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SACRAMENTO – The
California Geological Survey (CGS) is
expanding its inventory of seismic
instruments in Santa Rosa, an area that
has experienced significant shaking
during past earthquakes. These
“accelerographs” are not an early
warning system, but instead collect data
about the response of buildings and
structures to ground motion. That
information can improve public safety,
and help emergency responders and local
planners.
“We have relatively
few instruments around Santa Rosa, given
the population and the shaking we’d
anticipate there in the event of a large
earthquake in the Bay Area,” said State
Geologist Dr. John Parrish, chief of CGS.
“The information we gather with these
instruments ultimately can help make
buildings better able to withstand large
earthquakes, which are infrequent but
inevitable in California.”
CGS, part of the
California Department of Conservation,
established the Strong Motion
Instrumentation Program (SMIP) in 1971
after the damaging San Fernando
earthquake. SMIP has installed and
maintains recording instruments at more
than 1,000 locations statewide,
including the city halls of Los Angeles,
San Francisco and Oakland, and the state
capitol. A number of high-rises, dams
and all major bridges are instrumented.
In Santa Rosa, SMIP
currently has instruments in a bank, a
hospital, a retirement home, and in an
open field. The ongoing work will add
instruments at four fire stations in the
city and one in nearby Sebastopol.
“We place quite a few
instruments in fire stations, for a few
reasons,” explained Supervising
Geologist Dr. Anthony Shakal, who heads
SMIP. “For one thing, it’s a protected
environment. Also, fire departments are
in a state of readiness at all times in
case of emergency, so they understand
our mission and are very cooperative
with us. Finally, fire departments tend
to follow the pattern of population
density in a community, which is
important in monitoring the shaking
where people and buildings are.”
The accelerographs
measure the vertical and horizontal
response of buildings and soils. When
activated by earthquake shaking, the
devices produce a digital record from
which the critical characteristics
(acceleration, velocity, displacement)
of ground motion can be calculated. The
information gathered by SMIP is
processed and disseminated to
seismologists, engineers, building
officials, local governments and
emergency response personnel throughout
the state.
SMIP data has
verified the performance of new types of
earthquake-resistant construction, and
has resulted in improved formulas in the
Uniform Building Code for calculating
building vibration periods, which is
vital in earthquake-resistant design.
The goal of SMIP and
its partners in the California
Integrated Seismic Network (CISN) -- the
California Institute of Technology, UC
Berkeley and the U.S. Geological Survey
-- is to have at least one instrument in
every ZIP code in the state.
Within a few minutes
of a large earthquake, data from the
SMIP and CISN seismic instruments is
used to create a “ShakeMap.” Among other
things, the ShakeMap helps emergency
responders determine where the highest
levels of shaking have occurred and thus
where critical infrastructure – such as
transportation corridors and water lines
– is most likely to be damaged.
Santa Rosa suffered
severe damage during the Great San
Francisco Earthquake of 1906. While the
last major earthquake in the Bay Area –
the magnitude 6.9 Loma Prieta of October
17, 1989 – registered on SMIP’s
instruments in Santa Rosa, the epicenter
was too far away (about 110 miles) to do
any damage in the community.
Two moderate
earthquakes – magnitude 5.6 and 5.7 –
centered two miles north of Santa Rosa
on the Rodgers Creek fault system
occurred on October 1, 1969 and caused
significant damage. There were no
instruments in or near Santa Rosa at the
time, so relatively little is known
about the ground shaking that occurred.
“Santa Rosa tends to
have significant shaking because of the
underlying geology,” Shakal said. “There
are very deep sediments under the city,
and the Rodgers Creek and Maacama fault
systems are nearby. With more
instruments there, we will gain
understanding of how the structures and
the ground there perform in earthquakes.
The more we know, the better prepared
we’ll be.”
SMIP data is
available at the Department of
Conservation's Web site,
www.conservation.ca.gov, and at
quake.ca.gov. Data from many earthquakes
can be viewed and downloaded
conveniently for use in engineering and
scientific applications. There is also
information about how the public can
prepare for future earthquakes.
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