Seismic Swarm S20220806.1: Analysis of Activity South of Guánica, Puerto Rico

Seismic swarm S20220806.1 was recorded south of Guánica, Puerto Rico, beginning at 22:09 on 5 August 2022 and concluding at 04:27 on 19 August 2022. Over 318 hours and 18 minutes, the swarm produced 262 earthquakes. The events clustered within a compact area approximately 7 km south of the town, consistent with shallow crustal processes along regional fault systems. Analysis of the first 100 events reveals a predominance of low-to-moderate magnitudes, with values ranging from 1.7 to 4.2. Depths concentrated between 4 km and 21 km, averaging near 14 km. Early activity on 5–6 August featured events mostly below magnitude 3.0. A notable escalation occurred on the evening of 6 August, including a magnitude 4.2 earthquake at 12 km depth, preceded and followed by several magnitude 3+ shocks within minutes. Subsequent events through 9 August maintained similar depth ranges while showing a gradual decline in peak magnitudes after the initial cluster. Puerto Rico occupies a complex tectonic setting at the northeastern Caribbean plate boundary. The island experiences deformation from the interaction between the Caribbean and North American plates, expressed through the Puerto Rico Trench to the north and the Muertos Trough and associated thrust systems to the south. Guánica lies near the southwestern coast, where east-west trending normal and strike-slip faults accommodate extension and lateral shear. These structures have hosted recurrent seismic swarms, reflecting fluid migration or stress transfer within the upper crust. Historical records since 2000 indicate 14 documented swarms in the region, with increased frequency after 2018. Yearly counts include one swarm each in 2018 and 2019, five in 2020, four in 2021, and three in 2022. This pattern aligns with the broader seismic character of Puerto Rico, which has experienced significant historical events such as the 1918 magnitude 7.1 earthquake near the Mona Passage and the prolonged 2019–2020 southwestern Puerto Rico sequence. The 2022 swarm’s characteristics—shallow depths, rapid succession of similar-sized events, and absence of a single dominant mainshock—match typical swarm behavior driven by localized stress perturbations rather than classic mainshock-aftershock sequences. Such episodes contribute to ongoing hazard assessment for southern Puerto Rico communities.

References

USGS Earthquake Catalog
Puerto Rico Seismic Network (Red Sísmica de Puerto Rico)
SeismoSight internal swarm classification S20220806.1