Seismic Swarm South of Fiji Islands: November 9, 2022

A seismic swarm occurred south of the Fiji Islands on November 9, 2022, lasting approximately nine hours. The sequence began at 09:38 UTC and concluded at 18:41 UTC, during which five earthquakes were recorded. All events originated at depths between 624 and 660 kilometers, characteristic of deep-focus seismicity within the subducting Pacific Plate.

The individual events included a magnitude 6.8 earthquake at 09:38:42 UTC (630 km depth), followed by a magnitude 7.0 event at 09:51:04 UTC (660 km depth). Subsequent shocks registered magnitudes of 6.6 (10:14:33 UTC, 624 km), 5.1 (17:49:46 UTC, 650 km), and 4.9 (18:41:04 UTC, 656 km). The largest shock, magnitude 7.0, was located 13 km from the swarm centroid.

This activity took place in a tectonically complex region where the Pacific Plate subducts westward beneath the Australian Plate along the Tonga Trench system. Deep earthquakes in this zone result from phase transitions and dehydration embrittlement within the cold slab interior, allowing brittle failure at mantle depths. The Fiji-Tonga area experiences some of the world's deepest and most energetic intraslab events due to the rapid subduction rate exceeding 15 cm per year.

Historical records indicate recurrent deep seismicity south of Fiji, with the November 2022 swarm representing the only magnitude 7.0 event in the immediate vicinity since 2000. Such swarms at extreme depths are uncommon but provide valuable data on slab dynamics and stress transfer within the mantle transition zone.

The short temporal clustering of five events suggests localized stress release along pre-existing faults within the slab rather than a typical mainshock-aftershock sequence. Depths near 650 km align with the olivine-wadsleyite phase boundary, where transformational faulting may facilitate rupture.

SeismoSight internal classification identifies this sequence as Swarm PS20221109.1.

References

USGS Earthquake Catalog SeismoSight internal records