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SAN FRANCISCO -- It
has beauty and a brain. Inside beautiful
San Francisco City Hall, an electronic
brain with an army of sensors is ready
to respond to the next Bay Area
earthquake.
State officials today
demonstrated the new seismic monitoring
system at the historic structure as part
of an extensive earthquake retrofitting
project.
The California
Department of Conservation has installed
18 sensors. Divided among the four
levels and dome of City Hall, the
devices measure seismic waves shaking
the building. During an earthquake,
these "accelerometers" report to a
central computer the shaking that occurs
at several key points in the structure
from the foundation to the top of the
dome. The instruments provide valuable
data about earthquake shaking and the
building's response.
"We can't stop
earthquakes from happening, but we can
better prepare ourselves by improving
the way we build new structures and
retrofit older ones," said Darryl Young,
director of the Department of
Conservation. "These strong-motion
sensors provide the latest technology to
help us do that."
The installation
project conducted by DOC's Strong Motion
Instrumentation Program (SMIP) began in
1999 as part of the mammoth earthquake
retrofit of City Hall.
The sensors also
serve as watchdogs, automatically
phoning an alert to computers at SMIP
headquarters in Sacramento when strong
ground motion occurs (generally
magnitude 3.5 or greater in the San
Francisco Bay Area.)
In addition to City
Hall, accelerometers have been installed
on other structures in the San Francisco
area, such as the Golden Gate Bridge and
San Mateo Bridge. In the event of a
large earthquake, information gathered
by the sensors can be analyzed by DOC
computers and
seismologists and help emergency crews
determine the hardest-hit areas within
minutes after the quake. During the 1989
Loma Prieta earthquake, the City Hall
dome twisted like a bottle cap, moving
two inches. Walls and concrete floor
slabs cracked on all levels.
In 1995 engineers
began a "base isolation" retrofit of the
entire building. Base isolation helps
buffer a building from seismic waves.
There are 590 rubber cylinders at the
base of City Hall's support columns that
dampen the effect of the seismic waves.
Base isolation also allows the building
to move more than two feet in any
direction during an earthquake, further
minimizing quake damage.
Data gathered by SMIP
sensors on how structures react to
temblors is being applied by engineers
to create stronger, safer building plans
for new construction and retrofits.
The Strong Motion
Instrumentation Program is part of the
DOC's Division of Mines and Geology,
which is California's State Geological
Survey. SMIP instruments, in combination
with sensors from the U.S. Geological
Survey, California Institute of
Technology and UC Berkeley, are the
backbone of a developing integrated
seismic network that will cover most of
the state's earthquake-prone areas. This
consortium of institutions will jointly
produce maps of the shaking to help
guide emergency response. With more than
800 stations in place, the state's
Strong Motion Instrument Program,
established in 1971 following the San
Fernando earthquake, is one of the
largest of its kind in the world.
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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