Seismic Swarm S20210704.1: Analysis of Activity Near Vallenar, Chile

Seismic swarm S20210704.1 occurred 86 km west-southwest of Vallenar in Chile’s Atacama Region. The sequence began at 01:01 UTC on 4 July 2021 and concluded at 12:53 UTC on 6 July 2021, spanning 59 hours and 51 minutes. During this period, 72 earthquakes were recorded.

The swarm initiated with a magnitude 5.9 event at 24 km depth. Within the first hour, activity intensified, culminating in a magnitude 6.0 earthquake at 22 km depth. Subsequent events ranged between magnitudes 2.5 and 4.6, with depths predominantly between 10 km and 35 km. Notable later shocks included four magnitude 4.6 events at depths of 20 km, 10 km, 10 km, and 10 km. The final recorded event was a magnitude 2.5 earthquake at 35 km depth.

This swarm reflects typical behavior in a subduction-zone setting where the Nazca Plate converges with the South American Plate at approximately 6–7 cm per year. Northern Chile experiences frequent seismic sequences due to stress accumulation and release along the plate interface and within the overriding crust. Depths recorded during the swarm align with both interplate and intraplate sources common to the region.

Historical records maintained by SeismoSight indicate only two prior swarms in the immediate area since 2000. One occurred in 2015 and another in 2020, each consisting of a single documented sequence. The low frequency of swarms suggests that clustered activity remains episodic rather than persistent in this segment of the subduction zone.

The 2021 swarm did not produce a mainshock-aftershock pattern typical of isolated large events. Instead, the energy release was distributed across numerous moderate-magnitude earthquakes over nearly three days. Such swarms can occur when fluids or aseismic slip transiently alter fault friction, though specific causative mechanisms require further geophysical investigation.

No surface rupture or significant damage was associated with the sequence. The maximum magnitude of 6.0 remained below the threshold for widespread structural impact at the recorded distances from Vallenar.

References

SeismoSight internal swarm catalogue S20210704.1
USGS Earthquake Catalog (regional seismicity summaries)
Chilean National Seismological Center (CSN) annual reports on Atacama Region activity