The March 2012 Mw 7.4 Ometepec and February 2018 Mw 7.2 Pinotepa Earthquakes in Mexico Ruptured Small Patches of the Cocos Megathrust

Date: 4/24/2019

Time: 04:45 PM

Room: Cascade II

The megathrust of the Cocos Plate beneath Southern Mexico has major variations in the geometry and behavior. The segment beneath the Mexican state of Oaxaca has relatively frequent magnitude 7–7.5 earthquakes on the shallow part of the megathrust and within the subducting slab, and it also has large aseismic slow-slip events. The slab geometry includes part of the subhorizontal “flat-slab” zone extending far from the trench beneath Guerrero and the beginning of its transition to more regular subduction geometry to the southeast. We study the ruptures of the 20 March 2012 Mw 7.4 Ometepec earthquake near the Guerrero-Oaxaca border and the16 February 2018 Mw 7.2 Pinotepa earthquake near Pinotepa Nacional in Oaxaca that were both megathrust events. The 2012 Ometepec and 2018 Pinotepa earthquake epicenters were located about 45 km apart by the Mexican Servicio Sismológico Nacional (SSN).

We use geodetic measurements from interferometric analysis of synthetic aperture radar (InSAR) and GPS coseismic offsets to estimate finite-fault slip models for the M7.4 Ometepec and M7.2 Pinotepa earthquakes. We analyzed InSAR data from Copernicus Sentinel-1A and -1B satellites and JAXA ALOS-2 satellite for 2018 Pinotepa and Canadian RADARSAT-2 for 2012 Ometepec. Our Bayesian (AlTar) static slip models for both earthquakes shows all of the slip confined to very small (10-20 km diameter) ruptures, similar to some early seismic waveform fits, and the two ruptures do not overlap. The earthquakes ruptured part of the Cocos megathrust that has been previously mapped as partially coupled; at least small asperities in that zone of the subduction interface are fully coupled and fail in high stress-drop earthquakes. Preliminary analysis of GPS data after the Pinotepa earthquake indicates rapid afterslip on the megathrust in the region of coseismic slip. Previous analysis by Graham et al. (2014) and others showed afterslip after the 2012 Ometepec event, largely down-dip. This part of the megathrust may have areas of low coupling that have afterslip or slow slip between the locked patches.

Presenting Author: Eric J. Fielding

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