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1
Introduction and Background
Regional seismic networks are discrete arrays of tens to hundreds of
seismic stations targeted chiefly on seismically active regions. They are a
fundamental, multipurpose tool of observational seismology, providing a
broad range of data and information. Data acquired by these networks have
a host of applications, including but not restricted to public safety and
emergency management; quantification of hazards and risk associated with
both natural and human-induced earthquakes; surveillance of underground
nuclear explosions; and wide-ranging basic research encompassing earthquake
mechanics and dynamics, seismic wave propagation, seismotectonic processes,
earthquake forecasting and prediction, and properties and composition of
the crust and of the deeper internal structure of the earth (for a comprehen-
sive overview, see Heaton et al., 19891. Importantly, regional seismic net-
work facilities are also essential for the graduate education and training of
this country's professional seismologists, and they provide the most readily
available sources for public information and for expert assistance to public
policymakers, planners, designers, engineers, and safety officials on the
local and regional level.
Previous National Research Council reports (Committee on Seismology,
1980, 1983) have distinguished regional from local seismic networks on the
basis of scale, lifetime, and mission. In these reports, as in this one, "network"
means "a collection of seismic stations operated coherently, normally by
one organization, with a common basis for data collection and analysis"
(and typically with telemetry to a central recording and analysis facility).
Local networks are characterized by dimensions smaller than several tens of
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6
ASSESSING THE NATION'S EARTHQUAKES
kilometers, an operating lifetime of less than several years, and a special-
ized research and monitoring mission focused, for example, on a critical
facility (such as a dam or nuclear power plant) or a localized seismic source
zone (such as a volcano or geothermal area). Local networks are often
operated by private companies.
Regional networks operate on a scale ranging from hundreds of kilome-
ters to 1,000 km. They have an unspecified lifetime, but are commonly
assumed to be permanent facilities, and they are generally operated by government
agencies or universities. Figure 1 gives an overview of three fundamental
aspects of the role or mission of a regional seismic network (note that the
three functions are not mutually exclusive or in order of priority): earth-
quake monitoring and rapid emergency response; scientific research; and
the acquisition of information required for earthquake hazard and risk analyses
as well as for earthquake engineering. Efforts aimed at earthquake forecasting
and prediction apply to all three functions. Thus regional networks play an
essential, if unrecognized, role far beyond that of simply monitoring earth-
quake activity.
Currently, there are about 1,500 seismic stations operating in the United
States, forming parts of about 50 regional seismic networks (Appendix A).
Figure 2 shows the distribution of these stations, some of which may be
construed to be part of local networks. Because the panel supports the goal
of improving network seismology in the United States, it has not arbitrarily
excluded all consideration of local networks. Nevertheless, the panel's
recommendations chiefly address regional seismic networks as defined above.
The vast majority of current regional seismic network instruments are
substandard when compared with the needs of modern seismological practice
(see Appendix A). Specifically, they consist largely of vertical-motion-
only sensors, recorded over a narrow frequency band (~1-20 Hz) with limited
dynamic range (~40-60 dB). The desired operational characteristics of a
modern network would include full three-component recording with a much
higher dynamic range (>100 dB) and with at least a subset of broadband
stations. Not only is there no plan to modernize these networks to achieve
their full potential, but instead decreasing federal operating support is eroding
their capabilities.
The panel has found a crisis atmosphere affecting regional networks na-
tionwide. The decision of the U.S. Nuclear Regulatory Commission to
phase out its support of regional networks in the eastern United States and
to support instead the development of a U.S. National Seismic Network
(USNSN) by the U.S. Geological Survey (USGS) has already begun to
curtail network operation and student involvement (see Appendix B). In the
western United States, both federal and federally supported networks are
suffering because of inflation-eroded, no-growth funding of the USGS budget
for the last six years.
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INTRODUCTION AND BACKGROUND
Function: Scientific Research
Users: (Scientists and Engineers)
/ SEISMOLOGY \
Function: Earthquake Monitoring
& Rapid Emergency Response
Users: (Public Safety Officials,
News Media & General Public)
7
Function: Input to Earthquake
Hazard & Risk Analyses,
Earthquake Engineering
Users: (Engineers, Public
Officials ~ over Decision
Makers)
· Earthquake data base
· Seismotectonic framework
· Earthquake source
identification
Seismicity parameters &
earthquake occurrence
modeling
Information for predicting
strong ground motion (source
mechanics, attenuation)
Figure 1. The multifold practical functions of regional seismic networks.
Because large damaging earthquakes in the United States are episodic,
public attention and concern wax and wane, and the potential of earthquakes
to cause great sudden disasters is often ignored. As a result, earthquake
seismologists have been unable to gain adequate sustained support from
representatives and officials charged with taking a long-term view on society's
behalf. All of the major infusions of funds that have enabled seismology,
including regional networks, to grow have been the result of specific missions,
rather than a fundamental national commitment to the science. The major
missions e.g., nuclear test monitoring and the assessment of earthquake
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OCR for page 10
10
ASSESSING THE NATION'S EARTHQUAKES
hazards for siting of critical facilities, earthquake prediction, and hazardous
waste disposal have come at irregular intervals. The result has been the
lack of stability in support for regional networks, to a degree unique to the
United States among technologically advanced nations.
The U.S. seismological community is making coordinated efforts to modernize
and streamline the capabilities and effectiveness of regional seismic networks.
These efforts presuppose that stable, long-term funding can be secured from
funding agencies, when policymakers are convinced of the importance and
value of such investment. Only a modest amount of sustained support is
required (see Chapter 7~.
The development of the USNSN (see Chapter 5) has contributed to the
regional network crisis, as noted above. Funding for the limited deployment
of the USNSN has undercut and will soon eliminate the support currently
received by most of the central and eastern U.S. regional networks. The
sparse station spacing of the USNSN, however, means that many fewer
earthquakes will be recorded and that for those that are, the locations will
be determined with less accuracy than is possible when using regional net-
work data. Is such detailed information still needed, or are regional seismic
networks obsolete? This report is intended to answer that question. The
panel finds that the USNSN is essential to the nation's need for information
about earthquakes but that it is, by itself, insufficient to provide all of the
needed information. The panel also finds that the regional seismic networks
have been an inefficient means of producing the needed information because
they are regional and isolated and operate without adequate facilities and
support staff and without a unifying national support system.
The following chapters of this report present the basis for these findings.
They deal with the contributions to date of regional seismic networks (Chapter
2), problems and limitations of the networks in their present form (Chapter
3), the case for a continuance of regional network operation (Chapter 4),
and a specific initiative that holds great promise for revitalizing regional
networks (Chapter 5~. The panel envisions an essential and productive
future for regional networks as an integral partner with the developing U.S.
National Seismic Network (Chapter 6~. After a brief look at this future, the
report ends with a set of specific recommendations (Chapter 7), which if
followed, could make this partnership National Seismic System—a reality.
Representative terms from entire chapter:
seismic networks
