Location:

Period:

20 Apr 2021 14:17:57 - 24 Apr 2021 15:12:17 (4 days 54 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

9 swarms found nearby.

2001

S20011209.2(29.2km)

8 Dec

1 day 23 hours

34 earthquakes

2010

PS20100227.5(84.8km)

27 Feb

3 days 19 hours

62 earthquakes

PS20100227.1(110.3km)

27 Feb

15 hours

22 earthquakes

PS20100304.1(38.2km)

3 Mar

1 day 13 hours

7 earthquakes

PS20100305.1(80.0km)

4 Mar

1 day 22 hours

7 earthquakes

PS20100308.1(54.1km)

7 Mar

19 hours

6 earthquakes

PS20100311.1(62.2km)

11 Mar

2 days 16 hours

21 earthquakes

2017

PS20170428.1(98.6km)

28 Apr

2 hours

6 earthquakes

2021

PS20210419.1(7.4km)

19 Apr

10 hours

5 earthquakes

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Seismic Swarm Offshore O'Higgins Region, Chile: April 2021 Event Analysis

A seismic swarm designated S20210421.1 was recorded offshore the O'Higgins Region of Chile. The sequence initiated at 14:17 on 20 April 2021 and concluded at 15:12 on 24 April 2021, spanning 96 hours and 54 minutes. During this interval, 68 earthquakes were registered, with magnitudes ranging from 2.5 to 5.5 and focal depths between 6 km and 39 km.

The swarm exhibited several notable phases. Early activity on 20–21 April consisted primarily of events below magnitude 3.5 at shallow to intermediate depths. A cluster of larger shocks occurred on 21 April, including two events of magnitude 4.5 and 4.4 at 10 km depth. Peak intensity developed on 22 April with the largest event of the sequence, magnitude 5.5 at 13 km depth, followed by additional shocks reaching 4.4, 4.2, and 4.1. Activity continued at moderate levels through 23 April, featuring multiple magnitude 4+ events at depths of 10–14 km, before tapering on 24 April with smaller, deeper events.

Depth distribution indicates a predominance of shallow crustal seismicity (10–25 km) interspersed with events extending to 35–39 km. This pattern is consistent with activity along or near the plate interface in a subduction setting. Temporal clustering shows distinct bursts separated by quieter intervals, typical of swarm behavior rather than a classic mainshock-aftershock sequence.

The O'Higgins Region lies within the central Chilean subduction zone, where the Nazca Plate converges with the South American Plate at approximately 6–7 cm per year. This tectonic regime produces frequent seismicity, including both interface thrust events and intraslab normal-faulting earthquakes. Historical records document recurrent seismic swarms in the offshore domain, with nine episodes identified since 2000. Prior swarms occurred in 2001 (one event cluster), 2010 (six clusters), and 2017 (one cluster), indicating episodic unrest in the same sector.

Such swarms contribute to the overall seismic hazard assessment for central Chile, a region that experienced the 2010 Maule earthquake (magnitude 8.8) immediately to the south. Ongoing monitoring by national and international networks supports improved understanding of precursory patterns and stress transfer along the margin.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile
Global CMT Catalog (globalcmt.org)
Chilean National Seismic Network reports