SeismoSight Analysis: S20201224.1 Earthquake Swarm in Puerto Rico

The S20201224.1 earthquake swarm, recorded by SeismoSight, began at 09:21 on 23 December 2020 and concluded at 20:24 on 7 February 2021. Centered 7 km southeast of Maria Antonia, Puerto Rico, the sequence lasted 1115 hours and registered 2842 events. This swarm forms part of elevated seismic activity observed in the region during 2020.

Puerto Rico occupies a complex tectonic setting at the boundary between the North American and Caribbean plates. The Puerto Rico Trench to the north marks the subduction zone where the North American plate descends beneath the Caribbean plate at rates of approximately 2 cm per year. This interaction generates frequent shallow to intermediate-depth earthquakes across the island and surrounding waters. The island's geology includes volcanic arc remnants, sedimentary basins, and fault systems such as the Great Southern Puerto Rico Fault Zone and the Mona Passage extensional regime to the west.

Historical records show that Puerto Rico experiences recurrent seismic swarms linked to fluid migration and stress adjustments along these plate-boundary faults. Since 2000, six swarms have occurred according to SeismoSight internal classification, with one each in 2018 and 2019 plus four in 2020. The 2020 sequence aligns with broader Caribbean seismicity patterns influenced by regional plate motion and occasional volcanic unrest on nearby islands.

Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 1.2 to 4.8, with most events falling between 2.0 and 3.0. Depths clustered between 8 and 16 km, indicating shallow crustal sources consistent with the island's tectonic framework. Early events on 23 December showed magnitudes up to 3.6 at depths near 9 km. Subsequent activity on 24 December included several events above magnitude 3.5, peaking at 4.8 at 10 km depth. Temporal distribution indicated clusters of activity separated by quieter intervals, typical of swarm behavior rather than mainshock-aftershock sequences. Depths remained stable overall, with minor variations suggesting consistent fault activation levels.

This swarm's characteristics match documented patterns in Puerto Rico's seismic history, where swarms often precede or accompany larger tectonic adjustments without producing a single dominant event. No surface rupture or significant structural damage was associated with the sequence, reflecting the modest energy release distributed across thousands of small events.

Ongoing monitoring by regional networks continues to track after-activity and stress evolution in the area. Updated seismic catalogs confirm that such swarms contribute valuable data for refining hazard models in this high-exposure tectonic environment.

References:
USGS Earthquake Catalog (2020-2021 Puerto Rico data)
Puerto Rico Seismic Network annual reports
SeismoSight internal swarm classification database