Seismic Swarm PS20100308.2 Near La Ligua, Chile

On 8 March 2010, a seismic swarm designated PS20100308.2 was recorded approximately 40 km west-southwest of La Ligua in Chile’s Valparaíso Region. The sequence began at 16:49 UTC and concluded at 23:43 UTC, spanning 6 hours and 53 minutes. During this interval, six earthquakes with magnitudes between 5.0 and 5.3 were detected at focal depths ranging from 7 km to 28 km.

The events occurred in close temporal succession. The first shock registered magnitude 5.0 at 21 km depth. Subsequent events reached magnitudes of 5.3 at 23 km and 22 km, followed by additional magnitude 5.0 and 5.1 shocks, the shallowest at 7 km. The final event of magnitude 5.0 occurred at 28 km depth. Such rapid clustering of moderate-magnitude earthquakes within a confined time window characterizes swarm behavior in subduction-zone settings.

La Ligua lies along the active margin where the Nazca Plate subducts beneath the South American Plate. This tectonic configuration produces frequent seismicity, including both large megathrust events and smaller clustered sequences. The region’s geological record documents repeated episodes of strain release, consistent with the six swarms identified since 2000. Earlier swarms occurred in 2003 (two episodes), 2007 (one), 2008 (one), and 2010 (two, including the present sequence).

These swarms reflect localized stress adjustments along the plate interface and within the overriding crust. Depths between 7 km and 28 km align with the seismogenic zone where brittle failure predominates. The absence of a single dominant mainshock distinguishes the sequence from typical aftershock decay patterns and underscores the swarm’s independent character.

Continued monitoring of such episodes contributes to refined understanding of subduction-zone dynamics in central Chile. The data from PS20100308.2 provide a concise example of swarm activity within a historically active seismic corridor.

References
SeismoSight internal swarm classification records
USGS Earthquake Catalog (regional tectonics summary)
Chilean National Seismological Center (historical swarm context)