The 2:41 a.m. earthquake on the border of Fremont and Union City occurred on the Hayward Fault at a depth of 5 miles. The epicenter was at a spot just north of the intersection of Niles Canyon Road and Mission Boulevard.
While damage from the quake was minimal, scientists warn that a much larger one is expected on the Hayward Fault, which extends from San Pablo Bay in the north to Fremont in the south and passes through heavily populated areas including Berkeley, Oakland, Hayward and Fremont.
The last big earthquake on the fault, estimated to have a 6.8-magnitude, occurred in 1868, according to the USGS.
It killed about 30 people and caused extensive property damage in the Bay Area, particularly in the city of Hayward, from which the fault derives its name. Until the larger 1906 earthquake, it was widely referred to as the "Great San Francisco Earthquake."
"The population is now 100 times bigger in the East Bay, so we have many more people that will be impacted," said Tom Brocher, a research geophysicist with the USGS.
"We keep a close eye on the Hayward Fault because it does sit in the heart of the Bay Area and when we do get a big earthquake on it, it's going to have a big impact on the entire Bay Area," Brocher said.
While a 2008 report put the probability of a 6.7-magnitude or larger earthquake on the Hayward-Rodgers Creek Fault system over the next 30 years at 31 percent, Brocher said the reality is a major quake is expected on the fault "any day now."
"The past five major earthquakes 1/8 on the fault 3/8 have been about 140 years apart, and now we're 147 years from that 1868 earthquake, so we definitely feel that could happen any time," Brocher said.
Brocher urged residents to take steps to prepare for a major earthquake.
The USGS shake map shows residents in the areas close to Fremont and Union City experienced light shaking in this morning's event, while weaker shaking might have been felt in areas as far south as Santa Cruz, up the Peninsula and as far east as Livermore.
Residents throughout the Bay Area reported feeling the quake, with responses concentrated in the East and South Bay, according to the USGS.
Brocher said this morning's 4.0 earthquake was not likely to have much of an impact one way or the other on the likelihood of a major earthquake occurring on the same fault.
"It was on the Hayward fault, which is large fault that we expect to rupture in the next big earthquake. Since then, there have been 14 aftershocks, the largest which was a magnitude 2.7," explained AnneMarie Baltay with the U.S. Geological Survey.
@abc7newsBayArea and peeps..don't forget to report that you felt it on the USGS site. Short survey.— mary b (@zukeferd) July 21, 2015
Fremont police say they received numerous calls from scared and nervous residents.
There were some delays on BART. Around 4:30 a.m., BART reported 30-minute delays as the tracks were inspections. Just before 5 a.m., all of the tracks had been inspected and trains returned to normal speed.
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@abc7newsBayArea My Farmers Ins Office is on Niles Blvd. EQ knocked security cameras out of whack! The shaking knocked stuff off the counter— Jennifers Insurance (@jennifersins) July 21, 2015
@abc7newsBayArea felt it in south San Jose. My dog woke me up and then I felt a quick jolt. My dog is probably the best warning system yet!— Lorral_thebluecoat (@LorralWhaley) July 21, 2015
@abc7newsBayArea We felt it here in Downtown San Jose. One small shake followed by a slightly larger one.— rλan delucchi (@ryanonsrc) July 21, 2015
ABC7 News reporter Janet O contributed to this story.