In the first year (10/1/1999-9/30/2000) of ANSS funding, the California Integrated Seismic Network (CISN) installed 40 digital seismic stations in the San Francisco Bay region (Figure 1). The sites were chosen to address two critical seismic-monitoring needs in the region - construction of real-time ShakeMaps and monitoring of strong shaking in urban areas. To improve the ability of the CISN to create ShakeMaps, the USGS Northern California Seismic Network (NCSN) replaced obsolete analog seismic equipment at 19 existing regional network sites throughout the region with data loggers digitizing output from a tri-axial accelerometer (for strong motion) and a vertical seismometer (for monitoring microearthquakes). The remaining 21 sites were newly installed by the USGS National Strong Motion Program in fire stations located in metropolitan regions of the Bay Area where there were few other strong motion stations.
The equipment was installed by the end of the fiscal year at all 40 sites and configured to record on-site if strong shaking occurred. The NCSN was able to bring continuous data back from 6 sites via direct radio and microwave telemetry. However, continuous telemetry from the other stations was not completed at the time of installation because Pacific Bell was unable to install Frame Relay, DSL, and ATM circuits to the stations. Pacific Bell estimates that they will complete the engineering of this digital telemetry network in February 2001.
With the installation of these 40 stations and the integration of strong motion information in near real-time from other CISN participating institutions at UC Berkeley, California Division of Mines and Geology, and the Pacific Gas & Electric, there are now over 200 seismic stations in the San Francisco Bay region that are generating data for the creation of a ShakeMap (Figure 1). While this is a significant achievement relative to the state of seismic monitoring a decade ago, the density of stations in the urban region is still inadequate. The Advanced National Seismic Network plan (U.S. Geological Survey Circular 1188) recognizes the need for 1000 stations in the region. In the second year of ANSS funding the CISN plans to install an additional 21 urban sites and upgrade 10 regional stations. The CISN will also be installing strong motion instrumentation in 2 base-isolated buildings in Berkeley and Palo Alto.
The ability to record on-scale waveforms elsewhere in northern and Central California remains inadequate. The limited number of seismic instruments outside the San Francisco Bay region capable of recording on-scale strong motion data are shown by the blue circles in Figure 2. Even though many parts of the region are sparsely populated, this is where most of the large earthquakes have occurred during the past 30 years. Adequate instrumentation throughout the state is essential to collect data close to large earthquakes so that seismologists and seismic engineers may learn how to mitigate the damaging effects of future earthquakes. Through the coordinated efforts of all member institutions the CISN is working to improve the quality and density of seismic instrumentation statewide.