Seismic Swarm PS20160924.1 in the Fiji Region

The Fiji region lies within a tectonically complex area of the southwestern Pacific, positioned at the intersection of the Pacific, Australian, and several microplates. This setting produces frequent deep-focus earthquakes as the Pacific Plate subducts westward beneath the Tonga and Kermadec trenches, with remnants of the slab extending to depths exceeding 600 km. The region experiences both shallow crustal events and intermediate-to-deep seismicity associated with the subducting lithosphere.

Seismic swarm PS20160924.1 was recorded in this environment between 21:28 UTC on 24 September 2016 and 01:10 UTC on 25 September 2016. Six earthquakes occurred over 3 hours and 41 minutes. The sequence began with a magnitude 6.9 event at 596 km depth, followed by five additional events with magnitudes ranging from 4.6 to 5.2 and focal depths between 580 km and 591 km. All events clustered tightly in both time and hypocentral location, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

Such deep swarms reflect stress release within the cold, brittle core of the subducting slab. The Fiji Plateau and surrounding areas have long been recognized for this type of activity, where phase transitions and dehydration reactions at depth can trigger clusters of events without surface rupture. Historical records since 2000 indicate only two prior swarms in the immediate vicinity—one in 2004 and one in 2006—suggesting that swarm episodes remain relatively infrequent compared with isolated deep earthquakes.

Notable larger events have also occurred nearby. A magnitude 7.2 earthquake struck on 2 January 2006 approximately 17 km from the swarm center. Additional magnitude 7.1 and 7.0 events were recorded on 1 November 2014 (39 km distant) and 12 November 2022 (43 km distant), respectively. These events underscore the persistent seismic hazard associated with the steeply dipping slab beneath the region.

Analysis of the 2016 swarm shows a rapid onset followed by smaller events that decayed within hours, with depths remaining remarkably consistent. This pattern aligns with known characteristics of deep seismicity in subduction zones, where rupture occurs under high confining pressure and elevated temperatures. No significant shallow aftershocks or tsunami generation accompanied the sequence, as expected for events at such depths.

Continued monitoring of the Fiji region remains important for understanding slab dynamics and refining seismic hazard assessments for nearby island nations. The 2016 swarm provides a well-documented example of deep clustered seismicity that contributes to broader models of Pacific plate subduction.

References SeismoSight internal classification records for swarm PS20160924.1 USGS Earthquake Catalog for regional magnitude and location data since 2000