Parkfield, Focus of Earthquake Watchers, Stirred by 4.0 Temblor

The site of one of scientists’ most important earthquake prediction experiments, in Parkfield, Calif., was rattled by a magnitude 4.0 temblor Friday.

No damage or injuries were reported from the moderate quake, which struck at 7:26 a.m. It was centered 6.6 miles deep and five miles northwest of the sparsely populated hamlet in southeastern Monterey County.

No aftershocks were detected.

The quake was on the Middle Mountain segment of the San Andreas fault, where magnitude 6 temblors have erupted roughly every 22 years, the last time in 1966, said Pat Jorgenson, a spokeswoman for the U.S. Geological Survey in Menlo Park, Calif.

Such regularity has made the area halfway between Los Angeles and San Francisco a premier site for learning how to make long-term and short-term earthquake predictions.

“It’s just the perfect earthquake laboratory,” Jorgenson said.

It was impossible to say, however, if the moderate temblor might indicate that a long-overdue larger earthquake is on the way.

“There’s no way of knowing whether this could be a foreshock to this event,” Jorgenson said.

“There’s always continuing activity,” she added. “Any parts of the fault that are active are always subject to having movement.”

She noted that there was a magnitude 4.7 quake in the region in 1992, a magnitude 4.6 in 1993 and a magnitude 5 a year later.

Last fall there was a series of small earthquakes, including two of magnitude 3.5.

“Everybody thought, maybe this is it--and nothing,” Jorgenson said.

The latest temblor did prompt authorities to raise the alert level of an early warning system from its usual Level C to Level B. That was a “heads up” alert to scientists from around the world to monitor the 100 instruments they have placed at the site, Jorgenson said.

“It’s an ongoing experiment,” she said of prediction efforts.

Quake scientists had said it was 95% likely that a magnitude 6 quake would occur near Parkfield by 1993. No such quake has struck yet.