These accomplishments, and the research results they have spawned, attest to the success of
seismologists in the last decade in organizing and fielding new networks and experiments. Yet
there is a gap in the ability of current seismic instrumentation to resolve key questions about the
structure of the North American continent and probe more deeply into the mantle and core. This is
illustrated in
Figure 1,
which compares the station coverage across the United States achieved by
the old WWSSN network, compared to the GSN network (as of 1997) and current broadband
stations, including those of the United States National Seismic Network (USNSN) and assorted
regional seismic networks. Until very recently, only the GSN data have been available to
seismologists in standard format from the IRIS DMC. The station coverage of the WWSSN is far
superior to that of the GSN; current broadband stations provide coverage comparable to the old
WWSSN network, but with very non-uniform spacing across the United States.
Today, as we near the new millennium, seismic data sets for studying the overall structure of the North American continent have not improved significantly from the analog data of the WWSSN, recorded during the 1960s and archived in film chip libraries at Caltech and Lamont. The most comprehensive tomography models of the upper mantle beneath North America, obtained by Steve Grand at the University of Texas (e.g., Grand, 1994), were obtained by hand-digitizing thousands of WWSSN seismograms. The advantages of the WWSSN coverage are not limited to studies of North America; these data are also useful in probing deep Earth structure. The coverage provided by the current distribution of global seismic stations, while invaluable for constraining Earth's large-scale structure, is inadequate to resolve small-scale features at depth. For example, in studies of the core-mantle boundary (CMB) region, certain patches on the CMB are illuminated far more closely by WWSSN stations in the United States than they are with GSN data. For this reason, researchers have exploited WWSSN data for many years to study the CMB region (for a review, see Lay, Williams and Garnero, 1998). But even the WWSSN station coverage is inadequate to fully resolve many details of CMB structure, including the small-scale features implied by the observed variability in D" anisotropy and ultra-low velocity zones. To avoid spatial aliasing, much finer station spacing is required.
In some cases, finer station spacing is provided by regional seismic networks, such as the short- period arrays in the western United States. For example, John Vidale, Harley Benz and others have used the California networks of the U.S. Geological Survey to study both the CMB and upper mantle discontinuities (e.g., Benz, Vidale and Mori, 1994). Individual PASSCAL experiments also have proven valuable for probing both upper-mantle discontinuity structure and resolving upper mantle anisotropy via shear-wave splitting observations. But these studies are limited by the size of the experiments and do not provide a direct link to global-scale models. For example, receiver function analyses have recently been used to resolve topography on the 410- and 660-km discontinuities for two different PASSCAL profiles (500 to 1500 km long) in the United States (Dueker and Sheehan, 1997; Li et al., 1998). The results are very interesting, but it is difficult to connect these localized measures of discontinuity depth variations to global maps of discontinuity topography based on SS precursors (e.g., Flanagan and Shearer, 1998) due large differences in resolution and coverage between the experiments.
USArray is a proposed project that would fill the gap between the GSN and regional experiments.
It consists of several components, including: (1) An expansion and infilling of the USNSN to a
more uniform station spacing
(Figure 2, top),
(2) Systematic deployment of 100 to 200 portable
telemetered broadband instruments in a sequence of 6- to 12-month-long experiments to achieve, in
a decade, records from 2000 stations uniformly distributed across the United States
(Figure 2, bottom),
and 3) a group of high frequency and broadband instruments forming a flexible
component to the portable array. In local regions of particular interest, these additional stations
would be deployed in experiments designed to target specific problems. Lastly, a strong
educational and outreach program is planned to coordinate with local institutions and media outlets
as the experiments move to different parts of the country.
This article presents an overall outline of the proposed USArray facility; a companion article (Levander et al., 1999, this issue) focuses on the potential of USArray for investigations of the North American continent.
In addition to the uniformly spaced stations, the transportable USArray network will include broadband and short period instruments designed to densify parts of the array in local regions of interest. As the USArray network moves to different parts of the country, local experiments can be coordinated with the station deployments. This part of the USArray project will provide an opportunity for local universities and research groups to target specific geologic problems, and a regional, pre-deployment workshop would be held for each stage of the USArray deployment to help plan these experiments and coordinate educational and outreach efforts.
(1) The expansion of the USNSN will enable earthquake locations and focal mechanisms for continental U.S. events to be computed to much lower magnitude levels than are now possible. The impact of this will be greatest in the central and eastern United States where the station coverage is currently the sparsest. The USNSN observations will answer the question as to whether the lack of earthquakes in many of these regions is real or merely an artifact of the inadequate station coverage. The seismic hazard in the central and eastern United States is often under appreciated; an improved USNSN will provide the basis for expanded research into earthquake properties and strong motion potential in these regions.
(2) Vastly improved tomographic models of the crust and upper mantle beneath the United States will result from analyses of the 2000 sites covered by the transportable array, as well as the fixed stations of the USNSN. These models will involve P- and S-wave travel time inversions, surface wave analyses, and detailed waveform modeling. The Australian Skippy Project (van der Hilst, 1994), a smaller-scale version of the proposed USArray experiment, has already led to much higher resolution tomography models for the Australian continent than were previously available. The USArray will do even better, and help solve many of the key scientific issues regarding the history and evolution of North America.
(3) Maps of teleseismic shear-wave splitting orientations and delay times will permit the mapping of upper-mantle anisotropy in greater detail than is now possible. The orientation and strength of upper-mantle anisotropy is a key seismic constraint on the deep structure and history of the North American shield and its more tectonically active borders. Results from regional networks and individual PASSCAL experiments in the United States have produced intriguing patterns, but the results are too scattered to yield definitive interpretations across much of North America. The three-component, broad-band stations of the USArray will provide ideal data for shear-wave splitting studies.
(4) Receiver function analyses will map crustal thickness and upper mantle discontinuity topography at much greater resolution than is now possible. The PASSCAL studies by Dueker and Sheehan (1997) and Li et al. (1998) have shown that depth variations to the 410- and 660-km discontinuities can be mapped using temporary station deployments. Analyses of individual profiles, however, are complicated by the possible biasing effects of off-axis structure, which cannot be resolved with linear arrays. The full two-dimensional network of USArray stations will permit a more complete mapping of the discontinuity topography and increase the effectiveness of migration techniques in data processing.
(5) In addition to its advantages for local and regional studies, the broadband USArray will provide a powerful tool for probing the deep structure of the mantle and core. At long periods, the 2000 stations will form a coherent array suited to beam-forming, wavenumber analyses, downward continuation and other methods. At short periods, the individual waveforms will not be coherent, but analyses of travel times, polarizations and other observables will suffer from far less spatial aliasing than current studies.
(6) The USArray stations will record teleseisms at a variety of distances, but particularly
dense coverage will be achieved at ranges between 80 and 130 degrees
(see Figure 3).
These ranges are
ideal for studying the CMB region, using such seismic phases as S, ScS, P, PcP, Pdiff, SKS, and
SKKS. Some of the most important studies on the D" and CMB regions have analyzed old
WWSSN records from stations in the United States. These studies have suggested rapid lateral
variation in properties near the CMB that are difficult to image with the sparse coverage of the
available data. The USArray will provide a major increase in our ability to study the CMB region,
and resolve details of recent discoveries such as D" anisotropy and ultra-low velocity zones.
(7) Short-period PKP precursors provide constraints on deep scattering from small-scale structures in the mantle. The proposed USArray stations are favorably located to observe these precursors as they will record many teleseisms at ranges between 120 and 135 degrees (see Figure 3). The high station density should resolve ambiguities between source- and receiver-side scattering and image the locations of scatterers in the lower mantle and D" regions. These analyses will help to gauge the relative strengths of thermal and chemical heterogeneity in the lowermost mantle.
(1) An expanded USNSN, with close cooperation between IRIS and the USGS to ensure the timely flow of high-quality data to the IRIS DMC. This is vital to ensure the continued health of earthquake studies across the U.S. and to provide reference stations to calibrate the other USArray networks. The data will be immediately available to all researchers through the DMC.
(2) A transportable network of 100 to 200 broadband, telemetered, three-component stations fielded in an organized sequence of 6 to 12 month deployments over a ten year period. This will cover the United States with 2000 stations at nearly uniform density (this number assumes ten deployments of 200 instruments, the total number of sites will be between 1000 and 4000 depending upon the size of the network and the deployment period). The data will be immediately available to all researchers through the DMC.
(3) A flexible part of the transportable network, and additional high-frequency seismographs, to permit densification of the instruments to study special targets of interest during each deployment period. The details of how these experiments would be planned and organized remain to be determined, but strong involvement by individual investigators is expected.
At its heart, USArray will be a seismic experiment, organized by IRIS, that will provide a dramatic increase in the ability of seismologists to resolve key questions about the structure and evolution of the North American continent and the underlying mantle and core. It is also hoped to facilitate involvement with other Earth scientists in a systematic and comprehensive surveys that could be coordinated with the USArray deployments (see companion article). In addition, the USArray project can be used as a focus for educational and outreach efforts across the United States.
Many details concerning the USArray project remain to be decided, and, of course, much will depend upon the level of funding. A workshop is planned for March 15-17, 1999, to solicit input from a broad community of Earth scientists and formulate a specific set of recommendations for a proposal to NSF.
For more information, contact Peter Shearer, IGPP 0225, University of California, San Diego, La Jolla, CA 91093-0225 (pshearer@ucsd.edu).
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Flanagan, M.P. and P.M. Shearer, Global mapping of topography on transition zone velocity discontinuity by stacking SS precursors, J. Geophys. Res., 103, 2673-2692, 1998.
Grand, S.P., Mantle shear structure beneath the America and surrounding oceans, J. Geophys. Res., 99, 11,591Ð11,621, 1994.
Lay, T., Q. WIlliams and E.J. Garnero, The core-mantle boundary layer and deep Earth dynamics, Nature, 392, 461-468, 1998.
Li, A., K.M. Fischer, M.E. Wysession and T.J. Clarke, Mantle discontinuities and temperature under the North American continental keel, Nature, 395, 160-163, 1998.
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