Seismic Swarm S20030620.1: Analysis of Activity Near Ovalle, Chile

The seismic swarm designated S20030620.1 occurred in central Chile, centered 42 km west-southwest of Ovalle in the Coquimbo Region. It began at 23:04 on 19 June 2003 and concluded at 08:23 on 30 June 2003, spanning 249 hours and 18 minutes. During this interval, 233 earthquakes were recorded, providing a detailed record of clustered seismicity in a tectonically active subduction setting.

The sequence opened with a magnitude 5.6 event at 51 km depth, followed rapidly by numerous smaller shocks. Magnitudes in the initial phase ranged primarily between 2.0 and 4.2, with depths concentrated between 10 km and 45 km. A prominent feature was the magnitude 6.8 earthquake on 20 June at 13:30:41 UTC, occurring at 33 km depth. This event marked the peak of the swarm and was succeeded by continued moderate activity, including several magnitude 4+ aftershocks at similar depths.

Examination of the first 100 events reveals a typical swarm pattern: an initial energetic onset, a dominant mainshock, and a prolonged decay of smaller events. Depths remained consistent with the seismogenic zone above the subducting slab, while magnitudes showed a high b-value distribution indicative of fluid-influenced or heterogeneous stress release rather than a single large rupture.

The Coquimbo Region lies above the Nazca–South American plate boundary, where convergence occurs at approximately 6.5 cm per year. This setting produces both interface thrust earthquakes and intraslab events at intermediate depths. Ovalle and surrounding areas have experienced recurrent seismic sequences linked to the flat-slab segment and the transition to the Pampean flat slab further south. Historical records document major events in 1943 and 1997 in nearby segments, underscoring the persistent seismic hazard.

The 2003 swarm fits within this framework, illustrating how moderate-to-large events can trigger extensive aftershock sequences without necessarily heralding a great megathrust rupture. Depths recorded align with known slab geometry, and the spatial clustering near the coast reflects the downdip extent of interplate coupling.

Such sequences contribute to understanding strain accumulation and release along the Chilean margin. Continued monitoring in the region remains essential given its proximity to populated centers and critical infrastructure.

References

USGS Earthquake Catalog
Global CMT Catalog
Servicio Nacional de Geología y Minería (SERNAGEOMIN) reports on Coquimbo seismicity