Coseismic uplift and coral mortality caused by the 23 June 2020, Mw 7.4 La Crucecita, Oaxaca, Mexico earthquake and tsunami 

Abstract

<p>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 tide gauge stations and a DART off the coast of Mexico. We describe here details of the rapid response survey of vertical coseismic deformation, tsunami, geologic effects, ecological damage to coral reef ecosystems, and lessons from working in the field during the COVID-19 crisis. Moreover, ecological damage caused by earthquakes and tsunamis is an important problem barely addressed in recent studies. Coral reefs are some of the world’s most fragile ecosystems, they are highly sensitive to environmental changes. These communities had suffered a significant impact by both coseismic coastal uplift and the tsunami. We surveyed 44 km along the coast of Oaxaca. Because of the COVID-19 pandemic, some local communities enforced rules of confinement. They closed the access roads to their villages prohibiting the passage of outsiders to their community. We assessed coseismic coastal uplift by means of mortality caused by vertical displacement of intertidal organisms, bleaching of coral communities, resurveying of benchmarks, and measured tsunami runup. 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 fit well with 0.55 m of uplift reported by tide gauge data at Huatulco. These amounts of coastal uplift left coral communities exposed and not covered by seawater even at high tide, causing the death of these communities. 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. </p>