Seismic Swarm PS20120827.1: Insights from the August 2012 Sequence off El Salvador

The seismic swarm designated PS20120827.1 occurred approximately 127 km south of Puerto El Triunfo, El Salvador, within a tectonically active segment of the Middle America subduction zone. This region lies where the Cocos Plate subducts beneath the Caribbean Plate along the Middle America Trench, producing frequent seismic activity at varying depths. The swarm spanned 28 hours and 16 minutes, from 04:37 on 27 August 2012 to 08:53 on 28 August 2012, and included seven recorded earthquakes.

The sequence began with a magnitude 7.3 event at 04:37:19 on 27 August at a depth of 28 km. This was followed shortly afterward by a magnitude 5.1 earthquake at 04:39:27 (35 km depth). Subsequent events included a magnitude 5.5 quake at 05:38:04 (35 km), a magnitude 5.1 event at 09:05:03 (54 km), and a magnitude 4.6 shock at 10:59:42 (42 km). Activity continued into the next day with a magnitude 5.5 earthquake at 06:08:16 on 28 August (35 km) and a final magnitude 5.3 event at 08:53:36 (35 km). Most events clustered at intermediate depths between 28 and 54 km, consistent with slab-related seismicity in the subduction interface.

Such swarms reflect episodic stress release along the subducting slab rather than a single mainshock-aftershock pattern. The 2012 sequence took place in an area known for both shallow crustal events and deeper intraslab earthquakes. Historical records since 2000 indicate this was the only swarm in the immediate vicinity, with the prior comparable episode occurring in 2001.

The broader tectonic setting has produced several strong earthquakes in recent decades. Notable events include the magnitude 7.7 earthquake of 13 January 2001 located 28 km SSW of Puerto El Triunfo (approximately 99 km from the swarm center), the magnitude 7.3 event of 27 August 2012 itself (5 km from the swarm center), and another magnitude 7.3 earthquake on 14 October 2014 located 74 km south of Intipucá (69 km from the swarm center). These occurrences underscore the persistent seismic hazard along El Salvador’s Pacific margin.

Seismic monitoring in this zone contributes to improved understanding of subduction dynamics and helps refine hazard assessments for coastal communities. Continued observation of depth distributions and temporal clustering remains essential for distinguishing swarm behavior from larger mainshock sequences.

References

SeismoSight internal swarm classification PS20120827.1 (2012).
USGS Earthquake Catalog (events 2001–2014).