Seismic Swarm Near Bulnes, Chile: February 27, 2010

On February 27, 2010, a seismic swarm was recorded 27 km west-southwest of Bulnes in Chile’s Bío Bío Region. The sequence began at 08:35 UTC and concluded at 22:13 UTC, encompassing 10 earthquakes within a span of 13 hours and 38 minutes. All events originated at depths between 19 km and 38 km, consistent with activity along the subduction interface where the Nazca Plate descends beneath the South American Plate.

The swarm’s largest event reached magnitude 6.1 at 17:24 UTC and 19 km depth. Other notable shocks included magnitudes 5.5 at 15:44 UTC (35 km) and 17:43 UTC (38 km), with the remaining events ranging from 4.6 to 5.2. The tight temporal clustering and similar focal depths indicate a localized stress release along a segment of the megathrust, rather than widespread aftershock migration.

This swarm occurred on the same day as the M 8.8 Maule earthquake, whose epicenter lay approximately 31 km from the swarm centroid. The Maule event ruptured a roughly 500 km stretch of the plate boundary and remains one of the largest recorded in the region since modern instrumentation. The Bío Bío Region sits within the tectonically active Andean margin, where convergence rates exceed 6 cm per year, producing both great megathrust earthquakes and smaller swarm sequences.

Since January 1, 2000, five swarms have been identified in the broader area, with the February 2010 sequence being the earliest in the catalog. Such swarms typically reflect transient increases in pore-fluid pressure or aseismic slip that trigger multiple moderate events without a single dominant mainshock.

Geological studies of the central Chilean margin document a history of recurring large earthquakes, including the 1835 Concepción event and the 1960 Valdivia earthquake farther south. The 2010 Maule rupture filled a recognized seismic gap between these earlier ruptures, highlighting the region’s continued potential for significant seismic activity.

References
USGS Earthquake Catalog (events and magnitudes)
SeismoSight internal swarm classification PS20100227.6
Global CMT catalog (focal mechanisms and depths)