SAN FRANCISCO - Two earthquakes created large jolts in San Francisco Bay Area Thursday afternoon and night after millions of Californians ducked for cover in an earthquake drill earlier in the day.

According to the US Geological Survey (USGS), a quake with a magnitude of 3.8 hit at 8:16 pm local time (0316 GMT), about 1.6 km east of Berkeley, a city on the east shore of the San Francisco Bay in Northern California.

The USGS said the quake was the aftershock to another 4.0-magnitude quake, which occurred at 2:41 pm local time (2141 GMT) and centered some 3.2 km southeast of Berkeley.

The two quakes were widely felt in the San Francisco Bay Area. A local resident, who was some 48 km from Berkeley when the quakes occurred, told Xinhua that she felt a sudden jolt and a big shake for one of the quakes.

There have been no reports of damage or injuries from the two quakes.

Keith Knudsen, deputy director of the USGS Earthquake Science Center in Menlo Park, California, told local KTVU that Thursday's quake was a standard Hayward Fault Line quake, which is a geological fault zone on the east side of San Francisco Bay capable of generating significantly destructive earthquakes.

He said there is a roughly 5 percent chance that the afternoon's quake could be a foreshock to a larger seismic event.

At 10:20 am, more than eight million Californians participated in an earthquake drill called "the Great California Shakeout" to prepare for the Big One, a casual term used to describe the megathrust earthquake or other natural disaster likely striking the West Coast of the United States.

According to local TV reports, California seismologists say there is a 99 percent chance that a major earthquake will rattle California in the next 30 years.

The Thursday quakes hit almost 22 years after the Loma Prieta earthquake. The latter, which struck the Bay Area on October 17, 1989, measured a magnitude of 6.9 on the Richter scale, killing 63 people and injuring nearly 4,000.

San Francisco mayor Ed Lee released a statement about two hours after the first quake, calling it "another reminder that we have to be prepared."