Seismic Swarm PS20120320.1 Near San José del Progreso, Mexico

The seismic swarm PS20120320.1 occurred in the Oaxaca region of southern Mexico, centered 11 km west-southwest of San José del Progreso. It began at 18:02 UTC on 20 March 2012 and concluded at 11:36 UTC on 21 March 2012, spanning 17 hours and 33 minutes. During this period, 15 earthquakes were recorded, commencing with a magnitude 7.4 event at 20 km depth and followed by events ranging from magnitude 4.9 to 5.3 at depths between 3 km and 60 km.

This activity took place within the tectonically complex zone where the Cocos Plate subducts beneath the North American Plate along the Middle America Trench. Oaxaca lies in a region of high seismic hazard characterized by frequent moderate to large earthquakes, driven by the subduction interface and associated crustal faults. The 20 March 2012 magnitude 7.4 mainshock, located approximately 6 km east of Santiago Llano Grande and 20 km from the swarm centroid, aligns with the ongoing convergence rate of about 6–7 cm per year.

Subsequent events in the swarm exhibited variable focal depths, suggesting activation across multiple structural levels, including the subduction interface and overlying crustal blocks. Notable events included a pair of magnitude 5.2 shocks at 19:02 UTC on 20 March at depths of 3 km and 30 km, as well as several magnitude 5.0–5.1 events clustered between 10 km and 45 km depth through the night and into 21 March. Depths clustered around 10–20 km for many after-events, consistent with typical aftershock patterns in the upper plate and interface.

The swarm occurred in a corridor of elevated seismicity. A magnitude 7.2 earthquake on 16 February 2018, centered 4 km south of Pinotepa de Don Luis and 47 km from the swarm location, further illustrates the persistent activity along this segment of the subduction zone. Historical records document recurrent large events in Oaxaca, underscoring the region’s long-term role in accommodating plate motion through both interface thrust faulting and intraslab normal faulting.

Analysis of the 15 events reveals a rapid sequence without clear migration, typical of swarm-like behavior superimposed on a significant mainshock. Depths spanning 3–60 km indicate involvement of both shallow crustal structures and deeper portions of the subducting slab. Such sequences contribute to stress redistribution and can influence the probability of subsequent events in adjacent segments.

This swarm provides insight into the short-term clustering that accompanies major subduction-zone earthquakes in Oaxaca. Continued monitoring remains essential given the region’s history of producing damaging events and its proximity to populated areas and critical infrastructure.

References
SeismoSight internal classification data for swarm PS20120320.1
USGS Earthquake Catalog (events of 20 March 2012 and 16 February 2018)