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SAN FRANCISCO – The
California Department of Conservation
has just completed installing a group of
sensitive instruments on the western
portion of the Bay Bridge … instruments
it hopes will never be used.
The 88 seismic
sensors – 79 on the bridge itself and
nine in a subsurface array at the foot
of the bridge -- provide engineers with
information about the ground shaking and
structural response in the event of a
major earthquake. That data can be used
to assess the immediate safety of the
bridge and to design more
quake-resistant structures in the
future. Also, combined with readings
from other sites in the Bay Area, the
data can help guide emergency response
personnel to the hardest-hit areas in
the region after a quake.
“Obviously, we would
prefer to collect no data from these
instruments,” DOC Director Darryl Young
said. “However, the best information
science can provide indicates that there
will be a major earthquake in our
lifetime – perhaps not as destructive as
the Great San Francisco Earthquake, but
something very significant.”
The 98th anniversary
of the magnitude 7.8 earthquake that
killed about 3,000 people and leveled
much of San Francisco is April 18.
The sensors are part
of a California Department of
Transportation-funded project by DOC’s
Strong Motion Instrumentation Program (SMIP)
to instrument all toll bridges in
California.
Northern California
hasn’t experienced a damaging earthquake
since the magnitude 6.9 Loma Prieta of
1989, which caused part of the Bay
Bridge, as well as the Cypress Structure
on the Nimitz Freeway, to collapse.
“Damaging earthquakes
don’t happen every day, so the public
tends to forget about the risk,” Young
said. “But earthquakes are an everyday
concern to the Department of
Conservation and the California
Geological Survey. The instrumentation
of the Bay Bridge is just one example of
the work being done to ensure public
safety.”
DOC’s California
Geological Survey is also doing
extensive Seismic Hazard Zone mapping
work in the Bay Area, identifying areas
more likely to experience liquefaction
(ground temporarily behaving like
quicksand) and landslides during major
earthquakes.
The instrumentation
of the Bay Bridge has taken place as
Caltrans' retrofitting work has
progressed over the last two years.
Instrumentation is complete on the San
Mateo, Benicia-Martinez, eastbound
Carquinez, and Golden Gate bridges, and
is ongoing on the Richmond-San Rafael
Bridge, the new Benicia bridge, and the
new east span of the Bay Bridge.
Part of DOC's
California Geological Survey, SMIP is
the largest program of its kind in the
world. SMIP was established following
the 1971 San Fernando earthquake and has
sensors at more than 900 locations
throughout the state. The SMIP
instruments on the Bay Bridge
automatically dial up a central computer
when strong earth motion -- generally
from a magnitude 4.0 or greater temblor
-- is recorded.
SMIP is a member of
the California Integrated Seismic
Network, along with the U.S. Geological
Survey, the California Institute of
Technology and the University of
California at Berkeley. SMIP installs
seismic monitors on high-rise buildings,
dams, hospitals and industrial
facilities around the state. The
instruments are also placed in open land
to measure the effects of earthquakes on
soils.
Data from SMIP and
other network stations produce a
ShakeMap of ground shaking right after
an earthquake. The ShakeMap identifies
areas of the greatest potential damage
and is used by the Office of Emergency
Services and other emergency response
agencies to immediately direct
resources.
“The data these
instruments produce gives a clear
picture of where to concentrate initial
response efforts, saving lives and
property damage,” acting State Geologist
Michael Reichle said. “In the bigger
picture, it improves the building codes
and practices that make California
better prepared than anywhere else in
the world for earthquakes.”
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