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Earth Observations from Space: The First 50 Years of Scientific Achievements
11
Solid Earth
Today, solid Earth studies often involve spaceborne techniques, ranging from multispectral imaging to space-geodetic methods. Over the past several decades, space-based observations of the Earth have contributed powerfully to fields such as plate tectonics, seismology, and volcanology, as well as to studies of the geodynamo, mantle convection, and continental tectonics. Such investigations also provide insights into managing natural resources, understanding natural hazards, and predicting global environmental change.
Satellites have revealed Earth’s precise shape and how it changes subtly with time and have measured the spatial and temporal changes in mass distribution through measurements of its gravity. Thanks to space geodesy, Earth scientists benefit from an International Earth Reference System that is accurate to better than 1 cm in all components, including the time-dependent position of the geocenter. Even more impressive is the millimeter level relative positioning that is achievable anywhere on the surface of the planet, or in orbit. We can thus measure the movement of tectonic plates in real time and elucidate higher complexities such as the distribution of deformation within plate boundary zones. The transformative nature of this technology is demonstrated by the fact that, a mere 50 years ago, a traveler might not know his/her position on Earth to better than 500 m, even after expending considerable effort in tedious reduction of geodetic observations. Yet, today, an automobilist, aviator, or sailor can determine the vehicle’s position to meter precision in real time, anywhere on the planet, using an inexpensive Global Positioning System (GPS) receiver.
GEODESY
National Aeronautics and Space Administration (NASA) satellites have contributed substantially to improving our knowledge of Earth’s gravity field. Laser Geodynamics Satellites (LAGEOS) and the Gravity Recovery and Climate Experiment (GRACE) measure Earth’s gravity field to model the regional-scale shape of Earth. The shape is irregular and changes over many different timescales (Figures 11.1 and 11.2). The early geoid was described only to the third harmonic degree, revealing the “pear-shaped” departure from the ellipsoid. As more detailed information on Earth’s gravity field was made available by LAGEOS and follow-on missions (combined with the expansion to higher harmonic degrees), its precise geoid and topography on a global scale have been made accessible.
Owing to the modern, highly precise, and homogeneous data from satellites such as CHallenging Mission Payload (CHAMP) and GRACE, scientists have been able to derive improved high-resolution global mean gravity field models (Reigber et al. 2003). These models are needed in numerous geodetic-geophysical applications, including the precise orbit determination of Earth satellites, determination of ocean surface currents from altimetry, or GPS leveling. Scientists resolve the gravity anomalies relative to the “idealized” ellipsoidal Earth with the use of these mean gravity models, which have become more sophisticated since the low-orbiting satellites CHAMP and GRACE were able to provide more accurate data. Consequently, these improved gravity models can solve for gravity anomalies 10 times more accurately than before these satellite data became available with direct implications for the aforementioned applications.
STRUCTURE AND DYNAMICS OF EARTH’S DEEP INTERIOR
Satellite measurements of the geoid have provided crucial information to further the understanding of mantle convection. The main features visible in Figures 11.1 and 11.2 emerged in the early global estimates of spatial variations in Earth’s gravity field, which incorporated satellite tracking data (e.g., Gaposchkin and Lambeck 1971). Long-wavelength features such as the geoid highs over
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FIGURE 11.1 View of Earth’s geoid from the GRACE mission yields deep structures. Using an ellipsoid to approximate the bulk of the Earth’s shape and departures from the ellipsoid are represented by the geoid elevation above or below the ellipsoid. The geoid can be as low as 106 m (350 f) below the ellipsoid or as high as 85 m (280 f) above. SOURCE: NASA/Deutsches Zentrum für Luft-und Raumfahrt (DLR).
FIGURE 11.2 Earth’s gravity anomaly from the GRACE mission yields smaller-scale structures. Standard gravity is defined as the value of gravity for a perfectly smooth “idealized” Earth, and the gravity anomaly (expressed in units of milliGals [mGal]) is a measure of how actual gravity deviates from this standard. SOURCE: NASA/DLR.
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regions of plate convergence (New Zealand–New Guinea–Japan–Kamchatka–Aleutians and western South America) indicate mass variations associated with mantle convection and the variation of density and strength in Earth’s interior. For example, Hager (1984) demonstrated that these geoid highs over subduction zones require a substantial increase in viscosity with depth between the upper mantle and lower mantle, resulting in an impediment to convective mass transport across the boundary between these two layers. The gravity low over Hudson Bay (Figure 11.2) is due, in part, to a remaining depression in the surface caused by the weight of the great Laurentide ice sheet that melted at the end of the last ice age. The estimate that almost half of this gravity low is the result of ongoing post-glacial rebound again requires a substantial increase in the viscosity of the mantle with depth, otherwise the surface depression would have relaxed more by now (Simons and Hager 1997). The recent observation by GRACE of the rate at which this gravity low is decreasing in amplitude confirms that almost half of this gravity low is the remnant of the former ice sheet (Tamisiea et al. 2007).
THE GLOBAL POSITIONING SYSTEM
NASA missions provided major contributions to the development of the global reference frame through the GPS, Satellite Laser Ranging, and Very Long Baseline Interferometry technology. GPS and Interferometric synthetic aperture radar (InSAR) methods have provided precise measurements of Earth’s shape and surface positioning (Box 11.1), thus providing detailed local and global topographic and deformation information. Current InSAR satellites include the European Remote Sensing Satellite (ERS), the European Environmental Satellite (ENVISAT), the Japanese Advanced Land Observation Satellite (ALOS), and the Canadian Radarsat program. These satellites and the constellation of GPS satellites track current motions of Earth’s surface at centimeter precision over time and reveal many geophysical processes occurring on the surface and at depth, where they are generally inaccessible to surface observation. Ironically, the use of gravity and deformation data obtained from space has greatly improved our understanding of structure and change deep within the Earth (see below).
PLATE TECTONICS, TOPOGRAPHY, SEISMOLOGY, AND VOLCANOLOGY
The theory of plate tectonics was driven largely by observations in the 1950s from ocean vessels mapping the magnetic field and the seafloor shape, which can now be obtained more easily from satellite observations (Figure 11.3). Several decades later satellite observations enabled a scientific revolution in advancing the theory of plate tectonics by providing highly detailed, quantifiable measurements of Earth’s surface. GPS has enabled measurement of plate positioning and velocities, thus resolving geologic
BOX 11.1
Earth Reference Frame
Few scientific accomplishments are as “transformative” as the advances in space geodesy over the past five decades, particularly with the ubiquitous introduction of GPS devices. This breakthrough not only has transformed the field of geodesy but also provides vital information for studying global sea-level change, earthquakes, and volcanoes, as well as providing precise position information for all Earth science research.
At the time of the International Geophysical Year, the geolocation of most points at the surface of the Earth entailed errors that reached hundreds of meters in remote areas, even after much effort. Today, scientists rely on an International Earth Reference Frame from which geographical positions can be accurately described relative to the geocenter, in three-dimensional Cartesian coordinates to centimeter accuracy or better—a 2 to 3 orders-of-magnitude improvement compared to 50 years ago. This is true anywhere, on an active planet where every piece of real estate moves relative to every other. Geodesy observations from space have enabled modern measurements of Earth’s rotation. The change in position of the rotation axis (the poles) is determined daily to centimeter accuracy, and changes in the length of a day are determined to millisecond accuracy within a few hours. Improvements in GPS measurements over the past few decades have enabled instantaneous geodetic positioning (Genrich and Bock 2006)—a real-time GPS. GPS receivers are now available inexpensively to consumers, who are rapidly becoming accustomed to GPS navigation on the road, on the water, and in the air without realizing the enormous body of science behind this technological achievement: accurate ephemerides of the satellites, very stable clocks, well-calibrated atmospheric corrections, and even relativistic corrections.
and contemporary velocities. For example, Iaffaldano et al. (2006) found that the Nazca Plate moves at a velocity of 6.9 cm per year, compared to its geologic velocity of 10.1 cm per year 10 million years ago. Geologic timescale velocities typically disagree with present rates, with implications for crust-mantle interaction. Factors such as friction or time-dependent processes can be modeled if we understand how the rates vary with time.
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FIGURE 11.3 Map of seafloor topography derived from gravity measurements from satellite tracking and radar altimetry. SOURCE: Reprinted with permission of Dave Sandwell, University of California, San Diego.
Gravity and altimetry measurements from space have also led to discoveries in topography. The Shuttle Radar Topography Mission (SRTM) employed InSAR topography to produce the first (and only) fine-resolution, worldwide, consistent model of elevation. This discovery has mapped the world at 30-m posting, 10-m elevation accuracy; 90-m data are now openly available for Earth. Down-looking radar altimeters measuring ocean heights, which follow the geoid, yield sea-surface topography over the entire ocean at a data density unobtainable on a global scale from shipboard measurements. Applications of detailed gravity information include oil exploration and the location of undersea volcanoes (Smith and Sandwell 2003).
Gravity and topography anomalies relate to large-scale seismic risk and the geophysics of subduction zone boundaries (Song and Simons 2003). Finer-scale risk assessments follow from high-resolution observations of deformation along active faults, which reveal strain accumulations and can indicate stress transfer associated with seismic activity. Therefore, the scientific community took notice after an InSAR observation of the Landers earthquake of 1992, creating the first-ever detailed image of an earthquake and its effect on the crust (Massonnet et al. 1993; Figure 11.4).
Measuring surface displacement is now an important ingredient in seismic risk analysis. For example, Fialko et al. (2002) inferred stress change as a result of the Hector Mine earthquake and the resulting distribution of the stress in the upper crust suggests areas likely for further activity.
Other processes are occurring every day in the solid Earth, many of which escape our knowledge because they occur at a rate slow enough not to radiate seismic energy that can be detected with our present seismographs. Yet these mechanisms for the transfer of energy through the upper crust need to be observed and measured if we are to be able to explain many natural hazards. For example, GPS has enabled the discovery of aseismic (“slow”) earthquakes occurring in many subduction zones around the Earth and adding stress to subduction faults (Figure 11.5). The GPS time series for the Cascadia subduction zone shows the result of continual aseismic earthquakes (Melbourne and Webb 2003). Aseismic earthquakes may either dissipate or increase stress, affecting risk probabilities. Unknown until 5 years ago, aseismic earthquakes are a recent discovery dependent on satellite observations.
Inverse methods and the density of InSAR measurements permit a solution for fault slip at depth, giving a view of what is occurring underground as illustrated by images of the Hector Mine earthquake (see Zebker et al. 2000 and Figure 11.6). Such analyses are now commonplace over many terrains.
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FIGURE 11.4 Cover of the journal Nature showing the first-ever image of an earthquake. This interferogram was produced by combining the pair of ERS-1 SAR images taken before and after the Landers earthquake of June 28, 1992. Each cycle of colored shading represents a range difference of 28 mm between the before and after images, used to detect changes in the position of the ground surface. SOURCE: Massonnet et al. (1993). Reprinted with permission from Macmillan Publishers Ltd., copyright 1993.
Many processes on Earth leave strong signals in the deformation of the surface resulting from movements or changes in pressure far beneath the surface. For example, volcanoes cause surface deformation readily observed from satellites. Multiple deformation processes occur simultaneously during a volcanic eruption, prompting the need for volcanic mechanical modeling (Jonsson et al. 2005) in addition to simple mapping of the deformation signature. Detailed observations of the patterns of surficial change allow us to discriminate between many candidate effects and help us better understand the evolution and predictability of volcanoes.
Further applications of spaceborne geodesy follow from measurement of anthropogenic surface change. With implications for natural resource management and natural hazard response, satellites measure subsidence from petroleum extraction (e.g., Lost Hills, CA; Hooper 2005), landslides appearing clearly in InSAR maps (e.g., Berkeley Hills, CA; Hilley et al. 2004), and subsidence from groundwater extrac
FIGURE 11.5 GPS time series from Yreka, CA (YBHB), Newport, OR (NEWP), and Alberthead, BC (ALBH), and seismic tremor histogram from Yreka. (A) Blue points are daily GPS station positions in mm of the longitudinal component of station YBHB. Solid red line is a plot of the hours of tremor per week at seismic station YBH. Note the similarity of shape displayed by ALBH (C) and YBHB. The correlation between GPS offsets and increased tremor activity indicates that slow faulting occurs beneath Northern California. (B) Purple points represent daily solutions of station position for the longitudinal component of GPS station NEWP from Newport, Oregon. Note the similarity of NEWP offsets (dashed black lines) to those at ALBH. The lack of seismic and continuous GPS stations near NEWP precludes the definitive identification of slow earthquakes here at the present time. (C) Green points represent daily position solutions of the longitudinal component of ALBH. Note the characteristic sawtooth reset shape of the time series due to slow faulting events. Solid black lines denote times of known slow earthquakes at ALBH. SOURCE: Szeliga et al. (2004). Reproduced with permission from American Geophysical Union, copyright 2004.
tion (Figure 11.7; e.g., Las Vegas Valley, NV, 1992-1997; Amelung et al. 1999).
Water resource managers will be able to model aquifer storage extent (Hoffmann et al. 2003) and begin to map the direction and volume of water migrating through the aquifer system that feeds our cities and farms (see Chapter 6). As described in Chapter 7, gravity measurements have also been applied to studying continental ice sheets. The observed changes in ice flow velocity of glaciers have revolutionized the thinking in how climate change affects the ice sheet mass balance. Measurements of the ice sheet mass balance and maps of ice flow velocity provided by satellites (Rignot 2001) are making important contributions to improved accuracy in forecasting of sea-level rise. These estimates require combining the knowledge gained in solid-Earth geophysics and hydrology, with profound implications for accurately modeling and predicting the consequences of climate change.
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FIGURE 11.6 Hector Mine earthquake, view of fault slip at depth. SOURCE: Zebker et al. (2000). Reproduced with permission from American Geophysical Union, copyright 2000.
FIGURE 11.7 Subsidence from groundwater extraction in the Las Vegas Valley, 1992-1997. SOURCE: Amelung et al. (1999). Reprinted with permission from the Geological Society of America, copyright 1999.