Location:

Period:

23 Jul 2003 13:05:04 - 26 Jul 2003 11:50:10 (2 days 22 hours 45 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

6 swarms found nearby.

2007

PS20070329.1(95.1km)

29 Mar

3 hours

6 earthquakes

2008

PS20081219.1(22.6km)

18 Dec

1 day 14 hours

8 earthquakes

2010

PS20100308.2(48.1km)

8 Mar

6 hours

6 earthquakes

2011

PS20110317.2(36.8km)

16 Mar

16 hours

5 earthquakes

2015

PS20150917.2(19.5km)

17 Sep

21 hours

9 earthquakes

2017

PS20170428.1(93.2km)

28 Apr

2 hours

6 earthquakes

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Seismic Swarm S20030724.1: Analysis of Activity West of La Ligua, Chile

An earthquake swarm designated S20030724.1 occurred approximately 83 km west of La Ligua in Chile's Valparaíso Region. The sequence began at 13:05 on 23 July 2003 and concluded at 11:50 on 26 July 2003, spanning 70 hours and 45 minutes. During this interval, 48 earthquakes were recorded, with magnitudes ranging from 2.3 to 4.6 and focal depths between 0 and 39 km.

The swarm exhibited typical characteristics of clustered seismic activity in a subduction setting. Initial events on 23 July included a magnitude 4.2 quake at 24 km depth, followed by smaller tremors. Peak activity featured a magnitude 4.6 event at 24 km depth on 24 July and a magnitude 4.4 shock at 30 km later that day. Later phases on 25 and 26 July showed predominantly shallower events under 10 km, alongside a few deeper occurrences near 34–39 km. Magnitudes generally remained below 4.0 after the initial cluster, indicating a gradual decay in energy release without a dominant mainshock.

This region lies along the Peru-Chile Trench, where the Nazca Plate subducts beneath the South American Plate at a rate of approximately 6–7 cm per year. Such tectonic interaction produces both shallow crustal earthquakes and intermediate-depth events within the Wadati-Benioff zone. Depths observed in the swarm align with expected patterns for this margin, encompassing upper-plate deformation and slab-related seismicity.

Historically, the Valparaíso area has experienced significant earthquakes, including the 1906 Valparaíso event of magnitude 8.2 and the 1985 Algarrobo earthquake of magnitude 8.0. These large ruptures highlight the potential for megathrust activity, while swarms like S20030724.1 represent smaller-scale stress adjustments along the plate interface or within the overriding crust. Updated seismic monitoring since 2003 has improved detection of similar clusters, confirming their role in regional strain release.

The swarm's spatial concentration and temporal progression suggest fluid migration or aseismic slip as possible triggering mechanisms, common in Chilean subduction zones. No damage or felt reports beyond instrumental thresholds were associated with these events.

References

United States Geological Survey Earthquake Catalog
Servicio Nacional de Geología y Minería (SERNAGEOMIN) seismic reports
Global Centroid Moment Tensor database