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Hayward Fault's 'tectonic time bomb'
by Julie Sevrens Lyons
San Jose Mercury News, October 18, 2007

Studying layers of soil in a trench they dug near the Fremont BART station, geologists recently made a startling discovery: The Hayward Fault has had a big earthquake roughly every 140 years, on average, since 1315. And this Sunday marks year 139.

Calling the fault a "tectonic time bomb," scientists Wednesday urged Bay Area residents to put together an earthquake plan, stockpile supplies and consider having their older, two-story homes checked for structural weaknesses.

Researchers, gathered near the fault Wednesday, believe that it could produce quakes as large as a magnitude 7.0, even 7.3.

"It wouldn't be a surprise to any seismologist if it had a big earthquake tomorrow," said Tom Brocher, coordinator of Northern California earthquake hazards investigations for the U.S. Geological Survey in Menlo Park. "This is a real threat."

The announcement comes just days before the 139th anniversary of the 1868 Hayward earthquake. Known as the "Great San Francisco Earthquake" until the devastating 1906 temblor came along, the quake remains the nation's 12th most deadly earthquake despite the East Bay's sparse population at the time.

Scientists still don't know what sets the fault off, and have no way of predicting exactly when the next big quake will occur. But by studying layers of soil and dating them with radio-carbon methods, they've determined a loose pattern.

Sizable earthquakes occurred along the fault in Fremont in roughly 1315, 1470, 1630, 1725 and 1868. The briefest interval between quakes during that time was just 95 years, and the longest 160. The five earthquakes before those averaged about 170 years apart, telling scientists that recurrence of quakes on that fault line are surprisingly very regular.

"If we could say, 'There's going to be an earthquake in three days at 8:15 in the morning,' that would be helpful for a lot of folks, but we can't do that," Brocher said. Soil samples are the next best thing to a crystal ball that scientists have. "This fault has had plenty of time to prepare for the next earthquake," Brocher said. "It could go whenever it feels like it."  And leave a wide path of destruction.

With the number of densely built cities now straddling the dangerous fault, analysts at the Association of Bay Area Governments anticipate there could be 1,100 road closures and 94,000 destroyed homes and apartment units if a magnitude-6.7 quake struck.

Scientists have estimated that there is a 62 percent chance of a major earthquake striking the Bay Area by the year 2031. The Hayward Fault, they said, is the most likely culprit.

"It's not a question of if, it's a question of when," said Jim DeMersman, executive director of the Hayward Area Historical Society and Museum.

Scientists chose the museum as the spot for Wednesday's news conference for a reason: It's just 100 feet from the fault line. The active fault has been slowly creeping along over the years, cracking streets and sidewalks and leaving Hayward's picturesque old city hall uninhabitable. Since the 1970s, the landmark structure just off Main Street has been shuttered, with deep cracks slicing through its walls, floors and ceilings.

The last time it moved a lot was early in the morning on Oct. 21, 1868. Homes, church steeples, water towers and courthouses toppled from San Jose to Suisun City. In all, 30 people died, including five in San Francisco. At the time, the East Bay was largely made up of small towns, farms and ranches, and Alameda County's entire population was just 24,000.

By using historical records, old atlases and even cemetery markers to meticulously map the extent of the damage, Jack Boatwright, a geophysicist at the USGS, studied that quake in great detail. He created a preliminary map showing the intensity of the shaking felt between Santa Rosa and Gilroy. He was surprised to learn that the 1868 quake was twice as big as the standing model scientists had created for it.

Measuring the shakes

On a scientific range of 1 to 10-plus known as the Mercalli scale, Boatwright determined that Fremont, San Leandro and Hayward all ended up with 8s and 9s - violent shaking that caused heavy damage. Most of San Jose was a 7.5, with severe shaking and moderate damage. Even outlying areas like Livermore and Mountain View experienced more intense shaking than they did during 1989's Loma Prieta earthquake.

Comment from Friends of Coyote Hills

Given that the 520 acres in front of Coyote Hills is at high risk of liquefaction in a strong earthquake (Note: Fremont experienced a 5.7 magnitude earthquake on October 30, 2007), is in a 100-year floodplain, and is in the neighborhood with already the highest density in Fremont, should the Fremont city council risk public safety by approving an additional 800 housing units as proposed in the Patterson Ranch development?

Developers would like to diffuse the problem by saying they would do whatever they can to minimize the dangers. What technology is a match for the strong forces of nature?

 

 

Scientists still aren't certain what that quake's magnitude was or where it originated - possibly near Hayward - because modern instruments weren't around then to measure earthquakes.

What makes public safety officials nervous is the fact that there are now 60 times more people living in Alameda County than there were in 1868, according to Census records. The Hayward Fault snakes directly underneath major city infrastructure, even crossing smack through the middle of University of California-Berkeley's Memorial Stadium.

In 2003, a team of scientists put together their best predictions for a "Big One" on the Hayward Fault, figuring it could range from a Richter-scale magnitude 6.4 to a 6.9. The average estimate, 6.7, has been extensively used in disaster planning.

"But even a 6.5 would be very damaging," Brocher said. "That's a big earthquake right in our back yard."

The good news is that scientists think a large quake would not bring a high number of casualties, thanks to improved building codes and seismic restructuring of freeway overpasses and gas pipelines. Housing - or lack of it - is expected to be the biggest problem after a quake, with up to half a million people displaced.

The American Red Cross has determined that a whopping 83 percent of Bay Area residents are unprepared for the Big One and is encouraging people to making seismic improvements to their older homes.

Jean Quan, an Oakland councilwoman, emphasized that there is no time like the present to do so.

"We who live in earthquake country," she said, "live on borrowed time."

Click on the link below to see a simulation of 7.5 magnitude earthquake.

Trailer simulates earthquake (Then click on the small square on lower right to start video.)

 

 

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