Location:

Period:

11 Nov 2015 01:54:38 - 11 Nov 2015 03:07:49 (1 hour 13 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

5 swarms found nearby.

2015

PS20150917.1(164.1km)

16 Sep

4 hours

8 earthquakes

PS20150919.1(119.9km)

19 Sep

1 day 12 hours

10 earthquakes

2020

PS20200901.1(190.2km)

1 Sep

16 hours

10 earthquakes

2021

S20210704.1(90.8km)

4 Jul

2 days 11 hours

72 earthquakes

2026

S20260313.2(105.6km)

13 Mar

4 days 9 hours

62 earthquakes

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Seismic Swarm PS20151111.1: Analysis of Activity Near Coquimbo, Chile

On 11 November 2015, a seismic swarm designated PS20151111.1 was recorded approximately 78 km northwest of Coquimbo, Chile. The sequence began at 01:54 and concluded at 03:07, encompassing six earthquakes within a span of one hour and thirteen minutes. This event represents the first swarm documented in the region since 2000.

The earthquakes exhibited the following parameters:

01:54:38 UTC, magnitude 6.9 at 12 km depth
02:01:05 UTC, magnitude 5.1 at 10 km depth
02:06:36 UTC, magnitude 4.9 at 10 km depth
02:23:39 UTC, magnitude 5.1 at 10 km depth
02:46:19 UTC, magnitude 6.9 at 10 km depth
03:07:49 UTC, magnitude 5.1 at 10 km depth

All events clustered at shallow depths between 10 and 12 km, indicating a localized source within the upper crust.

Chile lies along the convergent boundary where the Nazca Plate subducts beneath the South American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismic activity, including both interplate thrust events and intraslab earthquakes. The Coquimbo region, situated in the northern segment of the country, forms part of the flat-slab subduction zone that transitions southward into steeper subduction angles. Historical records document major earthquakes in this area, such as the 1943 Illapel event of magnitude 8.2 and the 2015 Illapel mainshock of magnitude 8.3, which occurred two months prior to the swarm. These events highlight the persistent strain accumulation and release along the plate interface.

Seismic swarms in subduction zones often arise from fluid migration, stress triggering, or aftershock sequences following larger ruptures. The 2015 swarm parameters suggest a compact source volume consistent with triggered activity in a highly fractured forearc setting. Depths around 10 km align with the expected range for crustal events in this margin, where the plate interface lies at greater depths offshore.

Since 1 January 2000, only two swarms have been identified in the broader Coquimbo vicinity according to internal classification records, underscoring the relative rarity of such clustered sequences compared to isolated mainshock-aftershock patterns. The second swarm occurred after 2015, further illustrating episodic behavior within an otherwise steady background seismicity rate.

Ongoing monitoring by regional networks continues to refine understanding of swarm mechanics in this high-hazard corridor. Improved detection capabilities have enhanced the ability to distinguish swarm activity from typical aftershock decay, supporting more accurate hazard assessments for coastal communities.

References

SeismoSight internal classification records for swarm PS20151111.1.
USGS Earthquake Catalog (historical context on regional tectonics).
Global CMT Project (subduction zone parameters).