Seismic Swarm PS20081219.1: Analysis of Activity off Valparaíso, Chile

A seismic swarm designated PS20081219.1 was recorded 101 km northwest of Valparaíso, Chile, between 21:19 on 18 December 2008 and 11:29 on 20 December 2008. Over 38 hours and 10 minutes, eight earthquakes occurred in this offshore segment of the Peru-Chile Trench. The sequence began with a magnitude 6.2 event at 18 km depth, followed by a magnitude 6.0 shock at 25 km depth roughly 31 minutes later. Subsequent events included a magnitude 5.4 quake at 12 km depth on 19 December at 07:30, a small magnitude 3.5 event at 35 km depth, a magnitude 5.7 shock at 32 km depth, two magnitude 5.2 events at shallow depths of 13 km and 12 km, and a final magnitude 5.0 event at 39 km depth on 20 December.

This swarm exemplifies clustered seismicity typical of subduction-zone environments. Unlike mainshock-aftershock sequences, swarms lack a single dominant event and instead reflect distributed stress release along the plate interface or within the overriding crust. The depths ranging from 12 km to 39 km align with the expected seismogenic zone where the Nazca Plate subducts beneath the South American Plate at approximately 6–7 cm per year. Such activity can be driven by episodic slip, fluid migration, or localized changes in fault friction.

The Valparaíso region lies within one of Earth’s most active convergent margins. The Peru-Chile Trench has produced some of the largest recorded earthquakes, including the 1960 Valdivia event farther south and the 2010 Maule earthquake that ruptured adjacent segments. Historical records document repeated great earthquakes and associated tsunamis affecting central Chile, underscoring the persistent seismic hazard. The 2008 swarm fits within this broader tectonic framework, where strain accumulation along the megathrust is periodically relieved through both isolated large events and clustered smaller sequences.

SeismoSight internal classification identifies four swarms in the same offshore area since 1 January 2000. Earlier episodes occurred in 2001 (one swarm), 2003 (two swarms), and 2007 (one swarm). These recurrent clusters suggest that certain patches of the subduction interface may favor swarm-type behavior over classic aftershock decay, possibly due to heterogeneous frictional properties or fluid involvement.

Monitoring of such swarms contributes to improved understanding of short-term seismic patterns in central Chile. While individual events in the 2008 sequence remained moderate, their occurrence highlights the need for continued surveillance of the Valparaíso segment, where future larger ruptures remain possible given the ongoing plate convergence.

References
USGS Earthquake Catalog
Global CMT Catalog
SeismoSight internal swarm database