Seismic Swarm S20200115.1 in Oaxaca, Mexico: Analysis of January 2020 Activity

Oaxaca, located in southern Mexico along the Pacific coast, lies within a highly active tectonic zone characterized by the subduction of the Cocos Plate beneath the North American Plate. This convergence drives frequent seismic events, with the regional geology featuring the Sierra Madre del Sur mountains and coastal sedimentary basins that amplify ground shaking. The area experiences both shallow crustal earthquakes and deeper events associated with the subduction interface, typically ranging from 5 to 30 km depth.

The seismic swarm designated S20200115.1 began at 07:44 on 15 January 2020 and concluded at 06:35 on 19 January 2020, spanning 94 hours and 51 minutes. During this period, 52 earthquakes were recorded in Oaxaca. The sequence featured events with magnitudes between 3.0 and 4.8, predominantly at shallow depths of 1 to 15 km. Notable initial shocks included a magnitude 4.8 event at 10 km depth on 15 January at 07:44:33 UTC, followed closely by multiple magnitude 3+ tremors clustered within the first hours. Activity persisted with peaks such as a magnitude 4.2 event at 8 km on 15 January at 14:47:33 and a magnitude 4.1 event at 6 km on 19 January at 06:35:44. The swarm exhibited a typical pattern of clustered, moderate-magnitude quakes without a dominant mainshock-aftershock sequence.

Insights from the event reveal a concentrated release of tectonic stress along local faults, with many events occurring at depths under 10 km, suggesting involvement of the upper crust. Temporal distribution showed higher frequency on the first day, tapering over subsequent days, consistent with fluid migration or aseismic slip triggering mechanisms common in subduction-related swarms.

Historically, seismic swarms in Oaxaca remain infrequent. Records since 1 January 2000 indicate only two prior swarms: one in 2012 and another in 2018. This 2020 episode represents the third documented swarm in two decades, underscoring the episodic nature of such activity amid the region's steady background seismicity.

References

• SeismoSight internal classification and swarm dataset for event parameters and historical swarm counts.
• United States Geological Survey (USGS) Earthquake Hazards Program for regional tectonic framework and subduction zone characteristics.