Seismic Swarm PS20110317.2: Analysis of Activity Near Valparaíso, Chile

A seismic swarm designated PS20110317.2 occurred approximately 60 km NNW of Valparaíso, Chile, between 22:36 on 16 March 2011 and 15:25 on 17 March 2011. Over this 16-hour and 49-minute period, five earthquakes were recorded with magnitudes ranging from 4.9 to 5.5 and focal depths between 13 km and 39 km. The sequence began with a magnitude 5.5 event at 32 km depth, followed by events of magnitudes 5.0, 5.1, 5.2, and 4.9. This cluster represents a typical example of swarm behavior, where multiple events occur in close temporal and spatial proximity without a dominant mainshock-aftershock pattern.

The region lies along the active margin of central Chile, where the Nazca Plate subducts beneath the South American Plate at a rate of approximately 6.6 cm per year. This tectonic interaction produces the Peru-Chile Trench and drives intense seismicity across a broad zone extending from the trench to the Andean volcanic arc. The Valparaíso area sits above the seismogenic portion of the megathrust, which has hosted great earthquakes, including the 1906 Valparaíso event of magnitude 8.2 and the 1985 central Chile earthquake of magnitude 8.0. Shallower events in the recorded swarm likely originated within the overriding plate or near the plate interface, while deeper ones reflect intraslab deformation within the descending Nazca slab.

Historical records maintained by SeismoSight indicate nine swarms have occurred in the same region since 1 January 2000. These include one swarm in 2001, two in 2003, one each in 2007 and 2008, and four in 2010. The 2011 swarm followed a year of elevated swarm activity, suggesting episodic clustering that may relate to stress transfer along the subduction interface or fluid migration within the crust. Such patterns are common in subduction zones and do not necessarily indicate an imminent large earthquake, although they contribute to ongoing seismic hazard assessment for the densely populated Valparaíso–Viña del Mar metropolitan area.

In summary, swarm PS20110317.2 illustrates the persistent seismic productivity of the central Chilean margin. Continued monitoring of similar clusters remains essential for refining probabilistic forecasts and understanding the interplay between plate coupling and transient slip phenomena.

References

USGS Earthquake Catalog
Chilean National Seismological Center (CSN) reports
Global CMT catalog for focal mechanisms in the Nazca–South America subduction zone