Seismic Swarm S20200117.1: Geological Context and Event Analysis in Oaxaca, Mexico

Oaxaca, located in southern Mexico, occupies a highly active tectonic setting at the boundary between the North American and Cocos plates. The Cocos plate subducts beneath the North American plate along the Middle America Trench, generating frequent seismicity through thrust faulting and crustal deformation. This subduction zone has produced numerous moderate-to-large earthquakes historically, with Oaxaca experiencing recurrent events due to its position along the trench. Depths of regional seismicity commonly range from shallow crustal levels to intermediate depths exceeding 50 km, reflecting the geometry of the subducting slab.

Seismic swarms represent clusters of earthquakes occurring in close spatial and temporal proximity without a dominant mainshock-aftershock sequence. In Oaxaca, such swarms often arise from fluid migration along faults or stress perturbations within the overriding plate. Swarm S20200117.1, recorded between 02:18 on 17 January 2020 and 20:51 on 19 January 2020, lasted 66 hours and 32 minutes and comprised 60 events. Magnitudes reached a maximum of 4.3, with the majority of hypocenters at depths of 2–13 km and isolated events extending to 63 km. Activity peaked during the first day, featuring repeated events of magnitude 4.0–4.1, before tapering over the subsequent 48 hours.

The shallow focus of most events aligns with upper-crustal faulting typical of the region’s forearc domain. Historical records indicate limited swarm occurrences since 2000, with one prior episode documented in 2017. This low frequency underscores the episodic nature of swarm activity amid the steady background of subduction-driven earthquakes.

The 2020 swarm contributed to ongoing monitoring of strain accumulation along the Oaxaca segment of the subduction zone. No surface rupture or significant damage was associated with these moderate-magnitude events, consistent with their limited energy release compared to great subduction earthquakes that have affected the region in past centuries.

References

USGS Earthquake Catalog
Servicio Sismológico Nacional (Mexico)
Middle America Trench tectonic summaries (USGS)