Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 1
Executive Summary Regional seismic networks with centralized recording began in the late 1960s. Without an infusion of new instrumentation, commitment, and funding, most will either cease to exist or be technologically obsolete by the early 1990s, a brief lifetime indeed for such a major observational resource of the geosciences. Are regional seismic networks still necessary? If not, they should be phased out. But if they are, the means must be found not only to continue support for their operation, but also to modernize them so that their important future contributions to basic science and to seismic hazard mitigation can be fully realized. These issues and the various options for addressing them make up the substance of this report. The threat posed by earthquakes in the United States is actually a mosaic of different problems requiring different approaches to assessment and mitigation. Ours is the only country in the world that must deal with the diverse seismic hazards arising from the full range of earthquake environments, i.e., plate subduction zones (in the Aleutians and the Pacific Northwest), a transform plate boundary (the San Andreas fault in California), hot spots (beneath Hawaii and Yellowstone), distributed plate boundaries (along the Intermontane belt and the Basin and Range province), and major earthquakes of the stable continental interior (New Madrid, Missouri; Charleston, South Carolina). Such diversity presents both major problems in the context of earthquake hazards and major opportunities in terms of understanding the dynamics of the planet. A concerted national effort to systematically monitor the nation's earthquakes and to gain sufficient understanding to reduce their impact can be achieved. 7 1
OCR for page 2
2 ASSESSING THE NATION'S EARTHQUAKES A principal vehicle for reaching these goals would be a partnership between the U.S. National Seismic Network (USNSN)—planned by the U.S. Geological Survey for implementation in the early 1990s- and a group of streamlined and modernized independently operated regional seismic networks, sited in the important seismic zones of the nation. The combined facilities of the national and regional networks, as proposed in this report, would constitute a National Seismic System, a satellite-based network capable of systemati- cally monitoring and analyzing earthquakes throughout the nation within minutes of their occurrence. Such a system would maintain the vital regional research and response flexibility required by our nation's diverse seismic zones, and its dual components each would have significantly increased capabilities beyond those possible in isolation. Clearly, a National Seismic System can be a whole greater than the sum of its parts. The USNSN is designed and intended to detect and report on only those U.S. earthquakes above magnitude 2.5-3.0; research considerations are sec- ondary to this mainly operational intent. The addition of the regional network component to form a National Seismic System would expand USNSN's capability to be a national research facility of unprecedented effectiveness. Also, it is important to note that regional networks supply a continuity of seismicity data essential for seismic hazard evaluation, short-term earthquake forecasting, or even longer-range predicting. These data must be in place when the need arises they cannot be gathered after the fact. Two examples illustrate how the regional networks will augment the USNSN. First, the wide-aperture USNSN can provide three-dimensional locations of earthquake foci to within about +5-10 km; dense regional net- works can improve this to +1-3 km for earthquakes in their area. Over much of the United States, crustal faults capable of producing damaging earthquakes have minimum dimensions of less than 10 km. Only regional networks with closely spaced stations and microearthquake detection capa- bility have the resolving power necessary to delineate such features. Second, the powerful technique of seismic tomography developed during the 1980s is dependent on dense sampling of the earth's crust by seismic rays. Just as medical CAT-scans provide the surgeon with three-dimensional images of the interior of the human body, so also does seismic tomography provide the seismologist with three-dimensional images of the geologic structure of the earth's interior. Such high-resolution images are fundamental to achieving advances in understanding and dealing with all earth processes, including earthquakes. The USNSN, with an average station spacing of about 370 km, cannot adequately resolve the details of shallow earthquakes within the continental crust. Crustal tomography will require the operation of special arrays or the continued operation of regional seismic networks, which have the advantage of providing long-term recording. The panel considers that seismic tomography
OCR for page 3
EXECUTIVE SUMMARY 3 is a technique of such great promise that enabling its use alone justifies the operation and upgrading of regional seismic networks. Indeed, it is mainly because of the potential scientific gains afforded by seismic tomographic investigations that the panel foresees the need for an increase in the number of regional seismic network stations rather than the pending decimation that will result from withdrawal of seismic network support by federal agencies, principally the U.S. Nuclear Regulatory Commission. The current deployment of regional seismic networks in the United States is outlined in Appendix A. Nearly 50 organizations operate about 1,500 seismograph stations (roughly 40% of which are sited in California). Because of inadequate finances, fewer than 10% of the 1,500 stations record complete seismic waveforms, and fewer than 3% incorporate state-of-the-art design in their electronics. The panel recommends that a concerted program of regional network modernization be a high-priority objective of the proposed National Seismic System. A National Seismic System with the USNSN forming the backbone or framework would have operational advantages. The data communications, data management, and data distribution systems of the national network could be used by the regional networks. The regional networks in turn could provide local support for national stations within each region. The result would be greater efficiency in operations on both sides and more standardization in data collection, production of routine data-based products, and generation of software, thus making data exchange between networks easier. In addition to these tangible benefits, a National Seismic System would allow seismologists from both the regional or the national perspectives to speak and act from a stronger, more unified position. The quality and scope of both the national and the regional components of a National Seismic System will be controlled by financial considerations. For this reason the panel, in its "Findings and Recommendations" (Chapter 7), recommends a modest increase in the projected funding for the proposed National Seismic System and an increase in support for network operations from the current level of approximately $10 million per year from diverse sources to $12 million per year. Additionally, the panel recommends a one- time capital investment of $15 million spread over a five-year period. The $12 million incorporates the funds necessary to operate a complete National Seismic System and to continue the operation of regional networks in the principal seismic zones of the nation. The $15 million represents the funds necessary to (1) expand the USNSN from only the eastern United States to the entire nation, (2) provide satellite data links between the national center and regional network operation centers, and (3) provide for the needed gradual upgrading of regional network instrumentation and recording facilities. The recommended increase from $10 million to $12 million per year for operating a National Seismic System and the $15 million for capitalization
OCR for page 4
4 ASSESSING THE NATION'S EARTHQUAKES and modernization constitute funding that is modest considering the cost- benefit ratio. The proposed National Seismic System is an idea whose time has come. It should be fully implemented without delay. With it seismology can take a major step forward in the fundamental study of planet earth and in the determination of the earthquake hazard to which Americans are sub- lect.
Representative terms from entire chapter: