Scientists increased their watch along the San Andreas fault near Parkfield in Central California Monday after a 3.5 earthquake occurred in a particularly sensitive zone where a magnitude 6 quake has been predicted since 1985.
Bill Ellsworth of the U.S. Geological Survey said the quake at 4:28 p.m. Sunday was centered on the San Andreas fault under Middle Mountain, seven miles northwest of Parkfield. The predicted strong temblor under the mountain is at least four years overdue.
Ellsworth said the watch would last three days.
In other seismic activity, four moderate earthquakes, the strongest of them magnitude 5.1, rocked an area in the Pacific about 67 miles off the Humboldt County coast. The quake sequence began early Saturday, and continued Monday with jolts of 4.1 and 3.7. There was no damage on land.