M 7.1 - 208 km ENE of Levuka, Fiji
- 2004-07-15 04:27:14 (UTC)
- 17.656°S 178.760°W
- 565.5 km depth
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Origin - Review Status
- REVIEWED
- Magnitude
- 7.1 mwc
- Depth
- 565.5 km
- Time
- 2004-07-15 04:27:14 UTC
Moment Tensor Fault Plane Solution View Nearby Seismicity - Time Range
± Three Weeks - Search Radius
250.0 km - Magnitude Range
≥ 4.0
Contributors US
USGS National Earthquake Information Center, PDE
Tectonic Summary
The July 15, 2004, M 7.1 earthquake near Fiji occurred as the result of deep, oblique reverse faulting approximately 565 km beneath the South Pacific Ocean several hundred kilometers to the west of the Tonga Trench. Focal mechanism solutions indicate that rupture occurred on a moderately dipping reverse fault striking to the north-northwest or the southeast. Slip on a fault of either orientation would accommodate the down-dip compression of the Pacific slab that is implied by the reverse component of the faulting solution. At the location of this earthquake, the Pacific plate moves approximately due west relative to the Australia plate at a velocity of about 81 mm/yr. The eastern edge of the Australia plate may be viewed as a collection of microplates whose relative motions help to accommodate the overall Pacific-Australia convergence and associated back-arc spreading. The location, depth, and focal mechanism solutions of the July 15th event indicate that the earthquake is related to faulting within the subducted Pacific slab. The geometry of the Pacific plate at depth is known to be quite complex and may involve interactions between the Pacific plate proper and one or more relict slab fragments. The stresses generating the July 15th earthquake likely result from the slow deformation within the interior of one of these slabs, near the base of the mantle transition zone.
Earthquakes that have focal depths greater than 300 km are commonly termed “deep-focus” earthquakes. Deep-focus earthquakes cause less damage on the ground surface above their foci than similar-magnitude shallow-focus earthquakes, but large deep-focus earthquakes may be felt at great distance from their epicenters. The largest recorded deep-focus earthquake prior to this July 2004 earthquake was a M 8.2 event that occurred at a depth of 630 km within the subducted Nazca plate beneath South America near the northern Bolivian border in 1994. A larger event has since occurred—namely the M 8.3 earthquake that occurred at a depth of 600 km within the subducted Pacific plate beneath the Sea of Okhotsk offshore of northeastern Russia in 2013. The M 8.3 Sea of Okhotsk earthquake was felt all over Asia, as far away as Moscow, and across the Pacific Ocean along the western seaboard of the United States (though at distant locations, individuals reporting having felt the event were likely very favorably situated for the perception of small ground motions). The M 8.2 Bolivian deep-focus earthquake in 1994 had similarly been reported by individuals in North America at great distance from the epicenter.
Over the past century, 71 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 22 of these were located in the same region as July 15th event. The largest nearby event at these depths was a M 7.8 earthquake in January 1919, 200 km to the north and 80 km shallower than the July 15, 2004, event.
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)