Seismic Swarm Near Copiapó, Chile: April 30–May 1, 2006

A seismic swarm designated PS20060430.1 was recorded 78 km west-northwest of Copiapó, Chile, between 10:31 UTC on 30 April 2006 and 02:49 UTC on 1 May 2006. The sequence lasted 16 hours and 18 minutes and comprised 19 earthquakes. Magnitudes ranged from 5.0 to 6.7, with focal depths between 2 km and 48 km.

Two events exceeded magnitude 6.0. The largest, a magnitude 6.7 earthquake at 19:17 UTC on 30 April, occurred at 12 km depth. A magnitude 6.5 event followed at 21:40 UTC the same day, also at 12 km depth. Additional notable shocks included multiple magnitude 5.8–5.9 events clustered between 21:25 and 23:04 UTC. Depths showed considerable variation, with several events shallower than 10 km and others exceeding 30 km, indicating activity across different crustal levels.

The swarm occurred within the tectonically active Andean margin of northern Chile. This region lies above the subduction zone formed by the Nazca Plate descending beneath the South American Plate at a convergence rate of approximately 6.5–7 cm per year. Subduction generates frequent seismicity through megathrust ruptures, intraslab events, and crustal faulting. The Atacama Desert setting features a narrow continental shelf and rapid uplift of the Andean Cordillera, conditions that concentrate strain release near the plate interface and overlying crust.

Northern Chile has a well-documented history of large earthquakes driven by this subduction regime. Notable events include the 1877 magnitude 8.8 Iquique earthquake and the 1960 magnitude 9.5 Valdivia earthquake farther south. More recent activity near Copiapó includes the 2007 Tocopilla sequence and ongoing aftershocks from the 2015 Illapel earthquake, underscoring persistent seismic hazard along this segment of the margin.

Seismic swarms such as PS20060430.1 differ from typical mainshock-aftershock sequences by lacking a single dominant event followed by decaying aftershocks. Instead, they exhibit clustered activity over hours to days, often linked to fluid migration or aseismic slip along the subduction interface. The 2006 swarm displayed this pattern, with two near-simultaneous magnitude-6+ events and a rapid succession of moderate shocks distributed over a compact time window.

Monitoring networks operated by Chilean and international agencies continue to track seismicity in the region. The 2006 swarm contributed to refined understanding of strain accumulation along the Copiapó segment, which remains capable of producing larger earthquakes.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification PS20060430.1