Magnitude 7.8 Earthquake South of Fiji

Sunday, December 9, 2007 at 07:28:20 UTC

11:28:20 PM (December 8) Pacific Standard Time

Epicenter: Latitude 25.872°S, 177.517°W (indicated by stars on maps below).

Depth: 149 kilometers.

A large earthquake occurred last evening Portland time on the subduction zone at the Tonga Trench south of Fiji beneath the southwestern Pacific Ocean. Because this earthquake occurred deep below the surface of the Earth, no tsunami was produced. This earthquake occurred within the Pacific Plate that is subducting into the deeper mantle at the Tonga Trench where the Pacific and India-Australia plates converge. The Pacific Plate is over 100 million years old at the Tonga Trench where it subducts beneath the India-Australia Plate. The rate of convergence at the location of this earthquake is about 60 mm/yr. This is about twice the rate of convergence between the Juan de Fuca and North America plates at the Cascadia Subduction Zone off the coast of Oregon and Washington.


The record of the Fiji earthquake on the University of Portland seismometer is illustrated below. Portland is about 9600 km (~6000 miles) from the location of this earthquake. The waves labeled P, pP, S, and sS are waves that traveled through the Earth’s mantle from the earthquake to Portland. (P waves are compressional waves while S waves are shear waves). It took about 12 minutes for these fastest seismic waves to travel from the earthquake to Portland. The surface waves traveled from the earthquake to Portland around the perimeter of the Earth. Because the distance around the perimeter is longer and the speed of surface wave is slower than that of body waves, it takes surface waves much longer to travel from an earthquake to a distant seismic station. In this case, the first surface waves from the Fiji earthquake started arriving at the University of Portland about 35 minutes after the earthquake.



The recording from Corvallis is shown below: