Seismic Swarm PS20150916.2: Analysis of Aftershock Activity Following the 2015 Illapel Earthquake

The seismic swarm designated PS20150916.2 occurred 55 km west-southwest of Illapel, Chile, from 22:54 on 16 September 2015 to 13:51 on 18 September 2015. Over 38 hours and 56 minutes, the sequence registered 48 earthquakes. This activity unfolded in the Coquimbo Region along the Peru-Chile Trench, a convergent margin where the Nazca Plate subducts beneath the South American Plate at approximately 6–7 cm per year.

The swarm initiated with the Mw 8.3 mainshock at 22:54:32 UTC on 16 September 2015 (depth 22 km). Subsequent events included multiple aftershocks exceeding magnitude 6.0, such as an Mw 7.0 at 23:18:41 (28 km depth), an Mw 6.4 at 01:41:05 on 17 September (42 km depth), an Mw 6.5 at 03:55:15 (27 km depth), and an Mw 6.7 at 04:10:27 (23 km depth). Depths ranged from 4 km to 42 km, with the majority clustering between 10 km and 35 km, consistent with the megathrust interface and overlying crustal stresses.

Geologically, the Illapel area lies within the flat-slab segment of the subduction zone, where the Juan Fernández Ridge influences plate coupling and seismicity. This setting has produced recurrent large earthquakes, including the 1943 Mw 8.2 event near the same latitude. The 2015 sequence released accumulated strain along a roughly 200 km rupture zone, with aftershocks delineating both the main thrust plane and subsidiary faults in the forearc.

Historical records maintained by SeismoSight indicate six prior swarms in the region since 1 January 2000, occurring in 2003, 2006, 2007, 2008, 2010, and 2011. These episodes typically involved elevated rates of moderate-magnitude events without a dominant mainshock, contrasting with the 2015 sequence that accompanied a great earthquake. Two notable strong events since 2000 were the Mw 8.3 mainshock itself (48 km from swarm center) and an Mw 7.0 aftershock (25 km west-northwest of Illapel, 16 September 2015).

The swarm’s temporal decay followed a modified Omori law pattern typical of aftershock sequences in subduction zones, with the highest rates in the first 12 hours. Event magnitudes declined progressively, though several Mw 5.5+ shocks persisted into 18 September. Such activity highlights ongoing post-seismic relaxation and potential triggered slip on adjacent segments.

References

• SeismoSight internal swarm classification database (PS20150916.2 parameters and historical swarm counts).
• USGS Earthquake Catalog (mainshock and aftershock parameters for Illapel 2015 sequence).