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Contributions to Date of
Regional Seismic Networks
Most regional seismic networks currently in operation in this country
have been sited to monitor active seismic zones. Because they consist of
multiple sensors distributed over relatively small areas, they are essentially
telescopes focused downward into the earth to "see" the seismic source.
Such networks have been in operation for only about two decades but have
made extensive contributions to our knowledge of the spatial, temporal, and
physical characteristics of earthquake occurrences. Heaton et al. (1989)
recently reviewed these contributions and discussed the future of networks
in the context of a science plan for a National Seismic System. Briefly, the
contributions include the improved detection and more accurate location of
earthquakes, especially those of lower energy levels; greater precision in
focal depth determinations; enhanced monitoring of seismic energy release
as a function of space and time; refined determinations of the attenuation
characteristics of seismic waves; three-dimensional descriptions of the seismic
velocity structure of the interior of the earth; and more reliable specification
of the earthquake faulting process. Thus the fundamental contributions
from seismic networks are intrinsically observational, and these observational
data make possible a wide range of derived contributions that are of direct
benefit to both science and society.
Recent examples of such contributions with direct societal benefits are
described in two earthquake case studies in Chapter 4. Other examples
include contributions from networks associated with active volcanoes such
as Mount St. Helens in Washington and Kilauea in Hawaii. The networks
there track the subsurface motions of magma bodies and their associated
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ASSESSING THE NATION'S EARTHQUAKES
mechanical deformation and thereby provide invaluable data to emergency
preparedness agencies. Similar contributions are made when damaging
earthquakes occur in populated areas, such as happened in one of the case
study events, the Whittier Narrows shock in the Los Angeles basin. In
addition, data from regional seismic networks are essential to the safe siting
of nuclear and other hazardous waste repositories as well as large, unique
engineering structures such as the proposed Superconducting Super Collider.
Siting such structures safely requires an already-developed adequate seismicity
data base; once a site has been proposed, it is not possible to wait for data
to be gathered.
Seismic networks provide a major contribution to the estimation of U.S.
seismic hazards, which vary greatly across the nation: seismicity is highest
on the West Coast, but 37 states are in the two highest (out of four) risk
zones. The current federal National Earthquake Hazard Reduction Program
(NEHRP) recognizes this pervasive threat and seeks to mitigate it. The
program was created by the Earthquake Hazards Reduction Act of 1977; its
principal agencies are the Federal Emergency Management Agency, the
U.S. Geological Survey, the National Science Foundation, and the National
Institute of Standards and Technology. One of the major NEHRP elements
is "hazards delineation and assessment" (FEMA, 1988~. In particular, the
estimation of seismic hazard requires as input (1) spatial definitions of the
seismic source zones (especially important is the accurate definition of currently
active geologic structures as well as their seismotectonic host environment,
e.g., the thickness of the crustal seismogenic zone); (2) rates of occurrence
of earthquakes of various magnitudes for each zone; and (3) ground motion
estimation for seismic vibrations from earthquakes of varying magnitudes
and at varying distances. Clearly, only the highly accurate data from dense
regional seismic networks that are dedicated to the investigation of specific
seismic zones or regions can provide adequately for such specific requirements.
This is especially true for the eastern United States, where the seismic
station density before networks were established in the 1960s and 1970s
was lower than one per state. It is important to reemphasize that the required
input data from the regional networks cannot be obtained as the need arises
for their use; rather they must be obtained before such needs arise. It is
also important to note that the determination of seismic risk—i.e., the esti-
mation of probable consequences of earthquakes for life and property-
depends directly on the availability of accurate seismic hazard estimations,
which in turn are based largely on data from regional networks.
The technological growth of industry in this country, in concert with
increased land use during the past several decades, has resulted in a dramatic
increase in the elements of society at risk from earthquakes. Engineers
have constructed larger and more complex facilities, such as long bridges,
high dams, high-rise buildings, nuclear reactors, large computer centers,
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CONTRIBUTIONS OF REGIONAL SEISMIC NETWORKS
13
offshore drilling platforms, and concentrations of high-technology industry.
These and other critical facilities are often sited in areas of high population
density that are also earthquake-prone, e.g., the computer chip industry in
California. In addition to estimating the seismic hazard for such facilities,
it is also necessary to thoroughly evaluate the probable responses of the
structures themselves to seismic disturbances. Such studies are based directly
on the best possible estimations of the amplitudes and frequencies of ground
motions from both moderate and large earthquakes at distances ranging
from nearby to regional. Seismic networks, especially those that include
strong-moiion seismographs sited in He structures themselves as well as in
the free field, are the only source of the input data required for the necessary
estimations (Committee on Seismology, 1980~. Clearly, a lack of such monitoring
efforts exposes our society to increasingly unacceptable and unspecified
risks from future earthquakes.
One of the current frontiers of research in seismology involves the pre-
diction of earthquakes. For example, the U.S. Geological Survey has predicted
that a magnitude* 6 earthquake will occur on the Parkfield section of the
San Andreas fault in 1988 +5 years. In general, however, the present stage
of development of this research field is such that estimations of future
earthquake occurrences are generally derived in more probabilistic terms
and are based on detailed analyses of the spatial and temporal patterns of
earthquake activity in the forecast area. Both probabilistic and determinis-
tic analyses include the recognition of (1) "seismic gaps," i.e., locales that
are known from prior activity to be earthquake-prone but currently are seismically
quiescent, (2) repetitive "characteristic" earthquakes from a given fault segment,
and (3) "slip-deficient" fault segments. Only the resolving power of the
inward-looking regional seismic network "telescope" can provide data of
adequate precision, detail, and completeness to satisfy the requirements of
this most difficult and demanding seismological task—that of predicting
earthquakes in a quantitative manner. However, the benefits to society that
would result from this ability are so enormous that we must continue these
efforts.
The dense spatial coverage provided by regional seismic networks has
been directly exploited in recent studies of crustal velocity structure. Some
of these studies are similar in concept to computer-assisted X-ray tomography,
the CAT-scan in medical technology, which yields three-dimensional, com-
puter-generated "images" of the interior of a body without directly accessing
the volume being investigated. For example, Hearn and Clayton (1986)
have presented detailed images of lateral variations in the shallow crustal
*"Magnitude" as used throughout this report is a generic term for the relative
size of the earthquakes discussed. The term may refer variously to a body wave,
surface wave, moment, leg, local, or Richter scale magnitude.
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ASSESSING THE NATION'S EARTHQUAKES
velocity structure in southern California that they obtained from data de-
nved from the seismic network stations located there. These velocity variations
are associated with surface tectonic features such as the San Andreas fault.
Also in southern California, Humphreys et al. (1984) studied the deeper
mantle structure beneath the Transverse Ranges to image a spectacular,
high-velocity tabular root extending several hundred kilometers into the
mantle (see also Heaton et al., 1989~. In the Midwest, Al-Shukn and Mitchell
(1988) mapped a three-dimensional pattern of low velocities in the crustal
rocks of the active portions of the New Madrid fault system in southeastern
Missouri. The seismic velocities there are lowest in those subsurface volumes
exhibiting the greatest concentration of earthquake activity. The observed
several percent decrease in compressional wave velocity is consistent with a
source zone containing fluid-filled cracks. Studies such as the three mentioned
here have led to a markedly improved understanding of the physics and
architecture of the earth's crust. Again, the many stations of the regional
seismic networks are required to achieve the detail and resolution necessary
to accomplish such CAT-scans of the earth.
When large fault motions occur on the floors of oceans, they produce not
only earthquake vibrations but also energetic water waves, called tsunamis,
that travel across the oceans and run up on distant coastlines. Between
500,000 and 1 million residents along the coastlines of Hawaii, California,
Oregon, Washington, Alaska, and the U.S. Pacific Territories are at risk
from these rare but devastating waves. For example, the 1964 Alaskan
earthquake (magnitude 9.2) generated a tsunami that caused 122 fatalities in
Alaska, California, and Oregon and $100 million in damage in Alaska,
Hawaii, California, Oregon, and Canada. Tsunamis are predominantly, but
not exclusively, a Pacific hazard: in the Atlantic Ocean in 1929, the Grand
Banks earthquake off the coast of Newfoundland (magnitude 7.2) also resulted
in damage and fatalities (Committee on Seismology, 1980; Lander and Lockridge,
1989~. Additionally, submarine facilities, such as communications cables,
are at risk from these earthquakes as well as from submarine landslides
triggered by earthquakes. The Pacific Tsunami Warning Center at Honolulu,
Hawaii, is an international cooperative effort to provide tsunami watches
and warnings to the Pacific region. Onshore regional seismic networks
contribute to the detection and location of submarine earthquakes that are
potentially tsunamigenic. Needed, but not currently in place, are networks
of ocean-bottom seismographs on U.S. continental shelves to improve detection
and location capabilities in those nearshore areas. The combined onshore
and ocean-bottom seismic networks would allow for a more rapid determination
of focal mechanism and thus a more reliable assessment of the tsunami-
generating potential of shallow offshore events.
Earthquakes are common in volcanic areas, and seismic networks are the
fundamental tool for their study. Data from networks have shown that
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CONTRIBUTIONS OF REGIONAL SEISMIC NETWORKS
15
"volcanic earthquakes," those that result from the thermal and mechanical
forces of volcanic processes (volcanic a-type, high-frequency earthquakes),
are indistinguishable from tectonic earthquakes, which result from the mechanical
fracturing of rock due to tectonic forces. Other volcanic earthquakes (vol-
canic lo-type, low-frequency events) and harmonic tremor (vibrations due to
the shallow movements of magma) have distinctive properties. For example,
studies at Mount St. Helens indicate that harmonic tremor there consists of
a persistent sequence of lo-type earthquakes. Studies in Hawaii and Alaska
have resulted in the development of new models of the sources for the
volcanic shocks that include reverberations within the magma body triggered
by brittle failure of the adjacent rock as well as a point-force reaction to an
explosive eruption. The swarm-like series of magnitude 5.5-6.0 earthquakes
that occurred in 1978 near the Long Valley caldera in eastern California
raises the possibility of yet another type of volcanic earthquake, one due
either to the abrupt injection of magma into a dike or to a complex shear
failure on fault planes of differing orientations (Hill, 1987~. Clearly, much
work remains to be done to understand what the various types of volcanic
earthquakes imply about the volcanic processes that affect the westernmost
states.
The core of the earth has long held a particular fascination and position
of importance because of its inaccessibility and because it is the source of
the earth's magnetic field. Regional networks, when integrated within a
continent-wide National Seismic System, can contribute to its study. Recent
studies of the structure of the core and of its boundary with the mantle
using compressional waves that penetrate through the deep interior of the
earth suggest considerable complexity that could have important geodynamical
and geochemical consequences. It appears that topography of +8 km or so
may be present on the core-mantle boundary. Establishing whether that
boundary is thermal or chemical in nature is important for thermal modeling
of the earth's interior. Also, although the velocity gradients in the outermost
core appear not to be anomalous (as was once thought), and although the
inner core-outer core boundary may indeed be a simple discontinuity, the
first-generation three-dimensional core models indicate greater, not less,
complexity for core structure (Lay, 1987~. The rapid progress made in
imaging these most inaccessible regions testifies to the benefits that can be
reaped from the high-quality data derivable from the larger regional and
global networks.
Finally, the importance of seismological facilities for education deserves
emphasis. This includes not only the training of the nation's seismologists
but also the general education of a broad student population. Terminating
funding for some seismic networks will cause a certain number of research-
ers to seek new avenues of funding in more adequately supported areas of
research. Once these scientists are lost to other research fields, they cannot
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ASSESSING THE NATION'S EARTHQUAKES
easily be reclaimed for seismic network studies even if funding priorities
change. Thus, given the small number of network seismologists to begin
with, a short-term reduction of support will have long-lasting consequences.
Students at universities that operate regional seismic networks unques-
tionably have an enhanced educational experience. The incoming digital
data stream from multiple sensors provides hands-on opportunities to apply
and develop the seismological theories developed in the lecture hall and the
laboratory. Not only can near-real-time analyses be performed, but the
presence of a continually expanding archival digital data base also permits a
full range of thesis and dissertation investigations. The day-by-day, real-
time acquisition of seismic data provides an earth surveillance setting and
format that are particularly dynamic and impart to students an excitement
about earth processes that often lasts a lifetime.
In summary, regional seismic networks have made fundamental contribu-
tions to the estimation of national seismic hazards and strong earthquake
ground motions, the prediction and forecasting of earthquakes, the specification
of the three-dimensional internal structure of the earth, the surveillance for
tsunamis, the study of volcanic earthquakes, and the training of students.
Such worthwhile efforts should be continued and enhanced.
Representative terms from entire chapter:
regional seismic
