Seismic Swarm PS20070816.2 Near Subtanjalla, Peru

On 16 August 2007, a seismic swarm designated PS20070816.2 occurred 9 km west-northwest of Subtanjalla in Peru’s Ica region. The sequence began at 00:19 local time and concluded at 09:02, encompassing six earthquakes over 8 hours and 42 minutes.

The events unfolded as follows. The first shock registered magnitude 5.8 at 35 km depth. Five minutes later, a magnitude 5.3 event occurred at 41 km. Activity resumed at 04:23 with a magnitude 5.2 shock at 11 km depth. Two smaller events followed at 05:12 (magnitude 4.0, 35 km) and 05:16 (magnitude 6.4, 23 km). The final event, magnitude 5.0, struck at 09:02 from 35 km depth.

This swarm reflects typical behavior along the Peru-Chile subduction zone, where the Nazca Plate descends beneath the South American Plate at rates of approximately 6–7 cm per year. The resulting compressional stress produces both shallow crustal events and deeper intraslab seismicity. Depths recorded during the swarm span the upper 41 km, consistent with the transition from the plate interface to the overriding continental crust.

The Ica region has experienced recurrent seismic activity throughout recorded history. Notable events include the 2007 Pisco earthquake and earlier destructive shocks in 1868 and 1913. Since 1 January 2000, only one swarm has been documented in the immediate area, with the preceding episode also occurring in 2007. Such swarms often represent secondary stress adjustments following larger regional ruptures rather than independent mainshock-aftershock sequences.

Monitoring data indicate that the largest event (magnitude 6.4) occurred at an intermediate depth of 23 km, potentially linked to slab dehydration processes. Shallower events at 11 km may reflect brittle failure within the continental crust. The absence of additional swarms since 2007 underscores the episodic nature of clustered seismicity in this segment of the subduction zone.

Ongoing geophysical studies continue to refine models of plate coupling and fluid migration that influence swarm occurrence. These insights support improved hazard assessment for coastal communities in southern Peru.

References
USGS Earthquake Catalog
Global CMT Project
INGEMMET Geological Maps of Peru