Not Much Is Shaking on State’s Fault Lines

As of late Friday afternoon, it had been more than a week since California had an earthquake measuring magnitude 3.0 or larger, and that is unusual.

David Oppenheimer, who monitors the state’s earthquakes for the U.S. Geological Survey, said that on the average over the last three years California has had 327 quakes of magnitude 3.0 and larger per year, a little less than one a day, or more than six a week.

“So, it is correct to say that [the] last week was unusually low,” Oppenheimer said.

The Geological Survey’s Web site, which lists all quakes occurring in the state over the previous seven days, showed that the strongest temblor during the period was a 2.8 at 4:04 p.m. Wednesday. It was centered in the San Joaquin Valley, 16 miles east of Coalinga.

The last quake of any size in the state, a 4.4 jolt just off the Humboldt County coast, occurred at 1:52 p.m. July 4.

As of 5 p.m. Friday, 237 quakes had occurred in California in the previous seven days, most of them too small to be felt. That figure, comprising quakes of magnitudes less than 3.0, is also lower than normal.

So what does all this mean? According to scientists charged with keeping such records, not much.

“Earthquakes are a random process. The numbers rise and fall,” said Bill Ellsworth, head of earthquake research and monitoring for the Geological Survey in the Western states.

But, in general, more quakes occur during aftershock sequences that follow big ones, he said.

Aftershocks continue from the 1992 Landers and 1999 Hector Mine earthquakes, both of which were larger than magnitude 7.0.

But even those aftershocks have grown far less numerous than they were just after the earthquakes.

“We are pretty much at the background level for quakes, [exclusive of] aftershock sequences,” Caltech seismologist Kate Hutton said Friday.

But aftershocks frequently occur in clusters, and it is not all that rare that a week will go by without any sizable earthquake, she said.

Paul Reasenberg, a geophysicist with the Geological Survey in Menlo Park, dismissed suggestions that periods of comparative earthquake quiescence indicate that a large quake will occur in the near future.

“I did some research into this several years ago,” he said. “There are times when big earthquakes follow periods of quiescence, but there are also times when they follow active periods. There’s no pattern we can validate.

“There just are periods when quake activity is abnormally low,” he said.

“The fact that more or fewer quakes are taking place doesn’t seem to predict the future.”