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Latin America Subduction Zone Observatory (LASZO) Concept

Susan L. Beck, Anne Meltzer, Peter LaFemina, Jeff Freymueller, Andreas Rietbrock, Sergio Barrientos, Mario Ruiz, Alexandra Alvarado, Marino Protti, Lepolt Linkimer,

Xyoli Pérez-Campos, Michael Schmitz, Patricia Alvarado, Hernando Tavera, Carlos Vargas, Frederik Tilmann, Onno Oncken, Marcelo Bianchi, Marcelo Assumpcao, Gonzalo Fernandez,

Andy Frassetto

Why a Latin America SZO? The west coast of Latin America exhibits long-term subduction, high rates of seismicity,

damaging earthquakes and tsunamis, and active arc volcanism. A subduction zone observatory extending from Mexico to southern Chile utilizes the Earth’s largest long-lived ocean-continent subduction system to systematically study the breadth of subduction zone processes through space and time. Long-term international collaborations, investments in geophysical networks, and targeted study areas in a number of Latin American countries provide a solid foundation for enhancing, extending and developing new partnerships to engage a broad spectrum of the geosciences. Recent investments in infrastructure to better monitor hazards and study geophysical processes can be leveraged to establish an initial backbone of geophysical observations, serve as platforms for focused multidisciplinary investigations, and translate scientific advances to societal benefits. The subduction system in Latin America includes: • The largest magnitude and slowest recorded earthquakes, tsunamis, slow-slip events, tremors and

high rates of seismicity allowing us to observe the full earthquake cycle at multiple scales. • Frequent large earthquakes providing an opportunity to better understand rupture dynamics and

coupling along the length of the subduction zone interface. Along the Latin America SZ there is a magnitude 7.6 - 8 earthquake on average every decade with four Mw>7.8 events since 2010 (Fig. 1).

• Frequent damaging upper plate crustal earthquakes occurring close to population centers point to an urgent need to identify faults and estimate seismic hazard and risk.

• Active volcanoes, both active and eroded magmatic arcs, and backarc volcanism provide information on eruption dynamics, hazards, arc magmatic processes and their evolution through time (Fig. 2).

• Some of the largest ore deposits in the world related to the subduction system can provide clues on what factors control the formation of ore bodies.

• Long continuous subduction with along strike and temporal variations in the dip of the subducting slab, slab tears, multiple subducted oceanic ridges and plateaus, and slabs that extend into the lower mantle interacting with mantle convection that can be imaged and modeled in detail.

• Archetype Andean subduction with one of Earth’s largest continental plateaus (~4 km elevation and 300,000 km2 in the central Andes), high topography, forearc and backarc basins, a major retro-arc thrust belt with evidence of lithospheric removal and dynamic topography provide an opportunity to study the relationship between surface and deep earth processes along the subduction zone.

• Changes in surface elevation, climate and atmospheric circulation that allow the study of the interaction of climate and topography.

• Regions with land near or over the seismogenic zone (Mexico, Costa Rica, S. Ecuador, N. Perú). • Increased population and infrastructure at risk along the west coast of Central and South America

(>25M people living along or near the coast & important economic infrastructure) emphasize the need to translate scientific advances to societal benefits.

Science Targets for a LASZO The seismogenic zone, deformation, and associated hazards: • What is the range of spatial and temporal scales of deformation along this subduction zone? • How can we tell what part of the earthquake cycle a segment of the plate boundary is in? Can we

identify a pre-seismic phase distinct from interseismic strain accumulation?

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• What controls the size of the megathrust rupture and how do rupture dynamics relate to ground motion?

• What controls plate coupling and how do the locked and freely slipping portions of the subduction zone vary through time and space?

• How do large SZ earthquakes affect crustal stresses in the upper plate far into the back-arc or even in mid-plate regions where damaging earthquakes occur?

Magmatic arc processes • What is the plumbing system: where are the magma reservoirs at depth, how do they form and behave

from the granular to the crustal scale, and how do they evolve through time? • What pathways do fluids take from the subduction zone to the crust and eventually to the surface? • How are ore deposits formed? Lithospheric deformation and surface processes • Why do continental plateaus and high topography form in some places and not in others along the

LASZ? What are the feedbacks between the development of topography, climate, and erosion and how do they vary in time and space?

• How is convergence accommodated in the crust and lithospheric mantle? Are there feedbacks between shortening and lithospheric removal?

• How is the slab geometry related to the upper plate deformation? • How do slabs interact with other subducting slabs (e.g. Caribbean?) • What controls foreland basin formation and evolution through time? Earth’s water and carbon budget • How is water and carbon recycled in the SZ and how does it interact with the slab and over-riding

plate? How does water impact arc magmatism. • What is the role of water in enabling intermediate and deep focus earthquakes? • How much water is in the mantle and how deep does water go into the Earth? Deep subduction processes • What do subducting slabs do in the transition zone and below? • What is the relationship between plate motion and mantle convection?

International Partners, Community Data Collection, Modeling and Capacity Building Long-term international collaborations across many geoscience disciplines and emerging and expanding geodetic and seismic networks across Latin America make it an ideal focus for a LASZO (Figs. 3 and 4). Successful examples of international collaboration in Latin America related to geophysical observations include: COCONet and TLALOCNet in Central America and Chile GRO and IPOC. Multinational responses to megathrust earthquakes include Maule (2010), Iquique (2014), Illapel (2015) in Chile, and Pedernales (2016) in Ecuador. To significantly advance subduction zone science requires a real-time permanent backbone of on-shore and offshore seismic and geodetic observations; an appropriate suite of volcano observatories; an appropriate pool of robust portable instrumentation (i.e. seismometers, GPS, MT, near-surface geophysics, and more) to allow multiscale imaging, investigations, and rapid response to specific events; integration of InSAR and LIDAR; near real-time access to satellite remote sensing data and data products, and databases for geologic samples, geochemistry, geochronology and thermochronology, geomorphology, and mapping; geodynamic and fluid dynamics modeling; an ability to integrate diverse data into process models. A successful subduction zone observatory will require technology and IT development, open data, education and training, and a plan for long-term sustainability. Low-cost, permanent off-shore geophysical measurements are particularly critical for real time monitoring. Education, training, and capacity building at multiple levels (introductory to advanced study) must be an integral part of a LASZO. Such efforts could take the form of institutes, both physical and virtual, that focus on subduction zone science where participants can learn, collaborate, explore, and make new discoveries together.

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Figure 1. Recent and historic large (~M ≥ 8) earthquake ruptures along the South American subduction zone including the four large earthquakes since 2010 (Blue rupture zones). Yellow rupture zones are from 1930-2009, red rupture zones are prior to 1930, and white dashed lines are estimated rupture zones for pre-1870 events. In most cases we have very little resolution on the ruptures in the dip direction and we critically need off-shore observations. Note the large gap in northern Perú and southern Ecuador without a megathrust event in over 500 years suggesting the region may be uncoupled and not capable of a large event or the recurrence interval is longer than 500 years. Increased GPS measurements along the entire Latin America SZ will improve our ability to map out the interseismic coupling in the seismogenic zone.

Figure 2. (A) Plot of the uplift calculated from InSAR models between 2003 and 2014 for the large rhyolitic magma system at Laguna del Maule, Chile compared to other systems around the world (Singer et al., 2014). (B) Schematic diagram of the magma system for the Laguna del Maule region as shown in Singer et al., (2014). (C) Shear-wave velocity model of the Altiplano Puna Magma Body (APMB) from a joint inversion of ambient noise and receiver functions in southern Bolivia showing a very large mid-crustal low-velocity zone that likely has >20% partial melt (Ward et al., 2014). There are 320 active volcanoes in Central and South America (Smithsonian Institution, Global Volcanism Program, volcano.si.edu/region.cfm?rn=14).

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Figure 3. Example of existing seismic stations in Central and South America by type of instrument (Andy Frassetto at IRIS). There is the opportunity to leverage partnerships, resources, and data but this requires active engagement from all parties, an established agreed upon roadmap to follow, and appropriate MOUs regarding roles, responsibilities, data access, and collaborations. Similar maps could be made for other types of data.

Figure 4. (A) Example of velocities from existing GPS stations in Central America (Kobayashi et al., 2014). (B) Existing GPS stations as part of the GNSS in Chile. An additional 20 GPS stations are planned for Chile in the next year (S. Barrientos). There is a critical need for low cost permanent off-shore measurements of deformation (seismic and GPS) to go with the on-shore observations.

References Kobayashi, D., LaFemina, P. Geirsson, H. Chichaco, E. Abrego, A.A., Mora, H. and Camacho, E., 2014,

Kinematics of the western Caribbean: Collision of the Cocos Ridge and upper plate deformation, Geochem. Geophys. Geosyst., 15, 1671–1683, doi:10.1002/2014GC005234.

Singer, B.S., Andersen, N.L.,Le Mével,H., Feigl, K.L., DeMets,C., Tikoff, B., Thurber, C. H., Jicha, B.R., 2014, Dynamics of a large, restless, rhyolitic magma system at Laguna del Maule, southern Andes, Chile, GSA Today, v. 24, no. 12, doi: 10.1130/GSATG216A.1.

Ward, K.M., Zandt, G., Beck, S.L., Christensen, D.H., McFarlin, H., 2014, Seismic imaging of the magmatic underpinnings beneath the Altiplano-Puna Volcanic Complex from the joint inversion of surface wave dispersion and receiver functions, Earth and Planetary Science Letters, 404, 43-53, DOI: 10.1016/j.epsl.2014.07.022.