Seismic Swarm PS20150917.2 Near La Ligua, Chile

A seismic swarm occurred 44 km northwest of La Ligua, Chile, from 12:09 on 17 September 2015 to 09:10 on 18 September 2015. In just over 21 hours, nine earthquakes were recorded, with magnitudes ranging from 4.5 to 6.2 and focal depths between 6 and 35 km. The sequence began with a magnitude 5.3 event at 7 km depth, followed rapidly by two larger shocks of 5.9 and 5.4 within minutes. Subsequent events included additional magnitude 5.9 and 5.0–5.6 shocks, culminating in the largest event of magnitude 6.2 at 8 km depth on 18 September.

This swarm took place in a tectonically active segment of central Chile, where the Nazca Plate subducts beneath the South American Plate at rates of approximately 6–7 cm per year. The region experiences frequent seismicity due to this convergent margin, with thrust-faulting mechanisms dominating at shallow depths. The swarm’s shallow focal depths align with typical crustal deformation above the subduction interface.

Historical records since 2000 indicate 12 prior swarms in the vicinity, occurring in 2001, 2003 (two events), 2006, 2007, 2008, 2010 (two events), 2011, and 2015 (three events). These episodes reflect episodic stress release along the plate boundary without a single dominant mainshock.

One day before the swarm onset, a magnitude 8.3 earthquake struck 48 km west of Illapel, Chile, approximately 92 km from the swarm center. The proximity suggests possible dynamic triggering or afterslip effects influencing the subsequent swarm activity.

Seismic swarms in subduction zones often indicate fluid migration or aseismic slip rather than immediate foreshock sequences to larger events. Monitoring such patterns aids in understanding stress evolution along the megathrust.

References

USGS Earthquake Catalog (Illapel M8.3 event, 16 Sep 2015)
SeismoSight internal swarm classification data