Seismic Swarm Offshore Bio-Bio, Chile: March 2015 Analysis

Seismic swarms represent clusters of earthquakes occurring in a localized area over a short period without a clear mainshock-aftershock sequence. The swarm designated PS20150319.1 was recorded offshore Bio-Bio, Chile, beginning at 18:27 UTC on 18 March 2015 and concluding at 08:34 UTC on 19 March 2015. Within approximately 14 hours, five events were detected, with magnitudes ranging from 3.7 to 6.2 and focal depths between 3 km and 44 km.

The sequence initiated with a magnitude 6.2 event at 13 km depth, followed by a 5.4 event at 10 km roughly 40 minutes later. Subsequent activity included a magnitude 5.0 shock at 4 km depth early on 19 March, a smaller 3.7 event at 44 km, and a final 5.2 shock at 3 km depth. These shallow to intermediate depths align with the tectonic setting of the region, where the Nazca Plate subducts beneath the South American Plate along the Peru-Chile Trench.

The Bio-Bio offshore zone lies within one of the most seismically active segments of the subduction margin. Historical data indicate 11 swarms in the area since 2000, with notable activity including 10 swarms in 2010 and one in 2011. This pattern underscores episodic clustering that can occur independently of great earthquakes. The 27 February 2010 Maule earthquake (magnitude 8.8), centered approximately 74 km from the 2015 swarm epicenter, remains the dominant recent event shaping regional stress fields and aftershock productivity.

Such swarms provide valuable data for monitoring subduction dynamics, as variations in depth and magnitude distribution may reflect fluid migration or stress transfer along the plate interface. Continued observation in this segment supports improved understanding of precursory patterns in a margin capable of producing megathrust events.

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