Earthquake Is Not Related to Desert Temblor

Saturday’s magnitude 6.9 earthquake on the North Coast near Eureka was unrelated to last week’s weaker temblor that struck Southern California, according to scientists with the U.S. Geological Survey.

“This can be called random coincidence,” said Lucile M. Jones, a seismologist at the survey’s Pasadena office.

Like Wednesday’s magnitude-6.1 quake, the shaker centered south of Eureka occurred close to, but not on, the San Andreas Fault, the state’s largest and most famous fault.

“From what we’ve observed of behavior along the San Andreas, the various segments tend to behave independently of each other,” said Bruce Presgrave, a geophysicist at the survey’s National Earthquake Information Center in Golden, Colo.

The coastal region around Eureka is one of the most seismically active in the state and may be capable of producing stronger quakes than any of the faults that crisscross Southern California.

Last year, scientists said they had uncovered evidence of an earthquake of magnitude 7.6 to magnitude 8.4 that struck the vicinity about 300 years ago, and they have warned that a magnitude 9.5 temblor is possible off the coast of Oregon or Washington.

There have been at least 10 earthquakes over magnitude 5.0 near Eureka since 1975. Saturday’s quake was the fourth in the area at magnitude 6.5 or higher since 1980.

Presgrave said that Saturday’s event was centered northeast of the Cape Mendocino area, where the San Andreas turns west and goes out to sea. He called the area the “triple junction region” where three great tectonic plates--the Pacific, the North American and the Gorda--intersect.

Tectonic plates are large land masses that float on the molten core of the Earth. Their meeting grounds are often associated with volcanic and earthquake activity. Just to the north of Eureka begins the Cascadia subduction zone, where the Gorda plate is being pushed under the North American plate along the coast of the Pacific Northwest.

Indian tribes in the area have passed along stories of earthquakes in the distant past that produced huge drops in the elevation of coastal areas. Scientists have found evidence of great ancient tree trunks under the present ocean surface.

Last summer, three powerful earthquakes struck off the Oregon coast northwest of Crescent City, Calif. Robert Uhrhammer, a UC Berkeley seismologist, warned then that the area was “the most active (seismic) region in the vicinity of California.”

But Presgrave said Saturday it would take weeks or months for scientists to pinpoint which geological formations were involved in the latest quake. He said that because of its closeness to the northern land end of the San Andreas Fault, he suspected that movement of the Pacific and North American tectonic plates, more than the Gorda, could be responsible.

Presgrave also cautioned that the magnitude of Saturday’s temblor is likely to be revised as more seismological recording stations report. This process, too, can take months.

When the Loma Prieta earthquake occurred in the Bay Area on Oct. 17, 1989, the U.S. Geological Survey assigned it a preliminary 6.9 magnitude. Sometime later, this was revised to 7.1.

Each full point increase in magnitude represents a tenfold increase in quake strength. By this standard, Saturday’s quake south of Eureka was at least eight times more powerful than Wednesday night’s east of Desert Hot Springs.

California Quakes

Here is a list of California earthquakes since 1857 that were as strong as the quake that hit the north coast near Ferndale on Saturday:

Date Location Magnitude Lives Lost 1857 Ft. Tejon 7.9 1 1872 Owens Valley 7.8 27 1906 San Francisco 8.3 700 1923 Off north coast 7.2 0 1927 Lompoc 7.5 0 1940 Imperial Valley 7.1 7 1952 Tehachapi 7.7 14 1980 Off north coast 7.0 0 1989 Loma Prieta 7.1 67 1991 Off north coast 6.9 0 1992 North coast near Ferndale 6.9 *

* No deaths reported in immediate aftermath of the quake.