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5 The U.S. National Seismic Network In previous chapters the panel has alluded to a sense of crisis concerning regional seismic networks. The crisis is engendered by two factors: (1) inadequate instrumentation that causes regional networks to fall behind the forefront of seismological research at an increasing rate, and (2) diversion of funds to support the planned U.S. National Seismic Network (USNSN). Most of the initial capital costs of the USNSN are being supported by funds previously used to support regional seismic networks in the central and eastern United States through an interagency agreement between the U.S. Geological Survey (USGS) and the U.S. Nuclear Regulatory Commission (see Appendix B). Therefore, in this chapter the panel briefly examines the plans for the USNSN and its dramatic impact on the future of observational seismology in the United States. The USNSN is a new program being undertaken by the National Earth- quake Information Center (NEIC) of the USGS with start-up funding from the Nuclear Regulatory Commission. The immediate objective of this co- operative effort is to establish a network of some 60 modern seismograph stations more or less evenly spaced throughout the United States east of the Rocky Mountains with satellite communications links to the NEIC. The ultimate goal of the effort is to record ground motion across a wide range of frequencies and with high dynamic range from all earthquakes nationwide above magnitude 2.5-3.0. This network fills an immediate need for uniform monitoring of earthquakes of magnitude 2.5-3.0 and above in the eastern United States. Near this magnitude level, earthquakes in populated areas are usually felt by more than a few persons. It is the responsibility of the 32
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THE U.S. NATIONAL SEISMIC NETWORK 33 NEIC to provide a public statement on any earthquake felt in the United States. This network will allow NEIC to fulfill this function as well as to provide rapid reporting of damaging earthquakes and a high-quality data base for research on earthquake sources and the propagation of seismic waves. The funding provided by the Nuclear Regulatory Commission carries the stipulation that it be used only for the purchase of equipment for the network east of the Rocky Mountains. At this time there are no funds for completion of the network west of the Great Plains and in Hawaii and Alaska. If completed nationwide, the USNSN will consist of approximately 150 seismic stations distributed across the lower 48 states, Alaska, Hawaii, Puerto Rico, and the Virgin Islands. This station density should be adequate to provide the capability to detect, locate, and quantify the energy release of earthquakes of magnitude 2.5-3.0 in all states except possibly Alaska. Such a capability will exceed that which exists today in many regions of the United States not currently monitored by regional networks. However, the USNSN will not, even if completed nationwide, eliminate the need for the existing regional seismic networks in areas of moderate and high seismicity. The principal purposes of such networks, described earlier in this report, are to detect earthquakes with very low magnitudes, down to around magnitude 1 in many cases, and to achieve highly accurate determinations of locations. Other important uses for which high station density is essential include earthquake hazard mitigation, earthquake prediction, estimation of strong ground shaking, and studies of the earth's crust and deep interior. DESIGN OBJECTIVES The USNSN is being designed to meet the following objectives: · Detect and locate all earthquakes of magnitude 2.5-3.0 or greater within the United States; · Report to the public all earthquakes of magnitude 2.5-3.0 or greater within the United States within 30 minutes; · Minimize network development risk and cost Minimize operational cost of the network; Locate the stations where the seismic "noise" is low; a Measure the seismic signals over a wide range of frequencies and amplitudes; and · Provide rapid distribution of the data products. The capability of detecting and locating earthquakes of magnitude 2.5- 3.0 or greater will ensure that most felt events are located with modest (+5- 10 km) accuracy. The capability of reporting information on these earthquakes within 30 minutes is needed to allow the NEIC to issue rapid earthquake
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34 ASSESSING THE NATION'S EARTHQUAKES reports to emergency offices, government agencies, and the public. Re- gional seismic network centers provide similar information to the public and to emergency response centers in their areas. The USGS objective is to have a reliable network yielding high-quality data within the six-year design and implementation period that began in 1988. The greatest possibility of incurring delays or cost overruns in projects such as the USNSN is generally associated with implementation of the central processing facility. For the USNSN, the USGS will minimize this risk by using a state-of-the-art seismic processing system recently devel- oped for the NEIC. Only hardware that is currently available commercially will be used for the individual seismic stations. The most innovative feature of the USNSN, and therefore the one posing the most risk, is the use of satellite communications for the transmission of seismic data. From past experience, the decision of whether to implement and operate seismic networks and arrays generally turns on projections of operational costs. To ensure the long-term stability of the USNSN, it is important that the annual operating cost of the network be kept low. The chances of the network surviving in a period of reduced funding are inversely proportional to the operating costs. The NEIC plays a central role in distributing national and global seismic data to the scientific community. For data from the USNSN, the NEIC will establish procedures to ensure rapid distribution and equal access to the network data for all interested users. An example (in the lower 48 states) of a network configuration for the complete network of 150 stations is given in Figure 5. Only a few of these sites are fixed to date, but Figure 5 gives an idea of the station distribution and spacing that might be expected. The average station spacing is between 350 and 400 km, with a denser concentration of stations in the seismically active areas of the eastern and western United States. STATION CHARACTERISTICS Each USNSN station will be equipped with two sets of three-component seismometers, one set of conventional high-sensitivity instruments, and one set of instruments designed to respond linearly to strong ground motions experienced very near the epicenter of strong and moderate earthquakes. The system will provide 210 dB of dynamic range through 24-bit digitation at 80 samples per second (sps). The data will be recorded and transmitted in various bands. Not all of these bands will be recorded continuously; some will be "triggered" when the signal rises above a certain threshold. Each station will trigger independently as the signal conditions warrant. The various recording bands are characterized in Table 1. Individual sta- tions will be supported by a microcomputer, a clock, a satellite transmitter and antenna, and solar panels and batteries for power.
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36 ASSESSING THE NATION'S EARTHQUAKES Table 1. Recording Bandwidths for Seismic Stations Bandwidth No. Components Frequency Recording (samples per second) Broadband 3-Component 40 Triggered Strong motion 3-Component 80 Triggered Long period 3-Component 1 Continuous Short period Vertical 13 Continuous This station design should provide a wide range of seismic data useful for many purposes. In addition, the power and data transmission aspects of the design make the stations well suited for operations at sites remote from cultural activity, where seismic noise is likely to be low. DATA TRANSMISSION As mentioned above, data from individual stations will be sent to the NEIC via satellite. Each station will be equipped with a small satellite antenna less than 2 m in diameter. A master satellite receiving station will be located in the Denver area near the NEIC. The anticipated data capacity is a minimum of 2,400 bits/e per individual station (this can increase during peak periods) and 350,000 bits/e inbound and 50,000 bits/e outbound for the master stations at the NEIC. The data transmission protocol includes error detection, forward error correction, and packet retransmission. DATA PROCESSING A real-time seismic data processing system has recently been installed at the NEIC. This system is modular and will be expanded to meet the requirements of the national network through the use of additional hardware. Functions of the NEIC processing system include verifying and refining the triggered signal detections, determining the signal parameters, grouping or associating the signals from a single event, and determining a preliminary epicenter location. Other functions include maintaining an archive of waveform data with associated epicenter information, providing an interactive capability for a seismologist to review automated results, and producing final epicen- ter catalogs. In addition to epicenter catalogs, compact disks with read-only memory (CD-ROMs) containing all of the data collected by the USNSN will be produced and distributed routinely.
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THE U.S. NATIONAL SEISMIC NETWORK INTERFACE WITH NATIONAL AND REGIONAL STATIONS 37 The transmission of real-time seismic data by satellite from remote USNSN stations to the central processing facility at NEIC will be a major advance for U.S. observational seismology. Although satellite transmission of data is practiced on a limited scale by some Department of Energy and Department of Defense programs, the USNSN will be the first satellite-based instrument network available to the general seismological research community. One of the major advantages of a satellite-based system over land-link telemetry is the flexibility of station siting: the dual limitations of line-of- sight links for radio transmission or availability of telephone line drop points are eliminated. The beneficial result is that station sites can be selected on the basis of low background noise or optimum station distribution rather than data transmission feasibility. This new freedom in seismic network design and deployment raises a number of important issues concerning linking existing regional seismic network stations with the USNSN. Some of these are explored in an analysis by D.W. Simpson, which is reproduced in full as Appendix C to this report, and in the following chapter on the future of regional seismic networks.
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