Seismic Swarm Analysis: PS20150917.1 Near Illapel, Chile

A seismic swarm designated PS20150917.1 was recorded northwest of Illapel, Chile, between 23:13 UTC on 16 September 2015 and 03:55 UTC on 17 September 2015. Over 4 hours and 41 minutes, eight earthquakes were registered, with magnitudes ranging from 4.7 to 6.5 and focal depths between 16 km and 47 km. The events clustered approximately 66 km northwest of Illapel, a coastal city in Chile’s Coquimbo Region.

The sequence began with a magnitude 5.2 event at 29 km depth, followed rapidly by two magnitude 5.3 shocks at 35 km and 16 km depth. Subsequent activity included a magnitude 4.7 at 35 km, a magnitude 5.1 at 35 km, a magnitude 5.5 at 35 km, a magnitude 5.2 at 47 km, and culminated in a magnitude 6.5 event at 41 km depth. All events occurred within the same localized volume, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

Illapel lies above the subduction interface where the Nazca Plate descends beneath the South American Plate at a convergence rate of approximately 6.5–7 cm per year. This tectonic setting produces frequent megathrust earthquakes and associated aftershock sequences. The September 2015 swarm occurred immediately after the region’s major M8.3 mainshock of 16 September 2015, whose epicenter was located 48 km west of Illapel. A separate M7.0 event was also recorded 25 km west-northwest of the city on the same day, 76 km from the swarm centroid. These larger events likely altered local stress conditions, promoting the subsequent clustered activity.

Since 1 January 2000, seven swarms have been documented in the broader Illapel area, occurring in 2003, 2006, 2007, 2008, 2010, 2011, and 2015. This recurrence indicates episodic swarm activity superimposed on the region’s background seismicity. The 2015 swarm is the most recent and best-instrumented example in the catalog.

The swarm’s shallow-to-intermediate depths align with the expected range for intraslab and interface seismicity in the Coquimbo segment of the subduction zone. Historical records show that this segment has hosted multiple great earthquakes, underscoring its persistent seismic hazard. Post-2015 monitoring continues to track afterslip and viscoelastic relaxation that may influence future event rates.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Chilean Seismological Center (sismologia.cl)