Strong Motion Seismology

Finally, it should be mentioned that earthquake prediction relates not only to time, size, and place, but also to the forecasting of the strong ground motions that constitute an earthquake. Considerable work has been done by W. D. Smith, L. Drake, and others on the development of finite-element programs and algorithms for seismic wave propagation in realistic geologic structures such as continental boundaries, mountain roots, and alluvial valleys.

The testing of such numerical predictions of seismic shaking at the surface of complex structures depends on appropriate recordings of strong ground

Figure 8
The east-west component of ground motion at the Berkeley
station recorded by the Bosch-Omori seismograph on March
10, 1922, from an earthquake source near Parkfield, California.
The recording is part of the basis of the "Parkfield Prediction
Experiment" (1988 ± 5 years). Reproduced on a wine label
printed for the Centennial Symposium, May 28–30, 1987.

relatively inexpensive digital broadband network (BDBN) for the UCB network. The design philosophy was to use off-the-shelf items and PC computers of the XT/AT type. The operation at the Mt. Hamilton station (MHC) commenced in May 1987 and at Berkeley in June 1987. Records from this new system (ultimately to consist of ten digitally recording stations in central California) are shown in figures 6 and 7.

A recent innovation in microearthquake monitoring, achieved jointly with the Earth Sciences Division, Lawrence Berkeley Laboratory, has been the installation and operation of an array at Parkfield with digital telemetry recording 500 samples per second for each component. The ten stations, each with three components, are capable of being triggered by earthquakes as well as recording waves from controlled artificial sources.

motions. To this end, a program of instrumentation with strong motion accelerographs was commenced in 1967 and later strengthened with the appointment of W. K. Cloud, formerly head of the USCGS Field Investigation Office, as Research Seismologist at the Stations. In 1968, a cased bore-hole 500 feet deep was drilled in the gouge zone of the Hayward fault on the Berkeley campus for various instrumention. In 1976, downhole strong-motion accelerometers were installed in a hole bored in bay mud at the University's Richmond Field Station. This facility has yielded important new information on the depth variation of seismic ground motion (Johnson and Silva, 1981).

Background moderate seismicity along the Bear Valley segment of the San Andreas fault provided valuable measurements of near-field ground motions in an intensive study using a special network of strong-motion seismographs. In this "Near Field Project," funded under VELA UNIFORM, nine three-component broadband seismographs were operational from 1973 to early 1977 and resulted in improved methods for resolving hypocenters and fault mechanisms (see Johnson and McEvilly, 1985).

In 1979 a digital accelerometer system for a dense strong-motion array was designed and tested by the staff at the Seismographic Stations. The array, called the Strong Motion Array Taiwan-One (SMART 1), was installed in northeast Taiwan, which is currently much more seismically active than California. The project has been carried out in close cooperation with scientists at the Institute of Earth Sciences of the National Research Council in Taiwan. The results have been gratifying, with a substantial number (over fifty) of earthquakes triggering the array of thirty-seven digital instruments in seven years. Numerous research papers of both seismological interest and engineering applicability have been published to date (see Loh et al., 1982), and work on this important array is continuing.

Strong-motion field seismographs have also been used in the context of seismic source discrimination in an extended experimental program of near-source recordings at the Nevada Test Site, begun in 1969. Concomitant wave-propagation and moment-tensor modeling have resulted, and the work continues (Stump and Johnson, 1984).