North Coast Quake Activity May Produce More Aftershocks

A series of three earthquakes that rattled the Pacific Northwest over the weekend occurred in an unusually active and complex geological zone that could produce smaller aftershocks in coming days, according to Robert Uhrhammer, a UC Berkeley seismologist.

Two of the quakes, a magnitude 5.7 that hit about 3:30 p.m. Friday and a 6.9 temblor almost exactly 24 hours later, were centered on the Gorda Plate under the Pacific Ocean. The epicenter was roughly 65 miles west of Crescent City, a Del Norte County community about 20 miles south of the Oregon border. A six-inch tsunami wave followed the final quake but was too small to cause any damage--or even be noticed--when it reached shore at 4 p.m. Saturday at Crescent City.

The third earthquake, which Uhrhammer said may have been a “coincidence,” hit north of Shelter Cove, a small Humboldt County seaside community, roughly 115 miles south of Crescent City.

The ocean-covered site of the quakes is “the most active region in the vicinity of California, and apparently breaking up,” Uhrhammer said. The Gorda Plate stretches about 500 miles from Cape Mendocino in California north to Washington and is being pushed under the North American continent.

The Gorda Plate has accounted for 23--or about one-fourth--of all the magnitude 6 or greater earthquakes that have hit California since 1910, Uhrhammer said.

Saturday’s 6.9 quake was the largest on the plate since November, 1980, Uhrhammer said.

The timing of the Shelter Cove quake between the two others most likely was “pure chance,” Uhrhammer said. Shelter Cove lies in a junction of three major geological features, including the northern end of the San Andreas Fault.

Residents in Oregon and California’s Central Valley more than 200 miles from the epicenters reported that windows rattled and buildings shook.

The Shelter Cove quake struck about 12:30 p.m. Saturday and caused the most damage, but no reported injuries.