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Published November 13, 2019 | Accepted Version
Conference Paper Open

The February 2018 M_w 7.2 Pinotepa Earthquake in Mexico Ruptured a Small Patch of the Oaxaca Megathrust


The subduction zone of the Cocos Plate beneath Southern Mexico has major variations in the megathrust geometry and behavior. The subduction segment beneath the Oaxaca state of Mexico has relatively frequent large 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 under Oaxaca includes part of the subhorizontal "flat-slab" zone extending far from the trench beneath southern Mexico and the beginning of its transition to more regular subduction geometry to the southeast. We study the rupture of the 16 February 2018 Mw 7.2 Pinotepa earthquake near Pinotepa Nacional in Oaxaca that was a thrust event on the subduction interface. The Pinotepa earthquake was about 350 km away from the 8 September 2017 Mw 8.2 Tehuantepec earthquake in the subducting slab offshore Oaxaca and Chiapas; it was in an area of Coulomb stress decrease from the M8.2 quake, so it seems unlikely to be a regular aftershock and was not triggered by the static stress change. Geodetic measurements from interferometric analysis of synthetic aperture radar (InSAR) and time-series analysis of GPS station data constrain finite-fault slip models for the M7.2 Pinotepa earthquake. We analyzed InSAR data from Copernicus Sentinel-1A and -1B satellites and JAXA ALOS-2 satellite. Our Bayesian (AlTar) static slip model for the Pinotepa earthquake shows all of the slip confined to a very small (10-20 km diameter) rupture, similar to some early seismic waveform fits. The Pinotepa earthquake ruptured a portion of the Cocos megathrust that has been previously mapped as partially coupled and shows that at least small asperities in that zone of the subduction interface are fully coupled and fail in high-stress drop earthquakes. The previous 2012 Mw 7.4 Ometepec earthquake is another example of asperity in the partially coupled zone but was not imaged by InSAR so the rupture extent is not so well constrained. The preliminary NEIC epicenter for the Pinotepa earthquake was about 40 km away (NE) from the rupture imaged by InSAR, but the NEIC updated epicenter and Mexican SSN location are closer. Preliminary analysis of GPS data after the Pinotepa earthquake indicates rapid afterslip on the megathrust in the region of coseismic slip. Atmospheric noise masks the postseismic signal on early InSAR data.

Additional Information

We thank R. Jolivet for the CSI package used for the fault slip modeling and AlTar pre- and post-processing. InSAR processing was done with JPL InSAR Scientific Computing Environment (ISCE) software (Rosen et al., 2012) and Sentinel-1 interferograms were processed by the Advanced Rapid Imaging and Analysis (ARIA) project. Atmospheric corrections were done with the GACOS system Yu et al., 2017) and TRAIN (Bekaert et al., Part of this research was supported by the NASA Earth Surface and Interior focus area and performed at the Jet Propulsion Laboratory, California Institute of Technology. This work contains modified Copernicus Sentinel data from the Sentinel-1A and -1B satellites processed by the European Space Agency (ESA) and JPL. Original ALOS-2 is copyright JAXA (2017) and provided under JAXA RA4 PI Projects P1372002 and P1385002. Some figures were prepared with the Generic Mapping Tools (Wessel et al., 2013) and others with QGIS (QGIS, 2016).

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August 19, 2023
October 18, 2023