|
News
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?
|
