Scientific Rationale and Science Questions
The overriding question underpinning the EarthScope initiative is, “If we know in detail the anatomy of the United States lithosphere, and what lies beneath it, and how it is moving today, can we then understand how it works?” This is really equivalent to asking whether detailed knowledge of one place will provide an understanding of the general processes that operate on our planet. The answer to this broad question is almost certainly “yes”—earth scientists have always made general advances in this way—but here the approach being tried is on a much grander scale than was ever possible before. Examples of the more specific questions and processes that EarthScope seeks to address are:
How do earthquakes result from the accumulation of strain (the change in shape from an original configuration) in the earth? If we can see how strain accumulates and know the properties of materials at depth in a fault zone, can we recognize signals that tell us that an earthquake is imminent?
To what extent are the present motions on the surface influenced by the inherited effects of previous events over the earth’s long history as preserved in the rocks within and beneath the North American continent?
What is beneath the major geological features at the earth’s surface? Do they continue at depth? If so, what is their shape? Are they a result of processes occurring deep in the mantle beneath the
continents, or are they inherited from earlier plate boundary processes?
How is molten rock (magma) generated in the mantle? How does it migrate upward through the mantle and crust? How long does magma stay in shallow chambers in the crust prior to eruption?
With the knowledge of how elevations over the surface of a volcano vary with time, and an understanding of the earthquake activity and density variations at depth beneath it, are there characteristic features that might provide advanced warning of volcanic eruptions?
North America is the ideal natural laboratory to address these questions. It is a large, geologically varied continent with a complex active plate boundary along its Pacific margin and with many other sparsely but significantly active regions. It has a long geologic history extending back in time over 3 billion years, and boasts a robust, active, superbly qualified earth science community and the world’s most advanced electronic communication network. Because techniques are now being developed that will allow this earth science community to image the earth in unprecedented detail, the emerging challenge is to exploit the new imaging capabilities to develop as complete a picture as possible of the interior of our planet, its evolution, and its present-day deformation.
THE EARTHSCOPE INITIATIVE
EarthScope in its broadest sense is composed of two parts. The first part is the installation of equipment to gather data, and the development of the mechanisms to make those data widely available. The second part involves the exploitation of the accumulating data to address specific scientific questions of both national and regional importance that will enhance our scientific understanding of the North American continental lithosphere. The scientific goals and objectives of EarthScope are ambitious and challenging. Achieving them will require both a high degree of collaboration among diverse earth science disciplines, and the ability to interpret and synthesize a wide range of scientific evidence. That this collaborative process is already underway is clearly shown by the numerous community workshops and meetings that have been used to help develop and refine the EarthScope initiative.
The degree of collaboration to date is impressive and the committee is confident that this collaboration will continue and expand.
The primary tools of EarthScope mainly involve the fields of geophysics and remote sensing. The effectiveness of the scientific interpretation of the geophysical data, however, will depend upon the widest possible knowledge of geology, rock properties, geochronology (determination of the age of a given rock body), and the results from many other fields within earth science, including geomorphology, petrology, structural geology, tectonics, and soil science. The nature of the available geological information is quite variable throughout the United States. In most areas, modern geological maps are available. In some regions, however, the best available mapping is of only a reconnaissance nature; some maps date from before many modern geological concepts, especially the plate tectonics model, were formulated or widely adopted. The old aphorism “the eye seldom sees what the mind doesn’t anticipate” applies here. The proper interpretation of much of the data to be provided by the EarthScope facility will require detailed geological maps in many regions, and optimal siting of EarthScope instrumentation may also require improved geological maps. Much other new scientific input will be needed from the many subdisciplines of the earth sciences, and it is likely that knowledge and information from outside the conventional earth sciences, such as geotechnical engineering, may also be required. The committee suggests that one of the key challenges of the EarthScope program will be to ensure that this integration of information, knowledge, and expertise occurs in the most effective way possible.
In the remainder of this chapter, the committee presents comments concerning each of the science components of EarthScope— the United States Seismic Array, the Plate Boundary Observatory, the San Andreas Fault Observatory at Depth, the Interferometric Synthetic Aperture Radar mission—and the integrated EarthScope Education and Outreach Program.
EarthScope Components: United States Seismic Array (USArray)
USArray proposes to instrument the entire lower 48 states and Alaska with a grid of broadband seismometers with roughly 70-kilometer spacing, achieving this with a transportable array of 400 instruments
(informally called “Bigfoot”; see Figure 1) that will be deployed in successive north-south bands across the continent over a 10-year period. The results from USArray should allow mapping of the structure and thickness of the crustal and mantle lithosphere in unprecedented detail, as well as the mantle beneath the lithosphere as deep as the core-mantle boundary. This knowledge is one of the most obvious deliverables of the EarthScope project, and one that will present opportunities for all earth scientists concerned with the evolution and behavior of continents: they simply will be seeing the earth in a way that has never before been possible.
The committee notes that the EarthScope and USArray proponents had given much thought to how to deploy the transportable array, and to the geometry and sequence in which it rolls across the continent. The one-year deployment time and 70-kilometer spacing will necessarily place some limitations on the scale of observations that can be made with the transportable array, especially in regions with low natural seismicity. As discussed below, the portable seismometers are designed to address some of these limitations by examining particular areas and features in greater detail. It is important to recognize, however, that there are some geologic features that could only be well characterized using a seismometer grid with closer spacing and longer deployment times. Even so, the committee is satisfied that the operational decisions built into the implementation plans were justified in cost-effective terms that did not jeopardize the scientific goals. This element of USArray is a massive logistical task, but the committee is confident that the EarthScope seismologists have sufficient experience to carry it out successfully and efficiently.
In addition to the “Bigfoot” transportable array of 400 instruments, USArray also includes 2400 portable seismometers to be deployed in flexible network geometries to examine particular areas and features in great detail. The sheer number of these instruments should reveal images with a detail and accuracy that has never been seen before. It should allow, for example, entirely new and detailed knowledge of crustal structure and thickness variations in regions of active faulting. In addition, it will help address the question of how the distribution of deformation in the lower crust compares with that at the surface. Many other important and hitherto unanswerable questions could be addressed with networks of this density. The EarthScope proponents have indicated that this flexible capability will be allocated toward projects that are
chosen through the usual competitive grant-submission process at NSF This intent is admirable, fair, and sensible in principle, but it requires careful thought to ensure that the new capability is exploited to the maximum possible extent. Issues include: the realization that although the use of very large numbers of instruments simultaneously in a dense configuration is a formidable tool with great scientific problem-solving potential, such a deployment may be beyond the logistical capability of small investigator groups; and an appreciation that the rolling “Bigfoot” array, or any of the other three components of EarthScope (PBO, InSAR, SAFOD), probably will identify unexpected features or phenomena whose investigation would greatly benefit from the focused attention of the flexible US Array network. Some of these unexpected features may be urgent priorities, such as indications of imminent earthquakes or volcanic eruptions. Accordingly, the committee suggests that it will be important not to commit the whole flexible network to projects in advance in such a way that it cannot fulfill this role. The priority of the portable network goals should be flexible in both time and space.
Preparing for operational eventualities is a formidable management and planning challenge. Thus far, the attention of the EarthScope proponents has understandably been focused on instrument specification, deployment, and operation. The committee has confidence that the community concerned can effectively address these additional issues relating to management of the scientific objectives, but would urge them to start doing so now. Many of these management concerns apply equally to the flexible component of the GPS networks that form part of the PBO.
EarthScope Components: San Andreas Fault Observatory at Depth (SAFOD)
Apart from scientific ocean drilling during the Deep Sea Drilling Project and its successor, the Ocean Drilling Program, deep drilling in sedimentary rock sequences on continents and offshore has been carried out mainly in the context of hydrocarbon (oil and gas) exploration and recovery. Several deep holes have been drilled for scientific purposes in crystalline rocks in Russia, Germany, and Hawaii, and a number of deep boreholes (up to 1.8 kilometers) were drilled to intersect the Nojima Fault on Awaji Island in Japan. However, the EarthScope proposal to
drill and instrument the San Andreas Fault is unique, in that it seeks to intercept an active fault zone near the depth at which earthquakes are generated to study its physical and chemical characteristics. Although many active and inactive fault zones have been mapped at the surface and imaged at depth, no active fault has heretofore been examined or sampled directly at the hypocentral depths (the actual points of origin) for small earthquakes.
The chosen drill site is close to the hypocenter of the 1966 magnitude 6 Parkfield earthquake (Figure 2), in a region where the San Andreas Fault moves through a combination of slip associated with small-to-moderate magnitude earthquakes and aseismic creep. This segment of the San Andreas Fault is one of the most well documented and instrumented sites along its entire length, and thus is an area where information obtained from drilling can be leveraged to the greatest extent. The proposed facility will allow scientists to build on existing knowledge to address many fundamental questions about the physical and chemical processes acting within the San Andreas Fault and, by extension, to the faulting process in general. Drilling, sampling, and continuous measurements directly within the fault zone will test controversial scientific hypotheses about earthquakes by providing direct information on the composition and mechanical properties of active fault zone rocks, the nature of stresses (“normalized” force or force intensity) responsible for earthquakes, the role of fluids (particularly pressurized water) in controlling faulting and earthquake recurrence, and the physics of rupture-initiation and propagation that cause a given earthquake. In addition to recovery of fault zone rock and fluids for laboratory analyses, intensive down-hole geophysical measurements and long-term monitoring are planned within and adjacent to the active fault zone as an integral component of the EarthScope observatory. Monitoring experiments will include near-field, wide-dynamic-range seismological observations of earthquake nucleation and rupture and continuous monitoring of variations in pore pressure (pressure of fluid within the rocks), temperature, and crustal deformation (strain, fracturing and fault slip) during the earthquake cycle. By directly evaluating the roles of fluid pressure, intrinsic rock friction, chemical reactions, in situ stress (force intensity), and other parameters during generation of an earthquake, project scientists hope to simulate earthquakes in the laboratory and in computer models using representative fault zone properties and physical conditions.
The committee is very favorably impressed with the scientific objectives of the SAFOD facility. As a component of EarthScope with both sampling (during the drilling phase) and monitoring (during the observatory phase) elements, SAFOD is the tangible, three-dimensional exploration facility that relates to the USArray, the PBO, and InSAR. The USArray and SAFOD projects have fundamental goals in common. Both aim to provide information on the structure and rheology (firmness or viscosity) of the earth in three dimensions over a range of spatial scales. Both will investigate the processes and conditions that control deformation of the earth. SAFOD will provide critical in situ data on physical and rheological properties of earth materials at depth. USArray provides the tool for extrapolating the in situ data to a broader region and larger spatial scales. PBO and InSAR also will link directly with SAFOD in that determination of the rate and distribution of deformation in the crust surrounding the drill site are critical to the goals of SAFOD. Measurements made with GPS and InSAR are exactly the kind needed to determine the overall pattern and temporal variation of deformation. The committee notes that the Parkfield site for SAFOD is well known and characterized, and recognizes that there is a realistic chance of recording activity at this site in the near future because the fault both creeps and moves in relatively frequent, moderate-sized earthquakes (rather than in big earthquakes every few hundred years, as is the case at other sites). The committee accepts that attempting to sample and instrument faults at multiple locations, although an admirable long-term goal, is unrealistic because of cost at this time.
EarthScope Components: Plate Boundary Observatory (PBO)
The North American Pacific Plate Boundary Observatory (PBO) involves deployment and integration of a network of global positioning system (GPS) stations (at approximately 1000 sites), together with strainmeters, to monitor the motions in real time of the actively deforming region of the western North American continent (Figure 3). The GPS instruments use signals from the GPS satellite network to record the position of the individual locations with great accuracy. The strainmeters measure local strains and provide the sensitivity required to detect short-lived transients; as such, they will provide the shorter time-scale readings (days to weeks) to complement longer time-scale GPS and
InSAR data (weeks to months to years). The rationale for location of these instruments in the western United States is that this region is at the confluence of two great plates, the North American and Pacific plates, and a smaller plate, the Juan de Fuca plate, which itself is a remnant of the formerly extensive Farallon plate, much of which has disappeared beneath North America in the past 100 million years. It is a region that is actively undergoing deformation (change in shape and configuration) by faulting, uplift or sinking, and folding. In addition, this region experiences most of the earthquakes in the United States, all of its active volcanism, and many of its major related earth movements, such as landslides and mudflows (lahars), which can travel many miles down river valleys, through centers of population and industry. Many of the west coast cities lie along or at the mouths of such river valleys.
The PBO data, in conjunction with data from the other EarthScope components, should allow the construction of a detailed description of earth movements within western North America. This should in turn facilitate the development of realistic dynamic models to help explain why these movements occur, and stimulate related scientific research that should lead to a reduction in risks associated with earthquakes, volcanic eruptions, and associated land movements such as landslides.
Correlation of PBO observations with data from other EarthScope components will be invaluable. In particular, the combination of PBO and InSAR measurements shows great potential for understanding the location of future possible fault movements and impending volcanic activity at known or incipient centers. Areas of activity pinpointed by the PBO will serve as appropriate sites for possible deployment of the flexible components of the USArray. In addition, the San Andreas Fault Observatory will provide an important third dimensional view to PBO observations.
EarthScope Components: Interferometric Synthetic Aperature Radar (InSAR)
Interferometric Synthetic Aperture Radar (InSAR) provides a means of measuring and monitoring the motion of the earth’s surface in great detail over wide areas, and should be regarded as an essential component of EarthScope. InSAR works by comparing radar images of a
given area acquired at different times. Any motion of the earth’s surface occurring in the interval between recording of the two images produces changes in the radar signal from the first image to the second, which can be transformed into a map showing ground displacement. This technique allows observation of millimeter-level displacements at a resolution of 25 meters over broad swaths. Because it is a satellite-based instrument, these measurements may be made with global coverage at regular intervals.
Since its first demonstration to map the surface displacements associated with the 1992 Landers earthquake, InSAR has produced some spectacular images of deformation associated with earthquakes and volcanoes, and has been used to investigate a wide range of phenomena: the slow accumulation of crustal strain across fault zones, the motions that occur immediately following an earthquake and that allow the mechanical properties of the crust and uppermost mantle to be investigated, the inflation or deflation of volcanoes due to movement of magma at depth (e.g., Figure 4), subsidence in urban areas due to the extraction of oil or water, and the movement of Antarctic ice streams.
Despite the undoubted success of InSAR observations carried out with the European Space Agency’s (ESA) European Remote-Sensing Satellite (ERS) missions, these observations were subject to many constraints and limitations. The C-band (5.6-centimeter wavelength) of the ERS radars meant that radar coherence in vegetated areas decreased rapidly with time, preventing interferograms from being formed over the long time intervals needed to isolate small ground displacements. The ERS satellites carried multiple instruments with a wide range of objectives, and as a consequence the long, regular time series of SAR images needed to observe crustal strain accumulation were acquired only in limited areas. The cost of purchasing SAR images limited the range of problems open to individual investigators. Future missions with InSAR capabilities by overseas agencies (ASAR on ESA’s Envisat mission, PALSAR on NASDA’s ALOS mission, RADARSAT II) will be subject to many of the same limitations.
To realize its full potential, a satellite dedicated to InSAR is essential, enabling acquisition of synthetic aperture radar (SAR) images with good radar coherence over long time intervals. Use of the L-band (24-centimeter wavelength) radar will allow observations of all terrains, regardless of vegetation cover—a considerable improvement on the present C-band radars, and a real, dramatic improvement in InSAR’s
capability and applicability. These measurements will help provide a synoptic view of the accumulation of crustal strain across the whole of North America, strain that may eventually be released in earthquakes. Because such a satellite mission would regularly observe North America’s seismic and volcanic belts in their entirety, it should provide an early indication of new events, e.g., magma motion at depth beneath a volcano, or aseismic slip on a fault. Measurements such as these, made with complete coverage at high resolution, cannot be obtained in any other way, and hence InSAR must be regarded as an integral component of EarthScope.
In addition to contributing fundamentally to EarthScope objectives, the committee notes that InSAR, as a satellite-based instrument, could make similar measurements worldwide. Although the other components of EarthScope would not be available elsewhere, InSAR, by capturing events in other parts of the world that are similar to those in North America that can be investigated by the full suite of EarthScope facilities, would provide a starting point for applying knowledge gained by EarthScope to other parts of the world’s seismic and volcanic belts, and could stimulate similar activities in other countries.
EDUCATION AND OUTREACH
EarthScope provides a unique opportunity for education and outreach (E&O) efforts that not only will capitalize on the public’s natural curiosity about the planet on which they live, but also will engage them in scientific exploration through direct involvement in data collection and research. The instrument deployments and experiments that will bring earth science to local communities across the nation are the key elements that will provide opportunities for development of educational materials that are particularly relevant on a regional basis.
The committee strongly endorses the EarthScope Working Group’s intention to include an extensive E&O effort in their project plan. EarthScope expects to provide opportunities for participation at two levels: development of a core of materials and resources appropriate for national-scale outreach to a broad audience, and provision of more focused educational efforts for specific audiences and regions. Both of these are important to build on the national scale of the EarthScope
initiative, and the committee urges that sufficient personnel and funds be assigned to conduct these activities.
At present, the overall E&O plan is all-encompassing and includes a wide range of target audiences (K-12 and college students, earth science professionals, policy makers, the general public), and products (real-time data, analytical tools, curricular materials, and other publications). Further planning will have to focus on a subset of these audiences and products, depending upon available resources. Despite the great interest in knowledge about the earth, earth science has been generally neglected at the K-12 level for the past century. For the intellectual journey from data to information to knowledge to wisdom to occur, well-trained geoscientists are required. If students are given the opportunity to feel the excitement of discovering new information about our dynamic planet, there is an increased likelihood of training scientists who will be able to use these powerful tools of perception. The committee emphasizes that development of an effective educational outreach program for EarthScope is both highly desirable and challenging.
EarthScope is well poised to undertake a wide variety of outreach activities because several of its partners and collaborators (e.g. NASA, USGS, the Southern California Earthquake Center [SCEC]) have considerable experience in the development of successful outreach products, ranging from effective web sites to museum exhibits. In addition, education and outreach activities will benefit from the intention to make all EarthScope data freely and rapidly available—a policy the committee endorses wholeheartedly. EarthScope and its tools and techniques could furnish powerful data sets for future geoscientists. The committee encourages the involvement of all the EarthScope partners and collaborators in the E&O effort.
The committee reviewed a large number of documents that showed the evolution of the EarthScope initiative, and noted that the EarthScope concept and the plans for the facility have evolved fairly rapidly over the past year. As a result, the written description of the scientific program as currently envisaged is terse and lacks the detail that will be required once funding has been approved. However,
presentations to the committee indicated that the EarthScope Working Group has carefully considered the many things that will need to be addressed as the initiative moves forward. The overall assessment of the committee echoes that presented earlier this year by the Committee on Basic Research Opportunities in Earth Sciences1—the scientific vision and goals of EarthScope are well articulated and have been developed with a high degree of community involvement. The committee strongly endorses all four scientific components of the EarthScope initiative and the education and outreach plans.