Seismic Swarm Near Guayanilla, Puerto Rico: Analysis of October 2023 Activity

A seismic swarm designated S20231024.1 occurred 2 km west of Guayanilla, Puerto Rico, between 08:15 on 23 October 2023 and 21:40 on 24 October 2023. Over this 37-hour period, 25 earthquakes were recorded. The sequence began with a magnitude 2.7 event at 13 km depth and included a peak magnitude of 4.4 at 10 km depth shortly after midday on 23 October. Subsequent events ranged in magnitude from 1.8 to 3.4, with focal depths between 5 km and 18 km, indicating shallow crustal activity typical of the region.

The swarm exhibited a clustered temporal pattern, with the majority of events occurring within the first 12 hours. Magnitudes remained predominantly below 3.0 after the initial larger shocks, consistent with swarm behavior where energy release occurs through numerous smaller events rather than a single mainshock-aftershock sequence. Depths showed modest variation, averaging around 11 km, reflecting brittle failure within the upper crust.

Puerto Rico occupies a tectonically complex zone at the boundary between the North American and Caribbean plates. Convergence along the Puerto Rico Trench drives subduction-related deformation, while strike-slip motion on the Septentrional-Oriente fault system and the Enriquillo-Plantain Garden fault contributes to regional seismicity. Guayanilla lies within the southwestern portion of the island, an area influenced by both subduction and local fault systems such as the Great Southern Puerto Rico Fault Zone. Shallow events in the 5–18 km range align with known seismogenic depths in this setting.

Historical records indicate recurrent swarm activity in Puerto Rico since 2000, with a total of 18 documented swarms through 2023. Annual counts show elevated frequency in 2020 (5 swarms), 2021 (4), and 2022 (4), suggesting episodic clustering possibly linked to fluid migration or stress transfer along regional faults. The 2023 swarm fits within this pattern of moderate, short-duration sequences.

Such activity underscores the ongoing seismic hazard in southern Puerto Rico, where even modest-magnitude events can produce felt shaking due to proximity to populated areas and local soil conditions. Monitoring by regional networks continues to provide essential data for understanding these transient sequences.

References United States Geological Survey Earthquake Hazards Program Puerto Rico Seismic Network Annual Reports Caribbean Tectonic Setting Summaries, Geological Society of America