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  • richardmitnick 8:38 am on May 17, 2019 Permalink | Reply
    Tags: "From Earth’s deep mantle, Bermuda has a unique volcanic past., , Geochemical signatures, , scientists discover a new way volcanoes form", Subduction zones, The mantle’s transition zone – between 250 to 400 miles beneath our planet’s crust, The peculiar and extreme isotopes measured in the Bermuda lava core had not been observed before., There is enough water in the transition zone to form at least three oceans according to Gazel but it is the water that helps rock to melt in the transition zone.,   

    From Cornell Chronicle: “From Earth’s deep mantle, scientists discover a new way volcanoes form” 

    From Cornell Chronicle

    May 15, 2019
    Blaine Friedlander
    bpf2@cornell.edu

    1
    Bermuda has a unique volcanic past. About 30 million years ago, a disturbance in the mantle’s transition zone supplied the magma to form the now-dormant volcanic foundation on which the island sits. Wendy Kenigsberg/Clive Howard – Cornell University, modified from Mazza et al. (2019)

    Far below Bermuda’s pink sand beaches and turquoise tides, Cornell geoscientists have discovered the first direct evidence that material from deep within Earth’s mantle transition zone – a layer rich in water, crystals and melted rock – can percolate to the surface to form volcanoes.

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    In a cross-polarized microscopic slice of a core sample, the blue and yellow crystal is titanium-augite, surrounded by a ground mass of minerals, which include feldspars, phlogopite, spinel, perovskite and apatite. This assemblage suggests that the mantle source – rich in water – produced this lava. Gazel Lab/Provided

    Scientists have long known that volcanoes form when tectonic plates (traveling on top of the Earth’s mantle) converge, or as the result of mantle plumes that rise from the core-mantle boundary to make hotspots at Earth’s crust.

    The tectonic plates of the world were mapped in 1996, USGS.

    But obtaining evidence that material emanating from the mantle’s transition zone – between 250 to 400 miles beneath our planet’s crust – can cause volcanoes to form is new to geologists.

    “We found a new way to make volcanoes. This is the first time we found a clear indication from the transition zone deep in the Earth’s mantle that volcanoes can form this way,” said senior author Esteban Gazel, Cornell associate professor in the Department of Earth and Atmospheric Sciences. The research published in Nature on May 15.

    “We were expecting our data to show the volcano was a mantle plume formation – an upwelling from the deeper mantle – just like it is in Hawaii,” Gazel said. But 30 million years ago, a disturbance in the transition zone caused an upwelling of magma material to rise to the surface, form a now-dormant volcano under the Atlantic Ocean and then form Bermuda.

    Using a 2,600-foot core sample – drilled in 1972, housed at Dalhousie University, Nova Scotia – co-author Sarah Mazza of the University of Münster, Germany, assessed the cross-section for signature isotopes, trace elements, evidence of water content and other volatile material. The assessment provided a geologic, volcanic history of Bermuda.

    “I first suspected that Bermuda’s volcanic past was special as I sampled the core and noticed the diverse textures and mineralogy preserved in the different lava flows,” Mazza said. “We quickly confirmed extreme enrichments in trace element compositions. It was exciting going over our first results … the mysteries of Bermuda started to unfold.”

    From the core samples, the group detected geochemical signatures from the transition zone, which included larger amounts of water encased in the crystals than were found in subduction zones. Water in subduction zones recycles back to Earth’s surface. There is enough water in the transition zone to form at least three oceans, according to Gazel, but it is the water that helps rock to melt in the transition zone.

    The geoscientists developed numerical models with Robert Moucha, associate professor of Earth sciences at Syracuse University, to discover a disturbance in the transition zone that likely forced material from this deep mantle layer to melt and percolate to the surface.

    Despite more than 50 years of isotopic measurements in oceanic lavas, the peculiar and extreme isotopes measured in the Bermuda lava core had not been observed before. Yet, these extreme isotopic compositions allowed the scientists to identify the unique source of the lava.

    “If we start to look more carefully, I believe we’re going to find these geochemical signatures in more places,” said co-author Michael Bizimis, associate professor at the University of South Carolina.

    Gazel explained that this research provides a new connection between the transition zone layer and volcanoes on the surface of Earth. “With this work we can demonstrate that the Earth’s transition zone is an extreme chemical reservoir,” he said. “We are now just now beginning to recognize its importance in terms of global geodynamics and even volcanism.”

    Said Gazel: “Our next step is to examine more locations to determine the difference between geological processes that can result in intraplate volcanoes and determine the role of the mantle’s transition zone in the evolution of our planet.”

    Gazel is a fellow at Cornell’s Atkinson Center for a Sustainable Future and a fellow at Cornell’s Carl Sagan Institute. In addition to Gazel, Mazza, Bizimis and Moucha, co-authors of “Sampling the Volatile-Rich Transition Zone Beneath Bermuda,” are Paul Béguelin, University of South Carolina; Elizabeth A. Johnson, James Madison University; Ryan J. McAleer, United States Geological Survey; and Alexander V. Sobolev, the Russian Academy of Sciences.

    The National Science Foundation provided funding for this research.

    See the full article here .


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    Once called “the first American university” by educational historian Frederick Rudolph, Cornell University represents a distinctive mix of eminent scholarship and democratic ideals. Adding practical subjects to the classics and admitting qualified students regardless of nationality, race, social circumstance, gender, or religion was quite a departure when Cornell was founded in 1865.

    Today’s Cornell reflects this heritage of egalitarian excellence. It is home to the nation’s first colleges devoted to hotel administration, industrial and labor relations, and veterinary medicine. Both a private university and the land-grant institution of New York State, Cornell University is the most educationally diverse member of the Ivy League.

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  • richardmitnick 10:53 am on January 22, 2018 Permalink | Reply
    Tags: , , , , , Marine geodesy, Megathrust zone, , Subduction zones   

    From Eos: “Modeling Megathrust Zones” 

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    1.22.18
    Rob Govers

    A recent paper in Review of Geophysics built a unifying model to predict the surface characteristics of large earthquakes.

    1
    The Sendai coast of Japan approximately one year after the 2011 Tohoku earthquake. The harbor moorings and the quay show significant co-seismic subsidence. The dark band along the quay wall resulted from post-seismic uplift. Credit: Rob Govers.

    The past few decades have seen a number of very large earthquakes at subduction zones. Researchers now have an array of advanced technologies that provide insights into the processes of plate movement and crustal deformation. A review article recently published in Reviews of Geophysics pulled together observations from different locations worldwide to evaluate whether similar physical processes are active at different plate margins. The editors asked one of the authors to describe advances in our understanding and where additional research is still needed.

    What are “megathrust zones” and what are the main processes that occur there?

    A megathrust zone is a thin boundary layer between a tectonic plate that sinks into the Earth’s mantle and an overriding plate. The largest earthquakes and tsunamis are produced here. High friction in the shallow part of the megathrust zone effectively locks parts of the interface during decades to centuries. Ongoing plate motion slowly brings the shallow interface closer to failure, i.e., an earthquake. Other parts of the megathrust zone are mechanically weaker. They consequently attempt to creep at a rate that is required by plate tectonics, but are limited by being connected to the locked part of the interface.

    What insights have been learned from recent megathrust earthquakes at different margins?

    High magnitude earthquakes in Indonesia (2004), Chile (2010) and Japan (2011) were recorded by new networks utilizing Global Positioning System technology, which is capable of measuring ground displacements with millimeter accuracy. This complemented seismological observations of megathrust slip during these earthquakes. The crust turned out to deform significantly during and after these earthquakes. These observations indicated that slip on weak parts of the megathrust zone may be responsible, likely in combination with the more classical stress relaxation in the Earth’s mantle. In regions where megathrust earthquakes are anticipated, crustal deformation observations allowed researchers to identify parts of the megathrust zone that are currently locked. In our review article, we integrate these perspectives into a general framework for the earthquake cycle.

    How have models been used to complement observations and better understand these processes?

    Mechanical models are needed to tie the surface observations to their causative processes that take place from a few to hundreds of kilometers deep into the Earth, which is beyond what is directly accessible by drilling. Many of the published models focus on a single earthquake along a specific megathrust zone. We wondered what deep earth processes are common to these regions globally and built a unifying model to predict its surface expressions. Our model roughly reproduced the observed surface deformation, but it also became clear that some regional diversity would be required to match the data shortly after a major earthquake.

    What have been some of the recent significant scientific advances in understanding plate boundaries?

    Creep on weak parts of the megathrust zone is a very significant contributor to the surface measurements after an earthquake. Mantle relaxation is also relevant. We demonstrate that the surface deformation of these processes may give a biased impression of low friction on the megathrust zone. Creep on the megathrust zone downdip of a major earthquake may be responsible for observations that were puzzling thus far; in an overall context of convergence and compression, tension was observed in the overriding plate shortly after recent major earthquakes.

    What are some of the unresolved questions where additional research or modeling is needed?

    Marine geodesy is an exciting new field that aims to monitor deformation of the sea floor that already yielded important constraints on the deformation of the Japan megathrust. Measurements along various margins will tell whether all megathrusts are locked all the way up to the seafloor. A longstanding question is how observations on geological time scales of mountain building and deformation of the overriding plate are linked to the observations of active deformation. We think that the multi-earthquake cycle model that we present in this review article is a first step towards that goal.

    See the full article here .

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  • richardmitnick 7:56 pm on October 6, 2017 Permalink | Reply
    Tags: , , , Long floating arrays of hydrophones pick up the sound waves, Marine seismic surveys, Ocean bottom seismometers, , Seventy percent of Earth’s surface geology is under water, Subduction zones   

    From Eos: “Keeping Our Focus on the Subseafloor” 

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    Eos

    3 October 2017
    Nathan Bangs and
    James A. Austin Jr.

    1
    University of Texas at Austin Ph.D. students Kelly Olsen and Brooklyn Gose work on recovering, cleaning, and storing 1 of 50 streamer depth control birds on the back deck of the R/V Langseth during a marine seismic survey cruise. Although relatively few scientists go to sea to collect such data themselves, the data from these surveys provide valuable information on subseafloor structures to a much wider scientific community. Credit: Nathan Bangs

    Seventy percent of Earth’s surface geology is under water, but let’s face it: There are few options for exploring beneath the seafloor, and the limited number of techniques for subseafloor exploration presents a challenge. But with modern seismic imaging techniques it’s surmountable, and the opportunities are extremely exciting.

    The real challenge at present is finding financial support. We need a new funding commitment in the United States to address recent declines in National Science Foundation (NSF) seismic facilities support. We are at a juncture where an increment of additional support from U.S. funding agencies, academic institutions, and/or private contributions for seismic facilities will have a leveraging effect with tremendous science impact for a broad community. With less support, we will still progress, but with substantial new challenges and uncertainty throughout marine science.

    During marine seismic surveys, ships use pneumatic sound sources to generate acoustic waves beneath the water’s surface. Long floating arrays of hydrophones pick up the sound waves, reflected back by subsurface sedimentary layers and crustal structures, to provide a detailed picture of the geology below the ocean floor.

    This technique provides invaluable information for the scientific and petroleum exploration communities alike. However, research funding reductions continue to hamper the marine geoscience community’s ability to collect seismic data in areas of scientific interest.

    Although relatively few scientists are directly involved in collecting these data, a much larger community relies on the data they produce. Thus, a dwindling data stream produces ripple effects that extend far beyond the scientists and crews who go to sea.

    The science community needs to understand what those ripple effects mean. Below, we’ve outlined a few options that we see as the most likely going forward.

    Seismic Techniques Provide Context

    Data from beneath the ocean floor come from several sources, and each type has its own strengths and limitations.

    Drilling and piston coring provide valuable samples for establishing lithologies, ages, geochemistry, and physical properties, but they provide a keyhole view beneath the ocean floor. Cores are only a few centimeters wide, and the information we gain is generally limited by both depth of penetration and the cost of coring expeditions.

    Electromagnetic methods are designed for a broader view, but they provide only bulk property constraints, averaged over the sampling area.

    The reality is that only marine seismic data have the resolution to see into the subsurface, to reveal regional geologic structure, and to help us understand the broader context of “ground truth” sampling from cores.

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    Coherence volume derived from the Costa Rica 3-D seismic data acquired on the R/V Langseth in 2011. These data show amazing 3-D detail in the subseafloor geology. Dark lines indicate the disruption in the continuity of seismic reflections from a complex pattern of faulting (mostly normal faults here) within margin shelf and slope cover sediment sequences. Horizontal slices are at 455 and 1,060 m depth. Credit: Nathan Bangs

    We have some amazing seismic data analysis techniques available to us: 3-D imaging, full-waveform inversion, and vertical profiling. We also have technologies to gather these data: Large spatial deployments of ocean bottom seismometers (OBSs), for example, are now possible with governmentally supported OBS instrument pools in the United States and other countries. Advanced multichannel seismic (MCS) reflection platforms, like the United States’ R/V Marcus G. Langseth, are equipped with multiple hydrophone arrays and streamers, which can be as long as 15 kilometers. And we have improved computational facilities for data analyses.

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    United States’ R/V Marcus G. Langseth. https://marine.usgs.gov

    Together, these capabilities allow us to “see” into the subseafloor to address fundamental geologic questions on ocean crust formation and evolution, subduction zone earthquake genesis, fluid migration within the crust, continental rifting, and the list goes on.

    More Technology but Fewer Cruises

    However, the availability of high-quality seismic images is in decline. The International Ocean Discovery Program (IODP) Science Evaluation Panel (SEP), scientific ocean drilling’s primary review body, sent the following statement for programmatic review in 2015:

    “The SEP wishes to convey concern regarding the increased pressures on the acquisition of academic active-source seismic data, some of which by design is conducted in support of scientific ocean drilling. Continued reduction in the international marine geoscience communities’ ability to collect seismic data in areas of scientific interest is jeopardizing the scope and impact of IODP science. The SEP consensus is that the IODP should stress the importance, both to member country funding agencies and environmental permit organizations worldwide, of high-quality subsurface images for science and safety in connection with expected continuation of IODP.”

    The SEP concern is justified. From 1995 to 2005, the R/V Maurice Ewing, then the primary U.S. seismic acquisition facility, conducted on average 4.7 seismic (MCS and OBS) cruises each year (in addition to higher-resolution surveys) and participated in multiship vertical seismic profiling projects.

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    R/V Maurice-Ewing (Photo courtesy Lamont-Doherty Earth Observatory of Columbia University)

    In contrast, during the past decade, the R/V Marcus G. Langseth, the Maurice Ewing’s successor ship, has conducted an average of 3.2 cruises each year, with only 5 total in 2016 and 2017.

    This decline raised questions explicitly addressed in the 2015 National Academy of Sciences’ decadal survey of ocean sciences: Should the National Science Foundation consider divestiture of this expensive facility to maintain a healthy balance between spending on technology and infrastructure and spending on science? Are seismic facilities serving too few scientists to justify such infrastructure?

    Worth the Investment?

    To address these questions, the University–National Oceanographic Laboratory System (UNOLS) conducted a June 2016 survey to assess the community’s seismic needs. Response was excellent; in 2.5 weeks, 263 completed the survey. (As a comparison, the virtual town hall that surveyed the ocean sciences community as input to the National Academy of Sciences report generated about 400 responses over 20 weeks.)

    The UNOLS survey confirmed that although a minority of the respondents acquire data at sea, and most of those are senior scientists (at least 20 years post Ph.D.), the seismic data they collect are used by many more. Half of the respondents considered themselves nonspecialists who rely only on processed seismic data and interpretations, without other involvement.

    The majority of respondents had never submitted an NSF proposal to acquire seismic data; 75% had not served as a primary investigator (PI) or co-PI on a seismic acquisition and processing project in the previous 5 years. Many stated that they do not have the background to acquire seismic data. However, 94% said they plan to use seismic data in the future, primarily through collaborations.

    The most commonly cited reason for not serving as a PI on a Langseth-type acquisition cruise was a lack of background in seismology or know-how with acquisition, processing, and interpretation techniques. Processing seismic reflection data requires extensive technical knowledge, advanced computer systems equipped with appropriate (often expensive) software, and generally 1–2 years or more of postacquisition processing effort.

    Overcoming these challenges is not practical or even possible for many; the pool of seismic specialists will remain small. Yet the UNOLS survey confirms the demand for seismic data beyond the small number of PIs who acquire and process them. Therefore, a loss of acquisition facilities would have wide-reaching effects on Earth sciences.

    The survey also confirms that many seismic tools (Figure 1) are needed to address the diverse science goals of current and envisioned U.S. Earth science programs, some of which include international partners and span shore lines (e.g., IODP, Geodynamic Processes at Rifting and Subducting Margins (GeoPRISMS), and subduction zone observatory).

    Weighing the Options

    Can we maintain seismic acquisition capabilities with reduced funding? With the exception of the Scripps Institution of Oceanography high-resolution (short-streamer) portable MCS system, Langseth is the only U.S. seismic facility. Langseth can acquire 3-D volumes and 2-D data with streamers more than 6,000 meters long, an “aperture” long enough to image at crustal scales.

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    Fig. 1. Results of a June 2016 UNOLS survey to assess the scientific community’s needs for seismic data from the ocean floor and below. Shown here are the percentages of respondents for each type of primary interest and the facilities they use. Multiple responses were allowed. Credit: Nathan Bangs

    In August 2016, the NSF Division of Ocean Sciences distributed a “Dear Colleague” letter stating that “OCE anticipates spending an annual average of ~$8M for ship support and ~$2M for technical support, funding permitting, and supporting seismic infrastructure.” At current Langseth rates, this amounts to operations for some 90 to 112 days, only about 75% of the total 120–150 days per year considered viable for any UNOLS vessel.

    Operating fewer than 120–150 days in a given year actually increases the cost per day. This makes the Langseth operational costs stand out even more while limiting support for the highly experienced crew necessary for state-of-the-art seismic operations.

    Aside from making Langseth a general-purpose vessel and using it to serve other marine science programs, options are limited:

    Option 1. One option is to improve efficiency and increase funding. Since 2015, Langseth has operated according to a long-range framework to minimize transits and maximize opportunities through developing regional and international collaborations. In late 2017 and 2018, Langseth will operate offshore New Zealand, with support from that country, Japan, and the United Kingdom. Unfortunately, international support is limited because partner countries must also support their own facilities.

    Improved efficiency is also possible through sharing facilities internationally. Langseth is currently the most capable academic facility for crustal-scale 2-D/3-D imaging, but it is the only U.S. option for crustal-scale work. International collaboration to exchange Langseth with other international vessels would improve both opportunity and efficiency globally; the Ocean Facilities Exchange Group does this within the European Union.

    A scaled back seismic program will not only affect the PIs who responded to the UNOLS survey, but it will also affect U.S. institutions they represent through loss of NSF support: those who receive funding for seismic data acquisition and those relying on seismic results. Among international programs affected by scaled-back seismic facilities, IODP stands out. Broad support for seismic facilities from U.S. academic institutions would produce returns for these institutions.

    Institutional support could provide Langseth operational costs, but only through collaborative, multiyear commitments. As NSF described in their recent program solicitation “Provision of Marine Seismic Capabilities to the U. S. Research Community,” the agency is committed to providing Langseth or equivalent capabilities, but the proposed funding levels ($10 million per year) would be problematic for the current model. Is any other model viable?

    Option 2. Without Langseth, there will still be exciting science to do, using data from shorter streamers, archived data, and data available (occasionally and in certain areas) from industry.

    Unfortunately, this approach will change the foci of U.S. seismic studies. For example, reduced seismic imaging capability would limit large, successful international programs like IODP. Recent progress on understanding the largest earthquakes and tsunamis on Earth generated at subduction zones will be severely compromised in the future without an ability to see deep subduction zone structures and measure physical properties with seismic tools. Science would need to target shallower settings: submarine landslides, gas hydrates, fluid and gas migration, sea level change, and the like, using cheaper, portable (higher-resolution, smaller) seismic systems.

    Data would be less complicated and easier to process. Higher resolution could even be 3-D, using P-cable systems available from multiple U.S. institutions. However, these systems can rarely deploy streamers long enough or sources powerful enough to fully characterize even shallow stratigraphy and structure, and they can’t address crustal-scale problems at all.

    Going Commercial?

    In 2015, NSF conducted a workshop with invited members from the marine seismic community, primarily the now disbanded Marcus Langseth Scientific Oversight Committee, to consider a long-streamer portable system and commercial contracting. The workshop report concluded that the weight and size of portable systems incorporating long (6–8 kilometer) streamers with moderate-size, tuned acoustic sources made them impractical for current UNOLS vessels.

    Commercial contracting is viable for long-offset 2-D and 3-D acquisition; however, availability could be limited by high costs (especially mobilization costs for work far from oil and gas provinces, where commercial efforts tend to focus), cost volatility, and changes in ship availability due to hydrocarbon market cycles. These cruises would also lack simultaneous multibeam or gravity and magnetics data acquisition, which has historically been available on U.S. seismic vessels, and student training opportunities would be uncertain and problematic.

    Commercial contracting has been used successfully occasionally in the past, but it is risky to rely on contracting to maintain a regular, global crustal-scale acquisition program as we do now.

    Ironically, this development comes at a time when the IODP is increasing its number of operational days. Other exciting developments, such as seafloor geodesy, will also need an understanding of subsurface structure for tectonic context.

    There is considerable imaging science to be done, with or without Langseth. However, the broad impact of scaling back marine seismic facilities on Earth science makes it time to find more financial support for marine seismic acquisition.

    See the full article here .

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  • richardmitnick 11:31 am on May 20, 2016 Permalink | Reply
    Tags: , , , , , Possibility of a subduction zone observatory, Subduction zones,   

    From Eos: “Planning for a Subduction Zone Observatory” 

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    OPINION

    5.20.16

    By Joan Gomberg, Paul Bodin, Jody Bourgeois, Susan Cashman, Darrel Cowan, Ken Creager, Brendan Crowell, Alison Duvall, Arthur Frankel, Frank Gonzalez, Heidi Houston, Paul Johnson, Harvey Kelsey, Una Miller, Emily Roland, David Schmidt, Lydia Staisch, John Vidale, William Wilcock, and Erin Wirth

    1
    The eruption of Alaska’s Augustine Volcano on 27 March 2006, viewed from the M/V Maritime Maid. Credit: Cyrus Read, AVO/USGS

    Subduction zones contain many of Earth’s most remarkable geologic structures, from the deepest oceanic trenches to glacier-covered mountains and steaming volcanoes. These environments formed through spectacular events: Nature’s largest earthquakes, tsunamis, and volcanic eruptions are born here.

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    A scientist investigates dramatic subsidence—in places up to 2 meters—along the northwest coast of Sumatra in January 2005. Here tsunami waves spawned from the Mw9.1 earthquake that shook on 26 December 2004 snapped off tree tops; roots and lower trunks are now submerged in water. Credit: USGS

    Great subduction zone earthquakes can cause coastal subsidence in minutes that has impacts comparable to a century or more of climatically driven sea level rise. They can radiate shaking waves that trigger widespread landslides and submarine slope failures and can spawn tsunami waves that reach distant lands. Eruptions can produce fast-moving mudflows as well as high-lofted ash clouds complete with their own lightning systems.

    The geologic processes in subduction zones shape surface morphology, couple plate motions to mantle convection, control resource distributions, and even interact with Earth’s climate—volcanic gases can cool the climate, and sediments at subduction margins are important reservoirs for greenhouse gases. In short, the diverse environments in subduction zones are natural laboratories for investigating a wide range of interlinked processes.

    The myriad opportunities for collaborative work among scientific disciplines, institutions, and nations have inspired a grassroots movement to create a subduction zone observatory (SZO), now in a nascent stage of planning. Over the past few years, Earth scientists have been discussing options for its geographic scope and structure. For example, should such an observatory focus on one subduction zone or multiple zones? Should it be managed as a center or an umbrella organization? How should resources be balanced between new facilities and funds needed for research?

    As part of these discussions, we held a series of seminars focused on SZO topics. A more official and much broader workshop will occur in September 2016. Here we share ideas that have emerged from past talks in an effort to grow the movement, advance community discussion, and help shape a vision.

    Why Invest in an SZO Now? Lessons from Tohoku

    Scientific focus on subduction zones has increased dramatically in the past decade, accompanying a recognition that large numbers of people are migrating to urban areas that lie within subduction zones. Scientists recognize the hazards inherent in living near tectonically active margins [Bilham, 2009] and are motivated to understand them so that they may provide the best advice to protect communities.

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    Fig. 1. Fault slip from the Mw9 Tohoku, Japan, earthquake in 2011. Seismic and GPS waveform modeling revealed significant slip in patches with varying characteristics (amplitudes color coded), as if it were a composite of smaller earthquakes. The plate interface (fault) dips to the west. A downdip patch radiated the high-frequency waves that damaged buildings (right map), and the largest slip on an updip patch (left map) generated the enormous tsunami. Modified from Frankel [2013].

    Investments in onshore and offshore instrument arrays have enhanced the resolution of our understanding of subduction-related processes. Multifaceted analyses can make monitoring systems and hazard assessments more robust. For example, the Mw9.0 Tohoku earthquake, which shook Japan in 2011, occurred in an area with an abundance of onshore and offshore observations, including geodetic and seismic signals, marine seismic reflection profiles, bathymetry, tsunami wave heights, and novel seafloor acoustic GPS and pressure measurements. Offshore observations provided, for the first time, unambiguous evidence for why the tsunami was so large—they documented several tens of meters of fault slip near the Japan Trench (Figure 1). Rich data sets and multifaceted analyses also revealed that days before the Tohoku earthquake, the plate interface began slipping slowly, with slip propagating toward the initiation point of the Mw9.0 earthquake [Kato et al., 2012].

    The 2011 Tohoku earthquake illustrated the potential of an SZO to significantly advance understanding of the processes at work before, during, and after a major event. It also prompted sobering retrospectives on the difficulty of anticipating its enormous size, even after meticulous paleoseismological studies [Sawai et al., 2012], underscoring the need for further research investments that will make subduction zone populations more resilient.

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    Ideally, an SZO would produce research that is transformational, multidisciplinary, amphibious, and international. As illustrated by the Tohoku earthquake and tsunami, an SZO would enable transformational research by facilitating acquisition and analysis of disparate data with multiple synergistic applications considered from the start. An SZO must be multidisciplinary to understand linkages between processes within subduction zones and the transitions to adjacent tectonic environments.

    The tremendous diversity of processes and physical characteristics displayed by the world’s subduction zones begs for a global scope. Although the geographic scope of an SZO remains to be determined, we illustrate key features of an SZO with some examples of broad questions addressed in particular regional studies. Two of the three examples come from the Cascadia subduction zone (Figure 2a), only because it is where most of the authors reside and work.

    How do accretionary prism sedimentation, geochemistry, ocean currents, and climate change interact with gas hydrate stability? Subduction margin sediments contain a significant fraction of Earth’s greenhouse gas methane. This methane, derived from sediments of terrestrial origins and organisms at the sea surface, gets bound within the solid crystalline water lattices of gas hydrates. In Cascadia, multichannel seismic reflection profiles reveal a gas hydrate reservoir in a swath along the seafloor where the water depth exceeds 500 meters (depth contour in Figure 2a), above which hydrates decompose. However, seawater warming at these depths threatens to decompose some of this hydrate. This could increase bottom water acidity and lower the water’s oxygen content, which could have dramatic negative effects on organisms and accelerate climate change.

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    Fig. 2. The Cascadia subduction zone off the west coast of the United States and Canada provides examples of questions a subduction zone observatory could address. Merged bathymetry and topography (shaded) with superposed boundaries (white lines and arrows) illustrate the diversity of environments. (a) Southern and central Cascadia, from Pacific Ocean depths (dark blues) to high Cascade mountains and volcanoes (browns). Processes include formation of an accretionary prism, formation and dissolution of methane hydrates (wiggly white line), consumption of the Juan de Fuca plate at the deformation front (hatchured), and its descent beneath the North American plate. The plate interface is stuck (locked) at shallow depths and slips slowly at greater depths, generating tremors. Deeper, metamorphic changes lead to volcanic arc formation. (b) In northernmost Cascadia, the tectonic activity transitions from subduction to transform boundaries. The red rectangle outlines the rupture plane of the magnitude 7.8 Haida Gwaii earthquake.

    How do subduction zones transition to other types of tectonic regimes? At the northern end of the Cascadia subduction zone (Figure 2b), the plate motions change from converging toward one other with associated subduction and thrust faulting along shallowly dipping planes to moving parallel to each other, accommodated by steeply dipping strike-slip faulting, in which most of the movement is horizontal. The Queen Charlotte Fault system is this latter “transform” boundary. The 2012 M7.8 Haida Gwaii earthquake in British Columbia occurred on this system and illustrated the complexity of this transition, with surprisingly many of the characteristics of a subduction zone thrust event.

    Recent studies of the Alaska subduction zone demonstrate that during great subduction zone earthquakes, as ruptures approach the seafloor, they sometimes propagate from the plate interface to secondary faults, sometimes also deforming ductile overlying sediments. Uncertainties in seafloor fault displacements remain the most significant unknown in local tsunami hazard assessments [Geist, 2002]. These hazards could be reduced with an SZO monitoring system and seafloor mapping before and after events.

    How do subducting slabs and volcanism affect continental crust creation? Processes that generate magmas in subduction zones are the “factories” that create continents, based on the similarity between the compositions of continental crust and volcanic arcs. Numerous hypotheses exist about the processes by which slab materials accrete, subduct, melt, mix, and emerge at crustal depths or extrude from active volcanoes. Testing these hypotheses requires integrated studies spanning the disciplines of chemistry, petrology, fluid dynamics, geology, and seismology, with four-dimensional imaging.

    An SZO must be amphibious, and this requires new means of collecting and integrating offshore and onshore observations. Scientists need sustained marine geodetic observations to characterize subduction zone deformation processes over many time scales, from plate motions (decades) to earthquake fault slip (seconds). A geodetic component of an SZO could include high-resolution repeat seafloor mapping, pressure gauges, and novel tools like fiber-optic strain meters. These and other frontier offshore measurements are applicable to multiple scientific questions and practical applications.

    A High-Impact, Societally Relevant SZO

    The envisioned SZO would contribute to the scientific understanding needed to build societal resilience in the face of natural hazards. It would also serve as an international educational resource. For example, recording earthquake ground motions using densely spaced networks would enable us to properly calibrate the simulations that underlie seismic hazard assessments and building codes (Figure 1).

    An SZO should be built collaboratively with those responsible for hazard mitigation and response. Perhaps most important, it could offer opportunities to involve citizens in research, through educating the public and making them stakeholders. For example, local scientists, students, and interested citizenry could be engaged to make biological and survey observations before and after events and to document coastal uplift and subsidence, tsunami inundation, or storm surge impacts (Figure 3). Such engagement could extend understanding of the effects of major natural events in places where we lack prior baselines, and it would fit naturally into educational programs that teach by doing.

    6
    Fig. 3. This stray coral boulder was likely transported 230 meters inland by large waves or currents along the Atlantic shore of Anegada, British Virgin Islands. This and other geologic and geomorphic features are being used to assess earthquake and tsunami hazards of the subduction thrust and the adjoining outer rise along the Puerto Rico Trench. Credit: Brian Atwater

    A Path Toward an SZO

    Moving an SZO from concept to reality will require coordination within and among academic and hazards assessment communities. Individually and as members of professional entities, we must lead, advocate for, and contribute to proposals to governmental and private sector institutions to support, design, and build an SZO. If the Earth science community collaboratively pursues creative strategies, developing an SZO may simultaneously serve to enrich observational tools, improve hazard models, and enhance our basic understanding of one of Earth’s most dynamic environments.

    References

    Bilham, R. (2009), The seismic future of cities, Bull. Earthquake Eng., 7, 839–887, doi:10.1007/s10518-009-9147-0.

    Frankel, A. (2013), Rupture history of the 2011 M9 Tohoku Japan earthquake determined from strong-motion and high-rate GPS recordings: Subevents radiating energy in different frequency bands, Bull. Seismol. Soc. Am., 103, 1290–1306.

    Geist, E. L. (2002), Complex earthquake rupture and local tsunamis, J. Geophys. Res., 107(B5), 2086, doi:10.1029/2000JB000139.

    Kato, A., K. Obara, T. Igarishi, H. Tsuruoka, S. Nakagawa, and N. Hirata (2012), Propagation of slow slip leading up to the 2011 Mw 9.0 Tohoku-Oki earthquake, Science, 335, 705–708.

    Sawai, Y., Y. Namegaya, Y. Okamura, K. Satake, and M. Shishikura (2012), Challenges of anticipating the 2011 Tohoku earthquake and tsunami using coastal geology, Geophys. Res. Lett., 39, L21309, doi:10.1029/2012GL053692.

    —Joan Gomberg, U.S. Geological Survey, Seattle, Wash.; and Department of Earth and Space Sciences, University of Washington, Seattle; email: [email protected]; Paul Bodin and Jody Bourgeois, Department of Earth and Space Sciences, University of Washington, Seattle; Susan Cashman, Department of Geology, Humboldt State University, Arcata, Calif.; Darrel Cowan, Ken Creager, Brendan Crowell, and Alison Duvall, Department of Earth and Space Sciences, University of Washington, Seattle; Arthur Frankel, U.S. Geological Survey, Seattle, Wash.; and Department of Earth and Space Sciences, University of Washington, Seattle; Frank Gonzalez and Heidi Houston, Department of Earth and Space Sciences, University of Washington, Seattle; Paul Johnson, School of Oceanography, University of Washington, Seattle; Harvey Kelsey, Department of Geology, Humboldt State University, Arcata, Calif.; Una Miller and Emily Roland, School of Oceanography, University of Washington, Seattle; David Schmidt, Department of Earth and Space Sciences, University of Washington, Seattle; Lydia Staisch, U.S. Geological Survey, Seattle, Wash.; and Department of Earth and Space Sciences, University of Washington, Seattle; John Vidale, Department of Earth and Space Sciences, University of Washington, Seattle; William Wilcock, School of Oceanography, University of Washington, Seattle; and Erin Wirth, Department of Earth and Space Sciences, University of Washington, Seattle

    Citation: Gomberg, J., et al. (2016), Planning for a subduction zone observatory, Eos, 97, doi:10.1029/2016EO052635. Published on 20 May 2016.
    © 2016. The authors. CC BY-NC 3.0

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