Seismic Swarm in Guerrero, Mexico: February 2018 Analysis

Guerrero state lies along Mexico’s Pacific coast within the Middle America subduction zone, where the Cocos plate descends beneath the North American plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent moderate-to-large earthquakes and occasional seismic swarms. The region’s crust is characterized by a shallow-dipping slab that transitions to steeper angles inland, contributing to both interface and intraslab seismicity.

Between 23:39 UTC on 16 February 2018 and 21:41 UTC on 17 February 2018, a seismic swarm comprising five events was recorded near Pinotepa de Don Luis. The sequence began with a magnitude 7.2 earthquake at 22 km depth, followed within roughly two hours by a magnitude 5.8 event at only 3 km depth. Subsequent shocks included magnitudes 5.0 (34 km), 5.2 (19 km), and 4.2 (16 km). The entire swarm lasted 22 hours and 1 minute. All events occurred within a compact area whose center lay approximately 16 km from the epicenter of the initial magnitude 7.2 shock.

The February 2018 swarm is the second such episode documented in Guerrero since 1 January 2000; the only prior swarm occurred in 2012. That earlier sequence was centered 17 km from the 2018 activity and was associated with a magnitude 7.4 mainshock on 20 March 2012 near Santiago Llano Grande. Both swarms therefore clustered within the same segment of the subduction interface, highlighting localized stress heterogeneity along the plate boundary.

The 2018 magnitude 7.2 event itself ranks among the strongest shocks recorded in the region since 2000. Its aftershock distribution, combined with the smaller events that define the swarm, illustrates typical post-mainshock relaxation in a subduction setting. Shallow aftershocks (depths <20 km) likely reflect brittle failure in the upper plate or along the megathrust, while deeper events may indicate intraslab deformation.

Guerrero’s seismic history includes both great interface earthquakes and damaging intraslab events. The 2018 swarm occurred in a portion of the margin that last experienced a major rupture in the early twentieth century, leaving a persistent seismic gap farther to the southeast. Continued monitoring of microseismicity and geodetic strain in this gap remains essential for assessing future hazard.

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Servicio Sismológico Nacional, Universidad Nacional Autónoma de México (ssn.unam.mx)