Putting the “Community” in the Alaska Amphibious Community Seismic Experiment (AACSE): Alaska Peninsula and Western Gulf of Alaska, Summer 2018


The AACSE Team*

*This report was edited and compiled by Lindsay Worthington.

AACSE PI team: Geoff Abers (Lead PI, Cornell U.), Aubreya Adams (Colgate U.), Peter Haeussler (USGS), Emily Roland (U. of Washington), Susan Schwartz (U. of California Santa Cruz), Anne Sheehan (U. of Colorado), Donna Shillington (Lamont Doherty Earth Observatory), Spahr Webb (Lamont Doherty Earth Observatory), Doug Wiens (Washington U. St. Louis), Lindsay Worthington (U. of New Mexico).

2018 Apply-to-Sail Participants: Collin Brandl (Graduate Student, U. of New Mexico), Enrique Chon (Graduate Student, U. of Colorado), David Heath (Graduate Student, Colorado State U.), Robert Martin-Short (Graduate Student, U. of California Berkeley), Kelly Olsen (Graduate Student, U. of Texas), Holly Rotman (Postdoctoral Researcher, New Mexico Tech), Samantha Hansen (Associate Professor, U. of Alabama), Tiegan Hobbs (Graduate Student, Georgia Tech), Amanda Price (Graduate Student, Washington U. St. Louis), Heather Shaddox (Graduate Student, U. of California Santa Cruz), Jefferson Yarce (Graduate Student, U. of Colorado Boulder), Natalia Ruppert (Seismologist, U. of Alaska Fairbanks)

K-12 Educators On Board: Shannon Hendricks (High School Science Teacher, Anchorage School District), Bethany Essary (High School Science Teacher, Anchorage School District).

The shore-based field teams included graduate student Michael Mann (Lamont-Doherty Earth Observatory) and undergraduate student Jordan Tockstein (Colgate U.). We thank the captain and crew of the R/V Sikuliaq and the pilots, boat captains and land owners that made these deployments possible. Special thanks to Bill Danforth from the USGS for his bathymetric processing expertise aboard Leg 2 and Patrick Shore from Washington U. for coordinating onshore field logistics and preparing the data for delivery to the DMC.

If you visit Alaska and tell people that you are a seismologist, you are going to hear an earthquake story. The Alaska-Aleutian subduction system is arguably the most seismically active globally, producing more >M8 earthquakes over the last century than any other. As a result, earthquake and tsunami hazard are woven into daily life here. Near downtown Anchorage, you can visit Earthquake Park, occupying part of town that was decimated by a landslide during the 1964 M9.2 event that inspired the term “megathrust” earthquake. If you happen to be in Kodiak on a Wednesday afternoon, you will hear the weekly tsunami siren drill sound throughout the town. Earlier this year that drill was put in to practice as residents made their way through the tsunami evacuation process, meeting up at the school on high ground after midnight on January 29 following the M7.9 earthquake that occurred offshore.
So, how do you study an 800 km section of this subduction zone that is mostly offshore or only accessible via air or boat? Simple. Start with nine Principal Investigators (PIs) and dozens of conference calls; take 85 ocean bottom seismometers (OBS), thirty broadband seismometers, one fishing boat, two float planes, two fixed wing planes, a helicopter, and a 261-ft research ship; add a team of twelve OBS engineers, 24 ships crew, twelve Apply-to-Sail participants, two Alaskan K-12 teachers and two field technicians. Then make the data open and accessible as quickly as possible. This is the Alaska Amphibious Community Seismic Experiment (AACSE) and these are voices from the field.

The Alaska Amphibious Community Seismic Experiment (AACSE) deployment map prepared by Peter Haeussle

OBS Deployment Cruise Leg 1 | Seward, AK to Seward, AK – May 9-29, 2018

>> 9 May, 2018 | We are officially underway • It is 8:30am and we are departing Seward dock. We have donned our full-body immersion suits as part of a safety drill, and are now heading towards the first seismometer deployment site, lying in the Shelikof Strait just north of Kodiak Island. We are on one of the most modern and well-equipped scientific research ships in the world. The R/V Sikuliaq was built in 2014 and has a science lab, lounge, dining room, kitchen, gym, and the list goes on. There is even a sauna which apparently can double as a hypothermia recovery room – let’s hope we won’t be using it for that purpose. For cabins, we are treated to the height of oceanographic luxury. The rooms are practical and very comfortable. The Sikuliaq takes its name from the Inupiaq word that means “young sea ice”. Thanks to its round hull, the ship is capable of breaking ice up to 2.5 ft thick, which is essential on polar missions. This also gives it a tendency to move around more in high seas. As we travel, we will be collecting meteorological data such as pressure, temperature, and wind speed. We will also be recording bathymetry data to map the seafloor.

-Robert Martin-Short, University of California Berkeley

>> 10 May, 2018 | Deploying the first OBS instrument • The first OBS (Ocean Bottom Seismometer) is a shallow-water Trawl-Resistant Mounted Seismometer (TRMS), design to resist and deflect the lower leading line of bottom trawl nets. All of the OBSs are instrumented with a seismometer, batteries to last more than fifteen months, transponders to communicate with the ship and burn the wire to release the seismometer for recovery, data logger, temperature sensors, and other equipment necessary to collect these data. The shell for the TRMS itself weighs about 1,300 lbs, the whole instrument weighs about 1,800 lbs. The deployment is a success! After deploying the TRMS, we have to hide from foul weather in Larsen Bay, then assemble more TRMSs. This involves removing the doors and installing brackets to hold equipment, attaching hoods to the pop-up TRMS, checking the transponders to make sure they are properly communicating with the ship, and attaching the transponders. We will stay in the cove and work for a couple hours, then leave once the storm has passed.

-David Heath, Colorado State University

>> 12 May, 2018 | Waiting out the storm • Many of us are taking to personal hobbies and pastimes in between routine status logging. Some people are reading quietly. Others are attempting to catch up on emails, though the internet is particularly slow. Others are taking the opportunity to chat with shipmates, many of whom are still practically strangers after few days on the ship. I am learning that life on a ship provides a unique opportunity for people to connect with each other. I have spent part of the evening receiving a generous guitar lesson from the Chief Steward who is a skilled blues musician. He kindly reached out to play alongside me when he noticed me strumming out on deck. I’ve got to say, my experience thus far has been pretty great, despite the spotty weather and fits of acute nausea.

-Enrique Chon, University of Colorado

OBS Deployment Cruise Leg 2 | Seward, AK to Seward, AK; July 11-24, 2018

>> 11 July, 2018 | Educators Onboard • There are so many people involved in a research cruise like this. There is an entire ship crew, scientists, graduate students, USGS employees, OBS technicians, and, on this trip, there are even two high school science teachers and I am one of those. I am stoked to be on board. My colleague, Shannon Hendricks, and I were selected as part of the Educator Onboard K12 program. Through this program, educators are given the opportunity to participate in research to better understand current science practices. The goal is to use that knowledge to create engaging, authentic lesson plans to share with other educators. It is a little intimidating to meet all of these experts – as science teachers, we know a little bit about a lot of things, and we have a solid enough science foundation to understand what the experts are talking about (most of the time!). This also means we know enough to realize how much we don’t know! It is amazing to get to learn from scientists that have made this their life work. Getting to peek in on their ongoing research makes us better science teachers. And it is nice to know that, just like we tell our own students, there are no stupid questions.

-Bethany Essary, West High School science teacher, Anchorage, AK

>> 23 July, 2018 | The aftershock zone • Day 12 of the cruise, we have just successfully deployed our last OBS, 32 hours ahead of schedule! Half way through this cruise, we decided to move one of the instruments to near the aftershock zone of the M7.9 Offshore Kodiak earthquake. It struck about three hundred kilometers offshore Kodiak Island in the early morning hours of January 23, 2018, in the outer rise region of the Alaska-Aleutian subduction zone. It triggered tsunami warnings and prompted evacuations of thousands of people in Alaskan coastal communities. While the source parameters (such as seismic moment tensor) for the earthquake suggested strike-slip faulting (hence no significant tsunami generated), the true complexity of the source has only become evident through analysis of multiple datasets. At least four conjugate strike-slip faults were involved in the earthquake rupture. However, the distant location of the aftershock source region to the land-based stations made the data analysis and interpretation difficult. On the Leg 1 cruise, a couple of stations were serendipitously placed near or in the aftershock zone. After consultations with the PI group we moved this station to the aftershock cluster. This enhanced network of OBS sensors in the aftershock zone will help characterize the aftershock sequence with much better accuracy.

-Natalia Ruppert, University of Alaska

>> 24 July, 2018 | Good luck • For the past three years, I have been looking at OBS data off the east coast of New Zealand’s North Island, and I always wondered about the logistics behind the dataset of earthquakes. It turns out that deploying ocean bottom seismometers is a huge task that includes multiple people. This experience exceeds all my expectations. I imagined a repetitive process, but every single station has its own challenges: the bathymetry indicates a rough or steep relief so we have to move somewhere close by with a more flat and soft bathymetry; we need to be sure that the temperature sensors are the ideal for specific depths; we fill the sheets with station information and log it in our physical and digital forms, etc. This experience makes me really value all the effort that the science crew did for the deployment and recovery of the data that I am currently working on. For the future seismologists who are going to work with the data, I want to say that we did our best to make sure the seismometers were meticulously deployed and I am sure the recovery crew will be equally careful to collect the year-long log of wiggles from the stations deployed by the first and second legs. Good luck!

-Jefferson Yarce, University of Colorado

Onshore Deployment: Alaska Peninsula, Kodiak Island and Shumagin Islands; May-June 2018

>> 16 May, 2018 | A for Amphibious • The second A in AACSE stands for Amphibious – fully encompassing the entire subduction zone requires making measurements on land and at sea. The onshore part of the program involves installing instruments on Kodiak Island, the Shumagin Islands (southwest of Kodiak), the Alaska Peninsula and the region around Katmai National Park. These thirty instruments will be placed in remote locations (black circles on the map p.19) accessed by float planes or small fixed-wing planes. One team of three people is installing thirteen sites on Kodiak Island, and a second team is deploying the rest of the sites on the mainland and Shumagin Island. Today the Kodiak team started their first day of work! Like working at sea, the initial work involves unpacking all the gear shipped from across the country, and testing and assembling everything. To make sure everything is working properly, we do a “huddle test,” where we set up all of the seismometers and data loggers in one place and let them collect data for one day. We are fortunate to have been given access to some space in the Kodiak Alaska Fisheries Science Center, a research facility that provides valuable data to the fishing industry and that has a wonderful aquarium. This means we are sometimes sharing the space with sea life, like a large half-decomposed salmon shark! Tomorrow, if all goes well, we can start deploying!

– Geoff Abers, Cornell University

>> 21 May, 2018 | Kodiak Island • The road network on Kodiak Island is confined to the region around the town of Kodiak, so one must travel by boat or plane to reach other parts of this rugged and beautiful island. Eight of the thirteen seismic stations that we are installing here are both off the road system and far from towns with air strips, and we have been traveling to them by float plane. One limitation of using small planes for seismic installations is that there is a weight limit on what you can bring. The float plane we have been using, a de Havilland DHC-2 Beaver, can carry 1,200 lbs. Our field team and equipment for two stations weigh 1,175 lbs! We have to do a weigh-in before our first flight – fortunately they weighed our field team together and not individually. Flying also requires better weather than simply driving to a station. So far, we have found that the weather is worse on the eastern part of Kodiak near Kodiak town but improves to the west. We feel lucky to have had three days in a row where we could fly out to some of our sites. In the last three days, we have installed five stations that have taken us to many corners of Kodiak: McDonald lagoon on the southwestern coast, small Anvil Lake in far western Kodiak and the gorgeous Uyak Bay, a fjord that connects to the ocean in the north and cuts across two thirds of the island. This fjord is enabling us to deploy closely spaced stations over a part of the subduction zone fault where large earthquakes occur, one of the primary targets of this project. Traveling by plane across Kodiak is spectacular; you are treated to stunning views of snow-capped mountains and broad valleys. Sometimes you can see mountain goats lining steep slopes, bears meandering along the shore, and frolicking otters in the water. The views from our seismic sites are really amazing, too, when we look up from orienting sensors and plugging in data loggers. Six down, seven to go for the Kodiak team!

-Donna Shillington, Lamont-Doherty Earth Observatory

>> 30 May, 2018 | Challenging Conditions • The three members of the Sand Point team set sail on the Aleut Mistress to install two strong motion sites on Nagai Island. The day started with beautiful glassy-smooth seas and a calm two hour cruise to our first site on the north side of the island. We loaded our equipment into a skiff, hopped onboard and motored to our chosen landing site. This site was chosen by satellite imagery, and as always, conditions on the ground were a little different than expected. Our landing site was a bit marshy, and we had to lug the equipment uphill through marsh grasses and bushes, and then dig through a foot-thick mat of interwoven vegetation to find a suitably dry site for burial. Anything for good data! The equipment worked like a champ, so our time spent testing it in Sand Point paid off. We left the station after five hours of work – only two-and-a-half times longer than it has taken for any other station thus far! Back on the Aleut Mistress, our captain, Boomer, had boiled some Alaskan crab for our lunch. Hard to get it any fresher!

In the afternoon, the seas started picking up with swells a little over two fathoms (that’s a little over twelve feet for you land-lubbers). While none of our crew suffered from seasickness, there were some flying objects on deck and in the cabin! We hopped back in the skiff when we reached Nagai site #2, and headed toward shore. We got so close, but in the end the boat crew felt it was unsafe to land with the high seas and changing tide. Disappointed, we made the call to cancel the site. It is a hard decision to choose not to install a station. Fortunately, an excellent Plan B fell into our laps. As luck would have it, Boomer owns property near King Cove and offered his place as a home for our new station. So, a fairly tough first day in the field ended on a high note, with the formation of plans for the future. The next three days passed slowly, as our team waited on unanticipated repairs to the plane needed for other installations out of Sand Point. Everybody wants a well-maintained plane, so we waited patiently for the repairs and sorted through and retested equipment in Sand Point. By the time the plane was ready, our team was raring to hit the field again. We hammered out four more stations in just two days, and have nearly finished our work here in Sand Point.

-Aubreya Adams, Colgate University

Get involved

This project is intended to help grow the seismological community, and includes opportunities to sail on OBS cruises and short courses for undergraduates. Upcoming opportunities for 2019 will be announced in December on the project website.

Contact members of the PI team for more information. All seismic data from the project will be open to the community upon recovery and QA/QC efforts at the IRIS DMC (OBS array has network code XD (2018-2019) and land array has network code XO (2018-2019)). The first three months of onshore data is currently online. All underway data acquired by the Sikuliaq will be archived and available at the UNOLS rolling deck to repository server.

Check out the experiment blog for more stories from the field

Reference information

A continent-scale geodetic velocity field for East Africa. R. Bendick, M. Floyd, E. Lewi, G Kianji, R. King, E. Knappe
GeoPRISMS Newsletter, Issue No. 41, Fall 2018. Retrieved from http://geoprisms.org

A continent-scale geodetic velocity field for East Africa


Rebecca Bendick1, Mike Floyd2, Elias Lewi3, Gladys Kianji4, Robert King2, El Knappe1

1University of Montana, 2MIT, 3Addis Ababa University, 4University of Nairobi

The East African Rift System is a complicated set of extensional structures reaching from Malawi in the south to Eritrea and Djibouti in the north (Fig. 1)(e.g. Ebinger, 2005). These structures are broadly interpreted as the expression of the ongoing breakup of the African continent into a “Somali” block moving east or northeastward relative to a “Nubia” block, with perhaps additional smaller blocks (e.g. Saria et al., 2014) also involved. The details of the kinematics, the presence or importance of entrained microplates, and even the components of the force balance exciting relative block motions and extensional strains are all the subject of ongoing research and incompletely resolved scientific debates.

Figure 1. Overview of the EARS, with shaded topography in gray, major faults in red, recorded seismicity of Mw>5 as blue circles, and generalized kinematic velocities from Saria et al. (2014).

Several decades of geophysical and geologic research have contributed a large body of observational data related to the timing (Bosworth, 1992; Bosworth and Strecker, 1997; George et al., 1998; Wichura et al., 2010), chemistry (Aulbach et al., 2008; Bianchini et al., 2014; Chesley et al., 1999; Kaeser et al., 2009; Pik et al., 2006), mechanics (Buck, 2004; Calais et al., 2008; Corti et al., 2003; Weinstein et al., 2017), kinematics (Birhanu et al., 2015; Calais et al., 2008; Modisi et al., 2000; Saria et al., 2014), mantle involvement (Adams et al., 2012; Bastow et al., 2005; Bastow et al., 2008; Chang and Van der Lee, 2011; Fishwick, 2010; Hansen and Nyblade, 2013), magmatism (Bastow et al., 2010; Kendall et al., 2005) and natural hazards (Ayele, 2017) of continental extension in Africa. However, most of these studies are focused on a single “segment” of the larger rift system, hence on a distinct structural unit. Some work has been done to compare segments as a means of exploring the relative importance of contributing factors, such as the availability of fluids in magma-rich and magma-poor segments (Bialas et al., 2010; Hayward and Ebinger, 1996; Roecker et al., 2017; Rooney et al., 2011), the influence of total finite strain (Ebinger, 2005) on rift morphology, or the importance of sublithospheric plume impingement on the force balance (Ebinger and Sleep, 1998; Lin et al., 2005; Nyblade and Robinson, 1994). However, fully synoptic studies for the whole East African Rift System (EARS) are few in number.

A GeoPRISMS-supported collaboration between MIT and the University of Montana targeted the development of a comprehensive, consistent geodetic surface velocity solution for the entire EARS focus area (Fig. 2). This effort included several components:

  1. Collection of all publically available raw GPS observations from East Africa from 1992 to 2015;
  2. Negotiation for the release and inclusion of several additional restricted GPS observation data sets from European and African sources;
  3. Compilation and verification of all related metadata;
  4. Systematic assessment and quality control on all available data sets; and
  5. Processing of the merged data sets with a consistent processing strategy and reference frame.

Figure 2. The most recent community geodetic solution, using all available raw data from the EARS region, processed using GAMIT/GLOBK with a consistent quality standard and editing approach, in a single common reference frame. This solution, data sources, and relevant metadata are available from the GeoPRISMS data portal at http://www.marine-geo.org with doi:10.1594/IEDA/321764

The supported work addresses the GeoPRISMS Rift Initiation and Evolution (RIE) goal of synthesis, especially in the context of multiscale mechanics and controls on deformation and localization of strain.

During the period of support for this experiment, we also leveraged the NSF funding to invest in permanent geodetic instrumentation in Ethiopia and add new observations in the Turkana Depression of Ethiopia and Kenya, the part of the EARS with the fewest prior geodetic observations. In the first case, we extended operations of a previously-funded Ethiopian Highlands continuous GPS network for an additional year. That year allowed Addis Ababa University and the University of Montana to negotiate with several different stakeholders in the U.S. and Africa, with the end result that the Institute of Geophysics, Space Science, and Astronomy of Addis Ababa University adopted a fully operational, scientific-grade geodetic network of ten sites for permanent ongoing observations (Fig. 3). The network became the largest entirely African owned and operated geophysical system, and maintains operations and a fully open data policy to the present. In the second case, we added an additional epoch of campaign observations on six campaign GPS sites (Fig. 4) and added two continuous GPS systems in the Turkana Depression (Fig. 5). The continuous sites are located on either side of Lake Turkana and are hosted by the Turkana Basin Institute, a nonprofit entity supporting research through the region.

The primary purposes of the project were scientific and infrastructural capacity-building. The synoptic geodetic velocity field is intended for use by a wide range of researchers in many different disciplines within the rifting initiative and the EARS focus area. Many users will likely leverage the kinematic framework as boundary conditions, a priori constraints, or tectonic context for more focused studies without having to address data collection, standardization, quality control, metadata management, or processing strategies. We hope that the solution will inform other work and serve as an example of the value of a community commitment to open sharing of high-quality observations. In addition, the successful adoption of the instrumental array by African scientists sets a precedent for negotiated transfers of other instruments and capabilities throughout the region. African researchers and institutions can and should use such combinations of infrastructure and technical skills to pursue their own novel scientific targets and build indigenous training capabilities. Finally, the new Turkana Basin continuous sites are approaching a full year of operation, and will begin to yield usable scientific constraints on the most enigmatic part of the EARS very soon. ■

References

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Chang S., S. Van der Lee, (2011), Mantle plumes and associated flow beneath Arabia and East Africa. Earth Planet Sci Lett, 302(3), 448–454.
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Reference information

A continent-scale geodetic velocity field for East Africa. R. Bendick, M. Floyd, E. Lewi, G Kianji, R. King, E. Knappe
GeoPRISMS Newsletter, Issue No. 41, Fall 2018. Retrieved from http://geoprisms.org

Constraining variability in mantle CO2 flux along the East African Rift System


James D. Muirhead1, Tobias P. Fischer2, Amani Laizer3, Sarah J. Oliva4, Emily J. Judd1, Hyunwoo Lee5, Emmanuel Kazimoto3, Gladys Kianji6, Cynthia J. Ebinger4, Zachary D. Sharp2, Josef Dufek7

1Syracuse University, 2University of New Mexico, 3University of Dar es Salaam, 4Tulane University, 5Seoul National University, 6University of Nairobi, 7University of Oregon

Figure 1. Annotated SRTM map showing the extent of the rift basins in the current study. Filled circles show the location of sampling regions within each basin, and the dashed brown line delineates the eastward-dipping surface boundary between the Tanzanian craton and Proterozoic mobile belt rocks (from the geological map of Thiéblemont et al. (2016)). Also included is the mean flux of magmatic CO2 from sampling sites in each basin. Inset in the top left shows the location of the DEM map on the African continent. Red lines show the extent of the Eastern (EB) and Western (WB) branches of the EARS.

In May and June 2018, our team of researchers completed the longest along-strike magmatic CO2 degassing survey in the East African Rift System (EARS) to date. Our CO2 flux data now extend over four rift basins, from the Magadi basin (Kenya) southward to the Balangida basin (Tanzania) (Fig. 1). During the 25-day field campaign, we collected over one thousand diffuse soil degassing flux measurements, and sampled hydrothermal spring systems along major fault zones to analyze the sources and fluxes of different volatile species. Here we present preliminary results of diffuse CO2 flux in zones within one hundred meters of observed spring discharge and use these values to examine variations in magmatic CO2 discharges between basins. The spatial variability of these data reveal how mantle CO2 fluxes in the EARS may evolve over the course of rift basin development, and are impacted by the initial composition and structure of the East African lithosphere.
Continental rifts are sites of lithospheric thinning and heating, which is commonly accompanied by magmatism and volatile transfer from Earth’s mantle to the lithosphere and atmosphere (White and McKenzie, 1989; Ebinger, 2005; Rooney, 2010; Lee et al., 2017; Foley and Fischer, 2017). They represent a key tectonic setting for natural CO2 emissions and possibly modulate Earth’s climate on geological timescales (Brune et al., 2017; Foley and Fischer, 2017). However, the total volume of mantle CO2 emitted at rift settings is poorly constrained, as are the mechanisms that control variations in CO2 flux over the lifetime of rifting.

The original carbon content of cratonic lithosphere is expected to be relatively low (~0.25 Mt C km–3 for 2-3 Ga lithosphere; Foley and Fischer, 2017). However, abundant carbon may be sequestered in the mantle lithosphere during the infiltration of both plume melts (e.g., Thompson et al., 2015) and carbon-rich hydrous-silicate melts generated during subduction (Foley and Fischer, 2017; Malusà et al., 2018).

These processes can potentially enrich carbon contents in the mantle lithosphere up to a hundred times above background values (Foley and Fischer, 2017). The resulting carbon accumulated during these events may be released during the generation and ascent of magma at continental rift settings (Malusà et al., 2018) (Fig. 2).

Although continental rifts represent potentially key sites of CO2 release, measuring the flux of CO2 from these settings is challenging and requires direct measurements and observations of CO2 discharge from zones of active rifting. The magma-rich Eastern branch of the East African Rift System (EARS) represents an ideal location to investigate these processes. Earlier degassing studies focused on direct measurements of volcanic plumes emitted from active volcanoes, such as Nyiragongo (Sawyer et al., 2008) and Oldoinyo Lengai (Brantley and Koepenick, 1995). In addition to these plume sources, EARS volcanoes release mantle volatiles to the atmosphere via springs, fumaroles, and zones of diffuse soil degassing, as well as during eruptive episodes (Darling et al., 1995; Fischer et al., 2009; Barry et al., 2013; de Moor et al., 2013; Hutchison et al., 2015; Lee et al., 2017). More recent studies in the EARS have shown that large volumes of mantle carbon are also released to the atmosphere along extensional fault systems situated away from volcanoes (Lee et al., 2016, 2017; Hunt et al., 2017). During this process, termed “tectonic degassing” (Burton et al., 2013; Lee et al., 2016), mantle carbon ascends to the surface along permeable fault zones and exits via springs, diffuse soil degassing zones, and gas vents (Muirhead et al., 2016; Lee et al., 2016, 2017; Hunt et al., 2017). This mantle carbon is primarily sourced from an enriched sub-continental lithospheric mantle and released into the crust and atmosphere by magmas emplaced at lower crustal depths (Lee et al., 2017; Roecker et al., 2017).

Figure 2. Production and transport of magmatic CO2 at continental rift settings modified from Hunt et al. (2017). White arrows represent zones of CO2 fluid flow, yellow stars are hydrothermal springs, and orange stars are deep earthquakes. The CO2 depicted exsolves from cooling upper and lower crustal magmas. The distribution of crustal magma (red polygons) is based on seismicity from Weinstein et al., (2017) and the seismic tomography model of Roecker et al. (2017).

Given the large aerial extent, pervasive faulting, and widespread magma emplacement occurring at depth in the EARS (e.g., Keranen et al., 2004; Roecker et al., 2017; Plasman et al., 2017), quantifying the volumes of CO2 released requires observations from a wide variety of structural settings along the rift system. Results of diffuse soil degassing surveys have thus far been reported from the northern and central Main Ethiopian Rift (Hunt et al., 2017) and Magadi-Natron basin (Lee et al., 2016), with estimates of 0.52-4.36 Mt yr-1 and 2.15-5.95 Mt yr-1 for each rift sector, respectively.

Extrapolation of these estimates point to potential CO2 fluxes on the order of 10-100 Mt yr-1, particularly when accounting for dissolved CO2 volumes transported in springs (Lee et al., 2017). However, these estimates do not consider the spatial and temporal variations of mantle CO2 discharge expected along any active rift system. The flux of CO2 within any rift basin should depend on a number of critical factors, such as the volume of carbon trapped within the underlying mantle lithosphere, rates of magma production, and the dissolved CO2 contents of ascending rift magmas (Foley and Fischer, 2017; Hunt et al., 2017). These variables are expected to vary both spatially and temporally within any continental rift setting, and quantifying their importance for mantle CO2 release requires extensive along-strike sampling of zones of volatile discharge.

Our recent field campaign was specifically designed to fill in these critical gaps in our understanding of rift CO2 fluxes, through an investigation of four segments of the Eastern branch of the EARS: the Magadi, Natron, Manyara, and Balangida basins (Fig. 1). These basins encompass a ~350 km-long stretch of continental rifting and range in age between 1 and 7 Ma, and are thus currently at different stages of development. Furthermore, these basins exhibit varying volcanic/magmatic fluxes and histories, and even cross the boundary between Proterozoic mobile belt rocks and the Archean Tanzania Craton (Fig. 1). Therefore, from these data we can assess:

  1. How mantle CO2 fluxes may evolve over the course of basin development; and
  2. How CO2 fluxes are impacted by the initial lithospheric composition and structure of the East African lithosphere.

Given the inherent variability of CO2 flux within individual rift basins (e.g., Hunt et al., 2017), when comparing CO2 discharges between basins it is critical to compare data from sites exhibiting similar structure, substrate, and hydrology. Therefore, we present here a subset of our collected data, focusing specifically on flux data (1) from rift-graben sediments, (2) in the vicinity of faults, and (3) in areas within 100 m of observed spring discharge.

The sources for diffuse soil CO2 discharges in volcano-tectonic settings are typically characterized as either biogenic or magmatic, with flux data in each population exhibiting a log-normal distribution and the highest mean flux observed in the magmatic population (e.g., Chiodini et al., 1998, 2008). Data from each study site, presented as probability plots in Figure 3, were sub-divided into two distinct populations by adapting the methodology of Sinclair (1974) into a newly designed MATLAB® code. This code iteratively fits biogenic and magmatic regression lines to the log-transformed data. Based on these functions, synthetic data sets are generated for each population and plotted against observed data, with the final solution being that which produces the highest R-squared and smallest root-mean-squared error values between the compared datasets. Outputs from this procedure provide an estimate of the percent contribution of biogenic and magmatic sources and their mean flux values.

Figure 3. Probability plots of diffuse soil CO2 fluxes for each rift basin in the study. Note that the overall CO2 flux values decrease from north (Magadi) to south (Balangida). Flux values below the equipment detection limit (<0.24 g m-2 d-1) cannot be presented on the plots, but still affect the probability distribution of flux values above the detection limit.

Comparing data between basins, we observe a north to south decrease in both the percent contribution of the magmatic flux population and the mean magmatic flux value (see mean flux values in Figure 1). Lower magmatic CO2 flux values also correspond with younger rift basins (e.g., the Manyara and Balangida basins). These younger basins also exhibit lower volcanic/magmatic inputs (Le Gall et al., 2008; Albaric et al., 2014), which may relate to the low degree stretching and related decompression melting during this earlier stage of rifting, or to the relatively dry nature of thick Archean mantle that enables its preservation (e.g., Currie and van Wijk, 2016). Finally, as the locus of rifting gradually transitions from the Proterozoic mobile belt in the Natron basin, to the Tanzanian craton in the Balangida basin, we observe a significant reduction in the mean magmatic CO2 flux.

These preliminary results suggest that the volume of mantle CO2 discharge in the Eastern branch of the EARS is strongly dependent on the degree of lithospheric thinning, mantle hydration state, and related magmatism. The greatest mantle CO2 discharges in the EARS likely occur in more evolved systems outside the Archaean craton, such as the Kenya Rift (Lee et al., 2016) and Main Ethiopian Rift (Hunt et al., 2017). Furthermore, basins in their earliest rift stages (the ~1 Ma Manyara and Balangida basins) within Proterozoic mobile belt rocks exhibit higher CO2 fluxes than those in the Archean craton. This observation suggests that the Proterozoic lithosphere in East Africa may contain greater volumes of sequestered carbon, with its structure and composition suited for volumetrically significant CO2 discharges compared to the thick and probably dehydrated cratonic lithosphere. ■

References

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Barry, P.H., D.R. Hilton, T.P. Fischer, J.M. de Moor, F. Mangasini, C. Ramirez, (2013), Helium and carbon isotope systematics of cold “mazuku” CO2 vents and hydrothermal gases and fluids from Rungwe Volcanic Province, southern Tanzania. Chem Geol, 339, 141-156.
Brantley, S.L., K.W. Koepenick, (1995), Measured carbon dioxide emissions from Oldoinyo Lengai and the skewed distribution of passive volcanic fluxes. Geology, 23, 933-936.
Brune, S., S.E. Williams, R.D. Müller, (2017), Potential links between continental rifting, CO2 degassing and climate change through time. Nat Geosci, 10, 941.
Burton, M.R., G.M. Sawyer, D. Granieri, (2013), Deep Carbon Emissions from Volcanoes. Carbon in Earth, 75, 323-354.
Chiodini, G., R. Cioni, M. Guidi, B. Raco, L. Marini, (1998), Soil CO2 flux measurements in volcanic and geothermal areas. Appl Geochem, 13, 543-552.
Chiodini, G., S. Caliro, C. Cardellini, R. Avino, D. Granieri, A. Schmidt, (2008), Carbon isotopic composition of soil CO2 efflux, a powerful method to discriminate different sources feeding soil CO2 degassing in volcanic-hydrothermal areas. Earth Planet Sci Lett, 274, 372-379.
Currie, C.A., J. van Wijk, (2016), How craton margins are preserved: Insights from geodynamic models. J Geodyn, 100, 144-158.
Darling, W.G., E. Griesshaber, J.N. Andrews, H. Armannsson, R.K. O’Nions, (1995), The origin of hydrothermal and other gases in the Kenya Rift Valley. Geochim Cosmochim Acta, 59, 2501-2512.
de Moor, J.M., T.P. Fischer, Z.D. Sharp, D.R. Hilton, P.H. Barry, F. Mangasini, and C. Ramirez, (2013), Gas chemistry and nitrogen isotope compositions of cold mantle gases from Rungwe Volcanic Province, southern Tanzania. Chem Geol, 339, 30-42.
Ebinger, C., (2005), Continental break-up: The East African perspective. Astronomy & Geophysics, 46, 16-21.
Fischer, T.P., P. Burnard, B. Marty, D.R. Hilton, E. Furi, F. Palhol, Z.D. Sharp, and F. Mangasini, (2009), Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites. Nature, 459, 77-80.
Foley, S.F., T.P. Fischer, (2017), An essential role for continental rifts and lithosphere in the deep carbon cycle. Nat Geosci, 10, 897.
Hunt, J.A., A. Zafu, T.A. Mather, D.M. Pyle, P.H. Barry, (2017), Spatially Variable CO2 Degassing in the Main Ethiopian Rift: implications for magma storage, volatile transport, and rift‐related emissions. Geochem, Geophys Geosys, 18, 3714-3737.
Hutchison, W., T.A. Mather, D.M. Pyle, J. Biggs, G. Yirgu, (2015), Structural controls on fluid pathways in an active rift system: A case study of the Aluto volcanic complex. Geosphere, 11, 542-562.
Keranen, K., S.L. Klemperer, R. Gloaguen, E.W. Group, (2004), Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian rift. Geology, 32, 949-952.
Le Gall, B., P. Nonnotte, J. Rolet, M. Benoit, H. Guillou, M. Mousseau-Nonnotte, J. Albaric, J. Deverchere, (2008), Rift propagation at craton margin. Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times. Tectonophysics, 448, 1-19.
Lee, H., J.D. Muirhead, T.P. Fischer, C.J. Ebinger, S.A. Kattenhorn, Z.D. Sharp, G. Kianji, (2016), Massive and prolonged deep carbon emissions associated with continental rifting. Nat Geosci, 9, doi. 10.1038/NGEO2622.
Lee, H., T.P. Fischer, J.D. Muirhead, C.J. Ebinger, S.A. Kattenhorn, Z.D. Sharp, G. Kianji, N. Takahata, Y. Sano, (2017), Incipient rifting accompanied by the release of subcontinental lithospheric mantle volatiles in the Magadi and Natron basin, East Africa. J Volcanol Geotherm Res, 346, 118-133.
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Reference information

Constraining variability in mantle CO2 flux along the East African Rift System. J.D. Muirhead, T.P. Fischer, A. Laizer, S.J. Oliva, E.J. Judd, H. Lee, E. Kazimoto, G. Kianji, C.J. Ebinger, Z.D. Sharp, J. Dufek
GeoPRISMS Newsletter, Issue No. 41, Fall 2018. Retrieved from http://geoprisms.org

SISIE: South Island Subduction Initiation Experiment


Erin Hightower (Caltech) and Brandon Shuck (UT Austin)

The South Island Subduction Initiation Experiment (SISIE) was an international collaborative active-source seismic survey of the Puysegur subduction margin conducted aboard the R/V Marcus G. Langseth with researchers and graduate students from Caltech, the University of Texas, Texas A&M University, Victoria University of Wellington, and the University of Otago, NZ. The SISIE hopes to further our understanding of the processes controlling subduction initiation, which remains one of the last unsolved problems in plate tectonics. There are many existing hypotheses and models that attempt to quantify and understand these processes, but while many of them are plausible, our ideas far outrun our data. Geodynamic modeling of subduction initiation can only go so far in accurately explaining the mechanics and dynamics of the process. Therefore, without sufficient data to substantiate these models, there is no definitive answer to how subduction zones form.
The Puysegur Trench is part of the Pacific-Australian plate boundary and is a uniquely situated margin for such a survey because it is a young subduction zone with a well-constrained kinematic history that currently appears to be making the transition from a forced to a self-sustaining state, a development that is crucial in ensuring the longevity of a subduction system. The SISIE project aims to test this hypothesis with the marine geophysical data we recently collected. We will use these data to model the crustal structure across the margin, which will play an important role in constraining geodynamic models of subduction initiation.

The SISIE took place from mid-February to late March, 2018 and acquired high-quality geophysical data along and around the Puysegur-Fiordland plate boundary (Fig. 1). As we quickly learned, a research cruise in the Southern Ocean is no easy feat, and twice we had to take shelter from storms and relentless ten-plus meter swells behind Auckland and Stewart islands. We were able to collect multichannel seismic reflection, wide-angle seismic refraction, high-frequency chirp, multibeam bathymetry, magnetic, and gravity data across the margin. Students onboard participated in a daily Marine Geophysics Class, taught by the PIs, which familiarized us with the various data types we were collecting and the tectonic history of New Zealand. By combining theoretical lectures with hands-on applications, the class gave us practical skills in processing and analyzing multibeam and seismic data, which was an invaluable experience.

A total of 28 UTIG ocean-bottom seismometers (OBSs) were deployed on two key transects which span from the subducting Australian plate, across the Puysegur trench and ridge, over the Solander Basin, and onto the Campbell Plateau (Fig. 1). Students were involved with all OBS operations including programming, sealing and mounting, deployment, and recovery of the instruments (Fig. 2). The OBS records show coherent arrivals of crustal and mantle refractions and Moho reflections, and hints of reflections from the subduction interface. These data will help constrain the crustal thickness and seismic velocity structure across the margin, which will help guide gravity modeling.

Multichannel seismic (MCS) data were acquired with a 4 or 12 km long streamer, with channels spaced every 12.5 m, and recording airgun shots every 50 m. A standard processing sequence of trace editing, noise suppression, deconvolution, velocity analysis, mute, stacking, post-stack time migration, and multiple suppression was applied, with many of these steps performed as the data were coming in. The resulting subsurface images are of excellent quality, which will allow us to constrain the nature and geometry of the incoming oceanic plate, subduction interface, upper plate faulting, and stratigraphy of the Solander Basin (Fig. 3).

New multibeam bathymetry data provide high-resolution characterization of seafloor features and topography. Gravity and magnetic data obtained throughout the duration of the cruise will also help provide constraints on crustal densities and structure, and detailed estimates of plate ages and their thermal and kinematic histories, supplementing previous datasets for the region. The gravity data in particular will be integrated with the structural surfaces interpreted from the MCS lines and tomography models to develop a comprehensive view of crustal structure that will shed light on the isostatic state of the Puysegur margin and ridge.

The SISIE onshore seismic array was deployed by a small team comprising students from Victoria University of Wellington, GNS Science researchers, and an American student volunteer. The seismographs consisted of five broadbands and 37 short-period instruments deployed in Fiordland and Southland (Fig. 1). The short period array comprises two approximately north-south profiles and one east-west profile across the Winston and Waiau basins, which were designed to line up with several of the MCS lines shot by the Langseth offshore to provide continuous onshore-offshore coverage. The broadbands on the offshore Islands and in those deployed onshore in Fiordland will remain in the field for a year to record earthquakes, a number of which have already been recorded from the Fiordland area. With this array, we hope to record data that elucidates the nature of the crust and the plate geometry beneath southern New Zealand.

The SISIE MCS images are the highest quality data collected in the region, which gives an unprecedented view of the Puysegur subduction zone. In the marine seismic reflection images, we can identify a clear décollement extending from the trench. The image shows some sediment being subducting with the downgoing oceanic plate and some being underplated onto the Pacific plate (Fig. 3a). We were surprised to find stretched continental crust beneath the Solander Basin with the possibility of serpentinized upper mantle (Fig. 3b). Although more work is needed to determine the connection between this stretched crust and the Puysegur subduction system, this is already a major result that likely has great implications for understanding the mechanisms behind subduction initiation. In fact, our preliminary results leave almost no real example of ocean-ocean subduction globally, implying that some component of buoyant continental crust may be necessary for subduction initiation. In the future, we will integrate these data into a more complex and robust geodynamic model of subduction zone formation. SISIE highlights the need to continue marine seismic surveys of subduction margins, especially in areas that are not well explored, and the scientific impact that they bring to our community. Stay tuned for upcoming exciting results from the SISIE researchers and students! ■

References

Mitchell et al, (2012), Undersea New Zealand, 1:5,000,000. NIWA Chart, Miscellaneous Series No. 92.

Reference information
SISIE: South Island Subduction Initiation Experiment 
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org

Assessing changes in the state of a magma storage system over caldera-forming eruption cycles, a case study at Taupo Volcanic Zone, New Zealand


Kari Cooper (UC Davis), Adam Kent (Oregon State University), Chad Deering (Michigan Tech), and collaborator Darren Gravley (University of Canterbury, New Zealand)

The largest volcanic eruptions are rare events but when they occur can represent a global catastrophe. Relatively small eruptions may still have significant economic impacts (billions of dollars) and may affect the lives and livelihoods of large numbers of people – even in places quite distant from the erupting volcano (e.g., the relatively small Eyjafjallajokull eruption in Iceland in 2010). In an effort to study the processes that lead to large volcanic eruptions in more detail this project focuses on examining the highly active Taupo Volcanic Zone (TVZ) in New Zealand. Our goal is to develop a better understanding of how the temperature and mobility of a magma body below the surface changes before, during, and after a major eruption. As such the project contributes to an emerging understanding of the volcanoes and magmatic processes that can produce such large eruptions, and provides context for interpretation of hazard monitoring at these and other active volcanoes. The project also includes research experience for two K-12 teachers (one in the US and one in New Zealand), and will lead to development of new standard-based physics, chemistry and mathematics curricula.

Our approach is primarily a petrological and geochemical one and will focus on studying full caldera cycles – in addition to studying large eruptions themselves we will also focus on the smaller eruptions that occur before and after major episodes. We will couple age data with compositional data for both crystalline (plagioclase and zircon) and liquid (melt inclusions) parts of the erupted magma at the TVZ to develop constraints on the compositional and thermal variations within magma storage zones prior to eruptions. The project is at an early stage, but we have already compiled preliminary data and conducted a comprehensive sampling campaign during field work in December 2017. The field work was highly successful, bringing together PIs and graduate students from the three US institutions (UC Davis, OSU, and Michigan Tech) with our collaborator at University of Canterbury, along with K-12 science teachers Sara Moilanen (Houghton, MI) and Damien Canney (Christchurch, NZ). Field work also blended sample collection with filming videos of how we conduct field work and brief explanations of volcanic deposits and phenomena, which will be used to develop K-12 course content. The six graduate students in the group (Tyler Schlieder and Elizabeth Grant, UCD; Jordan Lubbers and Nicole Rocco, OSU; Olivia Barbee, MTU; and Lydia Harmon, Vanderbilt Univ.) also maintained a blog on the daily activities of the crew, and participated in the educational videos. The field work also set the stage for monthly video conferences among the graduate students, which helps to maintain coordination between individual thesis projects and the project as a whole.

Moving forward, we will collect a suite of data that will provide the foundation for a novel approach using two primary lines of investigation:

  1. Constraints on the thermal history of pre-eruptive magma storage by coupling absolute ages for plagioclase crystal populations derived from U-series measurements with trace element diffusion models to constrain the maximum residence time of crystals at a given temperature; and
  2. Quantification of the compositional heterogeneity of crystals and melt components, through in-situ measurements of trace-element and isotopic compositions in primary and accessory minerals and in melt inclusions (δ18O in zircon, εHf in zircon; Pb isotopes in plagioclase and melt inclusions), which will provide a measure of the degree to which the magma system is mixed across time and space within the reservoir as well as variations in the contributions of mantle and crustal sources to this reservoir.
  3. The unique strength of this approach is that it will allow simultaneous characterization of the thermal, compositional, and physical evolution of these silicic reservoirs. Therefore, the results of this study should be broadly relevant to other silicic volcanic systems and will represent an important step forward in improving our ability to interpret volcano monitoring data. Large silicic systems represent an end-member for volcanic activity globally, and more general models of the controls on the thermal conditions of magma storage beneath volcanoes will be developed by linking the results of this study with those from other ongoing projects. ■
Reference information
Assessing changes in the state of a magma storage system over caldera-forming eruption cycles, a case study at Taupo Volcanic Zone, New Zealand 
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org

Sizing up the Taniwha: Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE)


Jeff Marshall (Cal Poly Pomona) andJessica Pilarczyk (University of Southern Mississippi)

“A Live Dragon” Beneath the Sea

In Māori culture, the Taniwha is a dragon-like beast that lives beneath the water, sometimes protecting seafarers, while at other times wreaking disaster on coastal communities (King, 2007). Māori lore tells of Taniwha that cause sudden upheavals and changes in the coastline, altering the shape of the land-ocean interface. In the wake of New Zealand’s 2016 Mw7.8 Kaikōura Earthquake, the Taniwha was evoked as a supernatural force behind coastal uplift, tsunami, and landslides (Morton, 2018). For New Zealand, the Hikurangi subduction margin is a formidable Taniwha, a “live dragon” lurking just offshore, ready to unleash powerful forces locked within its seismogenic zone. With multiple collaborative research efforts now underway, geoscientists are shedding light on the habits of this secretive dragon, revealing new understandings of the earthquake and tsunami hazards that threaten New Zealand’s coastline.

The SHIRE Project

The Hikurangi margin along the east coast of New Zealand’s North Island (Fig. 1) provides an optimal venue for investigating megathrust behavior and controls on seismogenesis (e.g., Wallace et al., 2009 and 2014). Along-strike variations in multiple subduction parameters, such as interface coupling, fluid flow, and seafloor roughness, can be linked to observed differences in megathrust slip behavior (seismic vs. aseismic), forearc mass flux (accretion vs. erosion), and upper-plate deformation (contraction vs. extension). Much of the forearc is subaerial and therefore ideal for geodetic and geologic studies, while the submarine areas are easily accessible for geophysical imaging and monitoring. The SHIRE Project, funded by the NSF Integrated Earth Systems (IES) Program, is a four-year, multi-disciplinary, amphibious research effort involving a team of investigators at five US institutions, as well as multiple international collaborators from New Zealand, Japan, and the United Kingdom. This project is designed to evaluate system-level controls on subduction thrust behavior by combining both on and offshore active-source seismic imaging, with onshore paleoseismic, geomorphic, and geodetic investigations. The project results will be meshed with existing geophysical and geological datasets, and analyzed through the lens of state-of-the-art numerical modelling. The overarching goal is to develop an integrated perspective of the physical mechanisms controlling subduction thrust behavior and convergent margin tectonic evolution. Importantly, this perspective will help to elucidate a clearer picture of megathrust earthquake and tsunami hazards along New Zealand’s Hikurangi margin.

The SHIRE Project has three principal components:

Geophysical imaging: Harm van Avendonk (UT Austin) and David Okaya (Univ. of Southern California) are leading the shoreline-crossing geophysical imaging investigations. Marine seismic multi-channel reflection data (MCS) and seismic refraction data recorded by ocean-bottom seismometers (OBSs) are being used to characterize the incoming Hikurangi Plateau, map the structure of the offshore accretionary prism, and document subducted sediment variations. Onshore recordings of offshore airgun shots, explosive shots, and local earthquakes will determine the structure of the upper plate and properties of the deeper plate boundary zone.

Paleoseismology and morphotectonics: Paleoseismic and geomorphic studies led by Jeff Marshall (Cal Poly Pomona) and Jessica Pilarczyk (Univ. of Southern Mississippi) will collect new field data to supplement ongoing coastal tectonics investigations conducted by collaborators at New Zealand’s GNS Science. This integrated data set will help resolve megathrust slip behavior over several seismic cycles, and constrain long-term coastal uplift and subsidence patterns. This component of the project includes a Research Experience for Undergraduates (REU) program, supervised by Marshall, that engages US students in collaborative New Zealand fieldwork.

Numerical Modelling: Demian Saffer (Penn State) and Laura Wallace (UT and GNS Science) will coordinate the integration and analysis of project data through numerical modeling conducted by a team of U.S. and international investigators. The geophysical and geological results will be combined with a range of existing data sets from other projects to constrain numerical models of the physical state of the interface and evolution of the margin over both long and short (seismic cycle) timescales. Model results will also quantify linkages between in situ conditions, fluid flow, behavior of the subduction thrust, and subduction margin development.

SHIRE Spotlight: Geomorphic & Paleoseismic Studies

The SHIRE Project’s onshore geomorphic and paleoseismic fieldwork is investigating seismic cycle deformation in the coastal fore arc, focusing on geologic records of land level changes produced by episodes of tectonic uplift and subsidence. Jeff Marshall, Jessica Pilarczyk, and their students are targeting field sites along the North Island east coast (Fig. 2) that compliment ongoing investigations by GNS collaborators Nicola Litchfield, Kate Clark, and Ursula Cochran (e.g., Litchfield et al., 2016; Clark et al., 2015; Cochran et al., 2006). During field seasons in 2017 and 2018, the two research teams conducted parallel studies, with Marshall focused on marine terrace records of coastal uplift, and Pilarczyk on marsh stratigraphic records of subsidence and tsunami.

Marshall and students (Fig. 3A-F) are mapping, surveying, and sampling uplifted paleo-shorelines and marine terraces to identify past earthquakes, and to evaluate net coastal uplift patterns. Their efforts focus on several key locations along the Hikurangi margin, including the Raukumaura Peninsula, southern Hawkes Bay, and central Wairarapa coast. Coseismic uplift events are preserved along much of the Hikurangi coastline as elevated paleo-shore platforms and abandoned beach ridges. Marine shells collected from uplifted platforms and overlying beach sediments provide radiocarbon age constraints on prehistoric earthquakes. In addition to localized fieldwork, the Cal Poly Pomona team is using recently acquired airborne LiDAR imagery (provided by GNS) to correlate uplifted paleo-shorelines between field sites (both from this project and prior studies). The LiDAR data incorporates detailed altitude information, which can be used to track lateral variations in terrace uplift along the coast. Marshall and students are also mapping and sampling flights of uplifted Pleistocene marine terraces along the coast to evaluate longer-term fore arc uplift rates and deformation patterns.

Terrace cover beds have been sampled for optically stimulated luminescence (OSL) geochronology, and for the identification of volcanic tephra and loess deposits of known ages. During the next two years, terrace mapping and sampling will be expanded to new areas and drone imagery will be recorded for structure-from-motion studies. Project students will conduct digital terrain analyses using regional topographic data to evaluate net deformation patterns, calculate morphometric indices, and outline morphotectonic domains. Overall, the efforts of the coastal uplift team will provide new constraints on the timing and spatial distribution of both short-term seismic cycle events, as well as longer-term cumulative deformation.

Pilarczyk and students (Fig. 3 G-I) are using coastal sediments to develop long-term records of Hikurangi earthquakes and tsunamis. Microfossils such as foraminifera are used to recognize both subtle and abrupt changes in sea level along a coastline. An abrupt change in sea level, caused by coseismic subsidence, indicates the occurrence of an earthquake and can be recognized along the coastline as a soil buried beneath subtidal sediments. Because certain microfossils have fidelity to the tidal frame, they can be used to assess how much a coastline subsided during an earthquake. They can also be used to identify tsunami deposits because they indicate transport of marine sediment into a coastal setting where such sediment does not occur naturally. In this way, radiocarbon dating and microfossil analysis on coastal sediments can be used to understand the timing and magnitude of past Hikurangi earthquakes and tsunamis. In 2017 and 2018, Pilarczyk and students embarked on a sediment coring campaign that targeted low-energy depositional centers (i.e., marshes, lagoons) along the Hawke’s Bay coastline. Their mission was to find evidence for past Hikurangi earthquakes that would supplement the short-term observational record by expanding the age range of known events to include centennial and millennial timescales. The team’s ongoing investigations have led to the identification of newly discovered events that will help to better understand the seismic hazard for coastlines facing the Hikurangi margin. ■

References

Clark, K.J., B.W. Hayward, U.A. Cochran, L.M. Wallace, W.L. Power, A.T. Sabaa, (2015), Evidence for past subduction earthquakes at a plate boundary with widespread upper plate faulting: Southern Hikurangi Margin, New Zealand. Bull. Seismol. Soc. Am., 105. doi: 10.1785/0120140291
Cochran, U., K. Berryman, J. Zachariasen, D. Mildenhall, B. Hayward, K. Southall, C. Hollis, P. Barker, L.M. Wallace, B. Alloway, K. Wilson, (2006), Paleoecological insights into subduction zone earthquake occurrence, eastern North Island, New Zealand. Geol Soc Am Bull, 118, 1051-1074, doi:10.1130/B25761.1
Hayward, B.W., H.R. Grenfell, A.T. Sabaa, U.A. Cochran, K.J. Clark, L.M. Wallace, A.S. Palmer, A.S., (2016), Salt-marsh foraminiferal record of 10 large Holocene (last 7500 yr) earthquakes on a subducting plate margin, Hawkes Bay, New Zealand: Geological Society of America Bulletin, v.128, p. 896-915, doi:10.1130/B31295.1
King, D.N.T., J. Goff, A. Skipper, (2007), Māori environmental knowledge and natural hazards in Aotearoa-New Zealand. Journal of the Royal Society of New Zealand, 37, 59-73, doi.org/10.1080/03014220709510536.
Litchfield, N.J., U.A. Cochran, K.R. Berryman, K.J. Clark, B.G. McFadgen, R. Steele, (2016), Gisborne seismic and tsunami hazard: Constraints from marine terraces at Puatai Beach GNS Science Report 2016-21, 99
Morton, J., (2018), Our sleeping Taniwha: Hikurangi’s tsunami threat. New Zealand Herald, 10 March 2018, https://www.nzherald.co.nz
Wallace, L.M., M. Reyners, U. Cochran, S. Bannister, P.M. Barnes, K. Berryman, G. Downes, D. Eberhart-Phillips, A. Fagereng, S. Ellis, A. Nicol, R. McCaffrey, R.J. Beavan, S. Henrys, R. Sutherland, D.H.N. Barker, N. Litchfield, J. Townend, R. Robinson, R. Bell, K. Wilson, W. Power, (2009), Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand. Geochem Geophys Geosyst, 10, Q10006. doi:10010.11029/12009GC002610
Wallace, L.M., U.A. Cochran, W.L. Power, K.J. Clark, (2014), Earthquake and tsunami potential of the Hikurangi subduction thrust, New Zealand insight from paleoseismology, GPS, and tsunami modelling. Oceanography, 27, 104-117. doi:10.5670/oceanog.2014.46

Reference information
Sizing up the Taniwha: Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE)
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org

Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) – Revealing the environment of shallow slow slip


Susan Schwartz (UC Santa Cruz), Anne Sheehan (University Colorado, Boulder), Rachel Abercrombie (Boston University)

In the last fifteen years, it has become evident that slow slip events (SSEs) are a common and important part of the subduction process. They produce millimeters to centimeters of surface displacement over days to years that can be measured by geodesy and are often accompanied by seismic tremor and earthquake swarms. Slow slip and tremor have been observed in subduction zones in Cascadia, Japan, Mexico, Alaska, Ecuador, northern Peru, Costa Rica and New Zealand.

The 2014-15 HOBITSS deployment of 24 ocean bottom pressure sensors and fifteen ocean bottom seismometers (OBSs) at the northern Hikurangi margin, New Zealand captured a M7.0 SSE. The vertical deformation data collected were used to image one of the best-resolved slow slip distributions to date, and indicated slip very close, if not all the way to the trench (Wallace et al., 2016). The Fall 2016 GeoPRISMS Newsletter reported on this experiment and how for the first time, ocean bottom pressure recorders successfully mapped a SSE displacement field (Wallace et al., 2016). The HOBITSS results were instrumental in demonstrating that Absolute Pressure Gauges are a valuable tool for seafloor geodesy. Seismologists from UC Santa Cruz, University of Colorado Boulder and Boston University are now using the seismic data collected during the same experiment to evaluate the spatiotemporal relationship between seismicity (both earthquakes and tremor) and the slow slip event and the role that seismic structure plays in controlling slip behavior. One of our primary goals is to determine if slow and fast interplate slip modes spatially overlap or are segregated.

An initial catalog of local earthquakes was constructed and relocated in a New Zealand-derived velocity model to produce a catalog of 2,619 earthquakes ranging in magnitude between 0.5 and 4.7. Locations indicate that Hikurangi seismicity is concentrated in two NE-SW bands, one offshore beneath the Hikurangi trough and outer forearc wedge, and one onshore beneath the eastern Raukumara Peninsula, with a gap in seismicity between the two beneath the inner forearc wedge. We do not find an increase in seismicity during the 2014 slow slip event, though seismicity is slightly higher in the month following the SSE. The majority of earthquakes are within the subducting slab rather than at the plate interface. The few events that locate close to the plate interface were assumed to be thrust events and used as templates in a waveform matching technique to identify similarly located earthquake swarms within the entire dataset.

Like the general seismicity increase in the month following the SSE, repeating families of interplate events (Fig. 1) also cluster in time at the end of the SSE. They are spatially concentrated within the slow slip patch and associated with a well-imaged subducted seamount (Bell et al., 2010).

Tectonic tremor was also identified toward the end and continuing after the slow slip event. Like the interplate earthquake families, tremor is also co-located with slow slip and localized in the vicinity of subducted seamounts (Fig. 2). The subsequent, rather than synchronous occurrence of tremor and interplate earthquakes and slow slip suggests that seamount subduction plays the dominant role in the stress state of the shallow megathrust. While northern Hikurangi seamounts appear to primarily subduct aseismically, their subduction may generate elevated pore-fluid pressures in accumulated underplated sediment packages and a complex, interconnected fracture network such that tremor and microseismicity occur as seismic components of seamount subduction during shallow slow slip. This study indicates that the location of subducted seamounts is strongly correlated with the distribution of SSE-associated tectonic tremor and repeating earthquakes. The seamounts appear to be responsible for slow slip, tremor, and microseismicity rupturing adjacent regions in a range of slip processes.

Ongoing work more fully utilizes the rich data set of local earthquakes and includes analysis of seismic attenuation using the body wave spectra of local earthquakes, local earthquake seismic velocity tomography, and earthquake source parameter analysis including focal mechanisms and seismic moment. Knowing the physical state of the subducting plate interface is important for the slow slip modeling, and our attenuation and velocity tomography models will be key to infer the physical properties and structure in the area where slow slip occurs. For example, recent work revealed large differences between SSE slip inversions that assume homogeneous elastic properties versus those that utilize a more realistic elastic structure (Williams and Wallace, 2015). ■

References

Bell, R., R. Sutherland, D.H.N. Barker, S. Henrys, S. Bannister, L.M. Wallace, J. Beavan, (2010), Seismic reflection character of the Hikurangi subduction interface, New Zealand, in the region of repeated Gisborne slow slip events. Geophys. J. Int., 180(1), 34–48. doi.org/10.1111/j.1365-246X.2009.04401.x
Wallace, L.M., S.C. Webb, Y. Ito, K. Mochizuki, R. Hino, S. Henrys, S., Schwartz, A.F. Sheehan, (2016), Slow slip near the trench at the Hikurangi subduction zone, New Zealand. Science, 352(6286), 701–704. doi.org/10.1126/science.aaf2349
Williams, C.A., L.M. Wallace, (2015), Effects of material property variations on slip estimates for subduction interface slow slip events, Geophys. Res. Lett., 42(2), 1113-1121. doi.org/10.1016/j.epsl.2018.01.002

Reference information
Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) – Revealing the environment of shallow slow slip
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org

IODP tackles the Hikurangi Margin of New Zealand with two drilling expeditions to unlock the secrets of slow-slip events


MGL1801 Participants – Ryuta Aral (JAMSTEC), Stephen Ball (Univ. of Wisconsin, Madison), Nathan Bangs (UT Austin), Dan Barker (GNS Science), Joel Edwards (UC Santa Cruz), Melissa Gray (Imperial College London), Shuoshuo Han (UT Austin), Harold Leah (Cardiff Univ.), Tim Reston (Univ. of Birmingham), Hannah Tilley (Univ of Hawai’i), and Harold Tobin (Univ. of Wisconsin, Madison)

When the Pacific plate slips beneath the Australian plate along the northern Hikurangi margin, the subduction megathrust does not typically generate earthquakes as it often does in subduction zones. Instead, stress accumulated on the megathrust is released within patches every 2-4 years over a period of weeks in well documented slow slip events (SSEs) (i.e. Wallace and Bevean, 2010). While SSEs are not unique to the Hikurangi margin, SSEs often occur at depths of 30-40 km down dip of the seismogenic zone. This makes them difficult to access and thus difficult to examine the physical conditions that control whether the megathrust slips quickly in regular earthquakes or instead slips slowly. However, the Hikurangi margin has documented regular patterns of SSEs that extend updip along the megathrust to unusually shallow depths of ~2 km below the seafloor. This unusually shallow setting and the well documented distribution of slip makes these SSEs accessible with geophysical tools and even drilling.

From January 6th to February 9th 2018, a team of marine geologists and geophysicists from the US, UK, Japan, and New Zealand sailed on the R/V Langseth to acquire a new 3D seismic reflection data volume across the northern Hikurangi margin offshore of the North Island of New Zealand (Fig. 1). Previous seismic surveys have imaged the subsurface structures and offered hints into the unusual megathrust slip behavior along the Hikurangi margin. Bell et al. (2010) showed large seamounts on the subducting plate that can generate thrust faults within the upper plate, and entrain fluid rich sediments and carry them below the megathrust deep into the subduction zone. It is these impacts on the shallow subduction zone that are thought to generate conditions for high fluid content along the megathrust and fluid migration pathways from the megathrust through the upper plate. It is this fluid supply and flow system that is thought to lead to high fluid pressures and control effective stresses along the megathrust, which are also considered critical controls for slip behavior (Saffer and Wallace, 2015). However, it was also evident from earlier 2D seismic images that this complex setting required 3D data to correctly image the shallow megathrust and upper plate structures. Such high resolution 3D data can map out fluid content and faults to fully characterize this system.

The NZ3D experiment was designed to acquire 3D seismic images to map reflectivity and structures, and it provided an opportunity for a novel wide-angle seismic reflection and refraction component to measure seismic velocities in unprecedented detail and in 3D using full waveform inversion (FWI). The detailed seismic velocity data will reveal rock physical properties and will complement observations of reflectivity and structural geometry seen in 3D seismic images.

In most years this large ambitious geophysical experiment would by itself be a major achievement for any given site; however, the NZ3D project was designed to contribute to larger efforts on the New Zealand primary site that included: The NSF-funded SHIRE active source experiment (Nov–Dec 2017) to examine the crustal scale structure of the Hikurangi margin using ocean bottom seismometers, onland seismic receiver stations, and 2D seismic reflection imaging (p.22); IODP drilling to recover core samples, measure physical properties, and install observatories – Expeditions 372 (Nov 2017–Jan 2018) and 375 (Mar–Apr 2018) (p.16); and other related studies.

During the Langseth cruise we surveyed an area 14 x 60 km from the trench to the shelf across the Expedition 375 drilling transect (Fig. 1). Langseth fired one of two 3,300 in3 airgun arrays every 25 m in flip-flop mode and recorded returns on four 6-km-long, 468-channel seismic streamers spaced at 150 m. We made 62 passes through the survey area, fired 145,924 shots and recorded over 5Tbytes of seismic reflection data. With calm seas during most of the 35 days at sea, few equipment issues, and very few interruptions from protected species, we acquired a high-quality seismic data volume that will enable us to examine reflectivity of the megathrust down to more than 10 km in the area of SSEs and map the geometry of faults and stratigraphic horizons. In order to acquire the data needed for FWI, in December 2017, prior to NZ3D acquisition, the R/V Tangaroa deployed a hundred ocean bottom seismometers (OBSs) provided by JAMSTEC in a randomized grid with nominal 2 x 2 km spacing (Fig. 1). Shots for FWI were also recorded on stations deployed around the Gisborne area specifically for NZ3D and stations that had been deployed initially for SHIRE and remained for NZ3D (Fig. 1). A total of almost 300 onland stations recorded Langseth shots during NZ3D. We were also able to take advantage of the close line spacing during the 3D survey to increase the resolution of multibeam bathymetry and backscatter images across the margin. These data provide some of the best detail of the northern Hikurangi margin seafloor to date.

From here, we will spend the next few years processing the 3D volume (with emphasis on water column multiple removal) and OBS data sets to produce high-quality, detailed 3D images in depth, seismic velocity data, and interpret these results in the context of new results from the coordinated projects. Structures in 3D are already emerging from preliminary results (Fig. 1) and are only going to get better. There are lots of exciting results to come for studies of slow slip along the Hikurangi megathrust. ■

References

Araki, E., D.M. Saffer, A. Kopf, L.M. Wallace, T. Kimura, Y. Machida, S. Ide, (2017), Recurring and triggered slow slip events near the trench at the Nankai Trough subduction megathrust. Science, 356, 1157-1160. doi: 10.1126/science.aan3120
Barker, D.H.N., R. Sutherland, S. Henrys, S. Bannister, (2009), Geometry of the Hikurangi subduction thrust and upper plate, North Island, New Zealand. Geochem Geophys Geosyst, 10(2), Q02007. doi.org/10.1029/2008GC002153
Bell, R., R. Sutherland, D.H.N. Barker, S. Henrys, S. Bannister, L.M. Wallace, J. Beavan, (2010), Seismic reflection character of the Hikurangi subduction interface, New Zealand, in the region of repeated Gisborne slow slip events. Geophys. J. Int., 180(1), 34–48. doi.org/10.1111/j.1365-246X.2009.04401.x
Chang, C., L.C. McNeill, J.C. Moore, W. Lin, M. Conin, Y. Yamada, (2010), In situ stress state in the Nankai accretionary wedge estimated from borehole wall failures. Geochem Geophys Geosyst, 11:Q0AD04. doi.org/10.1029/2010GC003261
Davy, B., K. Hoernle, R. Werner, (2008), Hikurangi Plateau: crustal structure, rifted formation, and Gondwana subduction history. Geochem Geophys Geosyst, 9(7):Q07004. doi.org/10.1029/2007GC001855
Hensen, C., K. Wallmann, M. Schmidt, C.R. Ranero, E. Suess, (2004), Fluid expulsion related to mud extrusion off Costa Rica—a window to the subducting slab. Geology, 32(3), 201–204. doi.org/10.1130/G20119.1
Huffman, K.A., D.M. Saffer, (2016), In situ stress magnitudes at the toe of the Nankai Trough Accretionary Prism, offshore Shikoku Island, Japan. J. Geophys. Res.: Solid Earth, 121(2), 1202–1217. doi.org/10.1002/2015JB012415
Jannasch, H.W., C.G. Wheat, J.N. Plant, M. Kastner, D.S. Stakes, (2004), Continuous chemical monitoring with osmotically pumped water samplers: OsmoSampler design and applications. Limnology and Oceanography: Methods, 2(2), 102–113.
Kopf, A., G. Mora, A. Deyhle, S. Frape, R. Hesse, (2003), Fluid geochemistry in the Japan Trench forearc (ODP Leg 186): a synthesis. In Suyehiro, K., Sacks, I.S., Acton, G.D., and Oda, M. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 186: College Station, TX (Ocean Drilling Program), 1–23. doi.org/10.2973/odp.proc.sr.186.117.2003
Pecher, I.A., P.M. Barnes, L.J. LeVay, and the Expedition 372 Scientists, (2018), IODP Expedition 372 Preliminary Report, Creeping Gas Hydrate Slides and Hikurangi LWD. College Station, TX (International Ocean Discovery Program). doi.org/10.14379/iodp.pr.372.2018
Pedley, K.L., P.M. Barnes, J.R. Pettinga, K.B. Lewis, (2010), Seafloor structural geomorphic evolution of the accretionary frontal wedge in response to seamount subduction, Poverty Indentation, New Zealand. Marine Geology, 270(1–4), 119–138. doi.org/10.1016/j.margeo.2009.11.006
Peng, Z., J. Gomberg, (2010), An integrated perspective of the continuum between earthquakes and slow-slip phenomena. Nat. Geosci., 3(9), 599–607. doi.org/10.1038/ngeo940
Ranero, C.R., I. Grevemeyer, U. Sahling, U. Barckhausen, C. Hensen, K. Wallmann, W. Weinrebe, P. Vannucchi, R. von Huene, K. McIntosh, (2008), Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis. Geochem Geophys Geosyst, 9(3), Q03S04. doi.org/10.1029/2007GC001679
Saffer, D.M., L.M. Wallace, (2015), The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes. Nat. Geosci., 8(8), 594–600. doi.org/10.1038/ngeo2490
Saffer, D.M., L.M. Wallace, K. Petronotis, (2017), Hikurangi subduction margin coring and observatories: unlocking the secrets of slow slip through drilling to sample and monitor the forearc and subducting plate. International Ocean Discovery Program Expedition 375 Scientific Prospectus: College Station, TX (International Ocean Discovery Program). doi.org/10.14379/iodp.sp.375.2017
Saffer, D.M., M.B. Underwood, A.W. McKiernan, (2008), Evaluation of factors controlling smectite transformation and fluid production in subduction zones: application to the Nankai Trough. Island Arc, 17(2), 208–230. doi.org/10.1111/j.1440-1738.2008.00614.x
Schwartz, S.Y., J.M. Rokosky, (2007), Slow slip events and seismic tremor at circum-Pacific subduction zones. Reviews of Geophysics, 45:RG3004. doi.org/10.1029/2006RG000208
Solomon, E.A., M. Kastner, C.G. Wheat, H. Jannasch, G. Robertson, E.E. Davis, J.D. Morris, (2009), Long-term hydrogeochemical records in the oceanic basement and forearc prism at the Costa Rica subducti. Earth Planet. Sci. Lett., 282, 240–251. doi.org/10.1016/j.epsl.2009.03.022
Wallace, L.M., J. Beavan, (2010), Diverse slow slip behavior at the Hikurangi subduction margin, New Zealand. J. Geophys. Res.: Solid Earth, 115(B12):B12402. doi.org/10.1029/2010JB007717
Wallace, L.M., J. Beavan, R. McCaffrey, D. Darby, (2004), Subduction zone coupling and tectonic block rotations in the North Island, New Zealand. J. Geophys. Res.: Solid Earth, 109(B12):B12406. doi.org/10.1029/2004JB003241
Wallace, L.M., S.C. Webb, Y. Ito, K. Mochizuki, R. Hino, S. Henrys, S.Y. Schwartz, A.F. Sheehan, (2016), Slow slip near the trench at the Hikurangi subduction zone, New Zealand. Science, 352(6286), 701–704. doi.org/10.1126/science.aaf2349
Wallace, L.M., Y. Kaneko, S. Hreinsdottir, I. Hamling, Z. Peng, N. Bartlow, E. D’Anastasio, and B. Fry, (2017), Large-scale dynamic triggering of shallow slow slip enhanced by overlying sedimentary wedge. Nat. Geosci., 10, 765–770. doi: 10.1038/ngeo3021
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Reference information
IODP tackles the Hikurangi Margin of New Zealand with two drilling expeditions to unlock the secrets of slow-slip events
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org

The NZ3D Experiment – Adding a new dimension for understanding slow slip events


MGL1801 Participants – Ryuta Aral (JAMSTEC), Stephen Ball (Univ. of Wisconsin, Madison), Nathan Bangs (UT Austin), Dan Barker (GNS Science), Joel Edwards (UC Santa Cruz), Melissa Gray (Imperial College London), Shuoshuo Han (UT Austin), Harold Leah (Cardiff Univ.), Tim Reston (Univ. of Birmingham), Hannah Tilley (Univ of Hawai’i), and Harold Tobin (Univ. of Wisconsin, Madison)

When the Pacific plate slips beneath the Australian plate along the northern Hikurangi margin, the subduction megathrust does not typically generate earthquakes as it often does in subduction zones. Instead, stress accumulated on the megathrust is released within patches every 2-4 years over a period of weeks in well documented slow slip events (SSEs) (i.e. Wallace and Bevean, 2010). While SSEs are not unique to the Hikurangi margin, SSEs often occur at depths of 30-40 km down dip of the seismogenic zone. This makes them difficult to access and thus difficult to examine the physical conditions that control whether the megathrust slips quickly in regular earthquakes or instead slips slowly. However, the Hikurangi margin has documented regular patterns of SSEs that extend updip along the megathrust to unusually shallow depths of ~2 km below the seafloor. This unusually shallow setting and the well documented distribution of slip makes these SSEs accessible with geophysical tools and even drilling.

From January 6th to February 9th 2018, a team of marine geologists and geophysicists from the US, UK, Japan, and New Zealand sailed on the R/V Langseth to acquire a new 3D seismic reflection data volume across the northern Hikurangi margin offshore of the North Island of New Zealand (Fig. 1). Previous seismic surveys have imaged the subsurface structures and offered hints into the unusual megathrust slip behavior along the Hikurangi margin. Bell et al. (2010) showed large seamounts on the subducting plate that can generate thrust faults within the upper plate, and entrain fluid rich sediments and carry them below the megathrust deep into the subduction zone. It is these impacts on the shallow subduction zone that are thought to generate conditions for high fluid content along the megathrust and fluid migration pathways from the megathrust through the upper plate. It is this fluid supply and flow system that is thought to lead to high fluid pressures and control effective stresses along the megathrust, which are also considered critical controls for slip behavior (Saffer and Wallace, 2015). However, it was also evident from earlier 2D seismic images that this complex setting required 3D data to correctly image the shallow megathrust and upper plate structures. Such high resolution 3D data can map out fluid content and faults to fully characterize this system.

The NZ3D experiment was designed to acquire 3D seismic images to map reflectivity and structures, and it provided an opportunity for a novel wide-angle seismic reflection and refraction component to measure seismic velocities in unprecedented detail and in 3D using full waveform inversion (FWI). The detailed seismic velocity data will reveal rock physical properties and will complement observations of reflectivity and structural geometry seen in 3D seismic images.

In most years this large ambitious geophysical experiment would by itself be a major achievement for any given site; however, the NZ3D project was designed to contribute to larger efforts on the New Zealand primary site that included: The NSF-funded SHIRE active source experiment (Nov–Dec 2017) to examine the crustal scale structure of the Hikurangi margin using ocean bottom seismometers, onland seismic receiver stations, and 2D seismic reflection imaging (p.22); IODP drilling to recover core samples, measure physical properties, and install observatories – Expeditions 372 (Nov 2017–Jan 2018) and 375 (Mar–Apr 2018) (p.16); and other related studies.

During the Langseth cruise we surveyed an area 14 x 60 km from the trench to the shelf across the Expedition 375 drilling transect (Fig. 1). Langseth fired one of two 3,300 in3 airgun arrays every 25 m in flip-flop mode and recorded returns on four 6-km-long, 468-channel seismic streamers spaced at 150 m. We made 62 passes through the survey area, fired 145,924 shots and recorded over 5Tbytes of seismic reflection data. With calm seas during most of the 35 days at sea, few equipment issues, and very few interruptions from protected species, we acquired a high-quality seismic data volume that will enable us to examine reflectivity of the megathrust down to more than 10 km in the area of SSEs and map the geometry of faults and stratigraphic horizons. In order to acquire the data needed for FWI, in December 2017, prior to NZ3D acquisition, the R/V Tangaroa deployed a hundred ocean bottom seismometers (OBSs) provided by JAMSTEC in a randomized grid with nominal 2 x 2 km spacing (Fig. 1). Shots for FWI were also recorded on stations deployed around the Gisborne area specifically for NZ3D and stations that had been deployed initially for SHIRE and remained for NZ3D (Fig. 1). A total of almost 300 onland stations recorded Langseth shots during NZ3D. We were also able to take advantage of the close line spacing during the 3D survey to increase the resolution of multibeam bathymetry and backscatter images across the margin. These data provide some of the best detail of the northern Hikurangi margin seafloor to date.

From here, we will spend the next few years processing the 3D volume (with emphasis on water column multiple removal) and OBS data sets to produce high-quality, detailed 3D images in depth, seismic velocity data, and interpret these results in the context of new results from the coordinated projects. Structures in 3D are already emerging from preliminary results (Fig. 1) and are only going to get better. There are lots of exciting results to come for studies of slow slip along the Hikurangi megathrust. ■

References

Bell, R., R. Sutherland, D.H.N. Barker, S. Henrys, S. Bannister, L.M. Wallace, J. Beavan, (2010), Seismic reflection character of the Hikurangi subduction interface, New Zealand, in the region of repeated Gisborne slow slip events, Geophys. J. Int., 180(1), 34–48. doi.org/10.1111/j.1365-246X.2009.04401.x
Saffer, D. M, L.M. Wallace, (2015), The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes, Nat. Geosci., 8, 594–600. doi:10.1038/ngeo2490
Wallace, L.M., J. Beavan (2010), Diverse slow slip behavior at the Hikurangi subduction margin, New Zealand, J. Geophys. Res., 115, B12402. doi:10.1029/2010JB007717.

Reference information
The NZ3D Experiment – Adding a new dimension for understanding slow slip events
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org

Volatile cycling through the Hikurangi forearc, New Zealand


Jaime D. Barnes (UT Austin), Jeffrey Cullen (UT Austin), Shaun Barker (Univ. of Waikato), Samuele Agostini (Istituto di Geoscienze e Georisorse), Sarah Penniston-Dorland (Univ. of Maryland), John C. Lassiter (UT Austin), Andreas Klügel (Univ. of Bremen), Laura Wallace (UT Austin & GNS Science), Wolfgang Bach (Univ. of Bremen)

Much work has focused on defining the elemental concentration and isotopic composition of subduction zone inputs (e.g., sediments, altered oceanic crust, serpentinites), as well as outputs (e.g., volcanic gases and melt inclusions), in order to assess global cycling of volatiles. However, with most work focused on outputs from the volcanic front, poor constraints on the geochemistry of forearc outputs hamper global volatile flux calculations. In fact, most global flux calculations ignore contributions from the forearc because their outputs and the subducted sources contributing to the fore-arc outputs are poorly constrained (e.g., Barnes et al., 2018). The Hikurangi margin of New Zealand has a largely subaerial forearc which hosts numerous fore-arc seeps and springs (Fig. 1). This portion of the forearc is typically submerged beneath the ocean at most other subduction zones. Easy access to fore-arc springs along the margin allows for quantification of volatile flux and sources through the shallow portion of the subduction system (< 50 km).

In addition, there are dramatic along strike variations in subduction parameters along the length of the Hikurangi margin (Fig. 1). The northern portion of the margin has a thin layer of sediments (~ 1 km thick) and many seamounts on the subducting plate, a steep taper angle (7º to 10º) for the accretionary wedge, and shallow (< 15 km depth) aseismic creep. In marked contrast, the southern portion of the margin has a thick (3 to 6 km thick) package of sediments on the incoming plate, low taper angle (4º to 6º), and undergoes stick-slip behavior (e.g., Wallace et al., 2009). Interestingly, previous studies have documented an overall decrease in the Cl, B, Br, Na, and Sr concentrations in fore-arc spring waters from the north to the south (Giggenbach et al., 1995; Reyes et al., 2010). These observations raise numerous questions, such as: the amount of slab-derived fluid component to the springs; whether fluid sources vary along the length of the margin; and particularly whether any chemical variations in the spring fluids record dehydration metamorphic reactions that may be linked to changes in slip behavior along the margin. In order to address these questions, we have sampled fluids from sixteen cold and two thermal springs from along the Hikurangi margin and analyzed them for their cation and anion concentrations, as well as their B, Li, and Cl stable isotope compositions. Because Li, Cl, and B are highly fluid-mobile elements, their incompatibility limits modification by fluid-rock interaction making them excellent tracers of fluid source.

Data show that Cl, Br, I, Sr, B, Li, and Na concentrations are high in the forearc springs, consistent with previous studies. Most of these elements show a general decrease in concentration from north to south, a high in the central part of the margin, and limited variability through time. Despite the dramatic change in concentration along the margin, there is no corresponding trend in isotopic composition. Cl and B isotope compositions are remarkably consistent along the margin, suggesting fluids dominated by seawater and sedimentary pore fluids. Lithium isotope compositions are highly variable, suggesting fluids sourced from seawater and locally modified by interaction with host rock. High Br/Cl and I/Cl weight ratios also support a dominant seawater and pore fluid source.

The decrease in the absolute volatile concentrations along the margin is therefore not due to changes in subduction parameters (e.g., convergence rate, sediment subduction) altering the fluid source along the strike. In addition, the shift in seismic behavior along the margin is not linked to a change in fluid source within the forearc region. Instead, we hypothesize that the shift in volatile concentrations along the margin is controlled by fluid flux through the upper plate, due to increasing upper plate permeability from south to north. In the northern portion of the margin, the upper plate is undergoing extension, whereas in the southern portion, the upper plate is undergoing transpression (Fig. 1) (Wallace et al., 2004). The extension in the northern section of the margin could increase the permeability of the upper plate allowing for fluid loss along normal faults, and possibly lower fluid pressure within the forearc and near the interface. In contrast, the transpressional regime in the south could decrease the permeability of the upper plate, trapping fluids and increasing fluid pressure in the upper plate (Fagereng and Ellis, 2009). This model of changing permeability from north to south explains the decrease in volatile concentrations in spring fluids along strike. In the south, trapping of expelled seawater and pore fluids in the upper plate will allow the fluids to become diluted by meteoric groundwater, but their isotopic compositions will remain unchanged. Whereas in the north, the seawater and pore fluids will be able to pass through the upper plate with less dilution by groundwater. Seismic tomographic and attenuation data also suggest that more fluids are present in the northern and central portions of the upper plate of the Hikurangi subduction zone, compared to the south (Eberhart-Phillips et al., 2017; Eberhart-Phillips et al., 2005; Eberhart-Phillips et al., 2008). Springs with the highest concentrations of volatile elements are located in regions with some of the highest seismic attenuation (which can be interpreted as abundant inter-connected fluids). It is possible that the fluid pressure conditions in the upper plate may play an important role in seismic behavior along the Hikurangi margin. Higher fluid pressures in the south suppress the transition from brittle to viscous deformation, resulting in a deeper brittle-viscous transition and the occurrence of stick-slip behavior to greater depths (Fagereng and Ellis, 2009; Wallace et al., 2012). Greater structural permeability in the northern Hikurangi margin may allow fluids to bleed off, without building significant overpressures in the forearc — this could lead to a comparatively shallower brittle to viscous transition. This work highlights the role of the upper plate tectonics and permeability in controlling the flow of fluid through the forearc and the geochemical consequences on shallow outputs through the subduction system. ■

References

Barnes, J.D., C.E. Manning, M. Scambelluri, J. Selverstone, (2018), Behavior of halogens during subduction zone processes, in Harlov, D., and Aranovich, L., eds., The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes, Springer, 545-590.
Eberhart-Phillips, D., S. Bannister, M. Reyners, (2017), Deciphering the 3-D distribution of fluid along the shallow Hikurangi subduction zone using P-and S-wave attenuation. Geophys. J. Int., 211, 1054-1067.
Eberhart-Phillips, D., M. Reyners, M. Chadwick, J.M. Chiu, (2005), Crustal heterogeneity and subduction processes: 3-D Vp, Vp/Vs and Q in the southern North Island, New Zealand. Geophys. J. Int., 162, 270-288.
Eberhart-Phillips, D., M. Reyners, M. Chadwick, G. Stuart, (2008), Three-dimensional attenuation structure of the Hikurangi subduction zone in the central North Island, New Zealand. Geophys. J. Int., 174, 418-434.
Fagereng, A., S. Ellis, (2009), On factors controlling the depth of interseismic coupling on the Hikurangi subduction interface, New Zealand. Earth Planet. Sci. Lett., 278, 120-130.
Giggenbach, W. F., M.K. Stewart, Y. Sano, R.L. Goguel, G.L. Lyon, (1995), Isotopic and chemical composition of solutions and gases from the East Coast accretionary prism, New Zealand. Isotope and Geochemical Techniques Applied to Geothermal Investigations. IAEA-TECDOC, 788, 209–231.
Reyes, A.G., B.W. Christenson, K. Faure, (2010), Sources of solutes and heat in low-enthalpy mineral waters and their relation to tectonic setting, New Zealand. J. Volcanol. Geotherm. Res., 192, 117-141.
Wallace, L.M., J. Beavan, R. McCaffrey, D. Darby, (2004), Subduction zone coupling and tectonic block rotations in the North Island, New Zealand. J. Geophys. Res.: Solid Earth, 109(B12).
Wallace, L.M., A. Fagereng, S. Ellis, (2012), Upper plate tectonic stress state may influence interseismic coupling on subduction megathrusts. Geology, 40, 895-898.
Wallace, L.M., M. Reyners, U. Cochran, S. Bannister, P.M. Barnes, K. Berryman, G. Downes, D. Eberhart-Phillips, A. Fagereng, S. Ellis, A. Nicol, R. McCaffrey, R.J. Beavan, S. Henrys, R. Sutherland, D.H.N. Barker, N. Litchfield, J. Townend, R. Robinson, R. Bell, K. Wilson, W. Power, (2009), Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand. Geochem Geophys Geosyst, 10, Q10006, doi:10010.11029/12009GC002610.

Reference information
Volatile cycling through the Hikurangi forearc, New Zealand
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org