Subduction zones, where two tectonic plates converge, are generally dominated by large thrust earthquakes. Nonetheless, normal faulting from extensional stresses can occur as well. Rare large events of this kind in the instrumental record have typically nucleated in and ruptured the top half of old and cold lithosphere that is in a state of extension driven by flexure from plate bending. Such earthquakes are limited to regions of the subducting slab cooler than 650 °C and can be highly tsunamigenic, producing tsunamis similar in amplitude to those observed during large megathrust events. Here, we show from analyses of regional geophysical observations that normal faulting during the moment magnitude Mw 8.2 Tehuantepec earthquake ruptured the entire Cocos slab beneath the megathrust region. We find that the faulting reactivated a bend-fault fabric and ruptured to a depth well below the predicted brittle–ductile transition for the Cocos slab, including regions where temperature is expected to exceed 1,000 °C. Our findings suggest that young oceanic lithosphere is brittle to greater depths than previously assumed and that rupture is facilitated by wholesale deviatoric tension in the subducted slab, possibly due to fluid infiltration. We conclude that lithosphere can sustain brittle behaviour and fail in an earthquake at greater temperatures and ages than previously considered.
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Strong-motion data are available from the Strong Ground Motion Database System (http://aplicaciones.iingen.unam.mx/AcelerogramasRSM/) and by email request to SSNdata@sismologico.unam.mx. Receiver Independent Exchange Format files with the raw GPS observations from the TLALOCNet archive are open and freely available at http://tlalocnet.udg.mx as well as at the UNAVCO archive https://www.unavco.org/data/data.html. University of Nevada, Reno static offset solutions are available at http://geodesy.unr.edu/news_items/Offsets_Pijijiapan_rapid24hr.txt. Tide gauge data are provided by Servicio Mareográfico Nacional from Universidad Nacional Autónoma de México and are available at http://www.mareografico.unam.mx/ and at http://www.ioc-sealevelmonitoring.org/. DART buoy data can be obtained from https://www.ndbc.noaa.gov/dart.shtml.
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We thank D. Toomey, R. Burgmann and D. Sandwell for discussions. Strong-motion data used in this work were provided by Servicio Sismológico Nacional (SSN, Mexican National Seismological Service), and Unidad de Instrumentación Sismica (UIS) at the Instituto de Ingeniería (II), Universidad Nacional Autónoma de México (UNAM, National Autonomous University of Mexico). We thank their personnel for station maintenance, data acquisition and distribution. This material is partly based on data provided by the Transboundary, Land and Atmosphere Long-term Observational and Collaborative Network (TLALOCNet) operated by UNAVCO and the Servicio de Geodesia Satelital from Instituto de Geofísica-UNAM and supported by NSF grant EAR-1338091, CONACyT project 253760 and UNAM-PAPIIT projects IN104213 and IN109315-3. L. Salazar-Tlaczani at Servicio de Geodesia Satelital-UNAM provided invaluable support for the TLALOCNet field operations and stations maintenance. We are indebted to the staff and technicians of the Servicio Mareográfico Nacional who operate the tide gauge network. Some numerical computations were performed at the National Laboratory for Advanced Scientific Visualization at UNAM (LAVIS), and this work received support from LAVIS software engineers L. A. Aguilar Bautista, A. de León Cuevas and C. S. Flores Bautista. This research project was partially supported by the Japanese government through the programme Science and Technology Research Partnership for Sustainable Development (SATREPS) via the Japan International Cooperation Agency (JICA) and the Japan Science and Technology Agency (JST) with Grant Number 15543611. This work was also supported by a grant from the Romanian Ministry of National Education and Scientific Research, RDI Program for Space Technology and Advanced Research—STAR, project ID 513.