engaged in basic and applied research; (2) engineers, public officials, and other decision makers charged with the management of earthquake risk and emergency response; and (3) public safety officials, news media, and the general public. As information technology has transformed the regional networks into integrated monitoring systems, they have become centers for educating the general public about earthquake hazards, as well as key facilities for training graduate students in seismology (8).

The short-period, high-gain instruments historically used in regional networks (9) brought seismicity patterns into much clearer focus (Figure 4.6), but the dynamic range of these instruments was too low to furnish useful recordings of large regional events. In the last decade, deployments of broadband, high-dynamic-range seismometers have begun to transform the regional networks into much more powerful tools for investigating the basic physics of the earthquake source, the detailed structure of the Earth’s crust and deep interior, and the patterns of potentially destructive ground motions. With these data, seismologists can now map the patterns of slip during earthquakes using seismic tomography, just as they map Earth structure. Images of fault ruptures during the more recent earthquakes in the Los Angeles, San Francisco, and Seattle regions have all been captured by high-performance networks (Figure 4.7).

Long-term funding has been a persistent problem for regional network operators, and new investments in equipment are badly needed (10). In particular, the implementation of new broadband technologies in regional monitoring has been lagging in the United States, especially when compared to the investments made by other high-risk countries such as Japan (Box 4.1) and Taiwan. Two exceptions are the Berkeley Digital Seismic Network in northern California and Caltech’s TERRAscope Network in southern California. Both are equipped with a combination of three-component broadband seismometers and three-component strong-motion accelerometers; they have digital station processors and feed continuous data streams via real-time telemetry to central processing sites. Although these networks have developed independently, a major effort is under way, with some support from the State of California, to modernize the earthquake monitoring infrastructure throughout the region by integrating the regional networks into a California Integrated Seismic Network monitoring in the United States.

Local Networks Networks have been deployed with seismometers distributed over a few tens of kilometers or less for specialized purposes such as seismic monitoring of critical facilities (e.g., dams and nuclear power plants) or localized source zones (e.g., volcanoes or geothermal reservoirs). Local networks are important instruments for the study of natural earthquake laboratories such as deep mines. Digital arrays of very

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