The U.S. Geological Survey has downgraded Thursday night's earthquake from its original preliminary magnitude of 4.2, to 3.9 a short time later, and now experts report it was a 3.8-magnitude tremor.
The quake struck at 8:16 p.m. and was centered about one mile east of Berkley with a depth of 6 miles, the USGS said. The temblor created a large jolt in San Francisco and was felt throughout the region including Palo Alto, Danville, Santa Cruz, Oakland and Berkeley.
A 4.0-magnitude earthquake also struck the area at 2:41 p.m. Thursday. That quake was centered about two miles east-southeast of Berkeley and had a depth of 6.1 miles, according to the USGS. After Thursday's first quake, Keith Knudsen, deputy director of the USGS Earthquake Science Center in Menlo Park, said it was a standard Hayward Fault Line quake.
Knudsen said the temblor was of the typical "strike-slip" variety, in which two sides of the fault slide horizontally. Thursday's earthquakes occurred on the same day as the Great California ShakeOut, a statewide drill in which millions of Californians practiced ducking and covering at 10:20 a.m. Thursday morning.
Visit www.72hours.org for emergency planning resources and tips.