Seismic Swarm PS20041117.1 in the Fiji Region

The Fiji region lies within the southwestern Pacific Ocean at the complex intersection of the Pacific and Australian tectonic plates. This area forms part of the Pacific Ring of Fire and features the North Fiji Basin, Lau Basin, and Fiji Fracture Zone, where subduction, back-arc spreading, and strike-slip faulting interact. The subducting Pacific slab beneath the region produces both shallow crustal earthquakes and deep-focus events exceeding 600 km, reflecting the steep geometry of the Wadati-Benioff zone. Historical records document recurrent seismic activity, including major events in 1953 and 1979 that highlighted the area's potential for both shallow and intermediate-depth seismicity.

Between 21:03 UTC on 17 November 2004 and 12:27 UTC on 18 November 2004, a swarm of seven earthquakes was recorded in the Fiji region over a period of 15 hours and 23 minutes. The sequence began with a magnitude 5.6 event at 33 km depth, followed within minutes by a magnitude 5.2 shock at 627 km. Subsequent events included a magnitude 6.2 at 33 km and a magnitude 6.6 at 622 km, marking the largest shocks of the swarm. Later activity comprised a magnitude 4.3 event at 600 km depth, concluding with two closely spaced events of magnitudes 5.2 and 5.7 at depths of 600 km and 625 km, respectively.

The swarm exhibited a distinctive pattern of mixed focal depths, combining shallow crustal events with deep intraslab earthquakes. Such co-occurrence within a short temporal window is characteristic of Fiji's tectonic setting, where stress transfer along the subducting slab can trigger activity across multiple depth ranges. No significant surface damage was reported, consistent with the predominantly offshore and deep nature of the events.

This sequence underscores the ongoing tectonic deformation in the Fiji region driven by plate convergence rates of approximately 5–8 cm per year. Continued monitoring remains essential given the region's history of producing both shallow tsunamigenic earthquakes and deep-focus events that can be felt across the South Pacific.

References:
USGS Earthquake Catalog
Global CMT Catalog
Geological Survey of Fiji tectonic summaries