Fiji

Fiji sits at a complex tectonic junction between the Pacific Plate to the north and east and the Australian Plate to the west and south, with several microplates and back-arc systems adding to the complexity. The primary seismic drivers are the Tonga-Kermadec [[subduction-zone]] system to the east, where the Pacific Plate is consumed at extreme rates, and the Vanuatu (New Hebrides) [[subduction-zone]] to the west. Fiji itself occupies the Fiji Platform — a fragment of continental or thickened oceanic crust between these active subduction zones — and the Lau Basin to the east is an actively spreading back-arc basin whose extensional tectonics generate additional seismicity. The South Fiji Basin to the south is an older oceanic basin that is tectonically quieter. Deep intraslab earthquakes within the subducted Pacific slab produce some of the region's most powerful events, including deep-focus events at depths of 500–600 kilometres beneath Fiji that are felt across the island group. The volcanic islands of Viti Levu and Vanua Levu are geologically old enough that active volcanism has ceased, but the regional [[subduction-zone]] heritage is evident in the islands' andesitic and basaltic geology.

Fiji has experienced numerous significant earthquakes throughout its recorded history, though the remote location and limited built environment of many outer islands has reduced the human toll of many events. A magnitude 8.1 earthquake struck northeast of Fiji in 1953, generating a regionally significant [[tsunami]]. The 28 March 2007 Solomon Islands earthquake (magnitude 8.1) generated a [[tsunami]] that reached Fiji's northern islands. A magnitude 7.2 earthquake struck near Fiji in July 2018 at a depth of approximately 178 kilometres within the subducted slab, causing widespread felt shaking. In 2019, a magnitude 8.0 deep earthquake (depth 600 km) near Fiji was one of the deepest large earthquakes ever recorded and was felt across the South Pacific. Shallow crustal earthquakes near Viti Levu periodically cause structural damage in Suva and other populated areas. The compound hazard of earthquakes and cyclones means that Fiji's disaster management system addresses both threats with overlapping preparedness frameworks.

The geological foundation of Fiji reflects the complex accretionary history of the southwest Pacific. The Fiji Platform is interpreted as a fragment of arc material and thickened oceanic crust caught between the converging Tonga and Vanuatu subduction systems during a late Cenozoic reorganisation of Pacific plate boundaries. The oldest exposed rocks on Viti Levu are Eocene to Oligocene volcanic and intrusive rocks from an older arc system; these are overlain by Miocene to Pliocene sedimentary sequences and younger volcanic products. The shift from active volcanic arc to the current tectonically complex but non-volcanic setting occurred approximately 5 to 10 million years ago as the subduction geometry evolved. The Lau Basin back-arc spreading centre to the east represents the most recently active plate boundary nearest Fiji, and its extensional tectonics — including submarine volcanism along the Eastern Lau Spreading Centre — continue to generate seismicity detectable in the Fijian archipelago. [[tsunami]] monitoring for Fiji is coordinated through the Pacific Tsunami Warning Center in Hawaii.