Back-arc tsunami hazard in the Gulf of Mexico from Oaxaca earthquakes: Lessons from past events

Abstract:

The M_w=7.4 June 23, 2020 Oaxaca earthquake created a small tsunami in the Pacific Ocean. On-land location of the earthquake in Central America along with the shallow continental shelf caused the tsunami to be felt on a local scale and only registered on two Mexican tide gauges as well as questionably a single nearby DART station. However, numerical simulations show small but widespread tsunami waves in the back-arc Gulf of Mexico which were arguably recorded by tide gauges on the U.S. coastlines. Data from tide gauge stations also show the back-arc extent of tsunamis caused directly by coseismic deformation. Previous examples of coseismic back-arc excitation include the Sea of Japan following the 2011 Tohoku earthquake (particularly due to the large size of the rupture). The moderate size of the Oaxaca rupture, however, warrants an investigation of potential future tsunamis in case of large megathrusts filling the Oaxaca seismic gap. In this study, we have simulated a large number of tsunami propagation scenarios in the Gulf of Mexico from earthquake sources with various geometries and sizes that are consistent with the seismotectonics of the Oaxaca region. Our analysis of the tsunami amplitudes at the coastlines around the Gulf region reveals considerable hazard. We find that the amplitude of back-arc tsunamis not only depends on source geometry and location but is also affected by the shape of the back-arc basin due to resonance. An important result of this analysis – especially in the absence of active large thrust sources – is that back-arc basins, especially the Gulf of Mexico, are not immune to various modes of tsunami propagation. It also provides us with valuable insight into re-examining the tsunami hazard in the countries around the Gulf of Mexico.