Seismic Swarm S20201007.2 Near Guánica, Puerto Rico: Geological Context and Event Analysis

Puerto Rico lies at the convergent boundary between the Caribbean and North American tectonic plates. The island sits atop the Caribbean Plate, where oblique subduction along the Puerto Rico Trench and left-lateral strike-slip motion on the Bunce Fault and Septentrional Fault Zone generate frequent seismic activity. The region’s crust is fragmented by several active faults, including the North Coast Fault Zone and the Great Southern Fault, which accommodate regional strain. These structures have produced damaging historical earthquakes, most notably the 1918 Mw 7.1 event that struck the northwest coast and the January 2020 Mw 6.4 mainshock near Indios, which initiated an extended aftershock sequence across southern Puerto Rico.

Between 6 October and 28 October 2020, seismic network operators recorded Swarm S20201007.2 centered 7 km south of Guánica. The swarm began at 12:24 UTC on 6 October and concluded at 22:19 UTC on 28 October, spanning 537 hours and 55 minutes. During this interval, 229 earthquakes were detected, with the majority occurring at shallow to mid-crustal depths between 5 km and 17 km.

Analysis of the first 100 events reveals a typical swarm pattern dominated by low-to-moderate magnitudes. The largest event reached magnitude 4.0 at 23:03 UTC on 6 October at 6 km depth. Subsequent peaks included multiple magnitude 3.6 events on 11 October at depths near 11–12 km. Most events clustered between magnitudes 2.3 and 2.9, with focal depths predominantly 6–13 km, consistent with activation of shallow crustal faults south of the island. Activity showed temporal clustering, with notable bursts on 6–7 October, 10–11 October, and 13 October, followed by gradual decay.

This swarm was the fourth recorded in the Puerto Rico region since 2000. Earlier swarms occurred in 2018 (one swarm), 2019 (one swarm), and earlier in 2020 (one swarm), indicating episodic swarm behavior superimposed on the aftershock sequence of the January 2020 mainshock. Such swarms are commonly linked to fluid migration or slow slip along pre-existing faults rather than a single large rupture.

The October 2020 activity remained below levels that produced significant damage, yet it contributed to ongoing seismic hazard awareness for southern Puerto Rico communities. Continued monitoring by the Puerto Rico Seismic Network and the U.S. Geological Survey provides essential data for refining regional fault models and improving earthquake preparedness.

References

USGS Earthquake Catalog
Puerto Rico Seismic Network (Red Sísmica de Puerto Rico)
Tectonic framework summaries from the Puerto Rico Trench and Caribbean–North American plate boundary studies (published literature through 2023)