaviour. Participants agreed that deep drilling of an SSE source area is within reach and is the ultimate way to solve the mystery of why SSEs occur.

Figure 2. Oblique view of the Hikurangi subduction margin, including locations of slow slip (orange shaded), the location of the workshop and fieldtrip route, and the proposed slow slip event drilling targets offshore Gisborne.

Figure 2. Oblique view of the Hikurangi subduction margin, including locations of slow slip (orange shaded), the location of the workshop and fieldtrip route, and the proposed slow slip event drilling targets offshore Gisborne.

One of the interesting discoveries during the workshop was the realization by participants that the world’s best-documented areas of shallow (<20 km depth) slow slip events in Costa Rica, central Japan and New Zealand have some striking similarities. Specifically the 3 margins include: relatively cold temperatures on the interface in the SSE source regions, similar slow slip event durations (generally ~2-3 weeks), and comparable equivalent moment magnitudes per event (Mw ~6.5). Costa Rica and north Hikurangi have further similarities in that both margins are characterized by subduction erosion, and each exhibit a regular two-year SSE recurrence interval. We expect that continued comparison and contrasting of these 3 subduction zones and their shallow slow slip event behavior begun at the workshop will lead to new insights into the mechanisms behind shallow SSEs.

The main 3-day workshop was followed by a 2-day field trip, from Gisborne to Wellington. The fieldtrip was designed to expose participants to the onshore, uplifted components of the Hikurangi forearc and provide a complete transect of Hikurangi margin active tectonics. The fieldtrip also tracked above the slow slip event source areas of the Hikurangi subduction zone, and gave participants insights into the geological and tectonic context of slow slip in the North Island.

Just after the fieldtrip, on 6-7 Aug approximately 45 of the Gisborne workshop participants met to develop the full proposals and implementation plans for a proposed project to use IODP drilling to understand slow slip event processes offshore Gisborne, at the northern Hikurangi margin. We expect that these efforts will be of interest to the GeoPRISMS community, as New Zealand has been recently selected as one of the primary focus sites for the Subduction Cycles and Deformation (SCD) program. The evolving effort towards IODP drilling at north Hikurangi may provide an important focal point for SCD research in the New Zealand region in the coming years.

The workshop was supported by funding from IODP-MI, the New Zealand Ministry of Science and Innovation, the Consortium for Ocean Leadership, and GeoPRISMS. A full report on the workshop outcomes will be developed for IODP over the next few months, and will be publicly available once it is completed.

 Reference information
IODP Workshop on Using Ocean Drilling to Unlock the Secrets of Slow Slip Events, Wallace L. et al;

GeoPRISMS Newsletter, Issue No. 27, Fall 2011. Retrieved from http://geoprisms.nineplanetsllc.com

Deep Mapping of the Megathrust on Land and at Sea around the Alaska Peninsula

Donna Shillington (Lamont-Doherty Earth Observatory at Columbia University)

Figure 1. Simplified map of the Alaska Subduction Zone, showing distribution of catalog (white dots) and notable (yellow stars) earthquakes along the margin. Red arrows indicate absolute plate motions

Figure 1. Simplified map of the Alaska Subduction Zone, showing distribution of catalog (white dots) and notable (yellow stars) earthquakes along the margin. Red arrows indicate absolute plate motions

The Mission: Mapping the Alaska Megathrust

The 2500-km-long subduction zone offshore southern Alaska regularly produces large, destructive earthquakes. One of the big conundrums about these settings is how large of an area locks up on the contact between these plates (called the ‘megathrust’) and then ruptures in earthquakes. To tackle this question, my colleagues and I collected data on land and at sea in the summer of 2011 to produce an image of the megathrust, constrain the properties of rocks around and within the megathrust and link these fault properties to the earthquake history here. Our expedition focused on a part of the subduction zone off the Alaska Peninsula that exhibits very big changes in slip behavior. Some parts of this plate boundary lock up and then rupture catastrophically in big earthquakes. In other areas, the plates appear to be smoothly sliding by each other and thus do not produce great earthquakes. The Semidi segment last ruptured in a great earthquake (magnitude 8.3) 73 years ago in 1938. This area has an estimated recurrence interval of ~50-75 years, and thus might be due to produce another big earthquake soon. However, just to the west lies the Shumagin gap, an area that has not produced a great earthquake historically. Imaging a major fault boundary that lies tens of miles under the seafloor is not an easy task, but we had exceptional tools for the job. We used the R/V Marcus G. Langseth to acquire seismic reflection data and onshore/offshore wide-angle reflection/refraction data. Sound waves generated by an array of air guns were recorded on two 8-km-long streamers, an array of ocean bottom seismometers and onshore seismometers.

Figure 2. Katie Keranen and Guy Tytgat deploying a seismometer in Port Heiden

Figure 2. Katie Keranen and Guy Tytgat deploying a seismometer in Port Heiden

June 17-24: Installing seismic stations on the Alaska Peninsula

The first component of our program involved deploying seismometers onshore around the Alaska Peninsula with Katie Keranen (Univ. OK) and Guy Tytgat (PASSCAL). These instruments recorded small, local earthquakes, distant large earthquakes and (importantly for our project) the sound source of the R/V Langseth. The Alaska Peninsula is too rugged and expansive for a network of roads, so planes, helicopters or boats are the only transportation options. We decided to charter a plane based in Nelson Lagoon, a town of 80 people situated on a long, narrow sandy spit jutting out into the Bering Sea. The weather dictates when and where you can fly each day, and it varies dramatically. We were lucky enough to have several clear days (even saw some blue skies and sunshine!), but other days we were grounded by weather and wiled away the time indoors at our inn. While we were in the air, we saw majestic, snow capped volcanoes shrouded in clouds, expansive views of the sparsely vegetated Alaska Peninsula, which is riddled with rivers and lakes, and lots of wild life: caribou, bears, seals, walruses and eagles (just to name a few). It is a landscape that seems remarkably untouched by humanity.

Local communities were unwaveringly helpful and friendly in helping us find places for our stations. The two school districts here kindly granted us permission to install our seismic stations at any of their schools, and we also obtained permission to place equipment at various lodges and village offices. Residents volunteered to take our gear and us from the airstrip to our sites. In one town, our pilot made a general plea over the radio: “Is anyone listening on Channel 3? I’m here at the airstrip with scientists who need a ride to the school”. Someone answered immediately and picked us up 5 minutes later.

Many of our sites are in spectacular places near remote lodges or in towns nestled between mountains and the ocean. All of them are home to impressive wild life that poses a risk to our equipment, particularly bears. We protected the equipment against curious small animals but fully bear-proofing a station for a short (two-month-long) deployment was not feasible. Instead, we hoped that placing our stations in villages (rather than in the wild) would provide some protection, but we also needed good luck…

Figure 3. The R/V Langseth in port of Kodiak, with snowy mountains in the background

Figure 3. The R/V Langseth in port of Kodiak, with snowy mountains in the background

June 24-29: Transitioning from land to sea

Seven days and eleven flights after we arrived in Alaska, we finished deploying our seismic stations onshore. Our final constellation of stations differed a little from our original plan (as they always do), but achieved our main goal of instrumenting the part of the Alaska Peninsula nearest to our planned offshore work on the R/V Langseth. As luck would have it, we finished deploying our seismometers just in time to catch a large earthquake (magnitude 7.4) that occurred farther west in the Aleutians around the Fox Islands. After the onshore work was finished, Katie and Guy departed for home, and I flew to Kodiak to meet the R/V Langseth and our shipboard science party, including other chief scientists Mladen Nedimović (Dalhousie) and Spahr Webb (LDEO). Kodiak offered beautiful sights, delicious seafood and local beer (including Sarah Pale Ale!), but our science party was eager to leave for sea. We departed Kodiak on a sunny evening on June 29 for our 38-day-long research cruise.

June 29-July 11: Deploying and retrieving ocean bottom seismometers

The next part of our program involved using ocean bottom seismometers (OBS) to record seismic waves generated by the sound source of the Langseth. OBS’s are autonomous instruments that sit on the seafloor and record sounds waves traveling through the earth and the water. Floats made from glass balls and syntactic foam make each OBS buoyant, but an anchor holds it on the seafloor during the study. We placed OBS’s from Scripps Institution of Oceanography on the seafloor along two lines extending across the major offshore fault zone. The larger the distance between the sound source (earthquakes or air guns) and the seismometer, the deeper into the earth the recorded sound waves travel. OBS are not attached to the vessel and are also very sensitive, so they can record sound waves generated very far away (commonly >200 km). Because we want to examine deep fault zones that cause large earthquakes off Alaska, OBS are a critical part of our effort.

To deploy the OBS, we simply lifted them over the side of the ship with a large crane and gently dropped them in the water, after which they slowly sank to the seafloor. It never ceases to amaze me that we can throw a bundle of very sophisticated electronics over the side of the ship and hope to pick it up and retrieve information from it. Yet, it works! After leaving OBS on the seafloor along each line for ~3 days to record sound waves generated by the air guns of the Langseth, we returned to collect them. After receiving an acoustic signal to release from its anchor, the OBS rises through the water at 45 meters per minute. When the water is deep, it can be a long wait. Some of ours were 5500 m below the surface! The recovery of OBS always involves a certain amount of suspense. Despite all of the advanced engineering and planning that goes into these instruments, it is an inherently risky endeavor. Happily, we recovered 100% of our OBS.

Figure 4. Deploying an OBS

Figure 4. Deploying an OBS

Despite all the technology required to place a seismometer many miles below the ocean on the seafloor and summon it back to the surface, many aspects of actually plucking the OBS out of the ocean and pulling it on deck are remarkably low tech. Once the OBS is spotted floating on the surface, the ship drives along side. It is akin to driving your car up next to a ping-pong ball. Scientists and techs lean over the starboard side of the Langseth with large poles and attempt to attach a hook with rope to the top of the OBS. Its not always easy since the OBS is bobbing up and down on the waves. Once we hook it, we can attach a rope to the wench and haul the OBS onboard. Sometimes, OBS bring back surprises – an octopus returned with one of our OBS! He was alive and healthy, so we returned him to the ocean (though some lobbied that we keep him for lunch…)

July 11-August 5: Seismic reflection profiling with miles and miles of streamer

On July 11, we finished our OBS work, and began the second phase of the cruise: recording sound waves from the Langseth’s airgun array with two 8-km-long (5-mile-long) cables (or streamers) filled with pressure sensors. Changing gears in terms of scientific activities also involved changes to our science party; we swapped personnel by boat transfer in Sand Point on a beautiful sunny evening. The Scripps OBS team departed, and we were joined by new reinforcements, including five undergraduate students from Columbia University.

Our seismic streamers are stored on gigantic spools, which unreel cable off the back of the ship into the ocean. A large buoy is affixed to the end of the streamer, and ‘birds’ are attached along its length, which can be used to control the depth of the streamer. Large paravanes hold the streamers apart; these are like large kites flying off the back of the ship in the water. Deploying miles of streamer and the other attending gear is an impressively long and complicated undertaking, which also involves a fair amount of intense manual labor. But after 3 days, all of the gear was in the water. Once data acquisition began, we settled into a routine of watchstanding and standard shipboard data processing. Ship time is precious, so we collect data 24 hours a day, seven days a week.
One of the core objectives of our project is to image the deep parts of the plate tectonic boundary, which required us to go as far north (and as close to the coast) as possible. This was easier said that done! The southern edge of the Alaska Peninsula is rugged and flanked lots of small jagged islands and shallow features just below the surface of the ocean, and there is also more fishing activity close to the coast; both pose risks to the seismic gear. One of our closest approaches to land was near Unga, one of the Shumagin islands. At the apex of the turn, our streamers came within less than a mile of the coast. Due to some early difficulties with our equipment, we had to repeat this maneuver several times. I held my breath and watched our third (and final) pass from the bridge. After the ship and gear passed safely through the most harrowing part of the turn, the captain turned to me and asked, “We’re not going to do this again, are we?” Thankfully not! At least not there. But there were several other important parts of our survey that required close approaches to the coast to image critical parts of the boundary.

Figure 5. Watch-standers at work in the lab

Figure 5. Watch-standers at work in the lab

Over the course of our cruise, we were treated to amazing views of marine life, including fish, whales, seals and birds. On one memorable day, we found ourselves surrounded by three species of whales, including a rare North Pacific Right Whale. But we tried to keep our distance from marine mammals. Since we are creating sound waves to image the earth, and they use sound to navigate and communicate with one another, our activities might disturb them; we suspended operations if a mammal came too close.

We used our new data to create very preliminary images of the structures below the seafloor as we went. A regular sight in the main lab was a group of people gathered around a computer screen or a large paper plot, talking and pointing excitedly. It was exhilarating to glimpse faults, sediments and other structures in our data for the first time and ponder what they might be telling us about this active plate tectonic boundary. But we have a lot of hard work ahead after the cruise to obtain concrete results from our voluminous data – we acquired over 3 Tb (3000 gigabytes!) of raw seismic data during the cruise! At 6:30 am on August 5, the R/V Langseth pulled into port in Dutch Harbor, marking the end of our very successful research cruise. Our steam into port from our study area involved a trip through Unimak pass and beautiful views of Aleutian volcanoes, including majestic Shishaldin.

Figure 7. Bears at Bear Lake, Alaska

Figure 7. Bears at Bear Lake, Alaska

August 5-10: Back to the Alaska Peninsula

Many people flew home after our arrival in Dutch Harbor, but not me! (At least not immediately). Katie Keranen and I returned to the rugged Alaska peninsula to recover the land seismometers that we deployed way back at the beginning of the summer. An Anchorage-bound flight from Dutch Harbor dropped me off in Cold Bay to rendezvous with Katie. After the plane landed, the stewardess asked for our “Cold Bay passenger” to disembark. Passenger. Singular. I filed past all the folks heading to Anchorage and beyond. Katie and I returned to all of our sites by charter plane. According to our pilot, it was a very foggy summer on the Alaska Peninsula, but we were blessed with excellent weather, allowing us to pick up all of our instruments in just a day and a half. Multiple attempts were required to recover a seismometers we placed Heredeen Bay; on the first try, we saw a large brown bear only 20 feet away from the plane! But to our delight, none of the stations had been disturbed by wild life, and all of them recorded data for the entire summer. After recovering our last station at Bear Lake, we rewarded ourselves by lingering at beautiful lodge there. We tried (unsuccessfully) to catch some fish and watched bears pick through the brush on the other side of the river. And after an amazing 55 days on and around the spectacular Alaska Peninsula, I happily headed back to NYC.

Special thanks to the following:

Onshore Science Party: Katie Keranen (Univ. Oklahoma), Donna Shillington (LDEO), Guy Tytgat (Passcal Instrument Center)

Offshore Science Party: Donna Shillington (LDEO), Mladen Nedimovic’ (Dalhousie University), Spahr Webb (Lamont), along with Ann Bécel, Matthias Deleschluse, Harold Kuehn, Jiyao Li, Berta Biescas, Aaron Farkas, Andrew Wessbecher, Celia Eddy, Kelly Hostetler, Hannah Perls, Jack Zietman

Figure6_field_Alaska_Peninsula

Figure 6. Donna and Katie on their way to another station.

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.nineplanetsllc.com. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

 Reference information
Deep Mapping of the Megathrust on Land and at Sea around the Alaska Peninsula, Shillington D.;
GeoPRISMS Newsletter, Issue No. 27, Fall 2011. Retrieved from http://geoprisms.nineplanetsllc.com

Vignettes from the Salton Seismic Imaging Project: Student Field Work Experiences

Kathy Davenport (Virginia Tech) and members of the SSIP field crew

Figure 1. SSIP Project map. Red lines are faults; symbols (see index) are seismic sources or seismographs.

Figure 1. SSIP Project map. Red lines are faults; symbols (see index) are seismic sources or seismographs.

In early 2011, the Salton Seismic Imaging Project (SSIP) descended on Southern California. The Salton Trough was part of the Gulf of California focus area for MARGINS, and processes in this setting also address issues of rift initiation and evolution (RIE) important to GeoPRISMS. Over the course of three weeks, we acquired refraction and low-fold reflection seismic data along 7 lines totaling over 750 km, two 3D grids, and an offshore array. About 130 people participated in the data acquisition, including students from 31 different colleges and universities. During this time, 126 shots were fired, totaling 33,329 kg of explosives, and a 3.4-liter GI airgun was fired 2330 times in the Salton Sea. These sources were recorded on land on 2595 single-component seismographs and186 three-component seismographs at 4235 unique sites, as well as 48 three-component ocean bottom seismographs at 78 sites in the Salton Sea. A 42 station broadband deployment was also live during this time. We deployed instruments in sand dunes and snow, on bombing ranges and golf courses, beneath windmills and Joshua trees. We hiked through mesquite, avoided cactus and endangered lizards, and endured the stench of the Salton Sea. It took the best efforts of all the people involved to accomplish this massive data acquisition in the Salton Trough!

On January 23, Steve Skinner and I went to survey station locations along the San Andreas Fault east of Mecca. In this area of the desert few people have passed, so there are very few roads. We drove through washes and desert, looking for the easiest paths possible to reach our tentative waypoints. Jack rabbits and lizards tried to run away from us. When we finally stepped on the fault, with one foot on the Pacific Plate and the other on the North America Plate, looking at Salton Sea and the sunset, at that moment I felt that I was a real geologist.Liang Han, Virginia Tech. January 23, 2011

The Salton Trough is a prime target for investigating rift initiation and evolution and earthquake hazards because it is the northernmost extent of the Gulf of California extensional province. The San Andreas Fault ends in southern California, and strike-slip plate motion is transferred to the Imperial Fault. This step-over created the Salton Trough, a basin extending from Palm Springs to the Gulf of California. Previous studies suggest that North American lithosphere has rifted completely in the central Salton Trough. However, rifting here has been strongly affected by rapid sedimentation from the Colorado River, preventing the onset of seafloor spreading as has occurred in the southern Gulf of California. The 20-25 km thick crust in the central Salton Trough apparently is composed entirely of new crust created by magmatism from below and sedimentation from above. Between the major transform faults, active rifting is manifested by faults observed in modern sediment, abundant seismicity, minor volcanism, very high heat flow, and corresponding geothermal energy production.

Figure 2. Shot gather. The 911 kg shot was at the Imperial Fault. The 1142 seismograms (from Texans, plus vertical components from RT130's) were recorded along Line 2 that extends from the San Diego and Tijuana suburbs across the Peninsular Ranges, Salton Trough and Chocolate Mountains, to the Colorado River.

Figure 2. Shot gather. The 911 kg shot was at the Imperial Fault. The 1142 seismograms (from Texans, plus vertical components from RT130’s) were recorded along Line 2 that extends from the San Diego and Tijuana suburbs across the Peninsular Ranges, Salton Trough and Chocolate Mountains, to the Colorado River.

Based on the paleoseismic record, the southern San Andreas Fault is considered overdue for an earthquake of magnitude >7.5, and other nearby faults have had historic earthquakes with magnitudes >7. Earthquake hazard models and strong ground motion simulations require knowledge of the dip of the faults and the geometry and wavespeed of the adjacent sedimentary basins, but these parameters are currently poorly constrained.

SSIP ultimately will constrain the initiation and evolution of nearly complete continental rifting, including the emplacement of magmatism, effects of sedimentation upon extension and magmatism, and partitioning of strain during continental breakup. To improve earthquake hazard models, we will image the geometry of the San Andreas, Imperial and other faults, the structure of sedimentary basins in the Salton Trough, and the three-dimensional seismic wavespeed of the crust and uppermost mantle.

Constraining all these targets in the Salton Trough requires good instrument coverage in areas that are not always easily accessible. For instance, the deserts of Southern California are home to multiple military training facilities. These include the El Centro Naval Air Facility, whose bombing ranges are the winter training grounds for the Blue Angels, and the Chocolate Mountain Gunnery Range, Marine lands used for live munitions training. The Navy and Marine Corp were very accommodating to our project, providing safety training and time windows where we could safely cross the bombing ranges to deploy and pick up instruments. Of course, we had to work around the daily operations of these facilities, and that was not always easy.

Figure 3. Deploying a Texan seismograph on a wind farm near Palm Springs.

Figure 3. Deploying a Texan seismograph on a wind farm near Palm Springs.

The military assured us they had done sweep along our route so there shouldn’t be any live munitions on the ground. For safety, however, we were warned to avoid anything that appeared to be man-made. It was my role to drive into the desert, drop off the cross-country hikers, then drive around and pick them up on the other side of the bombing range. When I checked in at the operations center I was told that the Blue Angels were flying that day, and they don’t like moving objects on the ground. When I saw them I was to stop driving until they passed by. It seemed like I could drive for no more than a few minutes before the Blue Angels flew overhead and I would have to stop driving. It was pretty awesome to see them flying and executing their performance maneuvers right over our heads! As I stood by the truck awaiting the hikers, a solitary Blue Angel flew by, absolutely directly over my head. In the rush of noise and vibration of the flight, his elevation seemed like it was barely 30 meters. I decided to assume his flight path at that moment was a salute for the good work he thought we were doing.Janet Harvey, Caltech. March 2, 2011. El Centro NAF

Our access to the Chocolate Mountain marine bombing range was scheduled around daily munitions training. This meant we could only be on the range during hours when there was no chance of encountering one of the training groups, making this our earliest deployment – beginning at 3 am! We left the warehouse in El Centro hours before sunrise to give us enough time to get on and off the range before the firing started. Due to the extremely limited access, we could not survey the station locations ahead of time and instruments had to be deployed without precise GPS locations. We scurried around in the dark, planting seismometers as quickly as we could by flashlight, and left the base just as the sun came up. When we returned to retrieve the instruments we only had approximate station coordinates, so we had to scramble around, searching through the brush by flashlight for the buried instruments, with the imposing deadline of live ammunition flying through the air motivating us to find our instruments and get out by our sunrise deadline.Steve Skinner, Caltech. March 2, 2011. Chocolate Mountain Gunnery Range

Much of our work in the Imperial and Coachella Valleys was outside the urban areas and farmlands where the population is concentrated. We worked in the desert, the mountains, and on the Sea. Very often we found ourselves driving in washes or hiking because there were no roads where we needed to be. Bushwhacking, boating, and travelling cross-country led to many adventures for our deployment crews.

During surveying along Hwy 78 towards the Algodones sand dunes we chose a small, sandy side trail that was much safer than the main road. We tested the utility vans we would be using for deployment and learned that carefully driven, empty vans could successfully navigate the sandy road. Unfortunately, on deployment day I was the one driving the van loaded with instruments on this section. As we approached the dunes I saw the access to the side trail, took a deep breath, and began turning the van off the main road. 100 meters later, I learned that through either my lack of utility van experience or the weight of the fully loaded van, our test had failed… we were stuck. When we were pulled free we opted to work from the narrow shoulder on the main road. Later the trail looked more manageable, and much safer than pulling over on the half-shoulder of Hwy 78, so I gave it a second go… and 200 meters later became stuck again. After being pulled out for the second time, we finished our deployment from the main road. I would not try the van on the sandy trail again.Erin Carrick, Virginia Tech. March 1, 2011
Figure 4. Deploying an OBS into the shallow Salton Sea.

Figure 4. Deploying an OBS into the shallow Salton Sea.

The Salton Trough is often a barren and desolate place. Working on the Salton Sea, however, redefines desolate. I never saw another vessel on the water, despite a warning sign at the marina advising in case of emergency to flag down a passing boat, as there are no 911 services or coast guard rescue. We deployed our sound source and streamers off of a ~100’ barge towed behind a dual engine 40’ vessel. The water in the Sea is unbelievably hard on boat engines, precipitating salt quickly and preventing the internal cooling system from working. The Salton Sea also ‘blows out’ very quickly, going from dead calm to ocean size waves in 15 minutes. One nerve-wracking day, the water was as rough as I have ever seen it, one engine was out completely, and the other was screaming with warning sirens, close to overheating too. One may expect that this would be scary for fear of personal injury or lost data or ruined equipment, but the mind changes priorities on the Salton Sea. During the 4-hour ride back to the marina, I was only fearful of how utterly disgusting it would be to be in the water with the millions of dead tilapia. I would surely die from disgust! This particular evening, in true Salton Sea form, the water returned to glass 20 minutes out from the launch, and we enjoyed one of the most beautiful sunsets we had ever seen.Annie Kell, University of Nevada, Reno. March, 2011

The day’s assignment was to deploy two-dozen seismometers and geophones across the southern tip of the San Andreas Fault. We would drive as far as possible, and then pack in the instruments and equipment the rest of the way. Our crew had two extra members on this trip – a reporter and photographer from the Los Angeles Times. We drove into the field area on a path we blazed through the brush a month earlier. On the hike both of the media men were good sports, following us across the dry powdered mud in the heat, asking questions about regional tectonics and the SSIP experiment. After deploying the instruments we began the hike back to the vehicles along an abandoned railroad. All of a sudden we were stopped instantly in our tracks. An overwhelmingly close rattle sounded from just a few yards away and the biggest rattlesnake I have ever seen was coiled right off the tracks. We all backed away slowly. The cameraman, however, jumped into action, switching lenses and approaching the snake head-on until he was no more than a foot from its venomous fangs. Its head bobbed forward and back while he got his shots. This man who had fought in an infantry unit in Vietnam, covered troops in Iraq and Afghanistan, and won a Pulitzer Prize for following undocumented workers from Central America to the USA, had managed to find excitement and danger with a few geoscientists in the Salton Sea, California.Frank Sousa, Caltech. March 13, 2011
Figure 5. Backpacking seismographs across a Naval bombing range. Each person is carrying about 8 Texan seismographs and deployment equipment.

Figure 5. Backpacking seismographs across a Naval bombing range. Each person is carrying about 8 Texan seismographs and deployment equipment.

Onshore SSIP principal investigators are John Hole (Virginia Tech), Joann Stock (Caltech), and Gary Fuis (USGS, Menlo Park), working with Mexican collaborators Antonio Gonzalez-Fernandez (CICESE) and Octavio Lazaro-Mancilla (Univ. Autonoma de Baja California). The onshore work was funded by the NSF MARGINS Program (GeoPRISMS predecessor), the NSF EarthScope Program, and the USGS MultiHazards Program. The marine component, Wet-SSIP, is funded by an NSF Marine Geology and Geophysics Program grant to Neal Driscoll and Alistair Harding (Scripps Inst. Oceanography) and Graham Kent (Univ. Nevada, Reno). Broadband-SSIP is led by Simon Klemperer (Stanford Univ.) with funding from the NSF Geophysics Program. Onshore seismometers were provided by the EarthScope FlexArray and IRIS PASSCAL instrument pools with field support from PASSCAL. The OBSs were supplied by the OBSIP.

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.nineplanetsllc.com. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

Reference information
Vignettes from the Salton Seismic Imaging Project: Student Field Work Experiences, Davenport, K., and members of the SSIP field crew;

GeoPRISMS Newsletter, Issue No. 28, Spring 2012. Retrieved from http://geoprisms.nineplanetsllc.com

Report on GeoPRISMS Mini-Workshop at Fall 2011 AGU – “ExTerra: Understanding Convergent Margin Processes Through Studies of Exhumed Terranes”

AGU Fall Meeting 2011, San Francisco

M. Feineman1, S. Penniston-Dorland2, B. Savage3

1Pennsylvania State University; 2University of Maryland; 3University of Rhode Island

Figure 1. GeoPRISMS ExTerra mini-workshop participants.

Figure 1. GeoPRISMS ExTerra mini-workshop participants.

On the evening of December 7, 2011, about 35 geoscientists convened in the ExTerra mini-workshop during the fall AGU Meeting to discuss how to integrate the study of exhumed rocks into the GeoPRISMS Subduction Cycles and Deformation (SCD) initiative (Figure 1). After introductory presentations by the convenors and keynote speaker Brad Hacker (University of California, Santa Barbara), workshop participants divided into four groups based on different types of exhumed terranes: subducted slab, mantle wedge, arc crust, and fault systems. The group discussion was divided into two areas: identification of scientific objectives and organizational strategies. Details of the outcomes from each discussion group are outlined at http://www.geoprisms.nineplanetsllc.com/scd/exterra.html. This is an ongoing discussion leading to a white paper contribution to the GeoPRISMS SCD Science Plan, and we invite all interested parties to participate!

What is ExTerra?

The NSF GeoPRISMS Science Plan for the SCD Initiative identified the study of exhumed terranes as an important component of subduction zone research. It remains to be determined how to best integrate the study of exhumed terranes and high pressure rocks into GeoPRISMS SCD. GeoPRISMS largely follows the very effective model used previously by MARGINS of building a research program around a few locations, referred to as primary sites, atactive subduction, these features are buried deep beneath the surface. Of necessity, exhumation most often occurs during or following the death of a subduction zone. The nature of exhumation processes is such that entire subduction zones are rarely if ever exposed in a single location, requiring fieldwork to be conducted at multiple locations, and most often by multiple research groups using different techniques and approaches, before a comprehensive range of pressure and temperature conditions can be represented. Currently, the study of exhumed terranes is included in the GeoPRISMS Implementation Plan as a thematic study. The goal of this mini-workshop and the resulting white paper is to explore how we can best organize research on exhumed terranes under the umbrella of GeoPRISMS SCD such that we might accomplish more as a group than we could as individuals working independently.

Figure 2. SOTA fieldtrip to see Cycladic subduction zone rocks on the island of Syros, Greece.

Figure 2. SOTA fieldtrip to see Cycladic subduction zone rocks on the island of Syros, Greece.

What can studies of exhumed systems contribute to GeoPRISMS?

The integration of studies of exhumed systems through GeoPRISMS can organize individual efforts towards major interdisciplinary objectives. Integration of data from multiple sites allows coverage of a broad range of conditions not observable at a single site. Studies of exhumed systems under the umbrella of GeoPRISMS have the potential to link experiments and seismic observation to physical reality, adding the components of space and time. Collaboration and communication between different communities represented within GeoPRISMS allow sample and data collection to be tuned to serve the needs of other groups (geochemists helping seismologists, petrologists helping modelers, etc.).

Figure 3. ILP Subduction channel workshop fieldtrip to the Monviso Ophiolite, W. Alps, Italy.

Figure 3. ILP Subduction channel workshop fieldtrip to the Monviso Ophiolite, W. Alps, Italy.

Target areas

Four target areas have been identified as significant to improving our understanding active subduction processes by the study of exhumed terranes: 1) subducted slab, including HP and UHP rocks such as blueschists, eclogites, and metapelites; 2) mantle wedge, including serpentinites, ophiolites, and peridotites; 3) middle and lower arc crust, including granitoids, gabbros, migmatites, gneisses, amphibolites, granulites; and 4) exhumed fault systems, including accretionary prisms.

Figure 4. AGU Fieldtrip to see subduction zone rocks of the Franciscan Complex, CA.

Figure 4. AGU Fieldtrip to see subduction zone rocks of the Franciscan Complex, CA.

Fostering Interdisciplinary Communication

Several different ideas have been suggested in order to facilitate communication among different geoscientists. One idea is to hold focused, interdisciplinary field trips in order to provide the opportunity for non-field geologists to observe exhumed rocks and create an environment for exchange of ideas between field geologists and non-field geologists. Another idea is to create a sample repository and associated database that will allow sample collectors to connect with those who have use for rock samples. For example, experimental petrologists can make use of a sample repository to find materials for their experiments.

Challenges

We recognize that there are many challenges facing the integration of the study of exhumed terranes into GeoPRISMS. How do we open the dialog between petrologists, geophysicists, and modelers? How can studies of worldwide exhumed terranes be related to current GeoPRISMS focus sites? GeoPRISMS is a small program, and we will need to leverage with funds from outside sources.

 icon-chevron-right Go to the mini-workshop webpage

Reference information

Report on GeoPRISMS Mini-Workshop at Fall 2011 AGU – “ExTerra: Understanding Convergent Margin Processes Through Studies of Exhumed Terranes”, Feineman M., Penniston-Dorland S., Savage B.;

GeoPRISMS Newsletter, Issue No. 28, Spring 2012. Retrieved from http://geoprisms.nineplanetsllc.com

Report on GeoPRISMS Mini-Workshop at Fall 2011 AGU – “Using Geoinformatics Resources to Explore the Generation of Convergent Margin Magmas”

AGU Fall Meeting 2011, San Francisco

R. Stern1, M. Feigenson2, K. Lehnert3, A. Goodwillie3, P. van Keken4, J. Kimura5, B. Dreyer6, E. Jordan1,  W. Lieu1

1University of Texas, Dallas; 2Rutgers University; 3Lamont-Doherty Earth Observatory; 4University of Michigan; 5IFREE, JAMSTEC; 6University of California, Santa Cruz

Figure 1. Dec. 2011 GeoPRISMS geoinformatics workshop participants. Front row (L to R): M. Feigenson, Warren Lieu, Erika Jordan, Peter Michael, Megan Derrico, Nick Deems, Sara Douglas, Andrew Goodwillie, Jun-ichi Kimura. Back row: Charles Bopp, Bob Stern, Kerstin Lehnert, Guillaume Girard, Tyrone Rooney, Katherina Vogt, Osamu Ishizuka, Brian Dreyer, Julia Morgan, Peter van Keken

Figure 1. Dec. 2011 GeoPRISMS geoinformatics workshop participants. Front row (L to R): M. Feigenson, Warren Lieu, Erika Jordan, Peter Michael, Megan Derrico, Nick Deems, Sara Douglas, Andrew Goodwillie, Jun-ichi Kimura. Back row: Charles Bopp, Bob Stern, Kerstin Lehnert, Guillaume Girard, Tyrone Rooney, Katherina Vogt, Osamu Ishizuka, Brian Dreyer, Julia Morgan, Peter van Keken

Twenty geoscientists attending Fall AGU forsook the chance to enjoy a beautiful Sunday in San Francisco on December 4, 2011, and chose instead to descend into the bowels of the Grand Hyatt for the chance to explore how geoinformatics can help geoscientists understand the composition and generation of convergent margin magmas. The all-day workshop was organized in support of the science goals of the GeoPRISMS Subduction Cycles and Deformation (SCD) Initiative. SCD aims to understand how subduction zones work, from cold, shallow regimes (accretionary prism, forearc crust, and the seismogenic zone) to deeper, hotter regions where fluids and melts from the subducted slab trigger melting in the convecting asthenosphere above it. SCD builds on and integrates the successes of the predecessor MARGINS Seismogenic Zone and Subduction Factory experiments, targeting the Aleutian and Cascade arcs as community-chosen focus sites. The techniques and insights developed from studies of these arcs can be applied globally, and we hope to attract more geoscientists to join the “subduction parade”. In our efforts to involve a more diverse group of geoscientists in this effort – from students to university professors to expert researchers – we need to develop better, more accessible tools for this community to use. The Geoinformatics workshop was an effort to attract new members of the GeoPRISMS SCD team and help prepare these geoscientists.

Figure 2: Dec. 2011 GeoPRISMS Geoinformatics Workshop in action. Andrew Goodwillie explains GeoMapApp to workshop participants

Figure 2. Dec. 2011 GeoPRISMS Geoinformatics Workshop in action. Andrew Goodwillie explains GeoMapApp to workshop participants

These are lofty goals that can only be realized if interested geoscientists gather to explore effective tools and how they can be used. The most important community tools are databases, data visualizations, and data analysis software. Examples of each of these were highlighted in the workshop, as is clear from the agenda below (with pertinent links and other information). Each 50 minute long session encouraged questions and comments from workshop participants and was followed by a 10-minute break that allowed folks to stretch and refuel with plenty of food and beverages provided by GeoPRISMS.

  1. Geochemical databases and how to access them – Kerstin Lehnert (LDEO) explained how geochemical databases such as PetDB, Georoc, SedDB, and NAVDAT (all of which can be accessed via Earthchem) are increasingly important aspects of teaching and research (including GeoPRISMS SCD). Kerstin also explained how the new SESAR (System for Earth Sample Registration) can resolve problems of sample ambiguity (for example: how many samples have the same ID, e.g., how many “D1” samples are there in dredge collections around the world?) and data redundancy (for example: how many samples have been analyzed multiple times for different elements and isotopes, each reported with slightly different IDs that are entered separately into one or more databases?). Kerstin emphasized how important it was that samples studied as a result of GeoPRISMS SCD should each be registered for(International Geo Sample Number), a 9-digit alphanumeric code that uniquely identifies samples and provides information about where these can be found.
  2. Data Visualization Tool: GeoMapApp – GeoMapApp is an Earth science exploration and visualization application that is maintained and improved as part of the Marine Geoscience Data System at LDEO. There are several YouTube GeoMapApp multimedia tutorials that can be accessed. Of special interest to this workshop is the fact that GeoMapApp has a new feature that allows users to determine the depth of the subducted slab beneath a given arc volcano, and one tutorial shows how to do this, using the global compilations of Syracuse and Abers (2006).
  3. Central America and Izu-Bonin-Mariana arc geochemical databases – Erika Jordan and Warren Lieu (UT Dallas). CentAm and IBM were focus sites for the MARGINS Subduction Factory experiment and geochemical data for these from EarthChem have been compiled and are being filtered so that these can be made available as an Earthchem data library. Erika summarized the status of compilations for volcanic front lavas from these two focus sites, using graphs to show their geochemical similarities and differences. Once completed, these compilations will be available to anyone as a Geochemical Reference Library. One of the important issues related to such compilations is how to best show these data. There are hundreds to thousands of data points in these compilations, so individual points on graphs often lie on top of each other and it can be difficult to see underlying structure. Warren showed how large data sets can be contoured and it is clear that making such data visualization tools available will be key for exploiting large geochemical data sets.
  4. Thermal structure of subducted slabs – Modeling the thermal structure of subduction zones and using geochemical and geophysical data to test and refine these models is leading to some of the most rewarding collaborations between geodynamic modelers, geophysicists, and geochemists. After lunch, Peter van Keken (U. Michigan) presented some of the latest thinking about how lithosphere age and convergence rate control temperatures in subduction zones. It is the slightly (~1%) more dense nature of the ~100 km thick lithosphere relative to ambient mantle that makes plates subduct, but it is the thin veneer of subducted sediments and the upper, altered portion of the oceanic crust– typically only a few tenths of a percent of everything that is subducted – that controls the subduction zone incompatible element budget. We cannot understand the trace element and isotopic composition of arc lavas without understanding how sediments are cooked in the subduction zone kitchen. Subducted sediments lie athwart a very strong temperature gradient between hot convecting asthenosphere and cool conducting lithosphere, so it is not surprising that our thinking has fluctuated between a consensus that subducted sediments beneath the arc mostly are sufficiently cool that they only release hydrous fluids to the idea that they are mostly hot enough to melt. Thermal models for subducted sediments need to be tested and refined with geophysical and geochemical data and experiments.
  5. Introduction to Arc Basalt Simulator 3.1 – Jun-ichi Kimura (JAMSTEC/IFREE) presented the theoretical underpinnings of an evolving software package for understanding arc petrogenesis, called “Arc Basalt Simulator”, or “ABS”. ABS is a forward model designed to match the incompatible trace element and radiogenic isotopic composition of primitive (high Mg#) arc lava by inputting appropriate subducted sediment and altered oceanic crust compositions and compositions of unmodified mantle wedge, choosing an appropriate subduction zone thermal model (from Syracuse et al., 2010) and adjusting some other subduction zone parameters, such as where fluids or melts are extracted from the downgoing slab and the depth of mantle melting. ABS is an Excel-based spreadsheet that can be run on any PC or Mac, so all members of the GeoPRISMS SCD community can use it. A tutorial walks the new user through the various functions of ABS 3.1. The ABS spreadsheet and ABS tutorial can be downloaded from the Geochemical Resource Library. A recent paper (Kimura et al., 2010) uses ABS version 3.1 to investigate compositions of primitive magmas along the Izu arc, and a new version (ABS 4) is being developed by Dr. Kimura and colleagues.
  6. ABS exercise – Bob Stern (UTD) and Mark Feigenson (Rutgers) provided some practical experience with ABS3.1. Bob walked workshop participants through each of the ABS3.1 user-adjustable functions, then Mark showed the group how ABS 3.1 could be used to understand the composition of primitive Cerro Negro (Nicaragua) lavas.
Fig3_geoinfo_s2012

Figure 3. Two different ways to present large compilation geochemical datasets for volcanic rocks from the magmatic fronts of MARGINS Subduction Factory focus sites in Central America (CentAm, red, n=1,058) and Izu-Bonin-Mariana (IBM, blue, n=1,439), using K2O vs. SiO2 as an example. Most Central American lavas have higher K2O contents at a given SiO2 content than most Izu-Bonin-Mariana samples. Left panel presents contoured data, right panel shows individual data points. Contoured data draws attention to where data are concentrated, plots of individual data draws attention to outliers

There are some useful lessons to be learned for others considering proposing future workshops before or after national geoscientific meetings like AGU or GSA. It is important to start planning early so that possible participants have the workshop on their “radar screens” before they buy their air tickets for the meeting, and so these societies can include the workshops on meeting announcements. In addition, workshop conveners should make a strong push to advertise the workshop at least two months before the meeting, in order to maximize workshop attendance. The conveners would be happy to discuss other considerations with people thinking about proposing a GeoPRISMS workshop.

 icon-chevron-right Go to the mini-workshop webpage

References

Kimura, J.-I., Kent, A.J.R., Rowe, M.C., Katskuse, M., Nakano, F., B. R. Hacker, P. E. van Keken, H. Kawabata, and R. J. Stern, 2010. Origin of cross-chain geochemical variation in Quaternary lavas from the northern Izu arc: Using a quantitative mass balance approach to identify mantle sources and mantle wedge processes. Geochemistry, Geophysics, Geosystems 11, Q10011, doi:10.1029/2010GC003050

Syracuse, E.M., Abers, G.A., 2006. Global compilation of variations in slab depth beneath arc volcanoes and implications. Geochem. Geophys. Geosyst. 7, Q05017, doi:10.1029/2005GC001045.

Syracuse, E. M., van Keken, P.E., and Abers, G.A., 2010. The global range of subduction zone thermal models, Physics of the Earth and Planetary Interiors 183, 73-90. doi:10.1016/j.pepi.2010.1002.1004

Reference information

Report on GeoPRISMS Mini-Workshop at Fall 2011 AGU – “Using Geoinformatics Resources to Explore the Generation of Convergent Margin Magmas”, Stern B, et al;

GeoPRISMS Newsletter, Issue No. 28, Spring 2012. Retrieved from http://geoprisms.nineplanetsllc.com

Conference Report | Magmatic Rifting and Active Volcanism, Afar Rift Consortium (Addis Ababa, Ethiopia)

Anne Egger, Tyrone Rooney1, and Donna Shillington2

1Michigan State University, 2Lamont Doherty Earth Observatory

Figure 1. Map of the Afar rift region showing major tectonic and magmatic features from Ebinger et al., 2008.

Figure 1. Map of the Afar rift region showing major tectonic and magmatic features from Ebinger et al., 2008.

Conference Overview

The Magmatic Rifting and Active Volcanism (MRAV) Conference took place in Addis Ababa, Ethiopia January 10-13, 2012, convened by members of the Afar Rift Consortium, an international team investigating active magmatism and deformation in the Afar region. Over 200 people from around the world attended. The conference participants primarily presented the results of work on ongoing rifting processes in Afar, but work was also presented that addressed other portions of the East African Rift, comparable rift settings elsewhere, rifting processes in general, and the hazards and resources associated with the East African Rift.

The scientific program outlined the current state of knowledge in the East African rift and placed recent discoveries within the broader context of rift-related research globally. Central to the meeting was the presentation of results from thematic, multi-collaborator, international programs (e.g. Afar Consortium, RiftLink, Actions Marges), individual research groups, and industrial partners. The rich detail and modern datasets presented at the meeting highlight the importance of the existing infrastructure of international research in East Africa, which should be leveraged by GeoPRISMS to effectively focus resources in the extensive East African Rift System primary site.

Scientific Advances Related to GeoPRISMS Goals in East Africa

What follows is a brief summary of scientific results reported at the MRAV conference. A complete volume of abstracts and the program can be found at http://www.see.leeds.ac.uk/afar/new-afar/conference/conference.html. We present these results in the context of the questions outlined in the GeoPRISMS science Implementation Plan for the East Africa Rift System (EARS).

How is strain accommodated and partitioned throughout the lithosphere, and what are the controls on strain localization and migration?

A significant focus of the conference was the 2005 Dabbahu rifting event, which was dominated by a series of 14 dike intrusions and 4 eruptions with an estimated 2.5 km3 of magma intruded since September 2005. The initial Dabbahu diking events affected a large portion (60 km) of the magmatic segment, while subsequent activity was more localized. Several lines of evidence (including InSAR and seismicity) indicate that diking preceded and drove seismicity in the Dabbahu events. Importantly, the seismic moment and the associated slip along faults accounts for only 10% of the geodetic moment, indicating that most deformation in this rifting event was taken up aseismically, through dike injection or other igneous intrusion. Many aspects of this rifting resemble the 1974-89 rifting event at Krafla, in Iceland.

Additional recent tectonic activity reported on at the conference included the 2010 Gulf of Aden seismic swarm, which occurred along three segments of the rift at depths of less than 10 km. The 1989 Dobi earthquake swarm in central Afar appears to have followed a “bookshelf faulting” model, with slip occurring on at least 14 different faults during the earthquake sequence. The Asal rift was imaged with RADARSAT from 1997-2008; this time series showed 2-3 m of opening, accompanied by subsidence in the rift itself and uplift on the flanks with some component of shear.

What factors control the distribution and ponding of magmas and volatiles, and how are they related to extensional fault systems bounding the rift?

The Dabbahu event was dominantly a diking phenomenon, with magma playing a key role in crustal deformation. Similar to other portions of the rift, fractional crystallization processes and magmatic plumbing systems differ between axial and off-axis magmas. Resistivity surveys, surface velocity models, and receiver functions in the Dabbahu area all suggest that some 3000 km3 of magma remains in the crust, possibly stored in elongated magma chambers parallel to the rift axis, and that these may erupt on ~40 ka cycles. At upper mantle and lower crustal depths, the resistivity structure of active and inactive segments of the Afar rift are similar. The most significant heterogeneity exists at mid-crustal depths and is related to the presence or absence of melt.

Very high-resolution seismicity obtained through deployment of seismometer arrays helps detail the relationship between magmatic activity and faulting. While normal faulting occurs during the diking process, regions where magmatism has occurred are less seismically active. More broadly in the region, rift basalts show expected age progression with the youngest basalts at the center of the rift, and pointing to a spreading rate of 12 ± 1 mm/yr. However, less clear is off-axis magmatism, which shows no simple age progressive trend.

How does the mechanical heterogeneity of continental lithosphere influence rift initiation, morphology, and evolution?

Many presentations addressed aspects of the rift beyond the Dabbahu event. Comparing the recent, well-studied and well-constrained rifting event in Afar with the longer geologic record highlights that these processes change over time. Primarily, the asymmetry of the Afar rift suggests that the locus of rifting has migrated eastward. The orientation of different fault sets in the Asal-Danakil rift indicate two different directions of tension between 1.35 Ma and 0.3 Ma. This could be due to magmatic loading and flexure of the crust in addition to extension. Paleomagnetic data suggest minor block rotation (~7°) in Afar. The marginal grabens on the western edge of Afar are enigmatic: still seismically active, on top of the steepest gradient of crustal thickness. They are likely developed over crustal flexure, and the variability from north to south is controlled by migration of a wave of erosion. Farther south, thermochronology from the Albertine section of the rift show a complex, multi-stage cooling history and differential uplift within mountain blocks.

Several geophysical results suggest that structures at the surface mimic and reflect structures at depth in the lithosphere. Crustal anisotropy (fast direction) and the geoelectric strike both match the orientation of surface structures, with a transition zone in Afar. Both also increase in the magmatic segments of the rift: anisotropy is sensitive to strain fabrics, and MT to presence of melt. Shear-wave splitting directions in the mantle are different below mid-ocean ridges and the East African Rift. Below the Main Ethiopian Rift, they are parallel to rift axis; below the EPR, they are perpendicular to the rift axis. At slower-spreading ridges (mid-Atlantic and Gakkel), they are more variable. Gravity profiles across Dabbahu suggest a Moho depth of 19 or 23 km, and that faults at the surface may continue at depth.

How does the presence or absence of an upper-mantle plume influence extension?

At a wider scale, discussions focused on the lithosphere-asthenosphere boundary and how the thermo-chemical state of the East African upper mantle impacted the rifting process in East Africa. The nature of the lithosphere-asthenosphere boundary differs on the rift flanks in comparison to the central part of the rift. Beneath the flanks, velocities decrease with depth, suggesting melt pockets at the lithosphere-asthenosphere boundary, whereas velocities increase with depth beneath the main rift. These properties mean that at ~70 km depth, the rift in Afar resembles the East Pacific Rise. These observations are consistent with observations that at 50-150 km depth, the lowest seismic velocities follow the ridge structure. However, at 300 km depth, there is a very broad anomaly that lacks structure and extends down to the transition zone. Elevated mantle potential temperatures are detected in Afar and throughout the East African rift, supporting seismic evidence of a deep upwelling. Despite these elevated temperatures, the magnitude of the observed seismic anomalies cannot be explained solely by a thermal means and requires a chemical component within the upwelling.
How does rift topography, on either the continental- or basin-scale, influence regional climate, and what are the associated feedback processes?

Rifting affects climate through the construction of topography, which can have a significant effect on the local distribution of precipitation. Results of modeling experiments suggest that both tectonic events (the development of high topography associated with rifting) and orbital forcing (variability in insolation) are likely to have affected climate in eastern Africa over the last 20 million years. The East African Rift is also an excellent location to explore the mesoscale affects of orography, due to the presence of multiple lakes. Lakes generate their own weather, and interact with prevailing winds and local topographic features. There are coring efforts underway in Lake Malawi to test these effects. Rift lake sediments preserve unique records of climate and tectonics, including key time intervals in hominid evolution.

Figure 2. A fissure on the edge of Lake Besaka. Fantale volcano is in the background; it last erupted 170,000 years ago.

Figure 2. A fissure on the edge of Lake Besaka. Fantale volcano is in the background; it last erupted 170,000 years ago.

Broader Impacts

Hazards

Volcanic hazard risks associated with Ethiopian volcanoes are unexpectedly high, largely due to the uncertainties associated with individual volcanic centers. In particular, the geologic record is temporally limited. Of concern is that InSAR observations have shown that there are far more volcanoes that are currently deforming than have erupted historically, suggesting significant potential for future eruptions. To more broadly assess volcanic hazard potential, the NERC-funded ‘Global Volcano Model’, in cooperation with 12 international partners, seeks to better characterize potentially hazardous volcanoes.
Remote volcanic hazard monitoring through SO2 emissions, InSAR, thermal imaging, and infrasound, provide means to monitor volcanoes in difficult to access areas. Eruptions in remote regions may not have an immediate hazard impact due to sparse habitation, however the Nabro event in Eritrea was determined to have been the largest SO2 producer since 1991. These remote sensing techniques therefore have further application for global SO2 models with obvious implications for climate change studies.

Resources

The economic potential of East Africa is substantial; energy, commodity and tourism resources are clear growth areas. Epithermal gold deposits in Afar that are associated with geologically modern hydrothermal systems linked to rift magmatism are targets of active exploration. The gold potential of these systems is enhanced by the relatively low salinity magmatic environment in the rift. The resources being devoted to this epithermal play speak to the resource potential of currently active rifts (i.e. we do not have to wait for them to fill with sediments and develop oil).

There is extensive oil exploration in Lake Albert region in Uganda, and many boreholes have been drilled. Little production is occurring at this time, due to transport constraints, although estimates of the resources are substantial (~1000 million barrels). Oil exploration has also focused on the Lake Turkana region, where very detailed gravity, magnetic surveys and mapping have been completed.

Significant challenges remain in the electrification of East Africa. Only 15% of East Africans have access to electricity with an average consumption of 68 KwH/yr (compared with ~2500 KwH/yr per person globally). With current production, every East African could light a 60W bulb 3 hours/day. Energy production needs to expand 33 fold. So far, only ~1% of the geothermal potential of the Ethiopian Rift has been exploited. And while geothermal energy is a key area of exploration, there are inherent problems with power generation and cost scaling – small facilities are more costly to operate. There is also a drive to construct more dams for hydropower in Ethiopia, but the selection of dams is complicated by seismic and volcanic activity, which may be episodic.

One particularly interesting presentation addressed geotourism as a growing industry that should be examined in more detail, including prioritizing the generation of digestible information and graphics for visitor centers.

Figure 3. Field trip participants examine 'blister cave' in a welded tuff in the southern Afar.

Figure 3. Field trip participants examine ‘blister cave’ in a welded tuff in the southern Afar.

Future Opportunities and Challenges for GeoPRISMS

Attendees expressed strong interest in continuing research in the Afar region, as well as other parts of the East African Rift. Several projects are continuing or planned, and there are multiple opportunities for GeoPRISMS. Close collaborations with African scientists, particularly, will be essential to the success of GeoPRISMS work in the EAR, and many scientists from Ethiopia and elsewhere who attended the meeting expressed enthusiasm for such interactions.

The conference was opened by the Ethiopian Minister for Mines, who emphasized her desire to engage international scientists and the need to translate the scientific knowledge gained through research into economically useful information. The logistical, cultural, and administrative challenges of working in East Africa require and benefit from close collaboration with scientists from the host countries. Many of the participants from Africa were directly involved in the energy, commodity, or tourism industries, or other efforts that closely link to the scientific research being undertaken in the region. Another opportunity for GeoPRISMS scientists is to build successful cooperative efforts by linking the fundamental research to applications in energy, resource development, and hazards mitigation that can yield tangible benefits to the host country.

The conference was closed by the Dean of Research at Addis Ababa University, who articulated the need for a better understanding of the rift and its consequences for hazards and announced a new 5-year, $10 M Ethiopian birr (over $500,000 USD) initiative focused on hazards. Representatives from energy companies (including geothermal and hydrocarbon) and mining companies also attended the meeting and expressed interest in collaborating with international academic teams to better understand the tectonics and their consequences for resources. In January 2013, the 24th Colloquium of African Geology will be held in Addis Ababa, with sessions dedicated to the East African Rift, providing an additional opportunity to focus GeoPRISMS’ efforts.

Numerous graduate students from around the world were present at the meeting, as well as several undergraduates from Addis Ababa University. The opportunities to build research capacity in Africa by involving graduate and undergraduate students from the host countries in research are tremendous, and should be a part of any GeoPRISMS effort.

Ultimately, GeoPRISMS must work closely with East African scientists and develop a strategy that complements and capitalizes on existing initiatives. The opportunities for meaningful collaborations are significant.

Reference information
Report from the Magmatic Rifting and Active Volcanism Conference, Afar Rift Consortium (Addis Ababa, Ethiopia), Egger, A., Rooney T., Shillington D.;

GeoPRISMS Newsletter, Issue No. 28, Spring 2012. Retrieved from http://geoprisms.nineplanetsllc.com

U.S. Earth Scientists Prepare for a Community Seismic Experiment at the ENAM Primary Site

Harm Van Avendonk1, Beatrice Magnani2

1University of Texas at Austin, 2University of Memphis

Figure 1: Map of Discovery Corridors in ENAM focus area. The red shaded area is the target of the USGS seismic program on the U.S. Extended Continental Shelf. ECMA = East Coast Magnetic Anomaly, BSMA = Blake Spur Magnetic Anomaly.

Figure 1: Map of Discovery Corridors in ENAM focus area. The red shaded area is the target of the USGS seismic program on the U.S. Extended Continental Shelf. ECMA = East Coast Magnetic Anomaly, BSMA = Blake Spur Magnetic Anomaly.

Eastern North America (ENAM) was chosen as a GeoPRISMS Rift Initiation and Evolution primary site because it represents a mature rifted continental margin in which the entire record of continental break-up and rifting is preserved. The rifting history along ENAM is well recorded in basin stratigraphy and the underlying crustal structure, although subsidence, sediment transport and fluid flow are presently the dominant geological processes along the margin. The study of old rifted margins is often challenged by a thick cover of sediments, which masks much of the deep crustal structure. This is also true for ENAM; however, over the next few years, unprecedented opportunities exist to carry out focused geophysical studies, revealing both shallow and deep structures of ENAM in greater detail.

The convergence of two activities along ENAM serves to frame data-gathering opportunities. In 2013, the EarthScope Transportable Array (TA) will arrive in ENAM, and the USGS is planning a marine seismic reflection and a limited refraction study of the Extended Continental Shelf (ECS) along ENAM onboard the seismic vessel R/V Marcus Langseth, possibly as early as 2014. In addition, there is renewed interest from energy companies in the exploration of ENAM . At the joint Earthscope-GeoPRISMS Science Workshop on Eastern North America, held at Lehigh University in October 2011, discussions among various academic, government and industry scientists led to the suggestion that a community active-source seismic experiment could improve our understanding of the deep structure and evolution of ENAM, and make the best use of existing resources and upcoming opportunities. The planned USGS active-source seismic operations over the ECS provide part of the immediate impetus for such an experiment; however, the possibility exists to extend some of the proposed USGS profiles landward to image deep margin structures and obtain important seismic velocity constraints. Given the limited mission of USGS ECS surveys, funding to extend these profiles and record air-gun shots on-land must come from NSF, possibly with some industry sponsorship.

A GeoPRISMS-sponsored luncheon was held in San Francisco on December 8, 2011, during the AGU Fall Meeting. About 30 scientists met to discuss further the conceptual framework of a community proposal for an ENAM active-source seismic experiment. Several scenarios were discussed, from minimum-cost to comprehensive coverage. The latter could include onshore-offshore operations, e.g., air-gun shots from the R/V Marcus Langseth recorded not only by its 8-km-long multichannel seismic streamer, but also by co-linear OBSs and by EarthScope Flexible Array seismometers, deployed along on-land extensions of selected marine seismic transects. In addition, land-based shots along these transects could be recorded by Flexible Array seismometers as well as by OBSs, providing reverse coverage. Additional PI-driven piggyback deployments offshore and onshore could be designed to take further advantage of the community seismic effort. The consensus at the luncheon was that such a joint seismic experiment is feasible and opportune; however, the timing may depend on the final schedule for the USGS seismic program.

The GeoPRISMS ENAM primary site spans much of the U.S. and Canadian Atlantic margins, from Charleston to Nova Scotia. However, budgetary and logistical constraints require that the target area of a community seismic experiment be much smaller. The area of interest for the planned USGS ECS seismic study lies between the Outer Blake Ridge offshore South Carolina in the south and Cape Cod to the north (Figure 1). Within this region, the planned ECS seismic survey consists of profiles spaced 60 nautical miles apart, spanning the interval from the continental shelf break to the 200 nautical mile limit. To meet GeoPRISMS objectives, some of these profiles would be extended landward across the shelf, and onshore, where air-gun shots would be recorded by land stations.

At the EarthScope-GeoPRISMS Science Workshop at Lehigh, participants identified a few major corridors where dense data acquisition would benefit integrated studies of rifted margin processes (Figure 1). The “Philadelphia” and “Richmond” corridors exhibit pronounced along-strike structural variations in the Appalachians; thus, seismic transects that cross the shoreline in these two areas may yield insights into the role of inherited orogenic structure on the development of rift half-grabens, such as the Culpeper and Hartford basins, and the nature of syn-rift magmatic wedges that define the continent-ocean transition offshore. To the south, a transect in the vicinity of Charleston, SC, would image the transition between the Carolina Trough and the Blake Plateau, clarifying the structure and origin of basement in this area. In addition, the gas hydrate province of Blake Ridge is an important site for the assessment of geohazards on the continental slope. Comparisons of the deep-seismic structures along the northern and southern corridors would provide a view of regional differences in extension and magmatism during the opening of the Atlantic, helping to explain the linkages between these processes.

To have a true community experiment, broad participation from the U.S. scientific community is necessary. Researchers interested in participating in an ENAM community seismic experiment are invited to help with the (a) design of the active-source seismic data acquisition plan, (b) proposal writing, and (c) staffing of the data acquisition teams on-land and offshore. The involvement of graduate students and postdocs in this effort is very important, as these early-career scientists represent the core of the future GeoPRISMS and EarthScope communities. In the sprit of community science, we envision rapid data release and open data access following the experiment, enabling many members of the scientific community to participate in seismic data analysis and interpretation. Science proposals to use the seismic data could be submitted to NSF once the data are collected.

Although funding of the USGS seismic study of the ECS is currently uncertain, this field program is tentatively being planned for 2014. To create a successful partnership with the USGS in 2014, collaborative proposals must be submitted to the NSF GeoPRISMS and EarthScope Programs solicitations in 2012, on July 2nd and July 16th, respectively. Over the next few months, we hope to engage our colleagues in discussions about ENAM science priorities, and we welcome insights and contributions to the ENAM community seismic experiment proposal. Consider contributing through the GeoPRISMS forum site or by contacting us directly.

Reference information
U.S. Earth Scientists Prepare for a Community Seismic Experiment at the ENAM Primary Site, Van Avendonk H., Magnani B.;

GeoPRISMS Newsletter, Issue No. 28, Spring 2012. Retrieved from http://geoprisms.nineplanetsllc.com