Seismic Swarm S20210130.1: Analysis of Activity Near Palomas, Puerto Rico

A seismic swarm designated S20210130.1 occurred 2 km southeast of Palomas in southwestern Puerto Rico. The sequence began at 03:21 on 29 January 2021 and concluded at 04:48 on 31 January 2021, spanning 49 hours and 27 minutes. During this interval, 29 earthquakes were recorded.

Magnitudes ranged from 1.1 to 2.8, with the largest event measuring 2.8 at a depth of 3 km on 29 January at 11:18. Most events clustered between magnitudes 1.7 and 2.5. Depths varied from 3 km to 20 km, though the majority occurred between 8 km and 17 km, indicating shallow crustal activity typical of the region.

The swarm exhibited a rapid onset with multiple events within the first 12 hours, followed by a gradual decline. Notable clusters included several events above magnitude 2.0 on 29 January afternoon and isolated activity extending into 31 January. Such patterns reflect fluid migration or stress redistribution along local faults rather than a single mainshock-aftershock sequence.

Puerto Rico occupies a tectonically active zone at the boundary between the North American and Caribbean plates. The Puerto Rico Trench to the north accommodates oblique subduction, while the Mona Passage to the west features a complex network of normal and strike-slip faults. Southwestern Puerto Rico, including the vicinity of Palomas, lies near the intersection of these systems and the Great Southern Puerto Rico Fault Zone. This setting produces frequent low-to-moderate magnitude swarms alongside occasional larger events.

Historical records document eight swarms in the region since 2000, distributed as one in 2018, one in 2019, five in 2020, and the present sequence in 2021. The elevated swarm frequency in 2020 coincided with the prolonged 2019–2020 Puerto Rico seismic sequence, which included a magnitude 6.4 mainshock and thousands of aftershocks. The 2021 swarm near Palomas appears consistent with ongoing adjustment along the same fault network.

Seismic monitoring in Puerto Rico is conducted by the Puerto Rico Seismic Network and the U.S. Geological Survey. Data from these networks enable precise location and magnitude determination for events as small as magnitude 1.0. Continued surveillance remains essential given the potential for larger earthquakes in the plate-boundary region.

References

• Puerto Rico Seismic Network earthquake catalog
• U.S. Geological Survey Earthquake Hazards Program
• Caribbean tectonic framework summaries from peer-reviewed geophysical literature