New Quake Alert Issued for Parkfield

After last week’s false alarm, Parkfield is rumbling again, and scientists say there is a renewed--but smaller--chance that a long-predicted magnitude 6 earthquake will strike soon along a much-monitored portion of the San Andreas Fault.

The U.S. Geological Survey issued a Level B alert early Monday, after six small earthquakes rattled Middle Mountain, just northwest of the Central California ranching town. The largest, a magnitude 3.9, struck at 11:27 p.m. Sunday.

The alert “officially means a 10% to 37% chance (of the larger quake) within 72 hours,” said USGS spokeswoman Pat Jorgenson. “But we think it’s more like a 10% to 12% chance.”

Last week’s Level A alert was issued after a magnitude 4.7 quake and seven aftershocks persuaded seismologists that there was about a 40% chance of a larger temblor. The earthquake did not come, but a swarm of reporters did.

Because moderate earthquakes appear to hit Parkfield every 20 to 25 years, the USGS forecast in 1985 that a magnitude 6 would occur by January, 1993. Researchers installed instruments to measure everything from electrical resistance in rocks to the levels of underground water tables in an attempt to learn what happens before, during and after an earthquake.

The recent activity has been the most recorded along the Parkfield segment of the fault since the prediction. The USGS has spent about $19 million on the project.