Seismic Swarm PS20220920.1 in Michoacán, Mexico

A seismic swarm designated PS20220920.1 was recorded in Michoacán, Mexico, between 18:05 on 19 September 2022 and 08:24 on 20 September 2022. Over this 14-hour and 19-minute period, five earthquakes were registered. The events occurred in a tectonically active region characterized by ongoing subduction along the Middle America Trench, where the Cocos Plate descends beneath the North American Plate at rates of approximately 5–7 cm per year. This convergence generates frequent seismic activity, including both large thrust earthquakes and smaller swarm sequences.

The swarm began with a magnitude 7.6 earthquake at 18:05:08 on 19 September 2022, located at a depth of 26 km. Subsequent events included a magnitude 5.3 quake at 19:30:39 on the same day at 10 km depth, followed on 20 September by a magnitude 5.8 event at 08:17:10 (16 km depth), a magnitude 5.9 event at 08:17:18 (60 km depth), and a magnitude 3.5 event at 08:24:37 (11 km depth). The initial magnitude 7.6 shock was centered approximately 31 km from the swarm’s primary focus and 35 km south-southwest of Aguililla.

Michoacán’s geological setting features the Jalisco and Michoacán blocks, influenced by the Rivera and Cocos plates. The region has experienced significant historical seismicity, including the devastating 1985 Michoacán earthquake that affected Mexico City. Swarm activity remains uncommon; records since 1 January 2000 indicate only one prior swarm, which occurred in 2003. The 2022 sequence aligns with the area’s established pattern of clustered seismicity near the subduction interface and within the overriding continental crust.

Analysis of the depth distribution shows events spanning the shallow crust to intermediate depths, consistent with both interplate thrusting and intraslab processes. The swarm’s short duration and limited number of events distinguish it from aftershock sequences of major mainshocks, highlighting localized stress release along pre-existing faults.

References

SeismoSight internal swarm classification PS20220920.1
USGS Earthquake Catalog (event parameters for Michoacán, 19–20 September 2022)
Mexican National Seismological Service (SSN) regional tectonic summaries