Location:

Period:

17 Jul 2021 20:12:14 - 20 Jul 2021 21:35:43 (3 days 1 hour 23 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

19 swarms found nearby.

2018

S20181017.1(25.0km)

16 Oct

2 days 17 hours

72 earthquakes

2019

S20191229.1(19.1km)

28 Dec

244 days 6 hours

9147 earthquakes

2020

S20200107.1(23.7km)

7 Jan

1 day 21 hours

53 earthquakes

S20200502.3(16.1km)

1 May

2 days 18 hours

36 earthquakes

S20201118.1(17.1km)

17 Nov

35 days 20 hours

447 earthquakes

S20201224.1(15.8km)

23 Dec

46 days 11 hours

2842 earthquakes

2021

S20210108.1(5.3km)

7 Jan

13 days 20 hours

148 earthquakes

S20210130.1(27.3km)

29 Jan

2 days 1 hours

29 earthquakes

S20210625.1(21.7km)

24 Jun

6 days 15 hours

127 earthquakes

2022

S20220202.1(20.7km)

1 Feb

37 days 20 hours

630 earthquakes

S20220322.1(25.2km)

21 Mar

19 days 5 hours

207 earthquakes

S20220424.1(12.6km)

23 Apr

17 days 11 hours

189 earthquakes

S20220806.1(22.9km)

5 Aug

13 days 6 hours

262 earthquakes

2023

S20230315.1(16.3km)

14 Mar

6 days 9 hours

92 earthquakes

S20230409.1(14.3km)

8 Apr

10 days 15 hours

189 earthquakes

S20231001.1(23.3km)

30 Sep

3 days 22 hours

63 earthquakes

S20231024.1(23.7km)

23 Oct

1 day 13 hours

25 earthquakes

2025

S20250312.1(28.5km)

12 Mar

4 days 6 hours

74 earthquakes

S20250320.3(29.9km)

20 Mar

5 days 23 hours

70 earthquakes

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Seismic Swarm S20210718.1: Analysis of Activity Near La Parguera, Puerto Rico

Puerto Rico lies at the complex tectonic boundary between the Caribbean and North American plates. In the southwest, the region experiences strike-slip and oblique faulting associated with the Great Southern Puerto Rico Fault Zone and subsidiary structures linked to the Muertos Trough. These features accommodate lateral plate motion and produce frequent shallow seismicity at depths typically between 5 and 20 km. Historical records document recurrent moderate events, underscoring the area’s persistent seismic hazard.

The swarm designated S20210718.1 began at 20:12 on 17 July 2021 and concluded at 21:35 on 20 July 2021, spanning 73 hours and 23 minutes. Its epicenter was located 6 km west-southwest of La Parguera. During this interval, 37 earthquakes were recorded. Magnitudes ranged from 1.1 to 4.4, with the largest event (M4.4) occurring at 03:52 on 18 July at a depth of 11 km. A secondary peak reached M4.2 later that morning. Most events clustered between 9 and 13 km depth, consistent with the regional seismogenic zone.

The temporal pattern showed an initial rapid increase in activity within the first 12 hours, followed by a gradual decline punctuated by occasional larger aftershocks. This distribution is characteristic of swarm sequences rather than classical mainshock-aftershock sequences, suggesting distributed stress release along small fault segments or fluid-driven triggering within the fault network.

Since 1 January 2000, nine comparable swarms have been documented in the same general area. Their yearly counts are 2018 (1), 2019 (1), 2020 (4), and 2021 (3). The recent rise in swarm frequency aligns with elevated background seismicity observed after the 2020 M6.4 mainshock and its aftershock sequence, indicating that the crust in southwestern Puerto Rico remains in a state of heightened stress adjustment.

Collectively, the 2021 swarm provides further evidence of ongoing tectonic strain accumulation and release along the southern margin of Puerto Rico. Continued monitoring is essential for refining hazard assessments and understanding the interplay between regional fault systems.

References

USGS Earthquake Catalog (Puerto Rico region, 2000–2024)
Puerto Rico Seismic Network – Regional Tectonic Summary
SeismoSight internal swarm classification records