Seismic Swarm S20200817.1 Near San Vicente, Mexico

Seismic swarm S20200817.1 was recorded 56 km east-northeast of San Vicente, Mexico, beginning at 15:09 on 17 August 2020 and concluding at 11:25 on 21 August 2020. Over 92 hours and 16 minutes, the sequence produced 58 earthquakes. This event represents the second swarm documented in the region since 2000, following an earlier episode in 2010.

The swarm initiated with a magnitude 4.4 event at 10 km depth. Within the first hour, activity escalated to a peak magnitude of 5.1 at 16 km depth. Subsequent events ranged from magnitude 2.0 to 3.8, with the majority occurring at depths between 8 and 20 km. Shallower events, some as shallow as 1 km, were also recorded, though deeper foci near 20–22 km appeared toward the later stages of the sequence. The temporal distribution showed highest frequency during the initial 24 hours, with magnitudes generally declining after 18 August.

Earthquake swarms of this type reflect clustered seismic energy release without a single dominant mainshock-aftershock pattern. In this instance, multiple events clustered around magnitudes 3.0–3.8, interspersed with numerous smaller tremors. Depths predominantly in the upper to mid-crust suggest activation along pre-existing fault structures rather than a singular rupture plane.

The broader tectonic setting of southern Mexico involves convergence between the Cocos and North American plates along the Middle America Trench. Subduction generates frequent seismicity, including both interface thrust events and intraslab activity. Historical records indicate that swarm-like sequences can occur in areas of fluid migration or stress transfer within the overriding plate. Updated regional monitoring networks have improved detection of such low-to-moderate magnitude clusters since the early 2000s.

No damage or felt reports beyond instrumental recordings are associated with this swarm. The pattern of decreasing event rates after 19 August aligns with typical swarm decay, where fluid pressure or aseismic slip may sustain activity before gradual relaxation. Continued monitoring remains essential given the region’s history of both isolated events and episodic swarms along the subduction margin.

References:
USGS Earthquake Catalog
Servicio Sismológico Nacional (Mexico)
Global Centroid Moment Tensor Project