Seismic Swarm PS20100311.1: Analysis of Activity near Santa Cruz, Chile

Central Chile lies along the active subduction zone where the Nazca Plate converges with the South American Plate at rates exceeding 6 cm per year. This tectonic setting produces frequent seismicity, including both large megathrust events and episodic earthquake swarms. The region around Santa Cruz, in the O’Higgins Region, has experienced recurrent seismic clusters that reflect stress transfer and fluid migration within the overriding plate and along the plate interface.

Swarm PS20100311.1 began at 14:39 UTC on 11 March 2010 and concluded at 07:31 UTC on 14 March 2010. Over 64 hours and 51 minutes, 21 earthquakes were recorded at an average distance of 61 km west-northwest of Santa Cruz. The sequence included two events of magnitude 6.9 and 7.0, followed by numerous aftershocks ranging from magnitude 5.0 to 5.8. Depths varied from 3 km to 39 km, indicating activity both within the crust and near the subduction interface.

The largest event, magnitude 7.0 at 14:55 UTC on 11 March, occurred at 18 km depth and was located only 1 km from the swarm centroid. A second magnitude 6.9 event preceded it by 16 minutes at 11 km depth. Subsequent activity showed a gradual decline in magnitude, with the final recorded event of magnitude 5.2 at 35 km depth marking the swarm’s end. The temporal distribution—intense initial clustering followed by diminishing rates—typifies swarm behavior driven by aseismic slip or pore-pressure changes rather than a classic mainshock-aftershock sequence.

Historical records since 2000 indicate nine swarms in the same area. One swarm occurred in 2001, while eight took place in 2010, underscoring elevated seismic productivity during that year. The March 2010 swarm followed the great Maule earthquake of 27 February 2010 by roughly two weeks, consistent with regional stress perturbations that can trigger swarm activity hundreds of kilometers from a major rupture.

Seismic swarms in subduction zones such as this often signal transient deformation. Depths between 7 km and 36 km for most events align with the expected range of the seismogenic zone in central Chile. Shallower events (3–12 km) may reflect upper-crustal faulting, while deeper events (30–39 km) likely occurred near the plate interface. The concentration of 21 events within three days illustrates the rapid energy release characteristic of swarm episodes in this tectonic environment.

Continued monitoring of the Santa Cruz area remains important given its proximity to populated centers and critical infrastructure. The 2010 swarm sequence demonstrates how moderate-magnitude events can cluster tightly in both space and time, providing valuable data for refining seismic hazard models in subduction settings.

References

• SeismoSight internal swarm classification PS20100311.1
• United States Geological Survey earthquake catalog (events since 2000)