Deep-Seated Seismicity in the Fiji Region

The Fiji region occupies a tectonically intricate segment of the southwest Pacific where multiple plates converge. Situated near the boundary between the Pacific and Australian plates, the area experiences frequent deep-focus earthquakes driven by the descent of cold oceanic lithosphere into the mantle. Subduction along the Tonga and Vanuatu trenches produces slabs that penetrate well below 600 km, generating events whose focal depths routinely exceed those recorded in most other subduction zones. On 15 September 2011 at 19:31 UTC, a magnitude 7.3 earthquake struck the Fiji region at a depth of 644.6 km. The event originated within the subducted slab and produced no significant surface damage owing to its great depth and the buffering effect of the overlying mantle and ocean. Historical records document another major shock on 19 August 2002, a magnitude 7.7 earthquake located only 9 km from the 2011 hypocenter. Both events illustrate the persistent seismic productivity of the same deep slab segment. Geological studies attribute Fiji’s deep seismicity to the geometry of the Pacific slab, which bends sharply and steepens with depth. This configuration allows brittle failure to persist at temperatures and pressures that would normally favor ductile flow. The 2002 and 2011 ruptures occurred within a narrow corridor of the slab where stresses accumulate from both downdip pull and lateral compression against the Fiji Fracture Zone. Long-term monitoring since 2000 shows that the Fiji region has hosted multiple earthquakes above magnitude 7.0 at depths greater than 500 km. These events provide valuable data for mapping slab geometry and for understanding phase transitions in the mantle transition zone. The 644.6 km depth of the 2011 shock lies near the base of the transition zone, offering constraints on the depth at which olivine transforms to spinel structures. Regional tectonics also involve back-arc spreading in the North Fiji Basin and strike-slip motion along the Fiji Fracture Zone. These features accommodate relative motion between the Pacific and Australian plates and locally modulate stress within the subducting slab. Consequently, the Fiji region remains one of the most active sites for deep earthquakes on Earth. Continued seismic and geodetic observations will refine models of slab dynamics and improve hazard assessments for island nations across the southwest Pacific.

References

USGS Earthquake Catalog ISC-GEM Global Instrumental Earthquake Catalogue