Seismic Swarm Offshore Los Lagos, Chile: December 2016 Analysis

The seismic swarm designated S20161226.1 occurred offshore the Los Lagos region of Chile from 15:17 on 25 December 2016 to 18:18 on 27 December 2016. Over this 51-hour period, 32 earthquakes were recorded, with magnitudes ranging from 3.1 to 5.2 and focal depths between 4 km and 42 km. This sequence represents a typical cluster of moderate events in a tectonically active subduction setting.

The events began with a 4.3-magnitude shock at 29 km depth, followed rapidly by additional tremors including a 4.4 at 14 km and a peak 5.2 at 18 km depth later on 25 December. Subsequent activity included multiple events around 4.0–4.2 magnitude at varying depths, tapering toward the end with a 4.7 at 23 km and a final 4.5 at 25 km on 27 December. Depths showed no consistent shallowing or deepening trend, indicating distributed rupture within the crust and upper plate interface.

Los Lagos lies along the Peru-Chile Trench, where the Nazca Plate subducts beneath the South American Plate at rates of approximately 6–7 cm per year. This convergence drives the region’s high seismicity, including both interplate thrust events and intraslab activity. The area also features the north-south trending Liquiñe-Ofqui Fault Zone, which accommodates intra-arc deformation and can host strike-slip or oblique-slip earthquakes. Historical records document major events nearby, such as the 1960 Mw 9.5 Valdivia earthquake, whose rupture extended into adjacent segments and triggered widespread aftershock sequences.

Swarm activity of this type often occurs without a single dominant mainshock-aftershock pattern, reflecting fluid migration or stress transfer along pre-existing faults rather than a singular large rupture. In southern Chile, such swarms are monitored closely due to the potential for escalation into larger events, although this sequence remained moderate in scale.

Ongoing seismic networks operated by Chilean institutions continue to track activity in the region, contributing to refined hazard models for the Andean margin. The 2016 swarm underscores the persistent tectonic loading along this portion of the subduction zone.

References:
USGS Earthquake Hazards Program – Seismotectonics of South America
Servicio Nacional de Geología y Minería (SERNAGEOMIN) – Regional Geological Framework of Los Lagos
Global Centroid Moment Tensor Catalog – Subduction Zone Seismicity Studies