Survey (USGS), will upgrade the U.S. National Seismographic Network from 56 to 100 stations and modernize regional seismic networks (Box 6.1). These components of the ANSS plan, if brought into full operation, would furnish the instrumental system needed to satisfy the seismological objective stated above.
BOX 6.1 Advanced National Seismic System
A major initiative is under way to increase the number of seismographs deployed in the United States, with an emphasis on urban areas with significant seismic risk.1 Plans for the ANSS call for doubling the size the U.S. National Seismographic Network, upgrading regional earthquake monitoring with 1000 new stations, and installing 6000 strong-motion instruments in cities with moderate and high seismic risk. Half of the urban instruments would be ground based (free-field), and the other half would be located in buildings and other structures of engineering interest. Additional components include network operation and data distribution centers and an array of portable seismographs for targeted (e.g., postearthquake) studies.
The ANSS is designed to bridge the separation between strong-motion seismology and regional network seismology by recording ground motions over the broad range of frequencies and amplitudes required by both seismologists and engineers. The data collected in major earthquakes will allow engineers to study the strong-motion response of a diverse collection of structures, and they will permit seismologists to invert for the detailed slip history on a fault and thus constrain the fundamental processes involved in rupture dynamics. The high density of stations in urban areas, most of which are located on sedimentary basins, will enable seismologists to map site response at the fine scales needed to understand basin and near-surface effects. Moreover, the high density of stations proposed within certain buildings would calibrate and improve predictive engineering models of near-failure and nonlinear behaviors.
The ANSS will be a real-time network that will broadcast information about the location and magnitude of each earthquake. It will also provide near-real-time maps of ground shaking and spectral response parameters using the ShakeMap procedures,2 which are proving valuable for emergency response and rapid assessment of damage and losses. The regional and national committees implementing ANSS include representatives of regional networks, engineering groups, and other users.
The ANSS modernization effort will cost approximately $170 million, and its annual operational costs are estimated to be about $47 million. Congress appropriated $1.6 million in FY 2000 to improve real-time monitoring and reporting of earthquakes, $4 million in FY 2001 to install real-time instruments in several key cities, and $3.9 million in FY 2002. An additional $3.9 million has been requested in the President’s FY 2003 budget.