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11 Solid Earth Today, solid Earth studies often involve spaceborne tech- the regional-scale shape of Earth. The shape is irregular niques, ranging from multispectral imaging to space-geodetic and changes over many different timescales (Figures 11.1 methods. Over the past several decades, space-based observa- and 11.2). The early geoid was described only to the third tions of the Earth have contributed powerfully to fields such harmonic degree, revealing the âpear-shapedâ departure as plate tectonics, seismology, and volcanology, as well as to from the ellipsoid. As more detailed information on Earthâs studies of the geodynamo, mantle convection, and continen- gravity field was made available by LAGEOS and follow-on tal tectonics. Such investigations also provide insights into missions (combined with the expansion to higher harmonic managing natural resources, understanding Ânatural hazards, degrees), its precise geoid and topography on a global scale and predicting global environmental change. have been made accessible. Satellites have revealed Earthâs precise shape and how Owing to the modern, highly precise, and homogeneous it changes subtly with time and have measured the spatial data from satellites such as CHallenging Mission Payload and temporal changes in mass distribution through measure- (CHAMP) and GRACE, scientists have been able to derive ments of its gravity. Thanks to space geodesy, Earth scientists improved high-resolution global mean gravity field models benefit from an International Earth Reference System that (Reigber et al. 2003). These models are needed in numerous is accurate to better than 1 cm in all components, including geodetic-geophysical applications, including the precise the time-dependent position of the geocenter. Even more orbit determination of Earth satellites, determination of impressive is the millimeter level relative positioning that ocean surface currents from altimetry, or GPS leveling. is achievable anywhere on the surface of the planet, or in Scientists resolve the gravity anomalies relative to the âide- orbit. We can thus measure the movement of tectonic plates alizedâ ellipsoidal Earth with the use of these mean gravity in real time and elucidate higher complexities such as the models, which have become more sophisticated since the distribution of deformation within plate boundary zones. low-orbiting satellites CHAMP and GRACE were able to The transformative nature of this technology is demonstrated provide more accurate data. Consequently, these improved by the fact that, a mere 50 years ago, a traveler might not gravity models can solve for gravity anomalies 10 times know his/her position on Earth to better than 500 m, even more accurately than before these satellite data became after expending considerable effort in tedious reduction of available with direct implications for the aforementioned geodetic observations. Yet, today, an automobilist, aviator, or applications. sailor can determine the vehicleâs position to meter precision in real time, anywhere on the planet, using an inexpensive Structure and Dynamics of Global Positioning System (GPS) receiver. Earthâs Deep Interior Satellite measurements of the geoid have provided GEODESY crucial information to further the understanding of mantle National Aeronautics and Space Administration (NASA) convection. The main features visible in Figures 11.1 satellites have contributed substantially to improving our and 11.2 emerged in the early global estimates of spatial knowledge of Earthâs gravity field. Laser Geodynamics variations in Earthâs gravity field, which incorporated satel- Satellites (LAGEOS) and the Gravity Recovery and Climate lite tracking data (e.g., Gaposchkin and Lambeck 1971). Experiment (GRACE) measure Earthâs gravity field to model Long-wavelength features such as the geoid highs over 92
SOLID EARTH 93 Meters FIGURE 11.1â View of Earthâs geoid from the GRACE mission yields deep structures. Using an ellipsoid to approximate the bulk of the Earthâs 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 fÃ¼r Luft-und Raumfahrt (DLR). 11-1 FIGURE 11.2â Earthâs gravity anomaly from the GRACE mission yields smaller-scale structures. Standard gravity is defined as the value of gravity for a perfectly smooth âidealizedâ 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.
94 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS regions of plate convergence (New ZealandâNew Guineaâ JapanâÂKamchatkaâAleutians and western South America) BOX 11.1 indicate mass variations associated with mantle convection Earth Reference Frame and the variation of density and strength in Earthâs interior. For example, Hager (1984) demonstrated that these geoid Few scientific accomplishments are as âtransfor- highs over subduction zones require a substantial increase mativeâ as the advances in space geodesy over the in Âviscosity with depth between the upper mantle and lower past five decades, particularly with the ubiquitous mantle, resulting in an impediment to convective mass introduction of GPS devices. This breakthrough transport across the boundary between these two layers. The not only has transformed the field of geodesy but gravity low over Hudson Bay (Figure 11.2) is due, in part, to also provides vital information for studying global a remaining depression in the surface caused by the weight sea-level change, earthquakes, and volcanoes, as of the great Laurentide ice sheet that melted at the end of the well as providing precise position information for all last ice age. The estimate that almost half of this gravity low Earth science research. is the result of ongoing post-glacial rebound again requires a At the time of the International Geophysical substantial increase in the viscosity of the mantle with depth, Year, the geoÂlocation of most points at the surface otherwise the surface depression would have relaxed more of the Earth entailed errors that reached hundreds by now (Simons and Hager 1997). The recent observation by of meters in remote areas, even after much effort. GRACE of the rate at which this gravity low is decreasing in Today, scientists rely on an International Earth amplitude confirms that almost half of this gravity low is the Reference Frame from which geographical posi- remnant of the former ice sheet (Tamisiea et al. 2007). tions can be Â accurately Â described relative to the geocenter, in three-dimensional Â Cartesian coordi- The Global Positioning System nates to centimeter accuracy or betterâa 2 to 3 o Â rders-of-magnitude improvement compared to 50 NASA missions provided major contributions to the years ago. This is true anywhere, on an active planet development of the global reference frame through the GPS, where every piece of real estate moves relative to Satellite Laser Ranging, and Very Long Baseline Interferom- every other. Geodesy observations from space have etry technology. GPS and Interferometric synthetic aperture enabled modern measurements of Earthâs rotation. radar (InSAR) methods have provided precise measure- The change in position of the rotation axis (the ments of Earthâs shape and surface positioning (Box 11.1), poles) is determined daily to centimeter accuracy, thus providing detailed local and global topographic and and changes in the length of a day are determined deformation information. Current InSAR satellites include to millisecond accuracy within a few hours. ImproveÂ the European Remote Sensing Satellite (ERS), the European ments in GPS measurements over the past few Environmental Satellite (Envisat), the Japanese Advanced decades have enabled instantaneous geodetic posi- Land Observation Satellite (ALOS), and the Canadian tioning Â(Genrich and Bock 2006)âa real-time GPS. Radarsat program. These satellites and the constellation of GPS receivers are now available inexpensively to GPS satellites track current motions of Earthâs surface at consumers, who are rapidly becoming Âaccustomed centimeter precision over time and reveal many geophysical to GPS navigation on the road, on the water, and processes occurring on the surface and at depth, where they in the air without realizing the enormous body of are generally inaccessible to surface observation. Ironically, science behind this technological achievement: the use of gravity and deformation data obtained from space accurate ephemerides of the satellites, very stable has greatly improved our understanding of structure and clocks, well-calibrated atmospheric corrections, and change deep within the Earth (see below). even relativistic corrections. PLATE TECTONICS, TOPOGRAPHY, SEISMOLOGY, AND VOLCANOLOGY The theory of plate tectonics was driven largely by observations in the 1950s from ocean vessels mapping and contemporary velocities. For example, Iaffaldano et al. the magnetic field and the seafloor shape, which can now (2006) found that the Nazca Plate moves at a velocity of be obtained more easily from satellite observations (Fig- 6.9Â cm per year, compared to its geologic velocity of 10.1 cm ureÂ 11.3). Several decades later satellite observations enabled per year 10 million years ago. Geologic timescale velocities a scientific revolution in advancing the theory of plate typically disagree with present rates, with implications for t Âectonics by providing highly detailed, quantifiable measure- crust-mantle interaction. Factors such as friction or time- ments of Earthâs surface. GPS has enabled measurement dependent processes can be modeled if we understand how of plate positioning and velocities, thus resolving geologic the rates vary with time.
SOLID EARTH 95 FIGURE 11.3â Map of seafloor topography derived from gravity measurements from satellite tracking and radar altimetry. SOURCE: Re- printed with permission of Dave Sandwell, University of California, San Diego. 11-3 Gravity and altimetry measurements from space have (2002) inferred stress change as a result of the Hector Mine also led to discoveries in topography. The Shuttle Radar earthquake and the resulting distribution of the stress in the Topography Mission (SRTM) employed InSAR topography upper crust suggests areas likely for further activity. to produce the first (and only) fine-resolution, worldwide, Other processes are occurring every day in the solid consistent model of elevation. This discovery has mapped Earth, many of which escape our knowledge because they the world at 30-m posting, 10-m elevation accuracy; 90âm occur at a rate slow enough not to radiate seismic energy data are now openly available for Earth. Down-looking that can be detected with our present seismographs. Yet radar altimeters measuring ocean heights, which follow the these mechanisms for the transfer of energy through the geoid, yield sea-surface topography over the entire ocean at upper crust need to be observed and measured if we are to a data density unobtainable on a global scale from shipboard be able to explain many natural hazards. For example, GPS measurements. Applications of detailed gravity information has enabled the discovery of aseismic (âslowâ) earthquakes include oil exploration and the location of undersea volca- occurring in many subduction zones around the Earth and noes (Smith and Sandwell 2003). adding stress to subduction faults (Figure 11.5). The GPS Gravity and topography anomalies relate to large-scale time series for the Cascadia subduction zone shows the result seismic risk and the geophysics of subduction zone bound- of continual aseismic earthquakes (Melbourne and Webb aries (Song and Simons 2003). Finer-scale risk assessments 2003). Aseismic earthquakes may either dissipate or increase follow from high-resolution observations of deformation stress, affecting risk probabilities. Unknown until 5 years along active faults, which reveal strain accumulations and ago, aseismic earthquakes are a recent discovery dependent can indicate stress transfer associated with seismic activ- on satellite observations. ity. Therefore, the scientific community took notice after Inverse methods and the density of InSAR measure- an InSAR observation of the Landers earthquake of 1992, ments permit a solution for fault slip at depth, giving a view creating the first-ever detailed image of an earthquake and its of what is occurring underground as illustrated by images effect on the crust (Massonnet et al. 1993; Figure 11.4). of the Hector Mine earthquake (see Zebker et al. 2000 and Measuring surface displacement is now an important Figure 11.6). Such analyses are now commonplace over ingredient in seismic risk analysis. For example, Fialko et al. many terrains.
96 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 A Yreka, CA (YBHB) 100 GPS longitude (mm) 80 tremor (hr/week) 5 60 0 40 -5 20 5 B ? ? Newport, OR (NEWP) ? GPS longitude (mm) 0 -5 5 C Alberthead BC (ALBH) 0 -5 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 11-5 FIGURE 11.5â GPS time series from Yreka, CA (YBHB), ÂNewport, OR (NEWP), and Alberthead, BC (ALBH), and seismic tremor his- togram 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, FIGURE 11.4â Cover of the journal Nature showing the first-ever Oregon. Note the similarity of NEWP offsets (dashed black lines) image of an earthquake. This interferogram was produced by com- to those at ALBH. The lack of seismic and continuous GPS stations 11-4 bining the pair of ERS-1 SAR images taken before and after the near NEWP precludes the definitive identification of slow earth- Landers earthquake of June 28, 1992. Each cycle of colored shading quakes here at the present time. (C) Green points represent daily one column represents a range difference of 28 mm between the before and after position solutions of the longitudinal component of ALBH. Note images, used to detect changes in the position of the ground surface. the characteristic sawtooth reset shape of the time series due to slow SOURCE: Massonnet et al. (1993). Reprinted with permission from faulting events. Solid black lines denote times of known slow earth- Macmillan Publishers Ltd., copyright 1993. quakes at ALBH. SOURCE: Szeliga et al. (2004). Reproduced with permission from American Geophysical Union, copyright 2004. Many processes on Earth leave strong signals in the defor- tion (Figure 11.7; e.g., Las Vegas Valley, NV, 1992-1997; mation of the surface resulting from movements or changes in Amelung et al. 1999). pressure far beneath the surface. For example, volcanoes cause Water resource managers will be able to model aquifer surface deformation readily observed from satellites. Multiple storage extent (Hoffmann et al. 2003) and begin to map deformation processes occur simultaneously during a volcanic the direction and volume of water migrating through the eruption, prompting the need for volcanic mechanical model- aquifer system that feeds our cities and farms (see Chap- ing (Jonsson et al. 2005) in addition to simple mapping of the ter 6). As described in Chapter 7, gravity measurements deformation signature. Detailed observations of the patterns have also been applied to studying continental ice sheets. of surficial change allow us to discriminate between many The observed changes in ice flow velocity of glaciers have candidate effects and help us better understand the evolution revolutionized the thinking in how climate change affects and predictability of volcanoes. the ice sheet mass balance. Measurements of the ice sheet Further applications of spaceborne geodesy follow from mass balance and maps of ice flow velocity provided by measurement of anthropogenic surface change. With impli- satellites (Rignot 2001) are making important contributions cations for natural resource management and natural hazard to improved accuracy in forecasting of sea-level rise. These response, satellites measure subsidence from petroleum estimates require combining the knowledge gained in solid- extraction (e.g., Lost Hills, CA; Hooper 2005), landslides Earth geophysics and hydrology, with profound implications appearing clearly in InSAR maps (e.g., Berkeley Hills, CA; for accurately modeling and predicting the consequences of Hilley et al. 2004), and subsidence from groundwater extrac- climate change.
SOLID EARTH 97 FIGURE 11.6â Hector Mine earthquake, view of fault slip at depth. SOURCE: Zebker et al. (2000). Reproduced with permission from American Geophysical Union, copyright 2000. FIGURE 11.7â 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.