Location:

Period:

8 Dec 2001 07:29:46 - 10 Dec 2001 06:49:12 (1 day 23 hours 19 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

8 swarms found nearby.

2010

PS20100227.5(104.6km)

27 Feb

3 days 19 hours

62 earthquakes

PS20100304.1(18.2km)

3 Mar

1 day 13 hours

7 earthquakes

PS20100305.1(91.9km)

4 Mar

1 day 22 hours

7 earthquakes

PS20100308.1(40.6km)

7 Mar

19 hours

6 earthquakes

PS20100311.1(54.8km)

11 Mar

2 days 16 hours

21 earthquakes

2017

PS20170428.1(77.2km)

28 Apr

2 hours

6 earthquakes

2021

PS20210419.1(26.0km)

19 Apr

10 hours

5 earthquakes

S20210421.1(29.2km)

20 Apr

4 days 0 hours

68 earthquakes

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Earthquake Swarm S20011209.2 Southwest of San Antonio, Chile

An earthquake swarm designated S20011209.2 occurred 60 km southwest of San Antonio, Chile, from 07:29 on 8 December 2001 to 06:49 on 10 December 2001. Over 47 hours and 19 minutes, 34 events were recorded, with magnitudes ranging from 3.2 to 4.7 and focal depths between 4 km and 40 km. The sequence lacked a dominant mainshock, consistent with swarm behavior driven by fluid migration or localized stress adjustments along the plate interface.

Central Chile lies above the convergent boundary where the Nazca Plate subducts beneath the South American Plate at approximately 6–7 cm per year. This tectonic setting produces frequent seismicity, including both shallow crustal events and deeper interplate earthquakes. The San Antonio region sits near the transition between the flat-slab segment to the north and the more steeply dipping slab farther south, influencing rupture patterns and aftershock distributions.

The swarm initiated with a magnitude 4.1 event at 35 km depth, followed rapidly by a magnitude 4.7 shock at 32 km. Subsequent activity clustered at intermediate depths of 30–40 km, with occasional shallower events near 4–15 km. Depths remained predominantly in the 35–38 km range during the second day, suggesting activity along the subduction interface rather than within the overriding plate. Magnitudes stayed below 5.0 throughout, indicating modest energy release distributed across multiple small faults or asperities.

Historically, the area has experienced significant earthquakes, including the 1906 Valparaíso event and the 1985 central Chile mainshock, both linked to the same subduction segment. Swarms such as S20011209.2 provide insight into preparatory processes that may precede larger ruptures, although no immediate large event followed this sequence. Modern monitoring by national and international networks has improved detection of such low-magnitude clusters, aiding in the characterization of seismic hazard.

The distribution of events shows two depth populations: one tightly grouped around 35–38 km and another shallower group near 7–15 km. This bimodal pattern may reflect both interface slip and secondary faulting in the forearc crust. No surface rupture or volcanic association was noted, supporting a purely tectonic origin within the subduction zone.

Continued study of similar swarms contributes to refined models of stress transfer and fluid involvement along the Chilean margin. Updated seismic catalogs confirm that the region remains capable of producing moderate to large events, underscoring the importance of dense instrumentation for early identification of precursory activity.

References

SeismoSight internal swarm catalog
USGS Earthquake Catalog
Servicio Nacional de Geología y Minería (SERNAGEOMIN) reports on central Chile seismicity