Seismic Swarm PS20150921.1: Analysis of Events Southwest of Ovalle, Chile

A seismic swarm designated PS20150921.1 was recorded 64 km southwest of Ovalle in Chile’s Coquimbo Region. The sequence began at 05:39 UTC on 21 September 2015 and concluded at 07:13 UTC on 22 September 2015, encompassing seven earthquakes over 25 hours and 33 minutes. All events occurred within the shallow to intermediate depth range typical of the subduction interface between the Nazca and South American plates.

The sequence opened with a magnitude 6.1 event at 30 km depth. Subsequent shocks included a magnitude 5.4 at 25 km, followed by the largest event of the swarm—a magnitude 6.6 earthquake at 35 km depth. Later activity comprised magnitudes 4.5, 5.5, 5.6, and a final magnitude 6.0 event at 58 km depth. Depths remained consistent with the megathrust environment, where the Nazca plate descends beneath the South American continent at rates of approximately 6–7 cm per year.

This swarm occurred five days after the 16 September 2015 Illapel mainshock (magnitude 8.3, epicenter 48 km west of Illapel) and its magnitude 7.0 aftershock (25 km WNW of Illapel). Both large events nucleated within 12–23 km of the swarm centroid, indicating that the September 21–22 activity formed part of the post-seismic adjustment along the same rupture segment. The Coquimbo Region lies within a well-documented seismic corridor that has produced repeated great earthquakes, including the 1943 and 2015 Illapel events, underscoring the persistent strain accumulation along this portion of the plate boundary.

Historical records maintained since 2000 show eleven prior swarms in the immediate area, occurring in 2003, 2006, 2007, 2008, 2010, 2011, and five separate episodes in 2015. Such clustering is characteristic of subduction zones where heterogeneous frictional properties on the plate interface allow episodic slow-slip or afterslip to trigger localized earthquake sequences without necessarily leading to another great rupture.

The 2015 Illapel mainshock released a significant portion of accumulated strain, yet residual stress transfer and fluid migration along the slab interface continued to drive smaller events in the days that followed. Depths between 25 km and 58 km place the swarm firmly within the coupled seismogenic zone, consistent with the geometry of the Wadati-Benioff zone mapped beneath central Chile.

Continued monitoring of afterslip and microseismicity remains essential for assessing whether additional swarms or moderate aftershocks will occur as the fault system equilibrates following the 2015 sequence.

References:
USGS Earthquake Catalog (events of 16 and 21–22 September 2015)
Global CMT Catalog
SeismoSight internal swarm classification PS20150921.1