Disaster Prevention Research Institute, Kyoto University

Graduate School of Science, Kyoto University

Graduate School of Science, The University of Tokyo

Earthquake Research Institute, The University of Tokyo

Summary

Associate Professor Yoshihiro Ito of the Disaster Prevention Research Institute, Kyoto University Corporation, Prater Martinez Raimundo, a third-year PhD student in the Department of Earth and Planetary Science, Graduate School of Science, Kyoto University, Professor Satoshi Ide of the Graduate School of Science, the University of Tokyo, Professor Masanao Shinohara of the Earthquake Research Institute, the University of Tokyo, and others, in collaboration with the National Autonomous University of Mexico and the University of California, Berkeley, have analyzed seafloor seismograph records from the Mexican Pacific coast. The results of the analysis showed that the past seismic activity off the coast of Mexico's Pacific Ocean was not due to any earthquake. As a result, they revealed for the first time that slow earthquakes are occurring off the Pacific coast of Mexico in an area where no earthquakes have occurred in the past 100 years (seismic blank zone), where the plate boundary slowly shifts.

Slow earthquakes as well as major earthquakes have repeatedly occurred in the plate subduction zones of the Pacific Rim. Current global understanding of slow earthquakes, especially those occurring beneath the seafloor, has been limited to areas with well-developed seafloor observation networks. As part of an international joint Japan-Mexico research project (JST-JICA SATREP "Comprehensive Research for Mitigation of Mega Earthquake and Tsunami Hazards in Coastal Mexico" (Japanese representative: Yoshihiro Ito)), we have constructed a seafloor seismic observation network directly above the seismic void area off the Pacific Ocean in the Mexican state of Guerrero. Analysis of seismic records obtained from November 2017 to November 2018 confirmed the occurrence of slow earthquakes within the seismic blank area, where no major earthquakes have occurred in the past 100 years. The observed slow earthquake activity explains why the seismic blank area has less solidification between plates compared to the surrounding area, resulting in lower activity of large earthquakes with epicenters in the blank area compared to the surrounding area. It also indicates that the risk of a large earthquake occurring in this region may be lower than previously assumed.

This result has been scheduled to be published in the electronic version of the British scientific journal Nature Communications.

Figure: Shallow slow earthquake zone (tectonic microtremor area), silent zone (non-seismogenic area), and small and slow earthquake zone in the Guerrero seismic blank area identified in this study. The Silent Zone is likely to be constantly shifting slowly due to weak plate-to-plate solidification. The Silent Zone is also found in this study to possibly correspond to a concave plate boundary geometry. The present study indicates the possibility that the weakly adhered zone extends from the trench to near the coastline in the Guerrero seismic blank area.

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