Seismic Swarm S20210922.2 Near the Nicaraguan Coast: September 2021 Analysis

The seismic swarm designated S20210922.2 occurred off the Pacific coast of Nicaragua, a region defined by active subduction along the Middle America Trench. Here the Cocos Plate descends beneath the Caribbean Plate at rates of approximately 8–9 cm per year, generating frequent earthquakes and volcanic activity associated with the Central American Volcanic Arc. This tectonic setting produces both interplate thrust events and intraslab earthquakes, with hypocenters commonly distributed between 10 and 70 km depth.

The swarm began at 01:39 UTC on 22 September 2021 and concluded at 05:26 UTC on 26 September 2021, spanning 99 hours and 47 minutes. During this interval, 50 earthquakes were recorded. Magnitudes ranged from 2.7 to 6.5, with the majority occurring at depths of 10–35 km. Depths extended from as shallow as 1 km to a maximum of 70 km, reflecting activity across both the plate interface and the subducting slab.

Activity intensified shortly after initiation. A magnitude 6.5 event at 09:57 UTC on 22 September, located at 21 km depth, marked the largest shock and was followed within minutes by several events of magnitude 3.5–4.3. Subsequent hours on 22 September produced a sequence of moderate shocks, including magnitudes 4.5 and 4.2. On 23 September, two events of magnitude 4.6 and 4.8 occurred at depths of 37 km and 36 km, respectively. Later activity on 24–26 September consisted primarily of smaller events (magnitudes 2.7–4.4), with the final recorded shock of magnitude 3.2 at 05:26 UTC on 26 September.

The temporal distribution showed the highest rate of events during the first 24 hours, followed by a gradual decline. Depths remained broadly consistent across the swarm, with no clear migration pattern evident from the catalog. This behavior is characteristic of swarm sequences driven by fluid migration or slow slip rather than a single mainshock-aftershock cascade.

Historical records indicate that swarm-type sequences are infrequent in this segment of the subduction zone. Since 1 January 2000, only one prior swarm has been documented, occurring in 2012. The 2021 sequence therefore represents the second such episode in more than two decades, underscoring the episodic nature of clustered seismicity along this margin.

The Nicaraguan Pacific margin has experienced destructive earthquakes throughout recorded history, including the 1992 tsunami-generating event and earlier events in 1931 and 1972. These occurrences illustrate the persistent seismic hazard associated with the subduction interface. The 2021 swarm, while dominated by moderate magnitudes, contributed additional data on the spatial distribution of small-to-moderate events within the seismogenic zone.

Continued monitoring by regional and global networks remains essential for refining models of subduction-zone behavior and assessing the potential for larger events. The S20210922.2 swarm provides a well-documented example of clustered activity that can inform probabilistic seismic-hazard assessments for coastal Nicaragua.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification S20210922.2