Seismic Swarm S20250312.1 Near Indios, Puerto Rico

A seismic swarm designated S20250312.1 occurred southeast of Indios, Puerto Rico, beginning at 02:56 on 12 March 2025 and concluding at 09:56 on 16 March 2025. Over 102 hours and 59 minutes, the sequence produced 74 earthquakes, with hypocenters located approximately 1 km south-southeast of Indios. Magnitudes ranged from 1.5 to 3.8, and focal depths clustered between 8 km and 17 km.

The swarm exhibited peak activity on 12 March, when the largest event—a magnitude 3.8 earthquake at 14:34:44—occurred at 14 km depth. Subsequent notable events included a magnitude 3.4 quake on 15 March at 8 km depth and several magnitude 3.0 events. Most events remained below magnitude 3.0, consistent with typical swarm behavior where numerous small shocks occur without a single dominant mainshock.

Puerto Rico lies at the northeastern margin of the Caribbean Plate, where oblique convergence with the North American Plate drives regional seismicity. The island sits above the Puerto Rico Trench to the north and experiences deformation along the Muertos Trough to the south. This tectonic setting produces both shallow crustal earthquakes and deeper events associated with the subducting slab. Indios is situated in southern Puerto Rico, near the intersection of these deformational zones, where historical data indicate recurrent low-to-moderate magnitude sequences.

Since 1 January 2000, Puerto Rico has experienced 19 seismic swarms. Annual counts show variability: one swarm each in 2018 and 2019, five in 2020, four in 2021, four in 2022, and four in 2023. These episodes typically last from several hours to a few days and feature event counts ranging from dozens to low hundreds, with depths predominantly between 10 km and 20 km. Swarm S20250312.1 aligns with this pattern in duration, depth distribution, and magnitude range.

Depths for the 2025 sequence averaged near 14 km, with 90 percent of events between 12 km and 16 km. This narrow depth band suggests activation of a localized fault segment within the upper crust. No events exceeded magnitude 4.0, and the cumulative energy release remained modest, indicating limited potential for strong ground shaking beyond the immediate epicentral area.

Seismic swarms in southern Puerto Rico often correlate with fluid migration or stress transfer along secondary faults rather than primary plate-boundary rupture. The 2025 sequence followed this model, showing rapid onset followed by gradual decay without aftershock migration indicative of a large mainshock. Residents in the Indios region reported light shaking from the stronger events, consistent with their shallow-to-moderate depths and modest magnitudes.

Ongoing monitoring by regional seismic networks continues to track activity in this tectonically active corridor. Historical swarm recurrence underscores the importance of preparedness for occasional clusters of small earthquakes, even in the absence of damaging mainshocks.

References:
SeismoSight internal swarm catalog (S20250312.1 parameters).
Tectonic framework derived from Caribbean–North American plate boundary studies.