The Bay Area was rattled Tuesday by a magnitude 4.3 earthquake and a magnitude 3.5 aftershock that hit in the East Bay near Livermore, Calif., on Tuesday.
The magnitude 4.3 quake at 1:11 p.m. was felt over a wide area of the East Bay and as far away as San Francisco and San Jose, the USGS said. The epicenter was about 11 miles northeast of downtown Livermore, on the western edge of the Los Vaqueros Reservoir, which holds water from the Sacramento-San Joaquin Delta and is an important water source in the late summer and early fall when high levels of salt creep into the delta. The Contra Costa Water District said there were no reports of damage at the reservoir or dam.
The earthquakes hit near the northern side of Greenville fault, said David Schwartz, a U.S. Geological Survey earthquake geologist emeritus. The Greenville fault is the easternmost of the Bay Area’s seven significant earthquake fault zones, all roughly parallel to each other. Besides the better known San Andreas and Hayward, the region’s most important faults include the Calaveras, Concord-Green Valley, Rodgers Creek and San Gregorio.
Just north of the Greenville fault is the Mount Diablo thrust fault, and to the north of that is the Concord-Green Valley fault. The Greenville fault ends in the area around Henry W. Coe State Park, on the mountainous border between Santa Clara and Stanislaus counties.
The Greenville fault is capable of producing a magnitude 7 quake. The Greenville fault area ruptured with quakes of magnitudes 5.8 and 5.1 on Jan. 24, 1980 that were, respectively, 12 and 9 miles north of Livermore. Two days later, a magnitude 5.4 quake hit with an epicenter only 6 miles from downtown Livermore. Damage in 1980 was reported at Lawrence Livermore National Laboratory and a mobile home park.
Tuesday’s quake was calculated to have caused weak, light or moderate shaking — or levels 3 through 5 on the Modified Mercalli Intensity Scale — in Livermore, a city of 90,000 and home to the Lawrence Livermore National Laboratory, and Pleasanton, a city of 83,000. Police in both cities said there were no reports of damage.
“I don’t think it’s really anything out of the ordinary for this area,” Schwartz said. “It’s a small indication that people should think about preparations” for a larger quake.
Shaking in nearby suburban communities of Dublin, San Ramon, Danville, Walnut Creek, Concord, Brentwood, Fremont and Hayward were weak or light, according to USGS instruments. People in San Ramon said the quake caused some items in an office to rattle and shake.
The mainshock at 1:11 p.m. was followed by a magnitude 3.5 magnitude quake at 1:24 p.m. in the same area.
The quakes were about 30 miles east of Oakland, 30 miles west of Stockton, and 35 miles away from San Jose and San Francisco.
In the last 10 days, there have been two earthquakes of magnitude 3.0 or greater centered nearby.
An average of 25 earthquakes with magnitudes between 4.0 and 5.0 occur per year in California and Nevada, according to a recent three-year data sample.
Did you feel this earthquake? Consider reporting what you felt to the USGS.
Find out what to do before, and during, an earthquake near you by reading our five-step guide to coping with natural disasters.