Deep Earthquake South of Fiji Islands: Geological Context and the 2002 Event

On 19 August 2002 at 11:08 UTC, a magnitude 7.7 earthquake struck south of the Fiji Islands at a focal depth of 675.4 km. This event stands as the strongest recorded in the region since the start of 2000. Its great depth placed the hypocenter well within the mantle, where brittle failure occurs inside cold subducted lithosphere rather than at the surface plate boundary. The Fiji region sits at the northern end of the Tonga subduction zone, where the Pacific Plate descends beneath the Australian Plate. Convergence rates exceed 15 cm per year, driving one of Earth’s most active deep earthquake belts. Intermediate and deep-focus seismicity extends to nearly 700 km because the subducting slab remains sufficiently cold and stressed to generate brittle rupture at those pressures. The 2002 hypocenter lies inside this Wadati-Benioff zone, consistent with the geometry of Pacific lithosphere that has been sinking for roughly 10–15 million years. Historically, the same slab has produced other large deep events, including the 1994 M7.1 earthquake near the same latitude and comparable depth. These quakes rarely cause damage on land because their energy attenuates before reaching the surface, yet they supply critical data on slab mineralogy and phase transitions. The 675 km depth of the 2002 event coincides with the approximate location of the olivine–spinel transition, a region where transformational faulting may facilitate rupture. No significant aftershocks were reported at shallow depths, underscoring the isolated nature of deep intraslab failure. Modern seismic networks continue to monitor the Tonga–Fiji slab, refining tomographic images that reveal its penetration into the lower mantle. Such observations help constrain models of mantle convection and the long-term fate of subducted material.

References

USGS Earthquake Catalog, event 2002-08-19 south of Fiji Islands.
Global CMT Project, moment-tensor solution for 19 August 2002.
Syracuse, E. M., et al. (2010). “Tonga–Fiji subduction zone seismicity.” Journal of Geophysical Research.