Seismic Insights into Swarm S20181215.1: Earthquake Activity Near Tallaboa, Puerto Rico

Earthquake swarm S20181215.1 was recorded in southwestern Puerto Rico, centered 4 km southwest of Tallaboa. The sequence began at 23:02 on 14 December 2018 and concluded at 05:35 on 19 December 2018, spanning 102 hours and 33 minutes. During this interval, 109 earthquakes were registered.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.9 to 2.5, with the majority falling between 1.5 and 2.3. Depths varied from 3 km to 32 km, clustering most frequently between 13 km and 20 km. Early events on 14–15 December showed slightly deeper foci around 13–19 km, while subsequent activity on 16–18 December included shallower events near 4 km alongside deeper ones up to 30 km. Temporal distribution indicated sustained low-level energy release without a dominant mainshock, consistent with swarm behavior driven by fluid migration or stress redistribution along local faults.

Puerto Rico occupies a tectonically complex region at the boundary between the Caribbean and North American plates. Southwestern areas near Tallaboa lie adjacent to the Muertos Trough, where oblique convergence produces thrust and normal faulting. This setting generates frequent seismic swarms through crustal extension and fluid involvement in fault zones.

Historical records document recurrent strong earthquakes affecting the island. Notable events include the 1918 magnitude 7.1 San Fermín earthquake, which originated offshore northwest Puerto Rico and triggered a tsunami, and earlier shocks in 1787 and 1867 that caused widespread damage. The 2018 swarm occurred within this long-term pattern of clustered seismicity.

The region continues to experience elevated seismic rates due to ongoing plate interaction, underscoring the value of continuous monitoring for hazard assessment.