Seismic Swarm S20060501.1: Analysis of Activity West of Copiapó, Chile

The seismic swarm designated S20060501.1 occurred approximately 90 km west of Copiapó in Chile’s Atacama Region. Registered between 18:18 on 30 April 2006 and 11:12 on 4 May 2006, the sequence lasted 88 hours and 53 minutes and comprised 53 earthquakes. All events were recorded within a compact offshore area consistent with the subduction-related tectonic setting of northern Chile.

The swarm initiated with a magnitude 3.2 event at 3 km depth. Subsequent activity included multiple events of magnitude 4.0–4.8. Notable shocks reached magnitude 4.8 at 30 km depth on 30 April and again at 19 km depth on 3 May. Depths throughout the sequence ranged from near-surface (0 km) to 45 km, with many events clustered between 10 km and 30 km. Magnitudes generally remained below 4.5 after the first 24 hours, although isolated magnitude 4.0–4.1 events continued until the swarm’s termination.

This offshore segment lies above the convergent boundary where the Nazca Plate subducts beneath the South American Plate at rates of approximately 6–7 cm per year. The geometry produces frequent shallow to intermediate-depth seismicity along the plate interface and within the overriding crust. Copiapó and its western offshore zone form part of the northern Chile seismic gap, a region that has experienced both moderate swarms and great earthquakes historically, including the 1922 Mw 8.5 event and the 2015 Illapel Mw 8.3 earthquake farther south.

According to internal records maintained since 1 January 2000, S20060501.1 represents the sole swarm identified in the immediate area. No additional swarms have been classified in the same source catalog through the present.

Such sequences typically reflect localized stress adjustments along the subduction interface or within the forearc crust. The relatively short duration, moderate magnitudes, and absence of a dominant mainshock align with swarm characteristics rather than a classic foreshock–mainshock–aftershock pattern. Depths extending to 45 km are consistent with seismicity on or near the plate interface in this segment of the margin.

Continued monitoring by regional networks remains essential given the high seismic potential of the northern Chile subduction zone. The 2006 swarm provides a baseline for comparison with any future activity in the same offshore sector.

References

• USGS Earthquake Catalog (historical events and plate motion data)
• Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile – regional seismotectonic reports
• Internal SeismoSight swarm classification records (S20060501.1 parameters)