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Earthquake Hazards Program

Tectonic Summary

Friday, June 17, 2005 at 06:21:42 UTC

Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver

This earthquake is an aftershock to the June 15th, 2005 magnitude 7.2 mainshock. The epicenter lies to the west of the Cascadia Subduction Zone and to the south-west of the main shock. Both earthquakes occurred in a deformed section of the southernmost Juan de Fuca plate that is commonly called the Gorda plate. Light shaking from this aftershock was felt along the northern California - southern Oregon coastline.

Preliminary analysis of the aftershock indicates that it resulted from slip on a NE striking, left-lateral, strike-slip fault; this type of mechanism has been documented for other earthquakes having locations in the interior of the Gorda plate.

Earthquakes are common in the Gorda plate, which is subjected to north-south compression due to the north-west moving Pacific Plate that collides with the southern boundary of the Gorda Plate along the east-west Mendocino Fracture Zone. In addition to the June 15th mainshock, there have been 4 other quakes of magnitude M>6 that have occurred with 50 miles radius of the epicenter of this quake (November 8 1980, August 16 and 17 1991, and July 24 1996).


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