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BENICIA – The San
Francisco earthquake of 1906, a
remarkably powerful event in the
magnitude 8 range that occurred 96 years
ago tomorrow, caused unimaginable damage
in the Bay Area. Thanks to engineering
and scientific advances, the
significantly smaller Loma Prieta
earthquake of 1989 did much less damage.
Still, the magnitude 6.9 temblor did
cause part of the Bay Bridge to
collapse.
Since then, the state has not only been
retrofitting bridges to better withstand
earthquakes, but placing seismic
monitors on bridges and other critical
structures to verify the success of its
structural retrofitting and to learn how
to build even more earthquake-resistant
structures in the future. It also has
promoted public awareness by making
April Earthquake Preparedness Month.
Instrumentation work on the
Benicia-Martinez Bridge, built in 1962
and traveled by about 97,000 cars a day,
is essentially completed. The California
Department of Conservation has installed
81 sensors on the bridge to back up the
structural retrofitting work being
completed by Caltrans. The sensors,
called accelerometers, are part of a
Caltrans-funded project by DOC's Strong
Motion Instrumentation Program to
instrument all toll bridges in
California.
To the east of an existing railroad
bridge a new span that eventually will
carry the northbound traffic from
Martinez to Benicia is under
construction. When it is completed in
about three years, SMIP will place about
100 instruments on the bridge.
There will be about 65 sensors on the
I-80 eastbound span of the Carquinez
Bridge when work there is wrapped up in
the coming months. The westbound span of
the Carquinez Bridge was built in 1927,
31 years before the eastbound span, and
will not be retrofitted. Instead,
Caltrans is replacing it with a
suspension bridge, the first to be built
in California since the 1960s. The
Strong Motion Instrumentation Program
will place 108 accelerometers on the new
bridge when it is completed in a couple
of years. Currently, about 113,000 cars
cross the Carquinez spans each day.
The seismic instrumentation of the
Benicia-Martinez and Carquinez bridges
has taken place as Caltrans'
retrofitting work has progressed over
the last five years. Instrumentation is
complete on the San Mateo and Golden
Gate bridges, and is ongoing on the
Richmond-San Rafael Bridge and the Bay
Bridge, among others.
"The state's engineers are constantly
learning more about how to better design
for earthquake resistance, and the
state-of-the-art instruments that are
being placed on these bridges will go a
long way toward protecting people and
structures in the future," Department of
Conservation Director Darryl Young said.
While there are several major faults in
the greater Bay Area, including the
northern extension of the Hayward fault,
the closest to both the Benicia-Martinez
and Carquinez bridges is the Concord
Fault, approximately 1.5 and 6.5 miles
to the east, respectively. A magnitude
5.4 earthquake was centered between
Walnut Creek and Concord on or near that
fault on October 23, 1955.
The Strong Motion Instrumentation
Program 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.
Beginning this year, SMIP and CISN
became part of an effort by the Office
of Emergency Services to improve rapid
response in the event of a major
earthquake. SMIP will install 30 ground
monitors per year for the next five
years in addition to its work on bridges
and structures.
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 stations throughout the
state.
The most modern equipment, such as the
monitors installed on the
Benicia-Martinez and Carquinez bridges,
is designed to automatically dial up a
central computer when strong earth
motion (generally magnitude 4.0 or
greater) is recorded. Data gathered by
the sensors can be analyzed in the
minutes following a quake and used to
guide emergency response personnel to
the hardest-hit areas. Ultimately, the
data gathered about how bridges and
other structures react to the shaking is
applied to engineering principles that
are incorporated into new design codes
for better earthquake-resistant
construction.
In addition to studying and mapping
earthquakes and other geologic
phenomena, the Department of
Conservation maps and classifies areas
containing mineral deposits; ensures
reclamation of land used for mining;
regulates oil, gas and geothermal wells;
administers agricultural and open-space
land conservation programs; and promotes
beverage container recycling.
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