response to earthquake disasters. The idea for the USNSN dates back nearly 30 years (20); the concept was to complement the relatively dense coverage provided in selected areas by the regional seismic networks with a well-distributed but sparse permanent network of three-component, broadband stations. The USNSN currently maintains 32 complete broadband stations and some equipment at 96 cooperative broadband stations in North America (7 in Canada) from which it acquires real-time data. It also acquires real-time data from 82 short-period stations, 30 foreign broadband stations, and another 62 stations worldwide. Through participation of the Advanced National Seismic System (ANSS) and the planned EarthScope program, the USNSN will be expanded to 100 permanent broadband stations in North America and will serve as the “backbone” for both programs. Ten of the new stations will be built to GSN standards and, thus, be capable of high-quality recording at the low frequency of the Earth’s free oscillations.

Strong-Motion Seismology

Accurate recordings of strong motions near earthquake sources are crucial to both earthquake engineering and science, because they provide the forcing functions for structural design and testing, as well as valuable information on earthquake source processes. The motions are registered by triggered, three-component, low-gain accelerographs located at free-field sites and housed in important structures, such as dams, bridges, and high-rise buildings. Accelerographs are capable of recording 2g accelerations in the frequency band from 0.1 to 10 hertz. The attenuation relations derived from the free-field data are key components of seismic hazard analysis and mapping, while the housed recordings furnish ground truth for structural performance during earthquakes.

The USGS oversees a national network of about 900 strong-motion accelerographs through the National Strong-Motion Program (NSMP). The NSMP coordinates data collection by a variety of federal, state, and local agencies, companies, and academic institutions (21). The California Geological Survey (CGS) operates the California Strong-Motion Instrument Program with basic funding provided by state tax on permits for new construction; it comprises 910 analog and digital accelerographs in California, 255 of which are in extensively instrumented structures. Strong-motion databases are maintained by both the USGS and the CGS, as well as by the Southern California Earthquake Center (SCEC) and the Pacific Earthquake Engineering Research (PEER) Center (22). Coordination of the various organizations that collect, process, and distribute strong-motion data has been a long-standing issue (23), but the situation has benefited substantially from on-line access now offered by all data



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