Seismic Swarm S20191229.1: Prolonged Earthquake Activity Near María Antonia, Puerto Rico

Seismic swarm S20191229.1 began at 22:35 on 28 December 2019 and concluded at 04:42 on 29 August 2020. The sequence was centered 9 km south-southeast of María Antonia in southern Puerto Rico. Over 5,862 hours and 6 minutes, the swarm produced 9,147 earthquakes. This activity formed part of the broader 2019–2020 Puerto Rico earthquake sequence and represented the second swarm recorded in the region since 2000, following one in 2018.

Analysis of the first 100 events reveals a classic swarm pattern of clustered, low-to-moderate magnitude shocks with shallow focal depths. The initial event registered magnitude 4.7 at 6 km depth. Subsequent events remained predominantly below magnitude 3.0, although peaks reached 5.0 at 01:06 on 29 December and 4.7 at 01:21 the same day. Depths clustered between 3 km and 10 km, with occasional deeper occurrences up to 16 km. The temporal distribution showed high frequency in the first 12 hours, with multiple events per hour, followed by a gradual decline while maintaining elevated rates for weeks.

This pattern indicates fluid migration or aseismic slip along fault segments rather than a single mainshock-aftershock sequence. Most events occurred at crustal depths consistent with activation of local fault systems in the Puerto Rico–Virgin Islands block.

Puerto Rico lies at the complex northeastern boundary of the Caribbean plate, where the North American plate subducts beneath it along the Puerto Rico Trench to the north. To the south, the Muertos Trough accommodates convergence between the Caribbean plate and the Hispaniola–Puerto Rico microplate. The island’s geology reflects this setting through a combination of volcanic-arc remnants, sedimentary basins, and active strike-slip and thrust faults. The southwestern region near María Antonia features the Great Southern Puerto Rico Fault Zone and subsidiary structures that have historically hosted moderate seismicity.

Instrumental records document recurrent earthquake swarms in southern Puerto Rico, often linked to stress transfer along these faults. The 2019–2020 sequence followed this precedent but exceeded prior events in duration and total event count. Shallow focal depths amplified felt intensities across the island, prompting extensive aftershock monitoring by regional networks.

Ongoing tectonic loading from plate boundary forces ensures continued seismic hazard. The swarm data underscore the value of dense seismic networks for real-time characterization of such sequences and for refining probabilistic hazard assessments in Puerto Rico’s densely populated southern coastal zones.

References USGS Earthquake Hazards Program Puerto Rico Seismic Network (Red Sísmica de Puerto Rico) Caribbean Tsunami Warning Program geological summaries