A magnitude 4.5 earthquake was felt widely in the San Francisco Bay Area at 10:33 p.m. on Monday, with the epicenter in the Walnut Creek and Pleasant Hill areas.
Moderate shaking was felt in the Pleasant Hill, Walnut Creek and Concord areas of an intensity that can overturn unsecured objects, such as books and picture frames, and is capable of breaking dishes or windows. Weak shaking was felt in San Francisco, Oakland and San Jose, according to the U.S. Geological Survey.
Close to the epicenter in Walnut Creek, there was strong side-to-side shaking, strong enough to knock CDs off a shelf.
The USGS said there is a 2% chance of one or more aftershocks larger than magnitude 4.5 over the next week, and between zero and four aftershocks of magnitude 3 and higher. “The number of aftershocks will drop off over time, but a large aftershock can increase the numbers again, temporarily,” the USGS said in its aftershock forecast.
Based on the observed shaking intensity recorded by the USGS, scientists said they didn’t expect there to be major structural damage.
The earthquake had a preliminary depth of about 9 miles underneath the surface, fairly deep for this part of the world, Keith Knudsen, USGS geologist and deputy director of the agency’s Earthquake Science Center, said in an interview. Such a deep quake made it felt widely over a broad area, but lessened the shaking intensity felt at the surface than if it were shallow, scientists said.
The earthquake was not directly on top of any of the main Bay Area earthquake faults. The epicenter was about three miles west of the Concord fault, and farther than that off the northern end of the northern Calaveras fault, Knudsen said.
The epicenter is just northwest of Mt. Diablo, one of the Bay Area’s tallest peaks. The Mt. Diablo area is also a seismically active zone, “a region of uplift, folding and thrusting,” said David Schwartz, USGS scientist emeritus.