CALIFORNIA'S STRONG-MOTION INSTRUMENTATION PROGRAM
PHOTO
San Fernando Valley earthquake, February 9, 1971.
California has been struck by five major earthquakes---in northern California, the 1868 Hayward and 1906 San Francisco earthquakes . . . in southern California, the 1857 Ft. Tejon, 1872 Owens Valley, and 1952 Kern County earthquakes.
Recent damaging earthquakes near Coalinga, Oroville and San Fernando occurred in areas not previously recognized as seismically hazardous. These events demonstrate that the threat of destructive earthquakes is not confined to only a few places in California.
More than 500 potentially damaging earthquakes have occurred in California or near its borders since 1900. These earthquakes have been responsible for the death of almost 1,000 people and almost $2 billion in property damage.
Scientists agree that during the next 50 years California can expect at least one great earthquake and several smaller but destructive earthquakes.
Today, an event like the 1906 San Francisco earthquake could take many thousands of lives and cause billions in damage. One like the 1857 Fort Tejon earthquake near Los Angeles poses a similar threat.
The facts are clear---destructive earthquakes are certain to recur. They pose a continuing major threat to lives and property throughout California.
WHAT CAN BE DONE?
We cannot prevent earthquakes. However, since most earthquake losses result from the collapse and damage of structures, future losses of lives and property can be reduced significantly by improving the earthquake resistance of buildings and other structures.
To design for earthquake resistance engineers must have realistic estimates of the kind of ground-shaking future earthquakes can cause at the site of the structure. They must also know how various foundation materials and structures respond to strong shaking.
The Strong-Motion Instrumentation Program (SMIP) was established to obtain these vital earthquake data for the engineering-scientific community. Since 1972, the Program has been developing a statewide network of strong-motion instrumentation. Statewide coverage, when the planned network is completed, will ensure that strong ground motion for any moderate or larger size earthquake in the state will be recorded.
Strong-Motion Instrumentation
Strong-motion seismographs are being installed on various representative geologic foundation materials and in major structures such as buildings, dams, power plants, and transportation and utility systems. Instruments are currently located at more than 400 sites. Sites are selected by engineers and scientists representing industry, government, and the universities. At the onset of strong earthquake shaking, strong-motion seismographs are activated, producing a film or digital record from which the critical characteristics of ground motion (acceleration, velocity, displacement) can be calculated.
As the network of seismographs is developed statewide, new and important data become available on the occasion of each new earthquake. Some examples include:
The 1979 Imperial Valley earthquake. Instruments installed by SMIP at different levels of the Imperial County Services Building in El Centro recorded the motion of this modern multi-story building during the earthquake and resulting failure of the building. With these data, engineers have assessed the causes of the failure and reached a better understanding of the performance of this type of structure.
The Mammoth Lakes earthquakes. Seismic activity that began with four moderate earthquakes in May 1980 has provided strong-motion data allowing detailed analysis of the structural response of a high-school gymnasium and a large earthen dam.
The 1983 Coalinga earthquake. The earthquake activated a specially designed network of 45 strong-motion instruments located along the nearby San Andreas fault. The records from this network yielded an unprecedented suite of data for detailed study of the factors that control the character and distribution of ground shaking.
As these strong-motion records are analyzed and interpreted by engineers and seismologists, the resulting knowledge provides the basis for improvements in structured design and construction. In addition, the local building codes and ordinances that specify earthquake-resistant design and construction practices will be improved.
Earthquakes will continue to be a fact of life in California. Their effect on our lives and on future generations will gradually diminish, however, as we acquire and apply the knowledge necessary to build safer structures.
The Program
California's Strong-Motion Instrumentation Program was established by the California Legislature in 1971. Its mandate is to install and maintain strong-motion recorders in representative structures and geologic environments throughout the state and to process and disseminate the records. Since the programs inception, records from numerous earthquakes have been collected, processed, and distributed for analysis. All records are available upon request.
The program is managed by the California Division of Mines and Geology of the Department of Conservation and advised by a committee of the California Seismic Safety Commission. Current program funding is provided by an assessment of seven cents on each $1,000 of estimated construction costs on building permits issued by cities and counties.
PHOTO
San Fernando Valley earthquake, February 9, 1971.
PHOTO
San Andreas fault zone, Carrizo Plain.
PHOTO
Imperial Valley earthquake, October 15, 1979.