Seismic Swarm S20200107.1: Analysis of Activity Near Sabana Grande, Puerto Rico

Puerto Rico occupies a tectonically complex zone at the boundary between the Caribbean and North American plates. The island experiences frequent seismic events due to oblique subduction along the Puerto Rico Trench to the north and strike-slip faulting associated with the Septentrional-Oriente fault system. Shallow crustal seismicity is common, with most events occurring at depths less than 30 km. Historical records document destructive earthquakes, including the magnitude 7.5 event of 1918 that generated a tsunami and caused significant damage along the western coast.

Between 7 and 8 January 2020, a seismic swarm designated S20200107.1 was recorded 7 km northeast of Sabana Grande. The sequence began at 00:19 on 7 January and concluded at 22:05 on 8 January, spanning 45 hours and 46 minutes. During this period, 53 earthquakes were detected. Magnitudes ranged from 0.8 to 4.7, with the largest event occurring on 7 January at 12:54. Depths were predominantly shallow, between 2 km and 24 km, consistent with upper-crustal faulting in the region.

The swarm exhibited a typical pattern of clustered, low-to-moderate magnitude events without a single dominant mainshock. Activity peaked during the first day, featuring several events above magnitude 3.0, including magnitudes 3.2, 4.7, 3.1, and 3.0. On the second day, additional notable shocks reached magnitudes 3.8, 3.9, and 4.0. Most events clustered between 4 km and 13 km depth, suggesting activation of a localized fault segment or fluid-driven processes within the crust.

Such swarms are not uncommon in Puerto Rico. Since 2000, only two prior swarms have been identified in the SeismoSight internal classification: one in 2018 and one in 2019. The 2020 sequence adds to this sparse record, highlighting episodic, short-lived seismic clusters rather than prolonged aftershock sequences following major events.

Analysis of the temporal distribution shows rapid onset followed by gradual decay, with 40 events on 7 January and 13 on 8 January. This distribution aligns with swarm behavior driven by aseismic slip or pore-pressure changes rather than static stress transfer from a large rupture. Depths remained consistent across both days, reinforcing the interpretation of a confined source volume.

The swarm's location near Sabana Grande places it within the western volcanic province of Puerto Rico, an area underlain by Cretaceous to Tertiary igneous and sedimentary rocks cut by active faults. Ongoing plate-boundary deformation continues to load these structures, maintaining the island's elevated seismic hazard.

References

United States Geological Survey Earthquake Catalog
Puerto Rico Seismic Network historical data
SeismoSight internal swarm classification records