Seismic Swarm S20231001.1 Near María Antonia, Puerto Rico

A seismic swarm designated S20231001.1 was recorded 3 km south of María Antonia in Puerto Rico. The sequence began at 09:45 on 30 September 2023 and concluded at 07:59 on 4 October 2023, spanning 94 hours and 13 minutes. During this interval, 63 earthquakes were registered.

The events exhibited magnitudes primarily between 1.8 and 3.2, with the majority falling in the 2.2–2.8 range. Depths varied from 5 km to 17 km, indicating shallow crustal activity consistent with regional fault systems. Notable events included a magnitude 3.2 quake at 08:49 on 1 October and several magnitude 2.9 events distributed across the first two days. Activity peaked in the early hours of 1 October before gradually declining, with the final event measuring magnitude 2.3 at 16 km depth.

Puerto Rico lies at the complex boundary between the Caribbean and North American plates. This tectonic setting features subduction along the Puerto Rico Trench to the north and strike-slip faulting associated with the Septentrional-Oriente fault system. The island experiences frequent seismicity as the plates converge at rates of approximately 2 cm per year. Shallow crustal earthquakes, such as those in this swarm, commonly originate from local fault networks within the overriding Caribbean Plate.

The region has a well-documented history of seismic swarms. Since 2000, 17 such sequences have been identified, with notable concentrations in 2020 (five swarms), 2021 (four), and 2022 (four). The 2023 count reached two by October, underscoring ongoing tectonic unrest. These swarms typically involve low-to-moderate magnitude events clustered in time and space, often without a single dominant mainshock.

Analysis of S20231001.1 reveals a classic swarm pattern: rapid onset, high event rate over several days, and absence of a clear foreshock-mainshock-aftershock progression. Depths clustered around 10–13 km for most events, suggesting activation along a mid-crustal fault segment. Such sequences help delineate active structures and contribute to refined seismic hazard assessments for southern Puerto Rico.

Continued monitoring remains essential given the island’s exposure to both local crustal faults and the deeper subduction zone. Improved understanding of swarm characteristics supports better public preparedness in this tectonically active setting.

References
SeismoSight internal swarm catalog S20231001.1
USGS Earthquake Catalog (regional Puerto Rico events)
Puerto Rico Seismic Network historical swarm records