Whether one views with awe a sheer wall of rock in Yosemite Valley, Old Faithful geyser in Yellowstone, lava flows in Hawaii, or an image of Earth taken from an orbiting satellite, a sense of wonder about the forces that shape our world is all but inescapable. From earliest times, humans have demonstrated a deep hunger for knowledge about the earth, for an understanding of their surroundings, and how the earth works and affects their lives. This desire to understand the earth finds expression in the origin myths of the world’s cultures, as well as current widespread public interest in knowledge about the earth.1
Within the sciences, the fields known as “solid-earth sciences” have the enviable job of understanding how the earth “works.” The solid-earth sciences deal with the most tangible of objects—the earth beneath our feet, the landscape around us, and the source of the resource abundance of our lives.
“The goal of the solid-earth sciences is: to understand the past, present, and future behavior of the whole earth system. From the environments where life evolves on the surface to the interaction between the crust and its fluid envelopes (atmosphere and hydrosphere), this interest extends through the mantle and the outer core to the inner core. A major challenge is to use this understanding to maintain an environment in which the biosphere and humankind will continue to flourish.2”
The historian Will Durant is widely quoted as suggesting, “Civilization exists by geologic consent, subject to change without notice.” The solid-earth sciences play an essential role in developing the scientific understanding and knowledge that enable humankind to manage its environment wisely. Information about natural hazards, such as volcanoes, tsunamis, and landslides, are obviously critical to decisions about public health, safety, and security. As population has increased, this information has become increasingly important for the mitigation of potential damage from earthquakes, volcanic eruptions, and other earth movements that are the inevitable consequences of living on an active planet.
Many of the resources that make our civilization possible are found in the subsurface realm within the earth’s crust. From this realm we derive fresh water, oil and gas, coal, and minerals. Large areas of the crust contain sedimentary basins that are up to 16 kilometers deep and contain complex and dynamic fluid systems that are only poorly understood. These basins contain faults that cross rail and road systems, and lie beneath many of our major cities that were settled near river mouths—their soft sediments are subject to liquefaction in earthquakes.
“The world is rapidly changing; revolutionary technological advances, demographic growth, competing demands for resources, and increased awareness of the interconnectedness and global scale of many natural science issues are shaping tomorrow’s science needs. Managers, planners, and citizens are demanding more and better scientific information, delivered more rapidly, that will help them make decisions about the world around them.3”
For the past century or so, solid-earth scientists have known that the earth has a crust, averaging 5–10 km thick beneath the oceans and about 35 km thick in continental regions, composed chiefly of minerals containing oxygen and silicon (“silicate minerals”) with varying amounts of aluminum, iron, potassium, sodium, calcium, magnesium, and other elements. This is underlain by a mantle about 3000 km thick, composed chiefly of silicate minerals rich in magnesium and iron; and a core of iron-nickel alloys. In the last half of the 20th century, the most exciting developments in the solid-earth sciences centered on the theory
of plate tectonics. The concepts of sea floor spreading and continental drift changed our view of the earth from a static globe to a dynamic, live planet. We now recognize that the outer part of the earth (the crust and part of the upper mantle, called the lithosphere) is composed of a series of segments, or plates, that are in motion relative to each other. Plates move apart at ridges in the middle of oceans, they slide past each other along fault zones such as the San Andreas Fault of California, and they converge in regions such as the Pacific “Ring of Fire” where one plate descends beneath another. Nearly all earthquakes and volcanoes (with important exceptions such as Hawaii) occur at the boundaries between plates. Continents drift as plates move, coming apart in some places and coming together in others. As they move apart, features such as the Great Basin of the western United States or the Red Sea form, and oceans such as the Atlantic develop. Where continents come together and buckle, mountain ranges such as the Alps or the Himalaya are created. For example, the Appalachians developed some 300 million years ago as Africa approached and collided with North America. The western margin of North America has been an active plate boundary for at least 200 million years, and this activity is directly or indirectly responsible for the spectacular scenery of the region.
Since its inception, the theory of plate tectonics has dominated investigations of the earth and exploration for the mineral and petroleum deposits needed to supply our growing and increasingly technological society. In addition, plate tectonics has captured the imagination of nonscientists the world over. In recent years, technological advances have provided tools that have led to major improvements in our knowledge of the earth, increasing our understanding of the processes acting in the earth’s interior and our knowledge of plate tectonics and how it works. Techniques developed in the petroleum industry now make it possible to drill a deep borehole at an angle and target a particular area of a fault system for investigation with a precision of a few meters, even at a depth of 4 kilometers. Seismometers at the surface of the earth can be placed in arrays to generate high-resolution images of the internal structure of fault zones and volcanoes. Data from the global positioning system (GPS) offer the possibility of determining movements of the earth’s crust with great precision, both across fault zones and also in the vicinity of volcanoes as they become active. Satellites using sophisticated radars can map small positional changes with time, and thus document the slow movements of the earth’s crust (approximately at
the rate that a fingernail grows) that are the surface manifestations of the movements in the earth’s interior that drive the motions of the plates. The recent development of these tools of perception have enabled earth scientists to peer into the subsurface realm with much greater precision, and to significantly increase understanding of this dynamic region of the earth.
In the 1990s, a broad-based community of solid-earth scientists began to discuss how best to utilize these new technologies in an integrated way to advance earth science (“to understand the past, present, and future behavior of the whole earth system”4) and to provide information of critical concern to the public (“more and better scientific information, delivered more rapidly, that will help them make decisions about the world around them”5). This activity ultimately engaged a broad cross-section of the geophysical community in the solid-earth sciences and resulted in the NSF’s EarthScope initiative. With the coordinated and integrated EarthScope elements, the earth science community will have the ability to image earth movements in both the horizontal and vertical plane. By taking advantage of new technological capabilities, it should be possible to derive an image of the United States at a level of detail that has no equal anywhere else in the world.
“EarthScope is a bold undertaking to apply modern observational, analytical and telecommunications technologies to investigate the structure and evolution of the North American continent and the physical processes controlling earthquakes and volcanic eruptions. EarthScope will provide a foundation for fundamental and applied research throughout the United States that will contribute to the mitigation of risks from geological hazards, the development of natural resources, and the public’s understanding of the dynamic Earth.6”
The goals of EarthScope include: to produce the first high-resolution synoptic views of the crust and mantle beneath the United States to generate the first comprehensive maps of crustal deformation
(change in shape or displacement of one crustal block relative to another) in geologically active portions of the continent, and to provide the first look at the inner workings of an active fault system. The EarthScope facility has four interrelated parts: the United States Seismic Array (USArray; see Figure 1), the San Andreas Fault Observatory at Depth (SAFOD; see Figure 2), the Plate Boundary Observatory (PBO; see Figure 3), and the Interferometric Synthetic Aperture Radar (InSAR; see Figure 4). These facilities are described briefly in Boxes 1 to 4, quoted from EarthScope descriptive materials.7
USArray is a dense network of portable and permanent seismic stations that will allow scientists to image the details of Earth structure beneath North America. Over the course of a decade, using a rolling deployment, a transportable array of 400 broadband seismometers will cover the continent with a uniform grid of 2000 sites. As the array moves across the country, 2400 additional sensors will support high-resolution investigations of key geological features. At some sites, special instruments will record electrical and magnetic fields to provide constraints on temperatures and fluid content within the lithosphere. In coordination with the US Geological Survey’s Advanced National Seismic System, observatory-quality permanent stations will be installed to extend the temporal and spatial continuity of earthquake monitoring. By studying the recorded waveforms of hundreds of local, regional and global earthquakes, and large explosions from mines and quarries, scientists will be able to identify and map subtle differences in the velocity and amplitude of seismic energy traveling through Earth. These observations will result in a vastly improved ability to resolve geological structures throughout the entire crust and upper mantle and into Earth’s deepest interior.
SAFOD is a 4-km-deep observatory drilled directly into the San Andreas fault zone near the nucleation point of the 1966 magnitude 6 Parkfield earthquake. The project will reveal the physical and chemical processes acting deep within a seismically active fault. Initially, fault-zone rocks and fluids will be retrieved for laboratory analyses, and intensive downhole geophysical measurements will be taken within and adjacent to the active fault zone. The observatory’s long-term monitoring activities will include decades of detailed seismological observations of small- to moderate-sized earthquakes, and continuous measurement of pore pressure, temperature and strain during the earthquake cycle. SAFOD will provide direct information on the composition and mechanical properties of faulted rocks, the nature of stresses responsible for earthquakes, the role of fluids in controlling faulting and earthquake recurrence and the physics of earthquake initiation and growth. Drilling, sampling, downhole measurements and long-term monitoring will allow testing of a wide range of hypotheses
about faulting and earthquake generation, and the pursuit of a scientific basis for earthquake hazards assessment and prediction.
PBO is a distributed observatory of high-precision geodetic instruments designed to image the ongoing deformation of western North America. The geodetic network will extend from the Pacific coast to the eastern edge of the Rocky Mountains, and from Alaska to Mexico. Two complementary instrumentation systems—global positioning system (GPS) technology at about 1000 sites, and ultra-low-noise strainmeters at 200 locations—will provide superior time resolution. A sparse GPS network at 100–200 km spacing, connecting
At the request of the National Science Foundation, the National Research Council (NRC) appointed a committee to review the science objectives and implementation planning of the three NSF components of the EarthScope initiative: USArray, SAFOD, and PBO. Although not formally asked to examine the InSAR element, the committee was asked to assess the overall EarthScope scientific objectives after considering the integrated nature of the entire initiative. In particular, the committee was asked to consider the following questions:
Is the scientific rationale for EarthScope sound; are the scientific questions to be addressed of significant importance; and are the proposed methods appropriate?
Is there any additional component that should be added to the EarthScope initiative to ensure that it will achieve its objective of a vastly increased understanding of the structure, dynamics, and evolution of the continental crust of North America?
Are the implementation and management plans for the three elements of EarthScope reviewed here appropriate to achieve their objectives?
Have the appropriate partnerships required to maximize the scientific outcomes from EarthScope been identified in the planning documents?
The committee on the Review of EarthScope Science Objectives and Implementation Planning reviewed a great deal of written information on EarthScope, including workshop reports, proposals, and, most importantly, the July 2001 draft Project Plan. The committee had the benefit of a series of excellent briefings from members of the EarthScope Working Group (Box 5), who had recently prepared this draft Project Plan. In addition, the committee heard presentations and had discussions with other members of the solid-earth sciences community, with other interested scientists, and with NSF staff.
Although a final EarthScope scientific planning statement was not available at the time of the committee meeting, it was clear from presentations to the committee and from the extensive background scientific planning elements included within individual component white papers that scientific planning is well advanced, and accordingly the committee believed that it was able to assess the project and make recommendations.
Planning for the individual components of EarthScope had been taking place for as long as a decade prior to their integration by NSF into the EarthScope initiative in 1999. The SAFOD proposal was developed throughout the 1990s, with community workshops contributing to the proposals and expert panel evaluations that were submitted to the sponsoring agencies—the U.S. Geological Survey (USGS), the NSF, and the U.S. Department of Energy (DOE). Present plans and the project time-line are summarized in a SAFOD “fact sheet."a
USArray was first proposed in the mid-1990s under the auspices of IRIS (Incorporated Research Institutions for Seismology), and the experience of the IRIS community (made up of 93 member institutions and supported by NSF) in instrumentation development, data management, and instrument deployment and support is a key element of the EarthScope plan. The USArray Steering Committee has continued planning for USArray deployment, including presentation of a “white paper”b describing the science goals and facility implementation plans.
The Plate Boundary Observatory concept was developed over the past 4 years, again using NSF-funded workshops as the prime medium for project refinement. As a result of a workshop held in October 1999, the PBO Steering Committee produced a white paperc presenting a detailed scientific justification, deployment strategy, education and outreach plans, and an outline of the relationships between PBO and the other EarthScope initiatives.
Unlike the other three components of EarthScope, development of the InSAR concept from the early 1990s was largely supported as a NASA initiative, and NASA retains the lead role in its continuing development. The latest plans indicate that operations and science support costs for the InSAR component within an integrated EarthScope would require the contribution of $150 million by NASA together with $100 million by NSF.
After NSF integrated the individual components into the EarthScope initiative in 1999, an EarthScope Working Group was established to assume responsibility for organizing the community planning workshops that have refined the integrated science objectives and produced a draft Project Plan.d This project plan indicates that the total cost to the NSF Major Research Equipment (MRE) account for the 10-year duration of the initial three EarthScope elements—USArray, SAFOD, and PBO, will be $356 million, of which
$172 million will be for facilities, with the remainder budgeted for operations ($72 million) and science management ($112 million). The original MRE FY2001 budget request to Congress (at that time including only USArray and SAFOD) was not funded, although the notation “without prejudice” added by Congress encouraged NSF to prepare an FY2003 budget request, this time including the USArray, SAFOD, and PBO elements. Planning for the InSAR element continues, in a partnership between NSF and NASA.
The committee’s answers to the questions posed in the charge indicate a consensus that the EarthScope project should proceed apace. The committee concludes that the scientific rationale for EarthScope is sound, that the scientific questions posed are of great importance, and that the methods are appropriate. The committee considers that the components that have been identified are comprehensive, and that there are no additional components to be developed. The implementation and management plans at this early stage of the project are appropriate, although it will be critical to develop a detailed plan to address issues of integration as the project proceeds. Finally, the committee considers that appropriate partnerships have been identified by the EarthScope proponents, but encourages the continued development of these partnerships with the ocean science community and with programs and colleagues in Canada and Mexico.