The USArray Initiative

USArray Steering Committee: Anne Meltzer (Coordinator), Roberta Rudnick , Peter Zeitler, Alan Levander, Gene Humphreys, Karl Karlstrom, Göran Ekström, Rick Carlson, Tim Dixon, Michael Gurnis, Peter Shearer, Rob vander Hilst

Introduction. Images powerfully shape our thinking about nature. Consider what our understanding of North American tectonics would be like if our best image of the continent's topography was as blurred as the one shown below, at left (Figure 1). First-order features like the Cordillera are barely resolved and the characteristic topography within provinces like the Basin and Range and Great Valley are obscured beyond recognition. Yet it is precisely such a fuzzy view of the lithosphere and deeper mantle that we currently bring to the four-dimensional problem of understanding the structure, evolution, and dynamics of the North American continent.

At a workshop in Albuquerque last March, jointly sponsored by the National Science Foundation and IRIS (Incorporated Research Institutions for Seismology), seismologists and geologists discussed the design and implementation of an ambitious plan to explore, map, and develop an integrated understanding of North American geology. Proposed is the development of a facility, USArray, that amounts to a "Hubble telescope for the Earth sciences" (Levander, et al., 1999). In this article, we would like to introduce this initiative to the broad earth-science community represented by GSA and invite input and participation.

Overview of USArray. The USArray initiative will integrate a dramatic improvement in the resolution of seismic images of the continental lithosphere and deeper mantle with a diversity of geological data to address significant unresolved issues of continental structure, evolution, and dynamics. The USArray facility will consist of three major seismic components: (1) several dozen permanent high-quality seismic stations administered largely by the USGS within the context of the national seismic network; (2) a transportable array of broadband seismometers that will systematically cover the US one region at a time; and (3) ~2400 seismometers of various types designed to augment the transportable array so that a great range of specific targets can be addressed in a focused manner. The goal of this layered design is to achieve imaging capabilities that flexibly span the continuous range of scales from global, through lithospheric and crustal, to local.

The core of USArray is a transportable telemetered array of 400 broadband seismometers designed to provide real-time data from a regular grid with dense and uniform station spacing of ~50 km and an aperture of ~1000 km. The array will record local, regional, and teleseismic earthquakes, providing resolution of crustal and upper mantle structure on the order of tens of kilometers and increased resolution of structures in the lower mantle and core-mantle boundary. Approximately 50 magnetotelluric field systems will be embedded within the array to provide constraints on temperature and fluid content within the lithosphere. The transportable array will roll across the country with 1-2 year deployments at each site. Multiple deployments will cover the entire continental US over a period of 8-10 years, providing unprecedented 3-D seismic coverage. When completed, this will amount to systematic imaging from approximately 2000 seismograph stations (Figure 2). While the initial focus of USArray is coverage within the United States, clearly the continent does not stop at US borders or the coast. Extensions of the array into neighboring countries and onto the continental margins in collaboration with scientists from Canada, Mexico and the ocean science community are natural additions to the initiative (Figure 2).

An important second component of USArray is an additional pool of ~2400 instruments (broadband, short period, and high frequency) that can be deployed using flexible source-receiver geometries. These additional portable instruments will allow for high-density, shorter-term observations of key targets within the footprint of the larger transportable array using both natural and explosive sources. This component of USArray offers exciting opportunities for a variety of focused investigations requiring high-resolution images within the context provided by the larger array. Linked with coordinated geological, geochemical, and/or geodetic studies, this part of USArray can address a wide range of problems in continental geodynamics and tectonics. Examples include imaging and study of the continental arc system in the Cascades from slab to edifice; examination of the deep roots of the North American craton and the paleotectonics by which the craton was formed; imaging old and modern orogens and rifts to determine secular variation in continental tectonics; and identifying the role of the mantle lithosphere during orogenesis and rifting.

A third component of USArray is the installation of a permanent network to provide fixed reference points for calibration of the transportable array, covering the continent with a uniform spacing of 300-350 km. The permanent network also adds an important fourth dimension -- time -- to the USArray facility, by providing a platform for continuous long-term observations. This component of USArray will be undertaken in coordination with the US Geological Survey and complements the initiative underway at the Survey to install an Advanced National Seismic System. Some or all of the stations of the permanent component of USArray will be equipped as expanded geophysical observatories, with GPS receivers to provide direct real-time data on crustal deformation. The potential also exists for other geophysical instruments to take advantage of these platforms.

The elements of USArray can realize, individually and in concert, a number of scientific goals. A complete list is beyond the scope of this informational piece, but examples include:

o mechanisms of continent formation and breakup;

o relationship between crustal tectonic provinces and upper mantle structure;

o rheological stratification of the lithosphere with depth and its variation from orogenic belts to the cratonic interior of the continent;

o nature of the Moho, and mass transfer between the crust and mantle;

o lithospheric deformation and earthquake hazard assessment;

o variations in depth and sharpness of the 410 and 670 km seismic discontinuities and their relations to surface tectonics;

o heterogeneity, anisotropy, and flow in the mantle;

o role of fluids (magmas, partial melts, hydrothermal) in the crust;

o crustal recycling during subduction and orogenesis

USArray has been designed to provide a structured yet flexible foundation for integrated studies of the continental lithosphere and deep Earth structure over a wide range of scales. The outcome will be an integrated "whole continent" view of North America and improved understanding of the processes that have shaped and continue to shape the continent.

As a highly visible, science-driven initiative, USArray will play an important role in educating the public about the Earth sciences and science in general. As USArray moves across the country it will be accompanied by a comprehensive educational and outreach program highlighting both overarching and regional Earth-science issues (hazards, structures, resources) and links between Earth science and society. Specific initiatives will include coordination with local news media, museum displays, teacher workshops, and distribution of educational materials, including activities linked to USarray data and results made available over the Internet. Programs will be designed and targeted to engage communities in USArray before, during, and after passage of the array through specific regions of the country.

Status and Organization of the Initiative. An initiative of USArray's scope obviously requires a number of partnerships between the academic Earth Science community and a variety of organizations including the National Science Foundation, the U.S. Geological Survey, regional seismic networks, state geological surveys, IRIS, UNAVCO (University NAVSTAR Consortium), and EMSOC (Electro-Magnetic Sounding Consortium). International partnerships and collaborations with industry will also be important as the project matures.

At NSF, Program Officers of the Division of Earth Sciences (EAR) have united several intertwinning streams of research into a single integrated effort known as "EarthScope - A Look into Our Continent". EarthScope includes USArray and initiatives for a Plate Boundary Observatory (PBO; Silver, 1998), the San Andreas Fault Observatory at Depth (SAFOD); and Interferometric Synthetic Aperture Radar (InSAR).

An ambitious plan like USArray will require that significant new resources be allocated to the Earth sciences. An appropriate source of support for the facilities component of USArray is the MRE (Major Research Equipment) account, an NSF-wide program created in FY1995 to provide funding for the construction and acquisition of major research facilities that are beyond the funding resources of any one Directorate. MRE projects advanced by a Directorate are reviewed in a Foundation-wide competition and require approval by the National Science Board. The MRE account is funded as a separate item within the NSF budget, distinct from support for research programs, but it is important to note that research budgets at the Directorate or Division level are augmented to facilitate research to be carried out with these new facilities. Previous successful MRE proposals have included airplanes, radio telescopes, and similar large-ticket items. Other scientific disciplines (e.g., astronomy and physics) commonly champion successful MRE proposals for support in the range of tens to hundreds of million dollars, but to date no such proposal in support of Earth-science research has been forwarded or funded.

The EAR Division and GEO Directorate at NSF and members of the steering committees of the initiatives involved have worked together to develop what will hopefully be a successful proposal to the MRE competition. The good news to date is that the Earthscope initiative, of which USARRAY and SAFOD are Phase I components, has been advanced and received approval within NSF to move forward for consideration by the National Science Board.

Next Steps. How will the USArray initiative move forward? The challenge of developing the techhnical facility should be seen as only one component of USArray. There is broad interest in exploring ways to leverage the resources associated with the USArray facility to catalyze establishment of a fully multidisciplinary field laboratory and to determine the diverse data sets and measurements that should become part of an integrated North American geoscience information system. This is important because perhaps the most attractive part of the USArray initiative is its potential to unite North American geologists and geophysicists into a broad coalition of Earth scientists devoted to a decade or more of multidisciplinary studies of the continent. In so doing, in the spirit of the highly successful Lithoprobe program in Canada (Clowes, 1998), USArray stands to expand the culture of shared and coordinated resources within the Earth sciences as a whole. A further goal of USArray is to develop a geoscience information system including geophysical, geochemical and geological data that can be easily accessed by the Earth science community, educators, and government agencies. All data from USArray will be archived and available in near-real time to the community at large.

Over the coming year and beyond, the USArray steering committee will continue to seek community input and involvement in a variety of ways. The initial workshop held in Albuquerque led to substantial enthusiasm and momentum for USArray, increased recognition that this initiative needs to integrate geological and geophysical investigations into a single unified effort to best achieve its scientific goals, and formation of the steering committee, which comprises Earth scientists having a range of perspectives. A second workshop was held in September in Houston with the express purpose of involving a broad spectrum of Earth scientists in the early planning stages of USArray, in particular to define its multidisciplinary component and to identify ways in which USArray can best be used to advance Earth-science research, education and outreach. The USArray steering committee is coordinating workshop reports as well as a science and implementation plan that will be submitted to NSF as the community's input to USArray's role in the Earthscope MRE initiative. Forums are also planned at upcoming national meetings (autumn meetings of GSA and AGU) for further discussion of the opportunities provided by USArray to facilitate research in continental structure and dynamics across the geoscience communities. Finally, because the MRE competition will extend over the next 12-15 months and because USArray will take 3-4 years to come into operation if approved, there will be ample opportunity for all those having interests in the structure and evolution of North America to become engaged in the project.

For USArray to succeed and realize its full potential, the Earth-science community will have to provide enthusiastic, broad-based support for the initiative. All of us will have to come together to marshal compelling scientific arguments that this initiative can revolutionize our understanding of the continent on which we live. Our colleagues in other disciplines regularly organize themselves to their advantage, and it is our belief that USArray is a golden opportunity for Earth scientists to do the same.

References

Clowes, Ron M., Cook, Fred A., Ludden, John N, 1998, Lithoprobe leads to new perspectives on continental evolution, GSA Today, 8,1-7.

Ellsworth et al. REFERENCE

INSAR REFERENCE

Levander, Alan, Humphreys, Eugene D., Ekstrom, Goran, Meltzer, Anne S., Shearer, Peter M., 1999, Proposed project would give unprecedented look under North America, EOS, 80, 245, 250-251.

Silver, P., et al., 1998, A plate boundary observatory, IRIS Newsletter v. 16, n2, 3, 7-9.

Simpson, D. and Anders, M., 1992, Tectonics and topography of the westernUnited States - an application of digital

mapping, GSA Today, 2, 117-121

Figure Captions

Figure 1. Topography of western US at different degrees of resolution. Left, filtered at 500 km, a resolution similar to that of current global tomographic models. Right, at resolution of 1 km (Simpson and Anders, 1992). As designed, USArray will provide resolution of crustal and upper mantle structure on the order of tens of kilometers.

Figure 2 Coverage provided by the transportable component of USArray after installation of 2000 stations in the continental US and Alaska with augmentation in Canada, Mexico, and the continental margin.

Footnote

More information about USArray can be obtained from:

http://www.iris.edu;

http://www.iris.edu/newsletter/EE.Fall98.web/usarray.html and

Ekstrom et al., (1998; IRIS Newsletter v16, n2, 2-6),

Comments and suggestions about the initiative can be directed to any of the members of the USArray Steering committee:

Anne Meltzer(Coordinator), Lehigh University, asm3@lehigh.edu

Rick Carlson, Carnegie Institute of Washington, carlson@clrs1.ciw.edu

Tim Dixon, Miami University, tim@corsica.rsmas.miami.edu

Göran Ekström, Harvard University, ekstrom@seismology.harvard.edu

Michael Gurnis, California Institute of Technology, gurnis@gps.caltech.edu

Gene Humphreys, University of Oregon, gene@newberry.uoregon.edu

Karl Karlstrom, University of New Mexico, kek1@unm.edu

Alan Levander, Rice University, alan@geophysics.rice.edu

Roberta Rudnick, Harvard University, rudnick@eps.harvard.edu

Peter Shearer, University of California, San Diego, shearer@mahi.ucsd.edu

Paul Silver, Carnegie Institute of Washington, silver@clrs1.ciw.edu

Rob van der Hilst, Massachusettes Institute of Technology, hilst@quake.mit.edu

Peter Zeitler, Lehigh University, pkz0@lehigh.edu