Seismic Swarm Offshore Tarapacá, Chile: Analysis of the April 2014 Event

The offshore region of Tarapacá in northern Chile lies within one of the world's most active subduction zones, where the Nazca Plate converges with and subducts beneath the South American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismicity, including both shallow crustal events and deeper intraslab earthquakes, as well as occasional great megathrust ruptures. The area has a well-documented history of large earthquakes, with notable events such as the 2007 Tocopilla earthquake (Mw 7.7) and the 2014 Iquique earthquake (Mw 8.2) occurring nearby, highlighting the persistent strain accumulation along the plate interface.

Seismic swarms—clusters of earthquakes occurring in a localized area over a short time without a single dominant mainshock—are relatively uncommon in this segment of the subduction zone. Historical records since 2000 indicate only five such swarms in the broader region, with the earliest documented instance taking place in 2014. These episodes often reflect fluid migration, stress triggering, or aseismic slip processes rather than the classic foreshock-mainshock-aftershock sequence.

The swarm designated S20140402.1 began at 01:50 UTC on 2 April 2014 and concluded at 11:10 UTC on 10 April 2014, lasting 201 hours and 20 minutes. During this interval, 88 earthquakes were recorded offshore Tarapacá. Event magnitudes ranged from 2.5 to 4.9, with the majority occurring at depths between 10 and 40 km. The sequence featured several events of magnitude 4.5–4.9 clustered in the first two days, followed by a gradual decline in both frequency and size. Notable larger events included a 4.9 at 01:54 on 2 April (depth 10 km), another 4.9 at 02:32 the same day (depth 35 km), and subsequent 4.9 events on 3 April and 9 April. Depths showed a bimodal distribution, with many shallow events (under 20 km) interspersed with deeper activity extending to 45 km.

This temporal pattern—intense initial activity tapering over nine days—is consistent with swarm behavior driven by transient stress changes in the subduction environment. Depths predominantly in the 10–20 km range align with the seismogenic zone of the plate interface, while deeper events may relate to intraslab deformation. No damage or tsunami was associated with the swarm, distinguishing it from the preceding Mw 8.2 mainshock.

Geological monitoring in the region relies on networks operated by Chilean agencies and international partners, providing high-resolution data for such sequences. Continued observation remains essential given the area's potential for future large events.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile
• Global CMT Catalog (globalcmt.org)