USGS: Piedmont-based earthquake was shallow
OAKLAND -- A magnitude-4.0 temblor that rattled nerves in the East Bay on Monday morning was a very shallow but routine geologic shrug in an area notorious for seismic risk -- and yet another reminder of the threat lurking from a fault that's the Bay Area's most overdue for a major quake.
Monday's quake was the second magnitude-4.0 quake on the Hayward Fault in less than a month -- and comes almost a year after the magnitude-6.0 Napa temblor that was the region's biggest in 25 years -- but there is no indication it is a precursor of the Big One. While there was a flurry of aftershocks following Monday's 6:49 a.m. earthquake, there is only a 10 percent chance that it will be followed by a larger earthquake in the next week, Knudsen said.
Residents throughout Northern California took notice of the hiccup on the Hayward Fault near and far. "I was working on my computer and saw my monitor tilt and wobble," said Barbara Kossy of Moss Beach.
The earthquake occurred almost one year after the Aug. 24 South Napa temblor, which measured 6.0-magnitude on the West Napa Fault, noted USGS research geophysicist Tom Brocher. That event was the largest in the San Francisco Bay Area since the 1989 Loma Prieta earthquake, causing about $500 million in damage.
"Today is another reminder that we live in earthquake country," said Brocher. "These events remind us that we should be prepared -- because we can't predict earthquakes and don't know exactly when they'll happen."
![The "Faulted Floor" in the Fremont Community Center. Structural damage to the 1962 building was caused by the Hayward Fault.](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_uBMLYQxGwM3i_c12nG123o9NgiFCtkYa7QqmLfexQA-Ybhv0esWkFZFWWGGDDjgjvmomHfgif-LtzNeqLgug4DbW0kQWrzUTNeoedSES-3fO0cBv-IgUhvQA1krEkdYClnqfFSPSHyzWDiDMOLJGJ0th2BVPxykrH4ACkwJ6gVpEQ=s0-d "The "Faulted Floor" in the Fremont Community Center. Structural damage to the 1962 building was caused by the Hayward Fault.")
The "Faulted Floor" in the Fremont Community Center. Structural damage to the 1962 building was caused by the Hayward Fault. (Mercury News archives)
On July 21, the Hayward Fault produced another magnitude-4.0 earthquake at 2:41 a.m. on the border of Fremont and Union City. That temblor was 5 miles deep, located just north of the intersection of Niles Canyon Road and Mission Boulevard.
The largest earthquake on the fault, estimated to have a magnitude of 6.8, occurred in 1868, according to the USGS. It killed about 30 people and caused major property damage.
But the population of the East Bay is now about 100 times larger -- so many more people will be affected by the next major quake.
"Think about your home, your work and where your kids go to school," USGS' Knudsen said. "If there is an earthquake, are they likely to be safe? If you don't know, ask the experts."
For Northern Californians, the Hayward Fault is the most likely source of a dangerous quake, with a 31 percent chance in the next 30 years.
The Hayward Fault, part of the larger San Andreas Fault system, runs from San Pablo Bay in the north to Fremont in the south -- passing through the heart of Berkeley, Oakland, Hayward, Fremont and other East Bay cities.
The region is a place of ongoing and often imperceptible earthen creeping, as evidenced by routinely broken sidewalks in Hayward and Fremont. If you stand in the Bay Area and look toward the Sierra, over time you'd see the mountains move to the right.
The Hayward Fault creeps about one-fifth of an inch a year.
"If you live in the East Bay, you get used to the little earth shakes that occur from time to time," said Lafayette-based photographer Charlotte Gibb. "These do not bother me much. As a native Californian, I just accept them as part of living here."
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