Seismic Swarm S20181017.1: Analysis of Activity Near Puerto Real, Puerto Rico

A seismic swarm designated S20181017.1 was recorded north of Puerto Real in western Puerto Rico. The sequence began at 10:03 on 16 October 2018 and concluded at 03:19 on 19 October 2018, spanning 65 hours and 16 minutes. During this interval, 72 earthquakes were registered, with the majority occurring at depths between 14 and 21 km. Magnitudes ranged primarily from 1.3 to 2.8, indicating low-to-moderate energy release consistent with swarm behavior rather than a mainshock-aftershock sequence.

The swarm exhibited peak activity on 17 October, when numerous events clustered within a few hours. Notable shocks included a magnitude 2.8 event at 03:19 and another at 03:49, both at approximately 18 km depth. Later events on 18 and 19 October showed slightly shallower foci in some cases, down to 6–12 km, while maintaining similar magnitude ranges. This temporal and spatial pattern aligns with fluid migration or stress redistribution along local fault structures.

Puerto Rico lies at the complex tectonic boundary between the North American and Caribbean plates. The island sits on the Puerto Rico–Virgin Islands microplate, influenced by oblique subduction along the Puerto Rico Trench to the north and strike-slip faulting associated with the Septentrional–Oriente fault system. Western Puerto Rico, including the area near Puerto Real, experiences distributed deformation from these interactions, resulting in frequent small-magnitude seismicity. Crustal thickness in the region averages 20–30 km, consistent with the observed focal depths of the swarm events.

Historically, Puerto Rico has recorded significant earthquakes, including the 1918 Mw 7.1 event that caused widespread damage along the western coast and generated a local tsunami. Instrumental records since 2000 indicate that seismic swarms remain uncommon in this specific sector. Available statistics document only one prior swarm since 1 January 2000, occurring in 2011. The 2018 sequence therefore represents a rare clustered episode within an otherwise low-rate background of isolated events.

Geological mapping of western Puerto Rico reveals a mix of Cretaceous volcanic-arc rocks, Tertiary sedimentary units, and Quaternary alluvium. These formations are cut by northwest-trending faults that accommodate regional shear. The swarm location, approximately 4 km north of Puerto Real, places the hypocenters within or near these fault corridors, where minor stress perturbations can trigger prolonged microseismicity.

In summary, swarm S20181017.1 illustrates typical characteristics of tectonic swarm activity in a plate-boundary setting: modest magnitudes, limited duration, and clustered occurrence without a dominant mainshock. Continued monitoring of similar sequences contributes to refined understanding of fault behavior and seismic hazard in western Puerto Rico.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Puerto Rico Seismic Network historical bulletins
• Tectonic framework from published studies on Caribbean–North American plate boundary (e.g., Mann et al., Geological Society of America)