Seismic Swarm PS20070628.1: Geological Context and Event Analysis Near San Juan, Peru

The seismic swarm designated PS20070628.1 occurred approximately 65 km south-southeast of San Juan, Peru, within the tectonically active coastal margin of southern Peru. This sequence began at 08:44 on 28 June 2007 and concluded at 02:02 on 29 June 2007, encompassing six earthquakes over 17 hours and 17 minutes. The events were distributed across a range of magnitudes and focal depths, providing a snapshot of localized stress adjustment along the Peru-Chile subduction zone.

Southern Peru lies at the convergent boundary where the Nazca Plate subducts beneath the South American Plate at rates of approximately 6–7 cm per year. This setting produces frequent intermediate-depth and shallow seismicity, with the coastal region near Atiquipa and San Juan characterized by a combination of megathrust interfaces and crustal faults. Historical records indicate that the area has hosted multiple magnitude 7+ events since 2000, underscoring persistent strain accumulation and release.

The 2007 swarm sequence initiated with a magnitude 5.8 event at 12 km depth, followed by a cluster of four events between 12:39 and 15:43 on 28 June. These included two magnitude 5.0 shocks at depths of 24–25 km, a magnitude 5.2 event at 26 km, and a smaller magnitude 4.0 event at 48 km. The swarm concluded with a magnitude 5.6 earthquake at 16 km depth early on 29 June. Focal depths remained predominantly shallow to mid-crustal, consistent with activity near the plate interface or overlying crustal structures.

This swarm represents the second recorded instance since 2000, following an earlier episode in 2001. Its occurrence aligns with broader patterns of episodic seismicity in the region, where clusters of moderate events can precede or accompany larger ruptures. Notable subsequent strong earthquakes within tens of kilometers include the magnitude 7.2 event of 28 June 2024 located 51 km from the swarm centroid, the magnitude 7.1 event of 14 January 2018 at 23 km distance, and the magnitude 7.1 event of 25 September 2013 at 46 km distance. These larger shocks highlight the continued seismic productivity of the Atiquipa–San Juan corridor.

Analysis of the 2007 data shows a rapid decay in event rate after the initial mainshock, with magnitudes generally decreasing over the 17-hour window. Such temporal and magnitude patterns are typical of swarm-type sequences driven by localized stress perturbations rather than a single dominant rupture. Depths ranging from 12 km to 48 km suggest involvement of both shallow crustal volumes and slightly deeper portions of the subduction interface.

References

SeismoSight internal classification records for swarm PS20070628.1 parameters and historical swarm statistics since 2000.01.01.
USGS Earthquake Catalog for regional strong-event locations and magnitudes (2000–2024).
Global subduction-zone literature on Nazca–South American plate convergence rates and Peruvian coastal seismotectonics.