The U.S. Geological Survey has downgraded tonight'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 Berkeley with a depth of 6 miles, the USGS said.
BART officials said riders can expect 15-minute delays system-wide tonight.
The transit agency stopped all trains when the quake occurred and began track inspection. No injuries or damage was reported, according to BART spokesman Jim Allison.
The temblor created a large jolt in San Francisco, including the Civic Center area, Nob Hill, Richmond District, and Haight-Ashbury neighborhood, and was felt throughout the region including Danville, Santa Cruz, Palo Alto, Oakland and Berkeley.
A 4.0-magnitude earthquake struck the area at 2:41 p.m. today. That quake was centered about two miles east-southeast of Berkeley, and had a depth of 6.1 miles, according to the USGS.
After today'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.
He said the temblor was of the typical "strike-slip" variety, in which two sides of the fault slide horizontally, he said.
Today'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. today.
San Francisco Mayor Ed Lee today urged all Bay Area residents to visit www.72hours.org for emergency planning resources and tips.