Seismic Swarm PS20100316.1 Near Talcahuano, Chile: Event Analysis and Regional Context

Seismic swarm PS20100316.1 occurred 34 km northwest of Talcahuano, Chile, beginning at 11:08 on 15 March 2010 and concluding at 04:08 on 16 March 2010. Over 16 hours and 59 minutes, five earthquakes were recorded. The sequence is classified under SeismoSight internal protocols and reflects post-mainshock adjustment in a highly active subduction setting.

The events unfolded as follows. The initial shock on 15 March 2010 at 11:08:28 reached magnitude 6.2 at 14 km depth. A magnitude 5.0 event followed at 12:13:16, located at 35 km depth. Early on 16 March, a magnitude 6.7 earthquake struck at 02:21:57 with a focal depth of 18 km. This was succeeded by a magnitude 5.9 shock at 03:04:39 (42 km depth) and a final magnitude 4.0 event at 04:08:12 (35 km depth). Depths remained predominantly shallow to intermediate, consistent with activity along the plate interface and overlying crust.

The swarm took place in the tectonically complex zone where the Nazca plate subducts beneath the South American plate at rates of approximately 6–7 cm per year. This convergent margin generates frequent megathrust earthquakes and associated aftershock sequences. Talcahuano lies within the rupture area influenced by the 27 February 2010 Maule earthquake (Mw 8.8), whose epicenter was roughly 42 km from the swarm centroid. The Maule event released immense strain accumulated over centuries, producing widespread coastal uplift and triggering prolonged seismic readjustment across central Chile.

Since 1 January 2000, nine swarms have been documented in the broader region, with PS20100316.1 marking the first recorded instance. Such swarms often represent delayed responses to large mainshocks, involving fluid migration or aseismic slip along faults rather than progressive stress buildup toward another great earthquake. The 2010 sequence aligns with this pattern, occurring weeks after the Maule mainshock and exhibiting a mix of moderate magnitudes without escalation into a larger event.

Geological records indicate that central Chile has experienced repeated great earthquakes, including the 1835 Concepción event and earlier prehistoric ruptures identified through paleoseismic studies. The subduction interface here is segmented, with variable locking and creep behavior influencing swarm occurrence. Depths between 14 and 42 km place the activity within both the seismogenic zone and the transition to ductile behavior, where stress transfer can sustain clustered seismicity for days to weeks.

Monitoring data from regional networks confirm that no additional events above magnitude 4.0 extended the swarm beyond the stated termination. This limited duration and energy release underscore the swarm’s role as a localized relaxation process following the Maule rupture rather than an independent precursor.

References
USGS Earthquake Hazards Program – Maule, Chile Earthquake of 2010
Global CMT Catalog – Focal mechanisms and depths for 2010 central Chile sequence
Tectonic setting derived from published studies on Nazca–South America convergence (e.g., Contreras-Reyes et al., 2010)