Seismic Swarm S20170423.1: Offshore Valparaíso, Chile

The region offshore Valparaíso, Chile, lies along the Peru-Chile Trench, where the Nazca Plate subducts beneath the South American Plate at a rate of approximately 6–7 cm per year. This convergent margin produces frequent seismic activity, including large megathrust earthquakes and episodic earthquake swarms. The area has a well-documented history of destructive events, such as the 1906 Valparaíso earthquake (magnitude 8.2) and the 2010 Maule earthquake (magnitude 8.8), which released significant strain along adjacent segments of the subduction interface.

Seismic swarms in this zone often reflect fluid migration, stress transfer, or slow-slip phenomena rather than a single mainshock-aftershock sequence. Since 2000, six such swarms have been recorded in the offshore Valparaíso area: one each in 2003, 2008, 2011, and 2015, plus two in 2010. These episodes typically involve hundreds of events clustered over days to weeks, with magnitudes rarely exceeding 6.0 outside of major sequences.

Swarm S20170423.1 began at 22:46 UTC on 22 April 2017 and concluded at 21:23 UTC on 28 April 2017, lasting 142 hours and 37 minutes. During this period, 182 earthquakes were registered. The first 100 events displayed a broad magnitude range from 2.0 to 6.9, with hypocentral depths concentrated between 10 km and 33 km. Early activity included a magnitude 4.9 event at 9 km depth, followed by a rapid increase in rate that culminated in a magnitude 6.0 shock at 21 km depth roughly four hours after onset.

A notable escalation occurred on 24 April, when the largest event of the swarm—a magnitude 6.9 earthquake at 28 km depth—struck at 21:38 UTC. This was accompanied by immediate aftershocks of magnitude 6.9, 5.2, and 5.4 within minutes, all at depths of 13–28 km. Subsequent events showed a gradual decline in both frequency and magnitude, consistent with swarm decay patterns observed in prior episodes in the same region.

Depth distribution across the initial 100 events averaged approximately 20 km, aligning with the expected seismogenic zone of the subduction interface near Valparaíso. Magnitudes followed a typical Gutenberg-Richter distribution, with the majority below 4.0 and only a handful exceeding 5.0. The temporal clustering, particularly the intense burst on 24 April, suggests localized stress perturbations possibly linked to fluid movement or aseismic slip along the plate boundary.

Overall, Swarm S20170423.1 fits within the historical pattern of moderate seismic unrest offshore Valparaíso. While it did not produce a great earthquake, such episodes contribute to ongoing strain accumulation monitoring and improve understanding of subduction-zone dynamics in one of Earth’s most active margins.

References

USGS Earthquake Catalog (events 2017) SeismoSight internal swarm classification S20170423.1