A magnitude-5.1 earthquake struck about 19.8 kilometers (12 miles) southeast of San Jose on Tuesday. The Calaveras Fault is the likely source of this temblor.
By Meghomita Das, Palomar Fellow (@meghomita)
Citation: Das, M., 2022, Magnitude-5.1 earthquake strikes near San Jose, California, Temblor, http://doi.org/10.32858/temblor.278
A magnitude-5.1 earthquake rattled the San Francisco Bay Area on Tuesday. The epicenter was located within the Joseph D. Grant County Park, near Mount Hamilton in Santa Clara County. The earthquake, which likely occurred on the Calaveras Fault, struck at a depth of 8.4 kilometers (five miles) according to the United States Geological Survey (USGS). The earthquake struck on a strike slip fault, according to USGS data.
As of the time of this reporting, no incidents of damage or fatalities have been reported. As of 4:25 p.m. (PDT), more than 22,000 responses had been submitted in the USGS “Did You Feel It” system. Light shaking was felt throughout the Bay Area, including San Francisco, Salinas, Merced, Fresno and San Rafael. Weak shaking was felt as far as Stockton. The area immediately surrounding the epicenter experienced moderate shaking. The shallow depth of the earthquake, and its proximity to a major metro region likely contributed to the number of people that experienced shaking.
The ShakeAlert system detects an earthquake in the fractions of a second after it begins and alerts those that are likely to experience shaking above a certain intensity. A ShakeAlert earthquake early warning was issued for Tuesday’s quake. It was delivered via app and phone throughout the Bay Area. For some, the alert arrived just before shaking.
Good morning Bay Area, CA. Did you feel the magnitude 5.1 quake about 9 miles east of Seven Trees at 11:42 am? The #ShakeAlert system was activated. See: https://t.co/sab7S4wOJi@CAGeoSurvey @Cal_OES @CalConservation pic.twitter.com/nwG168f825
— USGS ShakeAlert (@USGS_ShakeAlert) October 25, 2022
Despite the temblor’s moderate size, some in the Bay Area felt no shaking, including Anuj Shetty, a student on Stanford’s campus. Others in his class felt shaking, he says.
Active faults bound the metro
The magnitude-5.1 struck along the Calaveras Fault, said Annemarie Baltay, a geophysicist at the USGS, in a recorded statement. The USGS is responding to the event and will continue to analyze data and aftershocks as they occur.
— USGS Earthquakes (@USGS_Quakes) October 25, 2022
The Calaveras Fault runs along the east side of the San Jose metro region. It is a zone of active, right-lateral, strike-slip faults, which compose a major branch of the San Andreas fault system. The San Andreas runs roughly parallel and to the west of the Calaveras Fault, up the San Francisco Peninsula. The Calaveras, together with the San Andreas, Concord, San Gregorio and the Hayward-Rodgers Creek faults, accommodate the northward motion of the Pacific tectonic plate past the North American Plate.
The Calaveras has a high probability of producing a surface-rupturing earthquake in the next 30 years, says Kim Blisniuk, an earthquake geologist at San Jose State University. The most notable recent earthquake on the Calaveras fault was the magnitude-6.2 1984 Morgan Hill earthquake, which caused severe damage to buildings in the area. The 1984 quake struck at approximately the same epicenter as today’s event. This location sits at the intersection between the Calaveras and the Hayward Fault, says Blisniuk. Some parts of the Hayward Fault tend to accommodate motion through tectonic creep, whereas the Calaveras ruptures in earthquakes, she says. The Hayward Fault does have a history of large earthquakes and studies suggest that it could again produce a large rupture. Some consider the two faults to be continuous and capable of generating a magnitude-7.5 earthquake. The Calaveras has also hosted more recent earthquakes, such as the magnitude-5.5 Alum Rock earthquake in 2007.
The Bay Area has a long history of moderate to large earthquakes due to the San Andreas fault and its branching fault systems. Last month, a magnitude-4.4 earthquake struck near Santa Rosa, California, (north of San Jose) on the Rodgers-Creek Fault. That widely felt temblor occurred 5.6 kilometers (three miles) below the surface and was followed less than a minute later by a magnitude-4.3 aftershock.
A magnitude 3.1 aftershock followed Tuesday’s mainshock near San Jose, and more aftershocks may follow. “As more time passes, the likelihood of a [larger] mainshock decreases, but small to moderate aftershocks are still likely in this area for the next couple of days,” adds Blisniuk.
In the event of a future earthquake, experts agree that the best action is to drop, cover and hold on.
Meghomita Das is Temblor’s Palomar Fellow. She is a Ph.D. candidate at McGill University in Montreal, Canada, where she studies the signals of ancient earthquakes and slow slip events (www.meghomita.com). Palomar Holdings is sponsoring a science writing fellow to cover important earthquake news across the U.S.
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