Seismic Swarm PS20100227.5: Analysis of Activity Southwest of Valparaíso, Chile

The seismic swarm designated PS20100227.5 occurred approximately 50 km southwest of Valparaíso, Chile, between 06:47 on 27 February 2010 and 01:51 on 3 March 2010. Over 91 hours and 3 minutes, the sequence registered 62 earthquakes, with magnitudes ranging from 4.8 to 6.2 and focal depths predominantly between 15 and 50 km. This activity unfolded within the tectonically active margin where the Nazca Plate subducts beneath the South American Plate.

Chile’s central coastal region lies along one of the world’s most seismically active subduction zones. Convergence rates of 6–7 cm per year generate frequent megathrust events and associated swarms. The swarm’s location places it near the northern extent of the rupture zone associated with the Mw 8.8 Maule earthquake of 27 February 2010, illustrating how post-mainshock stress redistribution can trigger clusters of moderate-magnitude events.

The sequence began with a 5.6 event at 35 km depth, followed rapidly by additional shocks of similar size. Peak activity included multiple 6.0–6.2 events on 27 and 28 February, occurring at depths of 30–46 km. Later events on 1–3 March maintained magnitudes above 5.0, with the final recorded shock reaching 5.3 at 32 km depth. Depths clustered around 30–40 km, consistent with the interface and intraslab seismicity typical of this subduction segment.

Historical records since 2000 indicate only one prior swarm in the immediate area, occurring in 2001. Stronger regional events since then include an Mw 7.0 earthquake on 11 March 2010 located 81 km from the swarm center and an Mw 7.1 event on 25 March 2012 situated 50 km from the center. These occurrences underscore the persistent seismic hazard along this portion of the margin.

Seismic swarms of this type provide insight into stress transfer following major subduction earthquakes. The dense sequence of moderate events, concentrated over several days, reflects aftershock triggering and possible fluid migration or aseismic slip along the plate interface. Depths between 15 and 50 km align with both the megathrust and the upper portion of the subducting slab, where brittle failure remains common.

Continued monitoring of such swarms contributes to refined seismic hazard models for central Chile. The Valparaíso region, with its proximity to major population centers and critical infrastructure, benefits from detailed analysis of these transient clusters to improve preparedness and early-warning capabilities.

References

United States Geological Survey Earthquake Catalog
Global Centroid Moment Tensor Project
SeismoSight internal swarm classification database