Published April 1, 2013 | Published
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Ometepec-­Pinotepa Nacional, Mexico Earthquake of 20 March 2012 (M_w 7.5): A preliminary report

Abstract

An analysis of local and regional data produced by the shallow, thrust Ometepec-Pinotepa Nacional earthquake (M_w 7.5) of 20 March 2012 shows that it nucleated at 16.254°N 98.531°W, about 5 km offshore at a depth of about 20 km. During de first 4 seconds the slip was relatively small. It was followed by rupture of two patches with large slip, one updip of the hypocenter to the SE and the other downdip to the north. Total rupture area, estimated from inversion of near-source strong-motion recordings, is ~25 km × 60 km. The earthquake was followed by an exceptionally large number of ftershocks. The aftershock area overlaps with that of the 1982 doublet (M_w 7.0, 6.9). However, the seismic moment of the 2012 earthquake is ~3 times the sum of the moments of the doublet, indicating that the gross rupture characteristics of the two earthquake episodes differ. The small-slip area near the hypocenter and large-slip areas of the two patches are characterized by relatively small aftershock activity. A striking, intense, linear NE alignment of the aftershocks is clearly seen. The radiated energy to seismic moment ratios, (E_s/M₀), of five earthquakes in the region reveal that they are an order of magnitude smaller for near-trench earthquakes than those that occur further downdip (e.g., 2012 and the 1995 Copala earthquakes). The near-trench earthquakes are known to produce low Amax. The available information suggests that the plate interface in the region can be divided in three domains. (1) From the trench to a distance of about 35 km downdip. In this domain M~6 to 7 earthquakes with low values of (E_s/M₀) occur. These events generate large number of aftershocks. It is not known whether the remaining area on this part of the interface slips aseismically (stable sliding) or is partially locked. (2) From 35 to 100 km from the trench. This domain is seismically coupled where stick-slip sliding occurs, generating large earthquakes. Part of the area is probably conditionally stable. (3) From 100 to 200 km from the trench. In this domain slow slip events (SSE) and nonvolcanic tremors (NVT) have been reported. The earthquake caused severe damage in and near the towns of Ometepec and Pinotepa Nacional. The PGA exceeded 1 g at a soft site in the epicentral region. Observed PGAs on hard sites as a function of distance are in reasonable agreement with the expected ones from ground motion prediction equations derived using data from Mexican interplate earthquakes. The earthquake was strongly felt in Mexico City. PGA at CU, a hard site in the city, was 12 gal. Strong-motion recordings in the city since 1985 demonstrate that PGAs during the 2012 earthquake were not exceptional, and that similar motion occurs about once in three years.

Additional Information

© 2013 Union Fisica Mexicana. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Universidad Nacional Autónoma de México Seismology Group (with contribution from Universidad Autónoma Metropolitana, Azcapozalco, México, D.F.). Received: February 02, 2013; accepted: February 15, 2013; published on line: March 22, 2013. UNAM Seismology Group includes all researchers and technicians, as well as students, of Institute of Geophysics and Institute of Engineering, UNAM, who work in the field of seismology and engineering seismology. In case of a significant earthquake in Mexico, the Group informally coordinates its efforts in data collection from autonomous stations, field deployment of seismographs and accelerographs, analysis of the data, and elaboration of a preliminary report. The credit of the work is shared by all. We thank Civil Protection authorities of the State of Oaxaca, Centro de Instrumentación y Registros Sísmicos (CIRES) and Centro Nacional de Prevención de Desastres (CENAPRED) for making available to us the recordings of the mainshock. We are indebted to H. Kanamori for sharing his unpublished results with us and for many fruitful discussions on the 2012 earthquake during his visit to UNAM in August 2012. We appreciate comments and suggestions by H. Kanamori, L. Astiz, R. Castro and C. Mendoza who were reviewers of the manuscript. The research was supported by Conacyt project 82599, and PAPIIT-­UNAM projects IN112411 and IN111411-­2.

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