M 7.7 - Fiji region

Tectonic Summary

The August 19, 2002, M 7.7 (11:01:01 UTC) earthquake in the region of Fiji occurred as the result of deep, oblique normal faulting approximately 580 km beneath the South Pacific Ocean in the Tonga-Fiji subduction zone. Focal mechanism solutions indicate that rupture occurred on either a steeply dipping normal fault or shallowly dipping normal fault. Slip on a fault of either orientation would accommodate the down-dip extension of the Pacific slab that is implied by the normal-component of the faulting solution. At the location of the earthquake, the Pacific plate moves west relative to the Australia plate, subducting at the Tonga Trench to the east of the August 19th earthquake at a velocity of about 75 mm/yr. The August 19, 11:01 UTC earthquake was the first of two M 7.7 events that occurred within 7 minutes and 315 km of one another; both are located within an area of deep-focus activity near the bottom of the subducted Pacific plate.

The pair of August 19 earthquakes was interesting in that the events occurred very close to one another in space and time, had large magnitudes (both M 7.7) and were at great depths (580 km and 675 km, respectively). Very large earthquakes are common in this area of the Pacific (35 other events greater than M 7.5 have been recorded since 1900), and deep-focus earthquakes are plentiful. What is unusual about this pair of earthquakes is the near coincidence in both time and space at the extreme lower depth limit of global earthquakes.

Earthquakes occur in this complex seismic zone because of the interaction of two tectonic plates, the Australia plate and the Pacific plate. Both plates are composed of relatively cold and rigid lithosphere (thin oceanic crust and uppermost mantle), moving towards one another at a rate of between 50 and 90 mm/yr. At the zone of contact, the cold Pacific plate subducts under the overlying Australia plate lithosphere and sinks into the hotter upper mantle. Where the rigid plates are in contact, a deep trench or trough forms—the Tonga Trench—and numerous shallow-focus earthquakes occur (occasionally large), typically demonstrating the thrust-type mechanism expected in this zone of strong horizontal compression and brittle deformation. As the subducting plate continues to greater depths, earthquakes indicate strain relief within the relatively cold slab. The inclined pattern of earthquake foci from shallow (typically 0 to 70 km) to intermediate (70 to 300 km) and deep depths (greater than 300 km) is called the Wadati-Benioff zone. Volcanoes are frequently found above the 100-km depth contour of the Wadati-Benioff zone.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)