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