The 23 June 2020 M-W 7.4 La Crucecita, Oaxaca, Mexico Earthquake And Tsunami: A Rapid Response Field Survey During Covid-19 Crisis

The 23 June 2020 La Crucecita earthquake occurred at 10:29 hr on the coast of Oaxaca in an M-w 7.4 megathrust event at 22.6 km depth and triggered a tsunami recorded at tide gauge stations and a Deep-ocean Assessment and Reporting of Tsunamis off the coast of Mexico. Immediately after the earthquake, a rapid response effort was coordinated by members of the Tsunami and Paleoseismology Laboratory, Universidad Nacional Autonoma de Mexico. Despite the challenges posed by the Coronavirus disease 2019 (COVID-19) pandemic crisis, a postearthquake and post-tsunami field survey went ahead two days after the event. We describe here the details of the rapid response survey of the vertical coseismic deformation, tsunami, geologic effects, and lessons from working in the field during the COVID-19 crisis. We surveyed 44 km along the coast of Oaxaca. Because of the COVID-19 pandemic, some local communities enforced rules of confinement. We solved most of the challenges faced during this crisis by rapidly networking with local organizations prior to surveying. We assessed coseismic uplift by means of mortality caused by vertical displacement of intertidal organisms and resurveying of benchmarks, and we measured tsunami runup. Our results show coastal uplift of 0.53 m near the epicenter and decreasing farther away from it; uplift was up to 0.8 m in areas related to exposure of the coast. Of our values of coastal uplift, about 0.53 m fit well with the 0.55 m of uplift reported by tide gauge data at Huatulco. Coastal uplift and low tide at the time of the event limited the tsunami inundation and runup on the Oaxaca coast. Nevertheless, we found tsunami inundation evidence at four confined coastal sites reaching a maximum runup of 1.5 m. The enclosed morphology of these sites determined higher runup and tsunami inundation. Local coastal morphology effects are not detected in tsunami models lacking detailed bathymetry and topography. This issue needs to be addressed during tsunami hazard assessments.