Seismic Swarm PS20030122.1: Analysis of January 22, 2003 Events Near El Naranjo, Mexico

The seismic swarm designated PS20030122.1 occurred in western Mexico on January 22, 2003, centered 52 km south-southwest of El Naranjo. Registered over a period of 19 hours and 23 minutes, the swarm comprised five earthquakes between 02:06 and 21:30 local time. This sequence unfolded within the tectonically active Colima region, part of the Middle America subduction zone where the Cocos Plate descends beneath the North American Plate at rates of approximately 5–7 cm per year.

The initial and largest event registered magnitude 7.6 at a depth of 24 km. Subsequent shocks included magnitudes 5.1, 5.9, 5.5, and 3.7, with focal depths ranging from 10 km to 16 km. These events clustered near the epicenter of a separate magnitude 7.6 earthquake recorded the same day 16 km south-southwest of Cuyutlán, approximately 42 km from the swarm centroid. The proximity underscores the interconnected fault systems in the area, including segments of the subduction interface and overlying crustal faults.

Western Mexico’s geology reflects long-term plate convergence that has shaped the region since the Miocene. The Colima Rift and adjacent Jalisco Block accommodate oblique subduction and crustal extension, contributing to elevated seismicity. Historical records document recurrent large earthquakes, with the 2003 sequence fitting patterns observed in prior events such as the 1932 Jalisco quake and the 1995 Colima-Jalisco event. The 2003 mainshock caused significant ground shaking across Colima and Jalisco states, triggering landslides and structural damage but limited fatalities due to relatively low population density near the epicenter.

Swarm activity of this type often signals stress transfer along the plate interface or within the overriding crust. Depths between 10 km and 24 km align with the seismogenic zone of the subduction megathrust and shallow crustal faults. Post-2000 monitoring has confirmed ongoing moderate seismicity in the broader region, consistent with the persistent convergence driving strain accumulation.

The 2003 swarm provides a case study in rapid energy release within a convergent margin setting. Integration of event timing and magnitude distribution illustrates how initial large ruptures can trigger secondary sequences along nearby structures. Continued instrumentation by regional networks supports refined hazard assessments for coastal communities in Colima and adjacent states.

References

USGS Earthquake Catalog
Servicio Sismológico Nacional (Mexico)
Global CMT Project catalog entries for 2003 Colima sequence