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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.
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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
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EXECUTIVE SUMMARY
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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
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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:
regional seismic
