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 32
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
OCR for page 33
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
OCR for page 34
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.
OCR for page 35
OCR for page 36
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.
OCR for page 37
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.
Representative terms from entire chapter:
regional seismic
