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multidisciplinary groups. Understanding        AGU Geophysical Monograph 138 (J. Eiler,          network comprising twelve leading inter-
of subduction dynamics has greatly ben-        ed., 2003). Major efforts similar to              national universities and research centers
efited from this approach, leading to an       MARGINS and GeoPRISMS have been                   and nine industrial partners. The U.S.
increasingly sophisticated view of subduc-     undertaken by subduction communities in           GeoPRISMS E-FIRE project (ExTerra
tion zones and how they evolve. More           Japan, Europe, New Zealand, Central and           Field Institute and Research Endeavor:
comparative studies have also helped to        South America, and southeast Asia, result-        Western Alps; http://geoprisms.org/
move the community away from thinking          ing in significant investment in subduction       exterra/e-fire/), funded by the NSF’s
that each convergent margin segment is         zone science by their respective national         PIRE program (Partnerships in
unique and toward seeing subduction as         governments.                                      International Research and Education),
the central problem, with individual mar-                                                        builds on the success of ZIP, providing
gins showing different variations—and            This is an exciting time for those inter-       support for eight Ph.D. students and two
thus opportunities for insights—on the         ested in understanding subduction margins!        postdoctoral fellows at 10 academic
unifying theme. As we’ve examined indi-        In addition to the GeoPRISMS initiative,          institutions.
vidual margins in greater detail and con-      the U.S. subduction science community is
trasted them with other margins, patterns      discussing the potential of a “Subduction         WHAT HAVE WE LEARNED
have emerged that reveal some of the con-      Zone Observatory,” which is presented in a        ABOUT SUBDUCTION IN THE
trols on convergent margin behavior and        “SZ4D Initiative” report aimed at reveal-         PAST TWENTY YEARS?
evolution. In fact, quite a number of mod-     ing the short- and long-term evolution of
ern subduction margins show dramatic           subduction margins. That report resulted            As a thought exercise for this report,
along-strike variations in key physical fac-   from an NSF-sponsored workshop in 2016            each of the co-authors assembled a list of
tors influencing their thermal and mechan-     that was attended by 250 scientists from          what they consider to be the greatest
ical evolution. A few of these factors are     the USA and 22 foreign countries (https://        advancements in subduction zone science
convergence rate and obliquity; age of         www.iris.edu/hq/workshops/2016/09/                in the past two decades since SUBCON/
incoming plate and the subduction zone         szo_16). The SZ4D Initiative, as presently        Big Purple. Several themes were repeated
itself; physical, thermal, and chemical state  configured, proposes three key compo-             on our lists and we agreed on the follow-
of the subducting oceanic lithosphere;         nents: a modeling component, an interdis-         ing. This list can of course be quibbled
presence of seamounts and other heteroge-      ciplinary science program, and a commu-           with, but it does capture how exciting
neities on the downgoing plate; the nature     nity infrastructure program (see McGuire          subduction science has been and the range
and thickness of subducting sediments;         et al., 2017). Its science “net” is cast widely,  of approaches that are being used and the
accretion versus erosion; and the composi-     with the aim of fostering integrated geo-         diverse research communities that have
tion and structure of the upper plate.         physics, geology, petrology, geochemistry,        been involved:
                                               and geodynamic modeling. Planning for
  Understanding of the dynamics of sub-        future subduction zone studies is also            Some Advances in Subduction Zone
duction is of particular importance in         being undertaken by the USGS, which has           Science in the Past 20 Years
assessing the associated hazards of earth-     recently announced a major directive,
quakes, tsunamis, and volcanic eruptions.      “Reducing Risk Where Tectonic Plates              • 	 Improved understanding of how new
The scientific community, governments,         Collide—A Plan to Advance Subduction                subduction zones form (see the review
and the broader public increasingly recog-     Zone Science” (https://www.usgs.gov/                by Stern and Gerya, 2018);
nize the need to assess hazards that sub-      news/usgs-publishes-a-new-blueprint-can-
duction margins pose, especially to regions    help-make-subduction-zone-areas-more-             • 	 The importance of outer rise normal
of high population densities around the        resilient; Gomberg et al., 2017). As its            faulting and deep hydration of subducted
Pacific and Indian Oceans (e.g., Japan,        name implies, this initiative aims to focus         lithospheric mantle, and the connection
Indonesia, the Cascadia margin). Recent        geological, geophysical, and petrologic/            with deep-seated seismicity (e.g.,
initiatives promoting this effort include      geochemical investigations and modeling             Ranero et al., 2005; Van Avendonk et al.,
community-led NSF initiatives such as          at understanding and forecasting hazards            2011; Fig. 1);
MARGINS and its successor GeoPRISMS,           associated with subduction plate boundar-
in each of which subduction-zone science       ies. Naturally, the Cascadia margin figures       • 	 Understanding the magnitude and signifi-
has constituted a major component. The         prominently in this planned endeavor                cance of subduction erosion; that most
two initiatives have identified “focus sites”  because of the large earthquake, tsunami,           convergent margins lose material from
where effort was concentrated, and these       and volcanic hazards it poses to the                the upper plate and only a minority add
included MARGINS Subduction Factory            increasingly populated Pacific Northwest            material by growing accretionary prisms
and SEIZE (SEIsmogenic ZonE) efforts in        region. Another example of an initiative            (e.g., Scholl and von Huene, 2007);
Central America, Izu-Bonin-Mariana, and        emphasizing study of subduction processes
Nankai, and GeoPRISMS Subduction               and hazards is the ZIP project (Zooming in        • 	 Exploration of submarine arc and back-
Cycles and Deformation focus sites of the      between Plates), which is a collaborative           arc basin volcanoes and associated
Cascadia, Aleutian/Alaska, and New             research and training project funded by the         hydrothermal systems and volcaniclastic
Zealand subduction zones. The MARGINS-         European community as a European Marie              sedimentation, especially in the Izu-
sponsored Theoretical Institute, “Inside the   Curie Initial Training Network (http://www          Bonin-Mariana and Tonga Kermadec
Subduction Factory,” in 2000 was a par-        .zip-itn.eu/). This project involves 12 Ph.D.       systems (e.g., Baker et al., 2008; Dziak
ticularly important milestone, resulting in    students and two postdoctoral fellows in a          et al., 2015);

                                                                                                 • 	 Recent major subduction earthquakes
                                                                                                   (such as the 2004 Sumatra and the 2011
                                                                                                   Tohoku-Oki earthquakes) have taught us
                                                                                                   important lessons about what is possible

                                               www.geosociety.org/gsatoday                                                                     5
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