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NR 2002-19
April 17, 2002

Contact: Carol Dahmen
Mark Oldfield
Don Drysdale
Ed Wilson
(916) 323-1886

ON THE ANNIVERSARY
OF THE GREAT SAN FRANCISCO EARTHQUAKE,
DOC FINISHING SEISMIC INSTRUMENTATION
OF TWO BAY AREA BRIDGES
 

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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