Seismic Swarm S20111217.1 Near Stella, Puerto Rico

The seismic swarm designated S20111217.1 occurred 16 km southwest of Stella in western Puerto Rico. It began at 06:06 on 17 December 2011 and concluded at 07:31 on 18 December 2011, spanning 25 hours and 25 minutes. During this period, 38 earthquakes were recorded, with magnitudes between 2.5 and 5.2 and focal depths ranging from 7 to 24 km. The sequence initiated with two larger events of magnitude 5.2 and 5.1, followed by numerous smaller aftershocks that gradually diminished in frequency.

This swarm unfolded within the tectonically active Mona Passage, a region shaped by the interaction between the North American and Caribbean plates. The passage lies along a complex boundary involving oblique subduction and strike-slip faulting, contributing to elevated seismicity in western Puerto Rico and adjacent offshore areas. Depths recorded during the swarm align with typical crustal activity in this zone, where shallow to intermediate events predominate due to the regional fault systems.

Puerto Rico’s seismic history reflects persistent tectonic stress. Major historical events include the 1918 magnitude 7.1 earthquake near the Mona Passage, which generated a destructive tsunami, and earlier shocks in the 19th century that affected the western coast. These episodes underscore the long-term potential for both moderate swarms and larger mainshock-aftershock sequences in the vicinity.

The 2011 swarm illustrates typical swarm behavior, in which clustered events occur without a single dominant mainshock. Most events remained below magnitude 3.2 after the initial pair, consistent with fluid migration or stress redistribution along local faults. No significant damage or casualties were associated with this particular sequence.

Geological monitoring in the region relies on networks operated by the Puerto Rico Seismic Network and the U.S. Geological Survey. Continued observation helps refine understanding of strain accumulation along the plate boundary.

References
U.S. Geological Survey Earthquake Catalog
Puerto Rico Seismic Network historical records
Tectonic summaries of the Caribbean-North American plate boundary (USGS Professional Papers)