Seismic Swarm S20210108.1 Near Guánica, Puerto Rico

A seismic swarm designated S20210108.1 occurred 7 km west-southwest of Guánica, Puerto Rico, beginning at 16:38 on 7 January 2021 and concluding at 12:55 on 21 January 2021. Over 332 hours and 16 minutes, the sequence produced 148 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude activity, with magnitudes ranging from 0.5 to 3.1 and focal depths between 2 km and 22 km. The initial event registered 3.1 at 10 km depth, followed by a rapid succession of smaller shocks clustered in the first 48 hours. Subsequent events maintained a pattern of shallow to intermediate depths, with the majority occurring between 4 km and 13 km.

Puerto Rico occupies a tectonically complex region at the northeastern Caribbean plate boundary, where the Caribbean Plate interacts with the North American Plate through a combination of subduction, oblique convergence, and strike-slip faulting. The island sits atop the Puerto Rico–Virgin Islands microplate, bounded by the Puerto Rico Trench to the north and the Muertos Trough to the south. This setting produces frequent seismic activity, including both mainshock-aftershock sequences and episodic swarms. The southwestern coast near Guánica lies along the western extension of the Great Southern Puerto Rico Fault Zone, a region historically prone to clustered seismicity.

Since 2000, seven seismic swarms have been recorded in the vicinity, with one in 2018, one in 2019, and five in 2020. The 2021 swarm fits within this established pattern of episodic unrest without a single dominant mainshock. Magnitudes remained modest, with only a few events exceeding 2.5, consistent with swarm behavior driven by fluid migration or stress redistribution along pre-existing faults rather than large-scale rupture.

The swarm's temporal evolution shows an initial burst of activity on 7–8 January, followed by a gradual decline punctuated by occasional bursts through mid-January. Depths clustered around 5–11 km, suggesting involvement of the upper crust where brittle failure is common. No events in the analyzed subset exceeded magnitude 3.1, indicating limited energy release compared with the more intense 2020 sequences.

This activity underscores the ongoing seismic hazard in southwestern Puerto Rico, where even moderate swarms can affect local infrastructure and communities. Continued monitoring remains essential given the region's position within an active plate-boundary system.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Puerto Rico Seismic Network historical records
SeismoSight internal swarm classification data