Maria Teresa Ramirez-Herrera, David Romero, Néstor Corona, Héctor Nava, Hamblet Torija, Felipe Hernández Maguey

The 23 June 2020 La Crucecita earthquake occurred at 10:29 hr on the coast of Oaxaca in a Mw 7.4 megathrust event at 22.6 km depth, and triggered a tsunami recorded at Huatulco and Salina Cruz tide gauge stations and a DART off the coast of Mexico. Immediately after the earthquake, a rapid response effort was coordinated by members of the Tsunami and Paleoseismology Laboratory UNAM, despite the challenges by the 23 June 2020 La Crucecita earthquake, a post-earthquake and post-tsunami field survey went ahead 2 days after the event. We describe here details of the rapid response survey focusing on evidence of 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 focusing on preselected sites. Because of COVID-19 pandemic, some local communities enforced rules of confinement. We solved most of the challenges faced during this crisis by rapid 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 bench marks, and measured tsunami runup using a laser ranger and GPS. Our results show coastal uplift of 0.53 m near the epicenter, decreasing farther away from it, and up to 0.8 m, the latest related to exposure of the coast. Our values of coastal uplift, ca. 0.53 m near the epicenter, fit well with 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.