Seismic Swarm PS20100227.7: Analysis of Activity Near Constitución, Chile

The coastal region northwest of Constitución, Chile, lies within the tectonically active Peru-Chile subduction zone, where the Nazca Plate converges with and subducts beneath the South American Plate at rates of approximately 6–7 cm per year. This setting produces frequent seismic events, including megathrust earthquakes capable of generating tsunamis. The local geology features a deep oceanic trench, accreted sedimentary wedges, and a volcanic arc inland, with crustal structures influenced by long-term plate boundary deformation.

Seismic history in central Chile includes major events such as the 1835 Concepción earthquake and the 2010 Maule earthquake (Mw 8.8), both of which originated nearby and caused widespread ground shaking and coastal uplift or subsidence. These episodes highlight the region's capacity for both large single ruptures and clustered aftershock sequences. Updated monitoring since 2000 has recorded three distinct earthquake swarms in the broader area, with the initial swarm occurring in 2010.

Swarm PS20100227.7 began at 08:28 on 27 February 2010 and concluded at 22:40 on 1 March 2010, spanning 62 hours and 11 minutes. The sequence was located 86 km NNW of Constitución and comprised 14 earthquakes. Magnitudes ranged from 4.9 to 5.5, with the majority clustered between 5.0 and 5.2. Depths were predominantly around 35 km, though several events occurred at shallower levels between 9 km and 30 km or slightly deeper at 37 km. The swarm initiated with two events of magnitude 5.4 within the first 25 minutes, followed by a magnitude 5.5 peak later that day. Subsequent activity showed a gradual decline in frequency, with the final event registering magnitude 5.0.

This swarm pattern reflects a concentrated release of strain along the subduction interface without progression to a larger mainshock. Depths near 35 km align with typical interface seismicity in the region, while shallower events may indicate activity within the overriding plate. The timing coincided with heightened regional stress following the 2010 Maule mainshock, illustrating how post-seismic relaxation can trigger clustered smaller-magnitude sequences.

Overall, the data underscore the value of continuous seismic monitoring for distinguishing swarm behavior from foreshock activity in subduction environments. Such insights support improved hazard assessment for central Chile's coastal communities.

References
USGS Earthquake Catalog
Global CMT Project
Chilean National Seismic Network (CSN) reports