Location:

Period:

16 Sep 2015 23:37:43 - 20 Sep 2015 01:26:23 (3 days 1 hour 48 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

7 swarms found nearby.

2006

S20061013.1(12.6km)

12 Oct

3 days 13 hours

50 earthquakes

2007

PS20070329.1(34.2km)

29 Mar

3 hours

6 earthquakes

2015

PS20150916.2(18.6km)

16 Sep

1 day 14 hours

48 earthquakes

PS20150917.1(59.4km)

16 Sep

4 hours

8 earthquakes

PS20150917.2(113.3km)

17 Sep

21 hours

9 earthquakes

PS20150919.1(89.6km)

19 Sep

1 day 12 hours

10 earthquakes

PS20150921.1(20.2km)

21 Sep

1 day 1 hours

7 earthquakes

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Seismic Swarm S20150917.1 Near Illapel, Chile: Geological Context and Event Analysis

Seismic swarm S20150917.1 occurred approximately 77 km west-northwest of Illapel in Chile’s Coquimbo Region. The sequence began at 23:37 UTC on 16 September 2015 and concluded at 01:26 UTC on 20 September 2015, spanning 73 hours and 48 minutes. During this interval, 40 earthquakes were recorded, with magnitudes ranging from 3.1 to 4.9 and focal depths between 5 km and 55 km.

The swarm took place along the Peru-Chile Trench, where the Nazca Plate subducts beneath the South American Plate at a rate of approximately 6–7 cm per year. This convergent margin produces frequent seismicity, including both interplate thrust events and intraslab activity. Illapel lies near the transition between the flat-slab segment to the north and the more steeply dipping slab to the south, a structural boundary that influences rupture segmentation and aftershock patterns.

The timing of the swarm immediately followed the magnitude 8.3 Illapel mainshock of 16 September 2015. The mainshock ruptured a roughly 200 km segment of the plate interface, releasing significant stress and triggering widespread aftershock activity. Swarm S20150917.1 represents a clustered subset of this aftershock sequence, characterized by moderate-magnitude events distributed across a range of depths consistent with both interface and intraslab sources.

Analysis of the provided event parameters shows the largest event reached magnitude 4.9 at 13 km depth on 16 September. Subsequent activity included multiple magnitude 4.8 events at depths of 14–17 km, with shallower events (5–11 km) occurring early in the sequence. Deeper events (30–55 km) became more prominent after 17 September, suggesting possible migration of stress or fluid involvement within the subducting slab.

Historical records indicate that four seismic swarms have occurred in the region since 2000. Prior swarms were documented in 2006 (one swarm) and 2007 (one swarm), with two additional swarms recorded in 2015. This pattern underscores the recurrent nature of clustered seismicity along this portion of the subduction zone, often linked to post-mainshock stress redistribution.

The Coquimbo Region has experienced several large earthquakes in the past century, including events in 1943 and 2015, both of which produced significant coastal uplift and tsunami impacts. Ongoing monitoring by Chilean and international networks continues to refine understanding of swarm behavior and its relation to megathrust cycles.

References
United States Geological Survey Earthquake Catalog
Global Centroid Moment Tensor Project
Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile