iMUSH: Imaging Magma Under St. Helens


iMUSH was a four year collaborative research project involving several institutions and supported by the GeoPRISMS and EarthScope Programs of the US National Science Foundation to illuminate the architecture of the greater Mount St. Helens magmatic system from slab to surface.

NEWS

May 18th 2020 marks the 40th anniversary of the big 1980 Mount Saint Helens’ eruption. See below a list of virtual events that are organized to commemorate the eruption and share what we have learnt since then.

 icon-chevron-right Mount St. Helens isn’t where it should be. Scientists may finally know why | National Geographic, May 18, 2020

 icon-chevron-right Mount St. Helens and the Cascade Range Volcanoes – The 40th Anniversary

May 18th 6:30 PM PDT, followed by live Q&A at 8:00pm

Live on Youtube at: https://bit.ly/2LmNj0v
and Facebook at: https://facebook.com/thePNSN

Four northwest scientists will present a review of Cascadia Region tectonics, volcanoes, volcanic hazards, and a summary of how the science and monitoring has evolved over the last 40 years. It will also include first person accounts of the buildup to the May 18, 1980 eruption as experienced by University of Washington seismologist Steve Malone.

Hosted by the Pacific Northwest Seismic Network, moderated by PNSN Director Harold Tobin, with presenters Dr Jackie Caplan-Auerbach, Dr Joe Dufek, Dr Seth Moran and Dr Steve Malone.

icon-chevron-right University of Washington Experts on Mount St Helens

About the iMUSH project

iMUSH was a four year collaborative research project involving several institutions and supported by the GeoPRISMS and EarthScope Programs of the US National Science Foundation to illuminate the architecture of the greater Mount St. Helens magmatic system from slab to surface.

To determine the architecture of magmatic systems in general, including the extent and characteristics of highly crystalline magma bodies, and to resolve major tectonic controls on volcanism along the Cascade arc, a variety of geophysical imaging techniques such as magnetotelluric, high-resolution active source seismic imaging and passive seismic monitoring and imaging, integrated with geochemical-petrological data, were used to image and interpret the crust and upper mantle in the greater Mount St. Helens area.

Left: Map of the active source deployment. In the summer of 2014, 23 shots were recorded by about 2500 Texan seismometers installed in two deployments, in addition to 950 Nodal seismometers. The black line shows the location of cross-sections in Figure 2. Right: Locations of permanent and temporary broadband seismometers used in the passive source experiment, magnetotelluric sites, and petrologic samples.

Participant list

Eight Principal Investigators (PIs) from six different institutions were collaborating on the iMUSH project. In addition to these NSF funded investigators, five Investigators from the three US Geological Survey research groups were closely involved with several parts of the project.

  • Geoffrey A Abers (Cornell University)
  • Olivier Bachmann (ETH-Zurich)
  • Paul Bedrosian (USGS)
  • Dawnika L Blatter (USGS)
  • Esteban Bowles-Martinez (Oregon State University)
  • Michael A Clynne (USGS)
  • Kenneth C Creager (University of Washington)
  • Kayla Crosbie (Cornell University)
  • Roger P Denlinger (USGS)
  • Margaret E Glasgow (University of New Mexico)
  • Jiangang Han (University of Washington)
  • Steven M Hansen (University of New Mexico)
  • Graham J Hill University of Canterbury
  • Eric Kiser (University of Arizona)
  • Alan Levander (Rice University)
  • Michael Mann (Cornell University)
  • Xiaofeng Meng (University of Washington)
  • Seth C Moran (USGS)
  • Jared Peacock (USGS)
  • Brandon Schmandt (University of New Mexico)
  • Adam Schultz (Oregon State University)
  • Thomas W Sisson (USGS)
  • Roque A Soto Castaneda (Cornell University)
  • Weston A Thelen (USGS)
  • Carl W Ulberg (University of Washington)
  • John E Vidale (University of Washington)
  • Maren Wanke (ETH-Zurich)

iMUSH is funded by NSF-GeoPRISMS, NSF-Earthscope with substantial in-kind support from the USGS.

Visit the iMUSH website for more information about the project.

PublicationsNewsletter and press reports

Ulberg, C.W., K.C. Creager, S.C. Moran, G.A. Abers, A. Levander, E. Kiser, B. Schmandt, S. Hansen, R. Crosson, 2020. Local source Vp and Vs tomography in the Mount St Helens region with the iMUSH broadband array. Geochemistry, Geophysics, Geosystems, 21, e2019GC00888. doi: 10.1029/2019GC008888

Crosbie, K.J., G.A. Abers, M.E. Mann, H.A. Janiszewski, K.C. Creager, C. Ulberg, S. Moran, 2019. Shear velocity structure from ambient noise and teleseismic surface wave tomography in the Cascades around Mount St. Helens. Journal of Geophysical Research, v. in press. https://doi.org/10.1029/2019JB017836

Eakin, C.M., E.A. Wirth, A. Wallace, C.W. Ulberg, K.C. Creager, G.A. Abers, 2019. SKS splitting beneath Mount St. Helens: Constraints on subslab mantle entrainment. Geochemistry, Geophysics, Geosystems, 20, 4202–4217. https://doi.org/10.1029/2019GC008433

Kiser, E., A. Levander, C. Zelt, B. Schmandt, S. Hansen, 2019. Upper crustal structure and magmatism in southwest Washington: Vp, Vs, and Vp/Vs results from the iMUSH active-source seismic experiment. Journal of Geophysical Research: Solid Earth, 124(7), 7067-7080. https://doi.org/10.1029/2018JB016203

Mann, M.E., G.A. Abers, K.J. Crosbie, K.C. Creager, C. Ulberg, S. Moran, S. Rondenay, 2019. Imaging subduction beneath Mount St. Helens: Implications for slab dehydration and magma transport. Geophysical Research Letters, 46, 3163–3171, https://doi.org/10.1029/2018GL081471

Wanke, M., M.A. Clynne, A. von Quadt, T.W. Vennemann, O. Bachmann, 2019. Geochemical and petrological diversity of mafic magmas from Mount St. Helens. Contributions to Mineralogy and Petrology, 174, 10. https://doi.org/10.1007/s00410-018-1544-4

Wanke, M., O. Karakas, O. Bachmann, 2019. The genesis of arc dacites: The case of Mount St. Helens, WA. Contributions to Mineralogy and Petrology, 174, 7. https://doi.org/10.1007/s00410-018-1542-6

Bedrosian, P.A., J.R. Peacock, E. Bowles-Martinez, A. Schultz, G.J. Hill, 2018. Crustal inheritance and a top-down control on arc magmatism at Mount St. Helens: Nature Geoscience, 11, 865–870. https://doi.org/10.1038/s41561-018-0217-2

Glasgow, M.E., B. Schmandt, S.M. Hansen, 2018. Upper crustal low-frequency seismicity at Mount St. Helens detected with a dense geophone array. Journal of Volcanology and Geothermal Research, 358, 329–341. https://doi.org/10.1016/j.jvolgeores.2018.06.006

Han, J., J.E. Vidale, H. Houston, D.A. Schmidt, K.C. Creager, 2018. Deep long‐period earthquakes beneath Mount St. Helens: Their relationship to tidal stress, episodic tremor and slip, and regular earthquakes. Geophysical Research Letters, 45, 2241–2247. https://doi.org/10.1002/2018GL077063

Kiser, E., A. Levander, C. Zelt, B. Schmandt, S. Hansen, 2018. Focusing of melt near the top of the Mount St. Helens (USA) magma reservoir and its relationship to major volcanic eruptions. Geology, 46, 775–778. https://doi.org/10.1130/G45140.1

Blatter, D.L., T.W. Sisson, W.B. Hankins, 2017. Voluminous arc dacites as amphibole reaction-boundary liquids. Contributions to Mineralogy and Petrology, 172, doi:10.1007/s00410-017-1340-6

Olson, S., 2017. The Next Big Bang. Scientific American, 317(5), 34-41 10.1038/scientificamerican1117-34

Wang, Y., F. Lin, B. Schmandt, J. Farrell, 2017. Ambient noise tomography across Mount St. Helens using a dense seismic array. Journal of Geophysical Research: Solid Earth, 122, 4492–4508. https://doi.org/10.1002/2016JB013769

Hansen, S.M., B. Schmandt, A. Levander, E. Kiser, J.E. Vidale, G.A. Abers, K.C. Creager, 2016. Seismic evidence for a cold serpentinized mantle wedge beneath Mount St Helens. Nature Communications, 7, 13242, doi:10.1038/ncomms13242.

Kiser, E., I. Palomeras, A. Levander, C. Zelt, S. Harder, B. Schmandt, S. Hansen, K. Creager, C. Ulberg, 2016. Magma reservoirs from the upper crust to the Moho inferred from high-resolution Vp and Vs models beneath Mount St. Helens, Washington State, USA. Geology, 44, 411–414. https://doi.org/10.1130/G37591.1

Hansen, S.M., B. Schmandt, 2015. Automated detection and location of microseismicity at Mount St. Helens with a large-N geophone array. Geophysical Research Letters, 42, 7390–7397. https://doi.org/10.1038/ncomms13242

icon-chevron-right Science Report – Imaging Magma Under Mount St. Helens with Geophysical and Petrologic Methods | Carl Ulberg and the iMUSH Team, GeoPRISMS Newsletter #39 – Fall 2017

 icon-chevron-right Report from the Field – iMUSH: Imaging Magma Under St. Helens | Carl Ulberg and the iMUSH Team, GeoPRISMS Newsletter #34 – Spring 2015

 icon-chevron-right Press reports are listed on the iMUSH project website

icon-chevron-right Brandon Schmandt video recordings, part of the GeoPRISMS Lecture Series. University of Oregon, Eugene, OR, May 2018


E-FIRE Field Institute Alps Summer 2017 | Erro Tobbio


Erro Tobbio

 icon-location-arrow 45.588˚N 8.790˚E

The Erro-Tobbio Unit in the Voltri Massif (Ligurian Alps, Italy) is mainly composed of high-pressure antigorite serpentinites and metaperidotites intruded by gabbroic and basaltic dikes. This unit has been interpreted as a slice of a variably serpentinized subcontinental mantle exposed on the Tethyan ocean floor that underwent partial dehydration during subduction in Cretaceous time. The peak metamorphic conditions of the Erro-Tobbio Unit were constrained to be 550–600 °C and 2.0–2.5 GPa based on the associated eclogitized metagabbros, metabasalts and metarodingites. The unit contains large and continuous outcrops of high-pressure antigorite serpentinites and metaperidotites with cross cutting veins containing secondary metamorphic olivine, magnetite and Ti-rich humite group minerals. The unique field relation and mineralogical assemblages in both the wall-rock and veins have been regarded as a snapshot of the early dehydration and subsequent fluid channelization that occurs in a subduction zone. Thus, studying the Erro-Tobbio Unit may provide key constraints in understanding those poorly understood deep Earth processes.

The E-FIRE group aims to understand fundamental questions on how and when water and other volatiles escape from a dehydrating rock, what the compositions of the liberated fluids are, and how such reactive fluids interact with and alter the wall-rock as it passes through. Those research questions will be addressed using a combination of phase petrology, whole-rock and mineral elemental and isotope geochemistry, and thermodynamic modeling of fluid-rock interactions. Samples collected mainly along the Gorzente River include deformed and undeformed partially dehydrated serpentinites/metaperidotites, cm-sized veins containing metamorphic olivine, magnetite and Ti-rich clinohumite assemblage, eclogitized metagabbro and metarodingite.

E-FIRE Field Institute Alps Summer 2017 | Voltri Massif


Voltri Massif

 icon-location-arrow 44.479˚N 8.601˚E

The Voltri massif is an exhumed meta-ophiolite that consist of portions of subcontinental lithospheric mantle, oceanic crust (predominantly metagabbroic dikes and rodingites), and minor sediments that formed part of the Ligurian Tethys Ocean that formed during the Jurassic. In different parts of the Voltri massif, these rocks variably record snapshots of their tectonic history from initial mantle processes and seafloor spreading, subduction up to eclogite facies conditions, and a greenschist facies overprint during exhumation. The entire massif has been interpreted as a tectonic mélange, with most structures forming during subduction and exhumation.

Our primary research goals in the Voltri massif are to constrain the PT conditions and timing of prograde subduction and the composition and movement of fluids during subduction. Current projects involve using thermodynamic and geochemical modeling, garnet geochronology, Li isotopes, and fluid inclusions to answer these questions.

E-FIRE Field Institute Alps Summer 2017 | Schistes Lustrés


Schistes Lustrés – Cottian Alps Transect

 icon-location-arrow 44.998N 6.883E

A former accretionary wedge, the Schistes Lustrés high-pressure (HP) unit in the Western Alps is composed of calcschists and metapelites subducted to blueschist facies conditions. Other lithologies entrained locally in the metasediments include serpentinites and, less often, metabasalts. Pressure and temperature conditions within the Schistes Lustrés vary from E-W and can show subtle, but distinct evidence for pressure jumps between individual subunits, which are interpreted to have been juxtaposed during subduction. The northern part of the Schistes Lustrés is overlain by the Dent Blanche unit. On the western side of the Dent Blanche, temperatures in the Schistes Lustrés decrease to the west from structurally deeper units in the east. In the southern section of the Western Alps, where the Schistes Lustrés borders the Monviso meta-ophiolite to the west, individual subunits grade continuously in temperature (350 – 500ºC), with the deepest, and hottest, units exposed in the west. A distinct jump in pressure of ~5 kbar between the Lower Unit and Median Unit likely indicates a post-peak pressure pairing of these two subunits. The whole unit was exhumed at 35 – 40 Ma based on Rb/Sr and Ar/Ar dating and reached peak pressure between 55 – 45 Ma, although the HP event is difficult to discern due to a widespread greenschist-facies overprint.

The Schistes Lustrés also represents an important locale as a key to quantifying the subduction component of the global carbon cycle. Stable isotope (δ18Ο and δ13C) data suggest externally derived H2O-rich fluids percolated into the unit. However, on average, the Schistes Lustrés experienced only minor decarbonation.

HT-RESIST Hikurangi Trench Regional Electromagnetic Survey to Image the Subduction Thrust


Christine Chesley with Samer Naif and Kerry Key
LDEO, Columbia University

Because New Zealand’s north island lies at the juncture between the converging Pacific and Indo-Australian plates, it is not surprising that the area experiences earthquakes. A unique feature of the Hikurangi margin, the name of New Zealand’s subduction zone, is that its earthquake slip behavior varies from north to south along strike. The northern Hikurangi margin is characterized by shallow slow slip events (SSEs) and weak seismic coupling while the southern margin exhibits deeper SSEs and stronger coupling. The host of other properties that change along this subduction zone have motivated the question, “What controls the along-strike variation in megathrust behavior at the Hikurangi margin?”
One key element of this question lies in quantifying the porosity and fluid budget along the margin. Marine electromagnetic (EM) methods are well-suited for imaging fluids and fluid pathways within the lithosphere. Of course, a major caveat to any geophysical survey of convergent margins is the challenge of collecting good data on the seafloor beneath a deep ocean. So that is what we set out to do on 16 December 2018.

Figure 1. Survey map showing location of leg 1 OBEM deployments (green triangles), leg 2 OBEM deployments (magenta squares), CSEM tows (peach lines), and GNS land receivers (white and blue circles). Inset shows regional tectonics (from http://volcano.oregonstate.edu).

“We know about earthquakes here in Wellington,” asserted a waiter at the Thistle Inn. After a satisfying meal, my colleague and I were giving an abridged rundown of our cruise objectives to this excited employee. It was a day or so before we would leave for a month-long voyage to deploy ocean bottom electromagnetometers (OBEMs) for controlled-source electromagnetic (CSEM) and magnetotelluric (MT) imaging of the subseafloor off New Zealand’s north island. Curious about our business in New Zealand, our waiter warned us that talking about earthquakes was making people anxious in his country. Somehow, it was refreshing to find a non-geophysicist who thought our work was important. But it also impressed upon me the urgency to make this cruise a success.

The cruise itself was divided into two legs, both of which were carried out on the R/V Roger Revelle. The first and longer of the two legs involved the collection of the four lines of CSEM data shown in Figure 1, in addition to the deployment of 42 OBEMs for collection of passive MT data. I had never been to sea for more than a few hours – as a geophysics PhD student, I would spend most of my days in front of a computer rather than performing manual labor. I am pretty accustomed to having stable ground beneath my feet and a bed that doesn’t rock at night.

Everything about the experience was new for me.

Before the cruise, I had only ever read about how our marine EM group collects data. Getting a firsthand look at the process has given me a tremendous amount of respect for how much effort goes into data collection, especially when things don’t go according to plan.

During the first leg of the cruise, the science crew consisted of eight researchers – five PhD students, two postdocs, and our Principal Investigator, Samer Naif, who led this cruise for the first time as Chief Scientist. The crew also included two Scripps EM Lab technicians and two Research Technicians to operate the cranes and supervise our actions on deck, making sure we were following safety protocols. Each twelve-hour shift counted six extremely hardworking individuals. Steady seas and mild to warm weather persisted for the majority of the first cruise, helping us ease into our sea legs and avoid seasickness.

Though we faced noteworthy obstacles in securing each line of CSEM data, the first line has given every one of us an answer to that age old interview question on describing a challenge we overcame. We began by deploying 38 of the Scripps OBEMs in just 24 hours, a nontrivial task as only five members of our entire team had ever assembled these receivers before the cruise. Receivers are the heart and soul of any data collection survey, and the Scripps OBEMs are broadband systems that continuously measure the horizontal components of natural and induced electromagnetic field energy. Such energy propagates through the Earth’s lithosphere in a manner that should depend on its electrical conductivity, which in turn depend in part on variations in fluid content. Proper assembly of the receivers is the first step to ensuring quality data recovery. I appreciated the inexhaustible patience shown by our Scripps EM Lab technicians, Jake Perez and Chris Armerding. From explaining to re-explaining how to use a torque wrench, test the acoustics on our receivers, properly affix electrodes, or attach a concrete block to the base of the receiver, Jake and Chris transformed our group of mostly inexperienced grad students into capable field workers. They showed us the multifaceted usefulness of 3M Scotch 35 electrical tape and cable ties that held electrodes, copper, or wires in place and always seemed to find a home in the pockets of my work pants.

Still jet-lagged and adjusting to twelve hours of manual labor per day, the first line of deployments was the most taxing. Nevertheless, the successful deployment of the receivers provided some reprieve as the next step was to tow our active source instrument, SUESI, the Scripps Undersea Electromagnetic Source Instrument. SUESI’s sharklike body tows behind it both long (~300 m) and short (~10 m) antennas terminated by thirty meter copper electrodes. By attaching SUESI to the ship’s winch using a standard oceanographic 0.680” coaxial deep-tow cable, we can send an alternating electric current from the ship to SUESI. SUESI then rectifies the signal and converts it from high voltage to a high current rectangular waveform that gets injected into the seawater across the copper electrodes. Thus, SUESI’s antennas behave as an EM dipole whose energy propagation can be used to probe the shallow lithosphere. As we started deploying SUESI, Poseidon decided it was time to pay for the nice weather and brisk pace we had enjoyed until then. After the arduous process of assembling, deploying, and lowering SUESI into the depths of the ocean, one of her copper antennas partially snapped. We had to haul SUESI back on board, repair the antenna, and deploy her down into the ocean again, a process that took several hours of deckwork. Hopefully, that was enough excitement to last the entire month. But no. The next day brought with it an inexplicable malfunction that led to yet another retrieval of SUESI. Perhaps she did not like the west Pacific water all that much. Thankfully, our Chief Scientist Samer Naif and lab techs Jake and Chris had planned for the unexpected and brought SUESI’s sister along, as a spare. We had better luck with the second SUESI and ended up relying on her for the remainder of the cruise.

Upon recovering SUESI at the end of the tow, it was time to retrieve the OBEMs to use them for the second line. Even with a heavy concrete block to carry the receivers to the seafloor (Fig. 1), ocean currents can move the OBEMs laterally away from the drop site during their descent through the water column. Once on the seafloor, it is necessary to know the exact location of the OBEM to accurately model the CSEM data. This is achieved by measuring the time it takes for an acoustic pulse sent from the ship to be repeated by the OBEM receiver. Similar to a game of “Marco Polo,” the ship sends and receives these acoustic signals at multiple locations until we have enough information to deduce where the receiver resides. We then send a specially coded acoustic signal to release the OBEM from its concrete block. Once the receiver floats to the surface, the team must act quickly to fish it out of the water. For me, retrieving the surfaced OBEMs was the most nerve wracking part of the process. What if we didn’t throw the grappling hook far enough? What if we couldn’t hook the receiver to the crane? What if the GPS buoy malfunctioned and the receiver couldn’t be located? Despite these worries, we managed to recover every single OBEM that we deployed for CSEM data, not only for the first line but for each of the next three as well – a total of 128 stations.

And what beautiful data we retrieved.

Between steak nights and fish tacos, rom coms and Coen Brothers movies, podcasts on olive oil and speculations about giant squids breaking our instruments, we collected three more lines of CSEM data following a similar routine of deploy-tow-recover. We learned to tie bowlines, clove hitches, and square knots. We watched sunrises, sunsets, witnessed dolphins playing with the bow and participated in safety drills of varying theatrics. And when all was said and done, we would manage to gather 20% more CSEM data than initially planned.

With the CSEM portion of the cruise over, we deployed all 42 OBEMs for the passive source MT portion of the project. Though broadband OBEMs can simultaneously collect CSEM and MT data, we left the receivers on the seafloor for about one month to collect higher quality, long-period MT data. This allowes us to look deeper into the Earth to learn about the lithosphere-asthenosphere system.

The second leg of the cruise in February 2019 involved recovering the OBEMs from the MT deployment phase. This leg included thirteen participants, five of whom were based in New Zealand. Though I did not participate in the second cruise, I was thrilled to hear that all 42 receivers were recovered despite the gnarly weather the team encountered. Taken together with the first cruise, it means a perfect recovery rate for all 170 deployments.

Combined data with the land MT sites collected by GNS Science, New Zealand, this is the largest amphibious EM dataset to date. I am thrilled to be working on this tremendous amount of data for the remainder of my PhD and excited to find what secrets they will unlock about the nature of the Hikurangi margin.

>> It was very educating and fun to work with instruments other than the ones I am used to from my institute. I also took home some ideas for organizing science on research vessels, which might benefit my work group.

– Gesa Franz

>> Even when the waves were high and we could surf in a chair inside the Roger Revelle it was an amazing personal and scientific experience. In my particular case, as a person used to coding and doing mathematics, to do ‘real’ science was very inspiring.

– Julen Alvarez-Aramberri

>> Doing fieldwork at sea gave me a whole new sense of what it means to do science, to be a scientist. It is so much more than analyzing or modeling data on a computer in the mundane safety of an office. We were out on deck in 40 knot winds and six meter seas. Science tests your body and your resolve, not just your mind. Just being on a research vessel dedicated solely to advancing our understanding of our amazing planet was inspiring. And then, of course, there were the sunrises, the stars, and the dolphins.

Daniel Blatter

Reference information

HT-RESIST Hikurangi Trench Regional Electromagnetic Survey to Image the Subduction Thrust. C. Chesley, S. Naif, K. Key
GeoPRISMS Newsletter, Issue No. 42, Spring 2019. Retrieved from http://geoprisms.org

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

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