Seismic Swarm PS20010623.2: Analysis of Southern Peru Earthquake Sequence

A seismic swarm designated PS20010623.2 was recorded in southern Peru, centered 68 km west-southwest of El Cardo. The sequence began at 20:56 on 23 June 2001 and concluded at 17:37 on 24 June 2001, spanning 20 hours and 41 minutes. During this period, 18 earthquakes were registered, providing insight into clustered seismic activity along the Peru-Chile subduction zone.

The events exhibited magnitudes ranging from 4.7 to 6.1, with the majority occurring at depths between 28 km and 38 km. The sequence opened with a magnitude 5.8 event at 33 km depth, followed rapidly by additional shocks including a magnitude 6.1 at 21:27 and a magnitude 5.5 at 22:32 on 23 June. Activity continued into 24 June with events such as a magnitude 5.6 at 02:31 and a final magnitude 5.1 at 17:37. Depths remained consistent in the upper to mid-crust, typical of interface seismicity in this tectonic setting.

This swarm represents the sole recorded swarm in the region since 1 January 2000. It preceded a strong magnitude 7.6 earthquake on 7 July 2001, located 51 km southwest of Punta de Bombón and approximately 19 km from the swarm epicentral area. Such sequences often reflect stress adjustments along locked segments of the plate boundary prior to larger ruptures.

Southern Peru lies at the convergent margin where the Nazca Plate subducts beneath the South American Plate at rates of approximately 6–7 cm per year. This ongoing subduction generates frequent seismicity, including both interface thrust events and intraslab activity. The regional geology features the Andean volcanic arc and forearc basins shaped by millions of years of plate convergence. Historical records document recurrent large earthquakes, with the 2001 swarm occurring amid heightened activity that included the major 23 June 2001 mainshock farther south.

The swarm events illustrate typical characteristics of subduction-related clusters: rapid succession of moderate-magnitude shocks at similar depths, likely triggered by fluid migration or aseismic slip. No surface rupture was associated, consistent with the moderate magnitudes and depths involved. Monitoring data indicate that such swarms can serve as indicators of evolving stress fields in highly active margins.

• References
  SeismoSight internal swarm classification records for PS20010623.2.
  USGS Earthquake Catalog for regional events since 2000.
  Global CMT catalog for focal mechanism context in the Peru subduction zone.