USArray draft manuscript

USARRAY DRAFT MANUSCRIPT (1 December, 1998)

USArray - Bridging the gap between regional seismic experiments and global networks

Peter Shearer, Anne Meltzer, Goran Ekstrom, Gene Humphreys and Alan Levander

Seismology is a data-driven science, in which important discoveries about the Earth typically follow the deployment of new instruments and networks. In the 1960s, the World Wide Standardized Seismograph Network (WWSSN) sparked breakthroughs in our understanding of earthquake sources, and led to greatly improved earthquake locations and much more detailed seismic velocity models. More recently, the digital stations of the Global Seismic Network (GSN) have revolutionized our understanding of deep Earth structure. The bulk of the GSN stations were deployed through the efforts of IRIS, a consortium of seismology researchers funded (primarily) by the U.S. National Science Foundation. IRIS also has provided a pool of portable instruments (the PASCALL program) that can be used by different groups for regional experiments. The value of seismic data is greatly increased if they are readily available to the seismological community in a standard format. Again IRIS has taken the lead, and data from both the GSN and the PASCALL experiments are archived and distributed from the IRIS Data Management Center (DMC) in Seattle. The IRIS archive currently contains about 7 terabytes of data and last year serviced over 45,000 data requests from seismologists all over the world.

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 WWSSN network, but data from many of these stations have only been available for a short time.

Today, as we near the new millennium, the best dataset for studying the overall structure of the North American continent remains 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) . Dense station coverage is required to fully understand small-scale structure in this region, in particular the structures implied by the observed variability in D" anisotropy and ultra-low velocity zones.

In some cases, regional seismic networks, such as the short-period arrays in the western United States, have been used to study deep Earth structure as well as resolve details of teleseismic sources. 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 near subduction zones (e.g., Benz, Vidale and Mori, 1994). Individual PASCALL 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 PASCALL 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 two main components: (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 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)

Expanding the USNSN

The United States National Seismic Network (USNSN) has never fully achieved its potential. Originally intended to span the United States with seismic stations, financial pressures limited the station coverage, particularly in the midwest, and prevented the timely distribution of the data to the seismological community. The network consists of high-quality, broadband stations, but because USNSN data have only recently begun to flow routinely to the IRIS DMC, the network has been an underutilized resource in seismic research. The USNSN, however, remains of key importance for monitoring U.S. seismicity and providing an intermediate-scale link between the dense networks in the western United States and the relatively sparse coverage of the global seismic networks. The USArray project hopes to revitalize the USNSN by adding stations to provide more complete coverage and working with the USGS to ensure that the data continue to flow to the IRIS DMC in a timely and reliable fashion. By adding 30 new stations to the current U.S. broadband stations, approximately uniform coverage could be achieved at 300 to 400 km spacing.

A transportable broadband network

The USNSN alone cannot provide the station density required to fully resolve the upper mantle structure beneath the United States. USArray proposes to cover the continental United States with 2000 stations in a ten year program of 6- to 12-month deployments of transportable, broadband networks. A similar strategy (on a much smaller scale) was recently used with great success by the Skippy Project in Australia. Each USArray network will record enough teleseisms to image crust and upper mantle structure using a variety of seismic techniques. The results from each deployment will be tied together, both by station overlap along the edges of the networks and by the permanent stations of the expanded USNSN. The final integrated data set will provide tomographic and other images of the North American upper mantle that will have unprecedented resolution.

In addition to the uniformly spaced stations, the transportable USArray network will also include broadband and/or 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 become involved with the program, and a regional, pre-deployment workshop would be held for each stage of the USArray deployment to help plan these experiments.

What we will learn

Whenever a new source of seismic data becomes available, unexpected new discoveries have often resulted that have overshadowed the original purpose of the data collection. Almost certainly USArray will yield many such bonuses. There are, however, a number of major results that we can be sure the USArray will provide.

(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 poorest. 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 far 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 PASCALL experiments in the United States have produced intriguing patterns (see Figure 3), 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 PASCALL 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 4 ). 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 work 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 4). 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 distinguish between the effects of thermal and chemical heterogeneity in the lowermost mantle.

How it will work

At this point the USArray idea is based on two key seismic facilities:

(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.

(2) A transportable network of 100 to 200 broadband, three-component stations to be fielded in an organized sequence of 6 to 12 month deployments that, over a ten year period, 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). In addition, a flexible part of the network will allow densification of the instruments to study special targets of interest during each deployment period. These experiments will be organized by IRIS, in coordination with local research institutions, and the data will be immediately available to all researchers through the DMC.

At its heart, USArray will be a seismic experiment, organized by IRIS, that will provide a huge increase in the ability of seismologists to resolve key questions about the structure and of the North American continent and the underlying mantle and core. But is also hoped to facilitate involvement with other Earth scientists in a systematic and comprehensive surveys that could be coordinated with the USArray deployments. In addition, the USArray project can be used as a focus for educational and outreach efforts across the United States.

Many details concerning the scope of the USArray project remain to be decided, and, of course, much will depend upon the level of funding. A workshop is planned in March, 1999, to solicit input from a broad community of Earth scientists and formulate a specific set of recommendations for a proposal to NSF.

References

Benz, H.M., J.E. Vidale and J. Mori, Using regional seismic networks to study the Earth's deep interior, EOS Trans. AGU, 75, 225-229, 1994.

Dueker, K.G. and A.F. Sheehan, Mantle discontinuity structure from midpoint stacks of converted P to S waves across the Yellowstone hotspot track, J. Geophys. Res., 102, 8313-8327, 1997.

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, 11,591�11,621, 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.

van der Hilst, R., B. Kennett, D. Christie and J. Grant, Project Skippy explores the lithosphere and mantle beneath Australia, EOS Trans. AGU, 75, 177-182, 1994.

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