The 23 June 2020 Mw 7.4 La Crucecita, Oaxaca, Mexico Earthquake and Tsunami: A Rapid Response Field Survey during COVID-19 Crisis

Abstract

The 23 June 2020 La Crucecita earthquake occurred at 10:29 hr on the coast of Oaxaca in an Mw 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 Autónoma de México. 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.

Do slow slip earthquakes inhibit catastrophes? Geologic evidence from the Guerrero segment, Mexican subduction zone

“…Nevertheless, the "1300 AD" event (unit 3) is likely a remarkable event that produced considerable coastal deformation (≥ 1 m subsidence) and may reflect a rupture closer to the trench. Most instrumentally recorded events on the MSZ rupturing near the coast produced coastal uplift for earthquakes in 1962, 1985, and 2020 16,27,28 . However, other large near-trench earthquake events (within 20 km) causing seismic slip in the shallow portion of the MSZ, such as Colima 1995 and Jalisco 1932, produced coseismic coastal subsidence 29,31,32 .…”

Behavior of RC Beam–Column Joints Strengthened with Modified Reinforcement Techniques

“…In most countries vulnerable to seismic events, pre-seismic codes do not meet present standards for reinforced concrete structures [10]. Earthquakes that have occurred recently (e.g., M7.4 Oaxaca (Mexico), M7.0 Aegean Sea (Turkey-Greece), M6.4 Croatia) have mostly caused failures of masonry buildings, but RC buildings were also, in some cases, heavily damaged [11][12][13]. One reason is that the beam-column joints in moment-resisting RC frame constructions have enough shear strength and ductility [14].…”