Seismic Swarm PS20100304.1: Analysis of Activity Near Cartagena, Chile

Seismic swarm PS20100304.1 occurred 63 km west-northwest of Cartagena, Chile, from 03:56 on 3 March 2010 to 17:37 on 4 March 2010. Over 37 hours and 41 minutes, the sequence produced seven earthquakes with magnitudes ranging from 5.0 to 6.0. The events were recorded at depths between 20 km and 38 km, consistent with the shallow to intermediate seismicity typical of the Peru-Chile Trench subduction zone.

The regional geology is defined by the convergence of the Nazca and South American plates at a rate of approximately 6–7 cm per year. This tectonic setting generates frequent megathrust earthquakes and associated seismic swarms along the plate interface and within the overriding plate. Cartagena lies within the Central Chile seismic gap, a segment that has experienced recurrent large-magnitude events throughout the Holocene. The 2010 sequence followed the Mw 8.8 Maule earthquake of 27 February 2010 by only days, placing the swarm within the broader aftershock zone of that great event.

The temporal progression of the swarm began with two events on 3 March: a magnitude 5.0 at 35 km depth at 03:56:20, followed by a magnitude 5.2 at the same depth at 06:16:22. Activity intensified later that day with a magnitude 5.8 event at 38 km depth at 19:58:29. On 4 March, the largest event—a magnitude 6.0 at 24 km depth—occurred at 01:59:48. Subsequent shocks included two magnitude 5.0 events at 38 km and 20 km depth at 02:14:39 and 02:23:41, respectively, and a final magnitude 5.4 at 35 km depth at 17:37:48. The clustering of events within a narrow spatial and temporal window, without a single dominant mainshock-aftershock decay, classifies the sequence as a swarm.

Historical records since 2000 indicate seven swarms in the immediate region. Prior episodes occurred in 2001 (one swarm), 2003 (one swarm), and 2008 (one swarm), with four swarms documented in 2010 alone. This elevated rate underscores the persistent stress accumulation along the subduction interface. Eleven days after the swarm concluded, a magnitude 7.0 earthquake struck 52 km northwest of Santa Cruz, Chile, approximately 71 km from the swarm centroid, further illustrating the continued seismic productivity of the area.

Seismic swarms in this setting commonly reflect fluid migration or aseismic slip transients that modulate frictional stability on nearby faults. Depths recorded during PS20100304.1 align with the expected range for interplate and intraslab events in central Chile, where the slab dips gently beneath the continent. Monitoring by regional networks continues to track such sequences to refine probabilistic forecasts for larger earthquakes.

References

• SeismoSight internal swarm classification database (prompt data)
• USGS Earthquake Catalog (regional tectonic parameters and historical events)
• National Seismological Center of Chile (subduction zone characteristics)