Seismic Analysis of Swarm S20201118.1 Near María Antonia, Puerto Rico

Puerto Rico occupies a tectonically active position at the boundary between the Caribbean and North American plates. The island experiences frequent seismicity driven by oblique convergence, with the Puerto Rico Trench to the north and the Muertos Trough to the south contributing to regional strain accumulation. Southwestern Puerto Rico, including areas near María Antonia, lies within a zone of diffuse faulting that has produced multiple historical earthquake sequences.

Swarm S20201118.1 was recorded 7 km south-southeast of María Antonia. The sequence began at 04:15 on 17 November 2020 and concluded at 00:51 on 23 December 2020, spanning 860 hours and 35 minutes. During this interval, 447 earthquakes were detected. The swarm represents one of five documented swarms in the region since 1 January 2000, with prior episodes occurring in 2018 (one swarm), 2019 (one swarm), and three additional swarms in 2020.

Examination of the first 100 events reveals a predominance of low-magnitude activity. Magnitudes ranged from 0.8 to 2.9, with the majority falling between 1.0 and 2.4. Depths varied from 2 km to 28 km, though most events clustered between 5 km and 16 km, consistent with shallow crustal faulting in the overriding plate. The initial event on 17 November at 04:15 registered magnitude 1.9 at 11 km depth. Subsequent activity included a magnitude 2.8 event at 14 km depth within the first hour and a magnitude 2.9 event at 8 km depth on 18 November. Temporal clustering was evident, with several hours showing multiple events of similar magnitude and depth, indicative of progressive stress release along local structures.

The swarm's characteristics align with known patterns of seismic swarms in Puerto Rico, where fluid migration or aseismic slip may trigger distributed microseismicity without a single dominant mainshock. Depths predominantly within the upper 15 km suggest involvement of the island-arc crust rather than deeper subduction-related processes.

This episode adds to the documented seismic history of southwestern Puerto Rico, where repeated swarm activity since 2000 highlights ongoing tectonic adjustment. Continued monitoring remains essential for understanding long-term strain patterns in this densely populated region.

References
SeismoSight internal swarm classification S20201118.1
USGS Earthquake Catalog (regional Puerto Rico seismicity)
Caribbean tectonic framework summaries from NOAA and academic plate-boundary studies