A small cluster of earthquakes began on December 12, 2000 about 18 miles west of the Tri-Cities (Richland, Pasco, and Kennewick, WA). The epicenters are near the top of the Horse Heaven Hills 8 miles east of Prosser.
So far (Dec 14) 10 earthquakes have occurred about 5-6 miles below the surface of the earth with magnitudes between 0.8 and 2.1. Persons do not generally feel earthquakes of this size.
Earthquakes in this general region are common and they sometimes occur in swarms (one of the better known areas of earthquake swarms occurs a few miles north of Richland seen on the map below).
The interpretation of the December earthquakes in terms of faulting is still ambiguous. The earthquakes occur on a NE trending zone of earthquakes in the Horse Heaven Hills. However, the mechanisms of the magnitude 2 earthquakes in the current cluster determined by P-wave first motions and hypocenter locations suggest that the earthquakes are occurring on a fault perpendicular to this trend, ie on a NW striking reverse fault dipping to the southwest.
The recent earthquake swarm is shown on the map below in red. Light grey symbols show historical earthquakes (1970-present) with magnitude M > 1.5.
A Seismogram is also available for viewing. It shows several earthquakes that initiated the swarm on December 12 recorded on a seismograph located on Rattlesnake Mountain, 12 miles north of the earthquake cluster. The 12-hour seismogram shows 5 earthquakes large enough to locate and several others that were too small to locate. The larger ones occurred at 14:13UT (M1.4), 14:14UT (M1.6), 14:40UT (M1.8), 14:48UT (M2.1), and 19:40UT (M2.1). UT is Universal time. Subtract 8 hours to get Pacific Standard time. For example the earthquake at 19:40UT occurred at 11:40AM PST.
To check for the most recent earthquakes near Prosser see the latest earthquake list.
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UW Dept. of Earth and Space Sciences ..... PNW EARTHQUAKES