Seismic Swarm S20030602.1: Analysis of the June 2003 Earthquake Sequence Near Valparaíso, Chile

The seismic swarm designated S20030602.1 occurred approximately 42 km west-northwest of Valparaíso, Chile, between 02:52 on 2 June 2003 and 13:55 on 5 June 2003. Over this 83-hour period, 65 earthquakes were recorded, providing a detailed record of clustered seismic activity along the subduction interface.

This region lies within the Peru-Chile Trench, where the Nazca Plate subducts beneath the South American Plate at a rate of approximately 6.5–7 cm per year. The resulting compressional tectonics generate frequent seismicity, including both interplate thrust events and intraslab activity. Valparaíso sits near the transition between the flat-slab segment to the north and the more steeply dipping slab to the south, a structural boundary that influences rupture propagation and aftershock patterns.

The swarm initiated with a magnitude 5.5 event at 33 km depth. Subsequent events ranged from magnitude 2.3 to 4.8, with the majority occurring at depths between 9 and 35 km. Focal depths clustered in two primary intervals: shallow crustal events (1–15 km) and deeper events near the plate interface (25–35 km). This bimodal distribution suggests activation of both the overriding plate and the subducting slab during the sequence.

Swarm behavior in subduction zones often reflects transient stress changes, possibly driven by aseismic slip or fluid migration along the megathrust. The 2003 sequence exhibited a rapid onset followed by sustained activity over three days, consistent with triggered seismicity rather than a classic mainshock-aftershock decay. No single dominant mainshock governed the energy release; instead, multiple events above magnitude 4.0 contributed to the total moment.

Historically, the Valparaíso area has experienced great earthquakes, notably the 1906 event (estimated magnitude 8.2) and the 1985 Mw 8.0 earthquake. Both ruptured adjacent segments of the plate boundary, leaving portions of the margin in varying states of coupling. The 2003 swarm occurred in a zone that had not hosted a major rupture since the mid-20th century, highlighting the persistent seismic hazard along this segment of the Chilean margin.

Post-2003 monitoring has confirmed continued low-level seismicity in the same offshore area, consistent with ongoing plate convergence. Modern networks, including those operated by the Centro Sismológico Nacional, provide improved resolution of similar swarms, aiding in the differentiation between background activity and potential precursory signals.

References
USGS Earthquake Catalog
Global CMT Catalog
Centro Sismológico Nacional, Universidad de Chile
Scholz, C. H. (2019). The Mechanics of Earthquakes and Faulting (3rd ed.). Cambridge University Press.