4
Hazards and Resources

Geological processes affect the sustenance and safety of the human population. For example, earthquakes and volcanic eruptions can cause widespread damage and loss of life. Mineral and water resources are necessary to maintain our complex societies, and waste products from resource use must somehow be returned to Earth in a manner that does not unnecessarily foul the environment and that can be sustained for the indefinite future. The catastrophic nature of some Earth processes and the wise use of our land, water, and mineral resources present a special set of challenging research issues. It is likely that Earth will remain habitable to humans for millions of years into the future, barring insurmountable environmental degradation or a catastrophe of the type that struck 65 million years ago. This last chapter deals with fundamental Earth science that must be advanced to ensure and enhance the future of humankind. We are most concerned as a society about the next decades and centuries, but it is interesting, instructive, and scientifically challenging to think much farther ahead (and to look back in deep geological time) to fully comprehend the range of possibilities.

This chapter comprises two grand questions. Question 9 addresses earthquakes and volcanic eruptions, essential planetary processes that sometimes are deleterious to humankind but are intrinsic to Earth and probably inseparable from its habitability. Both of these geological phenomena are catastrophic in the sense that they represent the sudden release of energy stored inside Earth. The best approach to minimize the loss of life and property from these events is to understand both the causative processes well enough to forecast or predict them and their effects well enough to address them. Question 10 is aimed at addressing the fundamental science that underlies many of the issues related to land, mineral, and water resource use, as well as waste disposal. We have focused this question on the science of Earth fluids, which arguably represents the single most central, fundamental, and crosscutting research area for environmental management and sustainability.

QUESTION 9:
CAN EARTHQUAKES, VOLCANIC ERUPTIONS, AND THEIR CONSEQUENCES BE PREDICTED?

Earthquakes and volcanic eruptions are sudden and hazardous manifestations of the normally gradual movements of Earth’s interior (Questions 4 and 5). Although much research is stimulated by the dangers these events pose to human populations, they are of special interest to Earth scientists for other reasons as well. They constitute a class of phenomena that can be observed in action and monitored at many different scales and that recur frequently. The fact that we know they will happen, and also (for the most part) where, creates a natural desire to be able to predict them. The imperative for improved predictive power is escalated as human populations increasingly concentrate in areas prone to earthquakes and volcanic eruptions. But prediction remains difficult because of the inherent complexity of the processes and the special demands imposed by attempting to specify exactly when these



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 95
4 Hazards and Resources G eological processes affect the sustenance and understand both the causative processes well enough to safety of the human population. For example, forecast or predict them and their effects well enough to earthquakes and volcanic eruptions can cause address them. Question 10 is aimed at addressing the widespread damage and loss of life. Mineral and wa- fundamental science that underlies many of the issues ter resources are necessary to maintain our complex related to land, mineral, and water resource use, as well societies, and waste products from resource use must as waste disposal. We have focused this question on somehow be returned to Earth in a manner that does the science of Earth fluids, which arguably represents not unnecessarily foul the environment and that can the single most central, fundamental, and crosscut- be sustained for the indefinite future. The catastrophic ting research area for environmental management and nature of some Earth processes and the wise use of our sustainability. land, water, and mineral resources present a special set of challenging research issues. It is likely that Earth QUESTION 9: CAN EARTHQUAKES, will remain habitable to humans for millions of years VOLCANIC ERUPTIONS, AND THEIR into the future, barring insurmountable environmental CONSEQUENCES BE PREDICTED? degradation or a catastrophe of the type that struck 65 million years ago. This last chapter deals with funda- Earthquakes and volcanic eruptions are sudden and mental Earth science that must be advanced to ensure hazardous manifestations of the normally gradual and enhance the future of humankind. We are most movements of Earth’s interior (Questions 4 and 5). concerned as a society about the next decades and cen- Although much research is stimulated by the dangers turies, but it is interesting, instructive, and scientifically these events pose to human populations, they are of challenging to think much farther ahead (and to look special interest to Earth scientists for other reasons as back in deep geological time) to fully comprehend the well. They constitute a class of phenomena that can be range of possibilities. observed in action and monitored at many different This chapter comprises two grand questions. scales and that recur frequently. The fact that we know Question 9 addresses earthquakes and volcanic erup- they will happen, and also (for the most part) where, tions, essential planetary processes that sometimes are creates a natural desire to be able to predict them. The deleterious to humankind but are intrinsic to Earth imperative for improved predictive power is escalated and probably inseparable from its habitability. Both as human populations increasingly concentrate in ar- of these geological phenomena are catastrophic in the eas prone to earthquakes and volcanic eruptions. But sense that they represent the sudden release of energy prediction remains difficult because of the inherent stored inside Earth. The best approach to minimize complexity of the processes and the special demands the loss of life and property from these events is to imposed by attempting to specify exactly when these 

OCR for page 95
6 ORIGIN AND EVOLUTION OF EARTH events will occur. And since even highly accurate Finally, even the word “earthquake” needs definition. predictions will not prevent widespread damage, an Scientists use the term to describe the fault rupture that improved ability to forecast the consequences of cata- generates seismic waves. However, the public views an strophic events, and hence to prepare for them, is at earthquake more broadly as both the faulting and the least as important as predicting them. waves. We will use this more general definition and discuss prediction of the faulting event and the shaking that accompanies it. earthquake hazards Earthquakes at their worst are extreme catastrophes. Predicting Where earthquakes happen The 1556 Shaanxi, China, earthquake killed over 800,000 people in a matter of minutes. By some es- Scientists have long recognized that some regions are timates the next large earthquake under Tokyo could seismically active, while others are not. By the middle cause trillions of dollars in direct economic losses. of the 20th century, seismologists had produced a re- These consequences could be mitigated if earthquakes markably complete earthquake atlas (Gutenberg and could be predicted over timescales short enough to al- Richter, 1954) that chronicled systematic features of low an effective response. However, this goal remains global seismicity, but they lacked a framework to under- elusive. stand those features. The advent of plate tectonics soon The nature of earthquakes makes them uniquely changed that and also enabled the first steps toward terrifying. No one sees an earthquake coming. It is a earthquake prediction to be taken. For example, plate matter of seconds from the time shaking first becomes tectonics theory led to the recognition that Cascadia perceptible until it becomes violent. At any locale, should be subject to large earthquakes, despite no his- damaging earthquakes occur infrequently on human tory of earthquake activity. This expectation was con- timescales, which means that most people caught in a firmed by the discovery, using stratigraphic and other major earthquake have no previous experience. It is also evidence, of a magnitude (M) ~ 9 earthquake in Casca- profoundly disturbing when our usually stable reference dia in January 1700 (Figure 4.1; Atwater, 1987). frame, the planet beneath us, does not hold still. The Plate tectonics holds that Earth’s lithosphere con- unpredictability and sudden onset of earthquakes also sists of large plates that move relative to one another mean that once an earthquake begins, it is generally at speeds of several centimeters per year (Question too late to do much more than duck and cover. The 5). Relative plate motion is accommodated on plate- combination of unpredictability, abrupt onset, rarity, boundary faults or, more typically, on complex fault and unfamiliarity means that the risk posed by earth- systems. These faults are frictionally locked between quakes is difficult to manage, for both individuals and earthquakes, causing ongoing plate motion to deform governments. the crust around them, storing elastic strain energy in the process. Once friction is overcome and the fault starts slipping in an earthquake, this stored energy is earthquake Prediction: Where, When, and how converted to other forms, most notably energy radiated Big? away from the fault as seismic waves. The goal of earthquake prediction is to specify where Most earthquakes occur at plate boundaries, and the and when a significant earthquake will occur. Where type of boundary plays a role in controlling the nature future earthquakes will occur is largely understood, with of earthquake activity. Extension at divergent boundar- some important exceptions (summarized in Beroza and ies is accommodated by normal faulting and formation Kanamori, 2007). Predicting when they will strike is of new crust through basaltic volcanism, with much of much more difficult, though progress has been made the deformation taking place aseismically. Horizontal and promising avenues of research have emerged. The motion across transcurrent plate boundaries takes place term “significant” is a subtle but important part of the on strike-slip faults. Most transcurrent boundaries are definition of earthquake prediction, and it brings up the oceanic, but when they traverse continents, they pose important question of what controls earthquake size. significant seismic hazard. All of the largest earth-

OCR for page 95
 HAZARDS AND RESOURCES FIGURE 4.1 A “ghost forest” near the mouth of the Copalis River, Washington, that was killed by saltwater tides after a M ~ 9 earthquake in January 1700 caused the land to subside. SOURCE: . See also Atwater et al. (2005). quakes and most of Earth’s seismicity occur at conver- as those across south Asia and western North America, gent plate boundaries. The subduction of relatively cool while others are not clearly associated with any plate oceanic crust increases the depth of the elastic-brittle boundary, such as those in Australia or eastern North regime in which earthquakes occur. Moreover, when America. Half of all intraplate earthquakes occur in the subducted slab crosses the elastic-brittle regime at failed continental rifts ( Johnston and Kanter, 1990), a shallow angle (e.g., in Sumatra, Alaska, and Chile), but their underlying cause remains a mystery. Putative the seismogenic zone can be hundreds of kilometers explanations for intraplate earthquakes include local- wide. The hazard at convergent boundaries is not ized stresses induced by emplacement and crystalliza- always commensurate with earthquake size because tion of magma below the surface, postglacial rebound, much of the faulting and the strongest shaking occur and weak zones in otherwise strong crust. underwater. However, the 2004 tsunami (Figure 4.2) Intermediate (70 to 300 km deep) and deep (300 illustrates the destructive potential and global reach of to 700 km deep) focus earthquakes occur at convergent large subduction zone earthquakes. plate boundaries within subducting lithosphere (Figure Earthquakes that occur within a tectonic plate 2.10). Although they pose less of a threat than shal- account for less than 1 percent of the world’s earth- low, plate-boundary earthquakes, intermediate-depth quakes, but they pose a significant seismic hazard and earthquakes can be quite destructive (Beck et al., 1998). can be quite large. For example, a sequence of strong What causes them is unclear because Earth materials earthquakes with magnitudes as high as 8 shook New are expected to deform plastically at the depths where Madrid, Missouri, for eight weeks in 1811-1812, de- they occur (Question 4). Candidate mechanisms to stroying the town and causing widespread destruction explain intermediate and deep earthquakes include across the central United States. Intraplate earthquakes elevated fluid pressures, accelerating deformation and are not readily explained by plate tectonics. Some occur frictional heating, and mineral phase changes (Kirby within broad plate boundary deformation zones, such et al., 1996).

OCR for page 95
 ORIGIN AND EVOLUTION OF EARTH FIGURE 4.2 Topography and bathymetry of the West Sunda subduction zone showing the location of the trench and the earthquake fracture zones (colored lines). The red dot shows the epicenter of the M 9.1 December 2004 earthquake that ruptured the India/Eurasia plate boundary (area between the trench and Sumatra) and caused the devastating Indian Ocean tsunami. Rupture propagation was primarily northward, toward the lower left of the figure, extending hundreds of kilometers beyond the figure. Subsequent earthquakes have ruptured parts of the plate boundary to the south. SOURCE: Courtesy of Mohamed Chlieh, Caltech. See also Chlieh et al. (2007) and . Used with permission. known, but even then the irregular recurrence of earth- Predicting When earthquakes Will happen quakes makes forecasting difficult. Earthquake predictions are commonly classified by Just as plate tectonics explains where most earthquakes time frame. The types of predictions discussed below occur, it has much to say about how often they occur. are: The velocity of relative motion across plate boundaries is known to within several millimeters per year, which 1. Long-term forecasts of eents of an uncertain provides a boundary condition on fault-slip rates, and thus on how frequently earthquakes must occur over magnitude that hae a low probability of occurrence oer a large window of time. Long-term forecasts based on the long term. The utility of this boundary condition probabilistic methods are an active area of research. for earthquake prediction is confounded by the fact 2. Short-term prediction of eents of a specific size that that plate boundaries typically comprise complex fault systems, and the partitioning of slip among faults is hae a high probability of occurrence within a narrow range of space and time, weeks or months in adance. There is difficult to unravel, even in well-studied systems such currently no way to predict the days or months when as the San Andreas. In some cases, slip rates are well

OCR for page 95
 HAZARDS AND RESOURCES an event will occur in any specific location, and it is not system behavior, long-term earthquake forecast models clear whether it will ever be possible. form a framework for integrative research. Paleoseis- 3. Early warning that seismic waes from a deelop- mology—the investigation of individual earthquakes in the geological record—provides critical information ing eent will arrie in seconds, ideally enabling an alert documenting the frequency and variability of earth- to be issued before damaging strong ground motions begin. This emerging area of research shows promise for re- quake occurrence over the long term. Geodetic mea- ducing seismic risk. surements (e.g., Figure 4.3) constrain the distribution of strain accumulation. Earthquake source models con- strain the amount of slip expected in large earthquakes. long-Term Forecasting Computer algorithms that scan seismicity catalogs and Earthquakes can be forecast by assuming that they occur account for fault slip and interaction, in concert with randomly in time but at known long-term rates. If no historical earthquake catalogs, constrain spatial and historical record of large earthquakes exists, the long- temporal earthquake probabilities. Finally, models of term rate is extrapolated from the frequency-magnitude static and dynamic triggering help us understand how relationship of small earthquakes. Unfortunately, the earthquakes interact, including how the probability of assumption that earthquakes occur randomly in time is one earthquake increases or lessens the probability of highly suspect. Earthquakes are observed to cluster in another (see below). A longstanding and difficult prob- both space and time. Moreover, the elastic-rebound hy- lem that would benefit from further research is how we pothesis suggests that once a large earthquake releases validate long-term earthquake forecasts. the accumulated stress on a fault, that same fault seg- The discussion above concerns shallow earth- ment is unlikely to experience a large earthquake until quakes for which we understand the factors influenc- strain has reaccumulated. Such considerations have led ing the frequency of recurrence, such as fault-slip to time-dependent earthquake forecasts. rates that must be satisfied over the long term. For Perhaps the simplest time-dependent, long-term both intermediate and deep-focus earthquakes, we forecast is that offered by the seismic gap hypothesis, lack the kinematic fault-slip boundary conditions that which posits that a future earthquake is more likely on enable us to constrain the long-term probabilities of a part of a fault that has not ruptured recently (Fedotov, shallow earthquakes. Until a better understanding of 1965). Implicit in this hypothesis is the notion of a intermediate and deep-focus earthquake recurrence “characteristic earthquake,” that is, a large earthquake is achieved, long-term forecasting of such events will that defines a fault segment and dominates the slip remain empirical. budget. According to the seismic gap hypothesis, the probability of an earthquake will be small immediately are short-Term Predictions Possible? after the previous earthquake, and the conditional probability that a characteristic earthquake will occur The challenge of short-term earthquake prediction can be determined from the time of the previous earth- can be illustrated by drawing an analogy with light- quake. The evidence for characteristic earthquake be- ning. Both phenomena involve the abrupt conversion havior is equivocal, and tests of the seismic gap theory of accumulated potential energy to kinetic energy. In have called its utility into question (Kagan and Jackson, the case of lightning, gradually accumulated electrical 1991). However, the notion that a fault must keep up charge suddenly flows as electric current in a lightning with geological slip rates over the long term seems bolt and radiates sound waves as thunder. For earth- inescapable, so there ought to be information in the quakes it is gradually accumulated elastic strain energy system that can inform earthquake forecasts. Develop- that accelerates the crust on both sides of the fault and ing accurate forecast models that use this information radiates energy as seismic waves. Based on the relative is an area of ongoing research. timescales, predicting the size, location, and time of an A number of time-dependent models take some earthquake to within a week corresponds to predicting account of how fault systems are thought to operate. the size, location, and time of a lightning bolt to within Because they need to draw on diverse aspects of fault a millisecond. The latter sounds hopeless but would ac-

OCR for page 95
00 ORIGIN AND EVOLUTION OF EARTH FIGURE 4.3 Estimates of crustal movement near the San Andreas fault based on decadal-scale measurements. Color combines Figure 4.3.eps information from Global Positioning System (GPS) measurements and spaceborne radar interferometry (InSAR). Scale at the top shows velocity in the satellite line of sight as it passes over the region. The blue region in the lower left, southwest of the San Andreas and San Jacinto (SJF) faults, is moving away from the satellite track at ~ 14 mm/yr, which corresponds to northwestward motion at ~ 45 mm/yr relative to the region in red to the northeast of the faults. This suggests that strain is accumulating on the San Andreas fault in this region, where no large earthquake has occurred in over 250 years. (CCF = Coyote Creek fault, EDM = Electro-optical Distance Measurement, SCEC = Southern California Earthquake Center, SCIGN = Southern California Integrated GPS Network, SHF = Supersti- tion Hills fault). SOURCE: Fialko (2006). Reprinted by permission from Macmillan Publishers, Ltd.: Nature, copyright 2006. tually be straightforward because lightning is preceded millisecond is easy once the stepped leader forms. Do by an easily observable nucleation process—the forma- earthquakes have a similar nucleation process? tion of a channel of ionized air, known as a stepped Laboratory studies (e.g., Dieterich, 1979) and leader, that provides a conductive path for the current theoretical models (e.g., Andrews, 1976) of earthquake in the main lightning bolt. The stepped leader precedes nucleation indicate that unstable fault slip should be the lightning strike over its entire length. Prediction preceded by an aseismic nucleation process. It seems of the time and location of a lightning bolt to within a likely that nucleation of some sort must occur before

OCR for page 95
0 HAZARDS AND RESOURCES earthquakes, but if so, how extensive is the nucleation earthquakes occurred more than 1,000 km away (Hill process? If it occurs over a limited part of the fault, and et al., 1993). Static stress changes over such distances thereafter earthquake rupture becomes self-sustaining, are negligible, but dynamic stress changes transmitted then earthquake prediction will be practically impos- by seismic waves have been documented in the 1999 sible. If, on the other hand, nucleation scales with the Izmit, 2002 Denali, and 2004 Sumatra earthquakes. size of the eventual earthquake (e.g., if the size of the In the case of Sumatra, earthquakes were triggered nucleation zone is proportional to the size of the even- in Alaska, a distance of 11,000 miles (West et al., tual earthquake), prediction would be a good deal more 2005). The Landers trigger was synchronous with the likely, though still extremely challenging (Ellsworth maximum vertical displacement of large and extremely and Beroza, 1995). The nature of earthquake nucle- long-period surface waves, indicating a direct role for ation is a key unknown that is central to the question dynamic stresses. This is a new concept of earthquake of short-term earthquake predictability. interaction, and dynamic stress changes can presumably Another possible way to predict earthquakes in the trigger earthquakes at short distances as well. short term is through patterns of earthquake interaction. Whether by static triggering, dynamic triggering, Large shallow earthquakes are immediately followed by or other mechanisms, the occurrence of an earthquake aftershocks that are triggered by the main shock. Large affects the probability of future earthquakes nearby. In earthquakes sometimes trigger other large earthquakes, aftershock sequences, triggered earthquakes can them- most famously in the sequence of large earthquakes that selves trigger other earthquakes in a cascade of failures. ruptured most of the North Anatolian fault during the Models that quantify these so-called epidemic-type 20th century (Toksöz et al., 1979). Thus, the study of aftershock sequence interactions form the basis for a aftershocks provides insight on how large earthquakes variety of probabilistic short-term earthquake forecasts. might interact and on how small earthquakes might Improving these forecasts and testing their skill at pre- trigger large earthquakes. diction are areas of active research (Field, 2007). Aftershocks reflect the response to a stress change imposed by the main shock (Scholz, 1990), but this is complexity not a complete explanation because static, elastic effects by themselves cannot explain the observed gradual de- Earthquake processes span a tremendous range of tem- cay of aftershock rates. The mechanisms proposed to poral and spatial scales, which makes them intrinsically explain aftershock decay include pore fluid flow, visco- difficult to characterize, let alone predict. Spatial scales elastic relaxation, and earthquake nucleation under range from the size of individual mineral grains to the rate- and state-variable friction. A better understand- size of tectonic plates. The smallest microearthquakes ing of these mechanisms would improve prediction of rupture faults for milliseconds, whereas strain accumu- earthquake interaction. lation during the earthquake cycle can be thousands of Less obvious than the triggering of earthquakes years. Physical mechanisms that are dominant at one by static stress changes is their suppression when the scale might become negligible at others. Superimposed stress necessary for earthquakes is relieved by a nearby on the scale variations is the complexity of geological earthquake. A well-known example of this stress structures and materials. shadow effect is the nearly complete absence of major Earthquakes and fault systems have been held up earthquakes in northern California following the 1906 as an example of a complex natural system that exhibits San Francisco earthquake (Ellsworth et al., 1981). self-organized criticality (Bak et al., 1988). Despite Dramatically fewer earthquakes have occurred in this the complexity, earthquake phenomena exhibit certain region in the 100+ years since that earthquake than types order. Earthquake stress drop and radiation effi- occurred in the 50 years leading up to it. An obvious ciency are similar for both large and small earthquakes. conclusion is that the 1906 earthquake suppressed Gutenberg-Richter statistics, a power-law description subsequent earthquakes by relieving the shear stress in of the relative number of large and small earthquakes, Earth’s crust. appear to apply for all earthquake populations. Omori’s Following the 1992 Landers earthquake, small Law provides a universal description of the rate of

OCR for page 95
0 ORIGIN AND EVOLUTION OF EARTH aftershock decay. The geometry of fault networks is earthquake prediction and risk mitigation. But even if typically treated using a fractal description. Methods short-term earthquake prediction ever became a reality, of statistical physics are used to understand how these it would still be impossible to protect most of the built relationships emerge from the earthquake process and environment from damage. Predicting strong ground to predict their behavior (e.g., Turcotte et al., 2007). If motion is itself a considerable scientific challenge. fault systems behave chaotically, as suggested by some Earth’s crust is strongly heterogeneous at all scales, so models, there may be an intrinsic limit to predictability earthquake waves are strongly distorted as they propa- (NRC, 2003b). This limit might be years or months gate through it. The faulting process is also complex if we could fill gaps in our knowledge of the physical and may represent the dominant source of uncertainty laws governing fault motion and if it were possible to in strong ground motion prediction. Predictions of measure accurately all the stresses and strains in and strong ground motions are generally made using proba- around the fault. bilistic methods and computer simulations. Probabilistic seismic hazard analysis. The probability of how much Warning can Be Given Before an strong ground motions is commonly calculated using earthquake? probabilistic seismic hazard analysis. The analysis may Real-time seismology, which became possible with the yield, for example, an estimate of ground motion inten- regional deployment of high-quality instrumentation sity that has a 2 percent probability of being exceeded and rapid, continuous telemetry, provides reliable es- over a 50-year time interval. Probabilistic seismic timates of the location and size of earthquakes within hazard analysis combines information on earthquake a few minutes of the initiation of rupture. For nearby likelihoods from long-term forecasts and data on peak earthquakes, ground shaking will have already begun ground acceleration, spectral acceleration, and peak before these estimates can be made. Earthquake early ground velocity to create a map of intensities at the warning systems focus on the seconds after an earth- specified exceedence probability. Such maps can be quake rupture has already started. These systems exploit used to develop design criteria for buildings and to set the fact that the speed of telecommunications exceeds priorities among risk reduction measures. As with other that of seismic waves. If seismographs can quickly earthquake prediction tools, it will be difficult to test determine that an earthquake is under way, and impor- the validity of these predictions until the instrumental tantly that it is a large earthquake, then regions likely to record is considerably longer. However, observations of be subject to dangerous shaking can be alerted before precariously balanced rocks (Figure 4.5) have recently the seismic waves arrive. Earthquake early warning been used to place bounds on maximum exceeded systems are operational in Japan (Figure 4.4), Mexico, ground motion amplitudes over time intervals as long and Taiwan and are in various stages of development as thousands of years (Brune, 1996). in Romania, Turkey, and the United States. The key to earthquake warning systems is rapid determination that Ground motion prediction through simulation. Very a large earthquake is under way before the earthquake few recordings exist of strong ground motion close to has fully developed (Allen and Kanamori, 2003). The large earthquakes. This is unfortunate because large extent to which this is possible is closely tied to the earthquakes often dominate seismic hazard. Computer nature of earthquake nucleation discussed above. The simulation of strong ground motion provides a pos- amount of warning that earthquake early warning sys- sible means to fill this data gap, as long as we can be tems can provide for large earthquakes can be tens of confident the simulations are accurate. Major sources of seconds under favorable circumstances. uncertainty in these calculations include characteriza- tion of the earthquake source (Figure 4.6), the ability to model the effects of wave propagation through Earth’s strong Ground motion Prediction crust, changes to the wavefield due to near-surface Predicting the level of damaging shaking from seismic nonlinearity, and earthquake-to-earthquake variability waves in an earthquake is a critical aspect of both in the rupture characteristics. Physics-based predic-

OCR for page 95
0 HAZARDS AND RESOURCES Estimating the focus, magnitude and seismic intensities using data from one seismograph An earthquake focus, magnitude & seismic intensities occurs! 5 upper Japan Meteorological Agency Immediately after occurrence P-wave Estimating the focus, magnitude S-wave and seismic intensities using data from two or three seismographs 4 focus, magnitude & More 5 lower seismic intensities accurate Japan Meteorological Agency estimate 5 upper 6 low er principal After ten 6 upper motion preliminary seconds tremors S-wave P-wave Estimating the focus, magnitude and seismic intensities using data from three to five seismographs 4 focus, magnitude & More 5 low er seismic intensities accurate Japan Meteorological Agency 5 upper estimate After 20 6 low er seconds principal preliminary 6 upper motion tremors S-wave P-wave FIGURE 4.4 Schematic representation of the Japanese earthquake early warning system developed by the Japan Meteorological Figure 4-4.eps Agency. This system calculates the location and magnitude of the earthquake from recordings near the epicenter and then estimates the distribution of shaking more widely before the arrival of the strongest shaking, which is typically comprised of S waves. Earthquake early warnings will be broadcast through media outlets such as TV and radio. The system went public in October 2007. SOURCE: Courtesy of the Japan Meteorological Agency. . Used with permission. FIGURE 4.5 The ground motion intensity thresholds at which precariously balanced rocks, such as the one shown at left, would be toppled provide an important test on ground motion exceedence probabilities determined from probabilistic seismic hazard analysis. SOURCE: . Used with permission.

OCR for page 95
0 ORIGIN AND EVOLUTION OF EARTH FIGURE 4.6 Ground motion intensities (warm colors correspond to high intensities) for a simulated M 7.7 earthquake with southeast to northwest rupture on 200-km section of the San Andreas fault. There is a strong rupture directivity effect and strong amplification due to funneling of seismic waves through sedimentary basins south of the San Bernardino and San Gabriel mountains. The simulation on the left assumes a kinematic rupture model, and the one on the right assumes a dynamic (physics-based) rupture model. The extreme difference in the predicted intensities underscores the importance of properly characterizing earthquake source processes. SOURCE: . Visualization courtesy of Amit Chourasia, San Diego Supercom- puter Center, based on data provided by Kim Olsen and colleagues, Southern California Earthquake Center. Used with permission. What is the role of slow earthquakes? tion of strong ground motions through simulations is an area of intense research. Creating simulations that reach high enough frequencies for structural engineer- Over the past decade seismologists and geodesists ing purposes requires high-performance computing. have discovered an entirely new family of unusual Further improvements, particularly in the validation of earthquakes that range in size from M 1 to at least simulation results, are required before they will have an M 7.5. They occur in diverse geological environ- impact on engineering practice. ments—from the subduction zones of Japan, Mexico, Cascadia, and Alaska, to the slopes of Kilauea volcano Dynamic rupture modeling. The evolution of rupture in Hawaii, to the San Andreas fault in California. on faults can be modeled either in terms of the dis- They appear to be caused by the same mechanism placements or as a function of the stresses. The former, as ordinary earthquakes but take such a long time the so-called kinematic description, is most common, to happen that they are described as “slow.” Because but the latter “dynamic” approach provides a more these earthquakes are slow, the waves they generate, if complete description of the process of fault failure they generate waves at all, are weak and were only de- and hence is an ongoing research focus. In dynamic tectable after highly sensitive earthquake monitoring rupture models the redistribution of stored strain en- networks were deployed. Unlike ordinary earthquakes ergy leads to shear failure that becomes unstable and that grow explosively in size with increasing dura- self-sustaining—the process that is believed to occur tion, slow earthquakes, whether large or small, grow in an earthquake. If the assumptions that go into them at a constant rate proportional to their duration. This are correct, dynamic models can serve as a foundation raises the interesting question: What puts the brake for better predictions of both fault behavior and strong on slow earthquakes? There are many other important ground motion (Figure 4.6). However, the models are unanswered questions about slow earthquakes, but the computationally intensive and require an understand- one most relevant for this discussion is their possible ing of fault behavior over a wide range of conditions relation to ordinary earthquakes. (e.g., slip, slip-rate, temperature, pressure, pore pres- Slow earthquakes occur on the deep extension of sure) and physical mechanisms (e.g., slip-weakening, large faults (Figure 4.7). This location is “strategic” for rate- and state-variable friction, thermal pressurization, earthquake prediction because the adjoining, shallower flash heating).

OCR for page 95
0 HAZARDS AND RESOURCES 35 M w5.8 M w6.0 M w6.2 M w3.5 M w3.1 M w6.0 M w3.3 M w6.8 M w5.9 34 M w3.2 M w3.3 Latitude (°N) M w3.5 M w3.4 2 M w3.4 2 M w3.3 4 4 6 2 10 8 33 km 1946 Nankai 30 km 20 4.3 cm yr –1 32 m 0k 200 km 1 132 133 134 135 136 Longitude (°E) FIGURE 4.7 Different types of earthquakes along the Nankai Trough, under Shikoku, Japan. Red and orange features show small Figure 4.7.eps low-frequency earthquakes (M < 2) and very-low-frequency earthquakes (magnitudes shown), respectively. Green rectangles and focal mechanisms show fault-slip models of larger slow-slip events (magnitudes shown). Purple features show the mechanism and slip of the M 8 1946 Nankai earthquake. The top of the Philippine Sea plate is shown by dashed contours. The blue arrow represents the direction of relative plate motion in this area. The slow earthquakes occur on the down-dip extension of the fault that ruptured in the 1946 earthquake. SOURCE: Ide et al. (2007). Reprinted by permission from Macmillan Publishers, Ltd.: Nature, copyright 2007. Tsunamis parts of these faults generate the dangerous earthquakes we are more familiar with. Because of their location Tsunamis are generated by shallow subduction zone and sense of slip, slow earthquakes ought to drive the earthquakes and large submarine landslides (Satake, dangerous part of the fault toward failure. At least in 2007). While extremely fast compared to wind-driven theory, slow earthquakes have the potential to trigger ocean waves, tsunami waves are more than 10 times large earthquakes. For this reason alone they merit slower than seismic waves. This is a big advantage for intense study. Their recent discovery also demonstrates early warning systems, which have been operational that there is still much to be learned about earthquakes for decades. Tsunami warnings rely on rapid analysis and that further fundamental discoveries are sure to lie of seismic waves and sea-level information from tide in our future.

OCR for page 95
 ORIGIN AND EVOLUTION OF EARTH to minimize or mitigate the undesirable consequences blocks to understanding in natural settings. We have of human activities. a general understanding of how fluid moves through Perhaps the most fundamental underlying scien- a granular solid (i.e., the mineral grains or rock frag- tific theme for resource and environmental issues is ments are packed together but separated by pore space), the behavior of fluids in the soils, sediments, and rocks based on models of fluid flow through a medium of that constitute Earth’s uppermost crust. Water is the homogeneous grain size and pore structure. Natural most common fluid of concern. Water in the ground materials are not homogeneous, however, especially on generally comes from water at the surface, and the the 100- to 100,000-m scale of groundwater systems, behavior of surface water and ultimately, precipitation, but even on scales of microns to meters. The rate of is an important aspect of environmental geology. In flow through porous materials varies exponentially with addition to water, various gases, organic liquids, and porosity and grain size, so predicting the spatial pattern both gaseous and liquid carbon dioxide are important of fluid flow even in a relatively simple, but heteroge- geological fluids. Mixtures of fluids—immiscible liq- neous, porous material can be difficult. At the pore uids like water and hydrocarbons, gas-liquid mixtures scale of individual mineral grains, surface tension also (two-phase fluids), and mixtures of a gas phase plus affects flow; the liquid phase present at the boundaries two immiscible liquids (multiphase fluids)—can be of multiple grains has different properties than a bulk particularly challenging materials to understand in liquid and can effectively be held in place by capillary natural underground settings. Some of the scientific forces. At larger scales, Earth’s subsurface is composed issues associated with fluids in shallow crustal environ- of a variety of rock types, with greatly varying porosity ments also apply to deeper-Earth processes, and many and permeability, that are further complicated by faults of them also overlap with issues of earthquake predic- and fractures. tion, climate prediction, the evolution of continents, the When a rock medium is not granular but crystal- behavior of volcanoes, the formation of ore deposits, line, the pore space is typically not visible to the naked and the properties of Earth materials. eye and its distribution within the rock is exceedingly Since water, as the best example, is a commodity variable. Most of the pore space in crystalline rock is of critical importance to humankind, and also an agent attributable to fractures, so the flow of fluid can be for so many important geological, chemical, physical, almost entirely limited to a few fractures that happen and biological processes, there is a continuing desire to be connected. Many geological media, especially to better understand how it works—especially under- volcanic rocks, are both porous and fractured. In these ground where we cannot see it directly, but also as an cases much of the fluid flow may be confined to frac- agent of erosion and sediment transport at the surface. tures, but there is also chemical and heat exchange by Ultimately, it is desirable to be able to manipulate water diffusion (and slow flow) between the fractures and the and other fluids in the environment. Such manipula- porous rock between the fractures. tion has been done for millennia in the case of surface Given this battery of uncertainties, geologists water and is also done in the subsurface, although still have developed a number of strategies to predict fluid with modest efficiency, in petroleum extraction and flow patterns in rocks, including some that are largely subsurface remediation of contaminants. To improve empirical. A more promising approach is to treat the our ability to control, or at least predict, the effects of structural variability with statistical methods, based on subsurface fluids, and to better manage surface wa- observations of analogous rocks that can be studied at ter and sediment, will require major advances in our the surface. But the flow of fluids through rocks under- understanding of how fluids transport materials and ground remains exceptionally difficult to predict. Gen- modify their environment by chemical and physical erally the best results are mere estimates, and even these interactions. are obtainable only from direct observations, usually by drilling into the subsurface and making measurements of returned fluids and rock cores. Still, there is cause how do Fluids Flow in Geological media? for optimism because increasingly powerful measuring The flow of fluids through soils and rocks is easily un- tools are being developed—using approaches such as derstood in the abstract but continues to present road- isotope geochemistry and geophysics—and more effec-

OCR for page 95
 HAZARDS AND RESOURCES tive mathematical modeling allows geologists to wring Other inputs come from benchtop experiments that more information from the data obtained. produce and observe coupled processes in a realistic, controlled environment. In addition to modeling, ef- forts are being made to document the role of microbes how do Fluid-rock chemical and Biological in altering mineral surfaces and chemical microenvi- interactions affect Fluid Flow? ronments (Figure 4.10). And the role of hydrology in As fluids flow through soils and rocks, chemical reac- chemical reactions is being approached with a combi- tions inevitably occur with the minerals of the rocks, nation of numerical models, such as approaches that sometimes catalyzed by microorganisms. The most include multiphase flow in complex geometries and familiar interaction is adsorption, or ion exchange, by microfluidic experiments, both of which can address which ions carried in solution in water are adsorbed and the roles of chemical transport and pore structure on desorbed from mineral surfaces. This process, which chemical reactions. For multiphase fluids there are ad- happens everywhere in nature, has been successfully ex- ditional considerations because the presence of each ploited by humans to create water purification systems. phase interferes with the flow of the other phases, and Fluids moving through rocks also act as weak acid solu- the detailed distribution of each phase within the pores tions, often due to dissolved carbon dioxide, that slowly can affect the surface area that is available for fluid- dissolve the original minerals, which are then replaced rock chemical interactions. There is also partitioning by secondary minerals such as rusty iron oxides and of chemical elements between multiple flowing phases clays. As rocks and soils chemically react with fluids, (e.g., gas, oil, water), which is important in many sub- changes occur not only in mineralogical and chemical surface processes but difficult to model because of its composition, but also in ion exchange and hydrological dependence on the physical relationships between the properties. For soils, the activities of plants, animals, phases. and microbes are important. In deeper groundwater systems, where temperatures are higher and fluids can how do Thermal and mechanical reactions affect be more corrosive, chemical reactions can be quite fast. Fluid Flow? But because chemical reactions between fluids and minerals occur only at mineral surfaces, the structure Chemical reactions are not the only processes that of the fluid flow through rocks and the geochemistry complicate fluid flow. As fluids move through rocks are inextricably linked. If fluid flow is confined to a few they redistribute heat as well as material, and both fractures, it may be fast, with little contact area between the heat and materials affect the subsequent fluid fluid and minerals and little chemical interaction. If flow. For example, buoyant upwelling of groundwa- there is grain-scale porous flow, however, flow veloc- ter heated by magma can cause rainwater that has ity will be low, the contact area large, and fluid-rock percolated into the ground to circulate to depths of interaction extensive. several kilometers in areas of active volcanism and Geological studies of fluid flow, chemical reactions, mountain building, as well as in sedimentary basins. and their interplay are grouped under the heading of At midocean ridges, cold seawater circulates through reactive chemical transport (e.g., Steefel et al., 2005; hot rocks to depths of several kilometers, and magma Figure 4.10). A major goal of this subfield is to de- at the shallow depths of midocean ridges causes such scribe, with advanced computational techniques, how rapid heating of water that it is expelled back into the characteristics of fluid-rock systems affect their the ocean at temperatures above 350°C. Base metal physical, chemical, and biological development. The ore deposits associated with magmatic intrusions in computer models require large inputs of basic materials the crust are products of “fossil” hydrothermal sys- property data, and the complexity of the interactions tems where circulating water attained temperatures of is a conceptual as well as a computing challenge. One 200°C to over 500°C (Hedenquist and Lowenstern, crucial feature of the models is mineral surface proper- 1994; Sillitoe and Hedenquist, 2003). Some of these ties and their role in chemical reaction kinetics, which systems persisted for tens to hundreds of thousands of are increasingly explored at synchrotron X-ray facilities. years at depths of 3 to 10 km. Any magma that makes

OCR for page 95
 ORIGIN AND EVOLUTION OF EARTH FIGURE 4.10 Schematic illustration of coupled transport, chemical, and biological processes in a hypothetical aquifer downstream of an organic-rich landfill. Closest to the landfill is a zone of methane generation, which is progressively followed downstream by sulfate reduction, iron reduction, denitrification, and aerobic respiration that develop as the flowing fluid becomes progressively oxi- dized by mixing with oxygenated water. Within the iron reduction zone, a pore-scale image (magnified about 10,000 times relative to the cross section) is shown in which the influx of dissolved organic molecules provides electrons for iron reduction mediated by a biofilm. Dissolution of the organic phase leads to the release of Fe2+, HCO3–, and OH– into the pore fluid, which then causes siderite or calcite to precipitate, reducing the porosity and permeability of the material. Sorption of Fe2+ may also occur on clays, displacing other cations originally present on the mineral surface. Where reactions are fast relative to local transport, gradients in concentration, and thus in reaction rates, may develop at the pore scale. SOURCE: Steefel et al. (2005). Copyright 2005 by Elsevier Science and Technology Journals. Reproduced with permission. its way to within several kilometers of Earth’s surface is warming is usually dissolving minerals; water that is will stimulate groundwater convection, and it is now cooling tends to precipitate minerals. Dissolution and believed that this convection plays a major role in ac- precipitation both affect permeability and compete with celerating the cooling and crystallization of magma in temperature changes and hydrofracture in modifying the crust (Fournier, 1999). fluid flow (Haneberg et al., 1999). If hot water is close Heat transfer can affect fluid flow in several ways. to Earth’s surface and hence at low confining pressure, Simple heating can cause the rocks to swell and may it can boil, and this phase change introduces additional cause fractures to close, decreasing permeability and complications. Water vapor cannot hold as much dis- slowing flow. Water that is heating, however, can also solved rock as hot water, so boiling tends to cause min- become pressurized as it expands, which can fracture the eral precipitation. Boiling also lowers a fluid’s viscosity rocks or expand and lengthen existing cracks, thereby and leads to a two-phase fluid. Some geothermal sys- increasing permeability and flow. Alternatively, ther- tems are hot and deep enough to support supercritical mal contraction of rocks due to cooling by infiltrating fluids, whose properties and behavior are much less well groundwater will also induce fracturing and promote known than those of their subcritical counterparts. Bet- permeability increases (Majer et al., 2007). Water that ter knowledge of these obscure regimes may be essential

OCR for page 95
 HAZARDS AND RESOURCES for understanding geothermal systems (Fridleifsson and cal requirement for addressing groundwater recharge, Elders, 2004). waste movement, and other issues. This limitation Geothermal fluid-rock circulation systems typically means that monitoring is required, but the effectiveness have scales of meters to tens of kilometers, which en- of subsurface monitoring systems is still limited. One sures that they will encounter a range of temperatures, way to track subsurface fluids and processes, and still pressures, rock compositions, and permeability. In real perhaps the most reliable approach, is to drill wells and systems the fluids are often saline, acidic, or toxic, and take samples of fluids and rock. Drilling is expensive high temperature and pressure gradients result in rapid and time consuming, however, and can never provide mineral precipitation that reduces porosity and fluid a complete picture of the subsurface. However, new flow. Nevertheless, it is highly desirable to understand methods are being developed to translate the chemistry and predict their behavior for a variety of practical and and isotopic composition of sampled fluids into physi- scientific reasons. For example, there is an abundance cal and chemical characteristics of the regions between of hot rock not far below the surface in the western the well samples. For example, fluid sampling now United States (Blackwell and Richards, 2004), and if makes possible estimates of in situ chemical weathering water, CO2, or some other fluid could be circulated rates; the sources, age, and velocity of the fluids; and through it and returned to the surface, a sizable amount the importance of fracture flow and even the spacing of thermal energy could be harvested. So far, efforts to between flowing fractures. do this have been only partially successful because the There is hope that noninvasive geophysical meth- evolution of this thermal-hydrological-mechanical- ods will yield increasing amounts of information about chemical (THMC) system is highly complex, and we subsurface fluid-rock systems. Geophysical methods lack the expertise to control these processes in a way can help detect subsurface fluids, either from the sur- that permits manipulation of fluid flow in the subsur- face or between bore holes. These methods combine face (MIT, 2006). electrical and mechanical signals with tomographic An interesting example of a proposed man-made analysis to provide three-dimensional maps of subsur- THMC system, which exhibits many of the complexities face properties. The challenge is to detect the relatively of fluid-rock systems in a compact form, is the planned weak signals and then convert them into reliable esti- underground nuclear waste repository at Yucca Moun- mates of hydrological quantities such as fluid content, tain, Nevada. The radioactive materials that would be fluid composition, and porosity. Figure 4.11 shows an stored in the ground produce heat, and both models and example of tomographic imaging, which can assess the experiments show that this heat will generate groundwa- connectivity of pore spaces or determine in situ spatially ter convection, boiling, mineral dissolution and precipita- distinct densities. Such imaging provides a powerful tion, as well as potentially corrosive conditions around the new tool for understanding the spatial characteristics waste canisters themselves (see Box 4.2). Each of these of Earth materials. effects is understood individually at a reasonable level, but Still, the uncertainties in predicting long-term the evolution of the overall system is sufficiently uncertain fluid-rock system performance are so daunting that that it affects our assessment of the risk of burying the we need much more accurate and efficient monitor- waste. Other examples of THMC systems are those used ing methods. The extent to which such monitoring to enhance petroleum recovery, where steam or other can be done remotely or by noninvasive methods will fluids are pumped into the ground to produce lower- determine just how useful they can be in monitoring viscosity oil, to enhance permeability, and to push residual contaminated groundwater sites and other systems. In oil toward existing wells. general, improvement of monitoring methods hinges on fundamental advances in the chemistry and physics of geological materials. This is because the chemical, can the Behavior of subsurface Fluids Be electrical, and seismic behavior of the bulk media is Predicted? often determined by the details of minerals, mineral Because of the complexity of fluid-rock systems, we can- surfaces, phase boundaries, and phase compositions. not yet predict how they will change over time—a criti- And these advances, in turn, must be optimized

OCR for page 95
6 ORIGIN AND EVOLUTION OF EARTH BOX 4.2 Thermal-Hydrological-Mechanical-Chemical Processes in Yucca Mountain An interesting example of coupled thermal-hydrological-mechanical-chemical (THMC) processes is the anticipated behavior of the water-under- saturated rock mass surrounding the proposed horizontal tunnels (drifts) at the Yucca Mountain site in southern Nevada. Small amounts of groundwater are present in the porous volcanic rocks, even above the water table, and the primary unknown is whether this water will enter the drifts and cause the waste canisters to corrode (Long and Ewing, 2004). Such corrosion could eventually allow the release of radioactive elements into solution, and the dissolved constituents could percolate slowly downward toward the water table and the regional groundwater aquifer. To test the likelihood of this outcome, mathematical models have been developed to simulate the combined effects of heat and material transport around the drifts (conceptual model shown below). For these models the radioactive waste containers are considered to be only a heat source, and the amount of heat they produce can be estimated accurately. This heat will keep rock temperatures near the drifts above the boiling point of water for a considerable period of time. Boiling of groundwater would generate vapor that migrates away from the drifts and then condenses in cooler regions and drains through the fracture network. This elevated temperature and moisture redistribution would cause changes in pore water and gas compositions, as well as mineral dissolution and precipitation. Mineral dissolution and precipitation can result in porosity and permeability changes in the rock, which lead to altered flow paths and flow focusing. The models suggest that in some circumstances very little water can enter the drift but that in other circumstances some water can enter. The scien- tific unknowns reflect the basic and overlapping questions that characterize the broader field of fluid flow: how to represent the processes in the model, how the rock properties change as mineral dissolution and precipitation proceed, how fast the minerals actually dissolve and precipitate, how water is distributed between fractures and the porous matrix rock separating them, and the effects of heating and cooling on fracture porosity and permeability. Conceptual model of processes in a fracture within volcanic tuff above a heat source with the properties of a radioactive waste container. Temperature is highest at the bottom of the fracture, nearest the heat source, and decreases upward. Heating of the fluid near the base of the fracture causes boiling (production of steam) and release of gaseous CO2. The vapor phase rises upward due to gravity, condenses at a higher level, and flows downward as a liquid. The CO2 dissolves into the colder groundwater near the top of the system, making it more acidic and causing it to dissolve silicates. The dis- solved constituents are carried downward in the liquid phase and precipitated, eventually causing the fractures to narrow or become sealed. Although the general features of this system can be established, the time evolution is highly dependent on the rates of flow, the rates of the dissolution reactions, and resultant changes in porosity and permeability. SOURCE: Sonnenthal et al. (2001).

OCR for page 95
 HAZARDS AND RESOURCES one scale—for example, faults and lithologic changes at scales of thousands of meters—are not present at smaller scales. Consequently, it is not justifiable to ex- trapolate material properties like hydraulic conductivity from small to large scales. With regard to fluid-mineral chemical reactions, reaction rates depend on the fluid’s chemical composition, the mineral-fluid contact area, and the microscopic characteristics of the mineral sur- faces (Question 6). All of these parameters can vary, and the range of variation can be large at both small and large spatial scales. Also, the chemical reactions that are significant at geological timescales proceed at ultraslow rates (Question 7), and it is obvious that the factors that control these rates are not the same as those FIGURE 4.11 Tomographic image of residual fluid saturation that control laboratory reactions that proceed a million in a sintered bead pack after free drainage. The beads have been or a billion times faster. rendered transparent and were 1.63 mm in diameter. SOURCE: Variation of material properties and reaction rates Sakellariou et al. (2004). Copyright 2004 by Elsevier Science and Technology Journals. Reproduced with permission. at different timescales and length scales is an important issue for large-scale geological sequestration of carbon dioxide (Box 4.3). Typical plans are to inject CO2 into sedimentary rock formations deep underground by improvements in data analysis and computing at hundreds of sites over periods of tens of years. The capabilities. injected CO2, which is lighter than saline aqueous fluid, can displace the fluids but at the same time will tend to mix with the ambient fluid to produce a carbonic What are the effects of multiple Timescales and acid-rich dense fluid. Since the CO2 must be retained length scales on Fluid-rock systems? underground for hundreds of years for geological se- As with other Earth materials and processes, the behav- questration to be effective, and because the fluids will ior of fluid-rock systems varies enormously with length be confined only by geological barriers, it is important scales and timescales. Although some processes can be to understand how the fluid will move and react with studied in the laboratory, experiments must generally the rocks and to have the capability to monitor the be limited to systems that are centimeters to meters movement and reactions (DOE, 2007). in size and days to months in duration. In this setting it is possible to characterize the average properties of can the effects of Water on earth Processes Be fluid flow and accurately predict both flow and chemi- Predicted? cal interactions, but as we have seen throughout this report, laboratory results cannot faithfully reflect those Water, in both gaseous and liquid forms, is a uniquely of natural systems that are much larger and persist for pervasive fluid in the ways it supports life and oth- thousands or millions of years. In general, larger sys- erwise influences the structure and evolution of the tems exhibit faster flow, greater dispersion, and much planet—and yet we only partially understand most of slower chemical interactions between fluids and solids these processes. For example, humans depend on the than we expect on the basis of laboratory experience. balance between the extraction of groundwater and the The problems of scale are more than technicalities; recharge of groundwater reservoirs, but the factors af- they are fundamental scientific challenges, as noted fecting this balance are complex. They depend on how also for material properties (Question 6), earthquake rainfall is partitioned between evaporation back to the prediction (Question 9), and global weathering rates atmosphere, surface runoff, and infiltration into deeper (Question 7). Geological features that are present at reservoirs where evaporation is no longer important. In

OCR for page 95
 ORIGIN AND EVOLUTION OF EARTH BOX 4.3 Geological Sequestration of CO2 Several techniques have been proposed to capture CO2 at the source and sequester it from the atmosphere. One approach under active investi- gation is storage in geological reservoirs. Current feasible options for geological sequestration include oil and gas reservoirs, coal beds, and saline formations (i.e., saline aquifers and brine-saturated sedimentary rock). Although nature has stored CO2 in these geological structures for millions of years, the human use of this technique has other advantages. For example, injecting CO2 into an oil or a natural gas reservoir can enhance hydrocarbon production, and about 35 million tons of CO2 per year is already used for this purpose in the United States (Stevens et al., 2000). Limited field tests suggest that CO2 injection would also enhance extraction of methane from coal beds by displacing methane with CO2. Although these techniques have the potential to enhance resource recovery and offset the costs of CO2 capture, transport, and injection, questions of reservoir availability (for oil and gas) and technological readiness (for coal beds) limit their widespread use. CO2 can also be injected into saline formations in sedimentary basins and on the continental shelves and trapped by displacement or compression of brine in the porous rocks. Disposal under several hundred meters of deep-sea sediment is another option. The limiting factors on storage volume include sediment layer thickness and permeability, as well as the potential for ground perturbations, such as landslides (House et al., 2006). To what extent will CO2 move within or beyond the geological formation? What physical and chemical changes are likely to occur in the formation when CO2 is injected? A better understanding of the implications of CO2 injection and sequestration is critical to determining its viability as a mitigation option for atmospheric CO2 emissions. Key questions that must be answered include the location, capacity, and availability of storage sites; the perma- nence of storage; and the risks to humans and ecosystems. Long-term monitoring, measurement, and verification technologies must also be developed to improve the storage prediction models used to estimate storage capacity and to design storage areas. Leakage of CO2 from storage sites, as well as migration within the sites, could pose local and global environmental hazards; in 1986 the sudden venting of CO2 from the bottom of Lake Nyos, Cameroons, killed 1,800 people. Escaped CO2 can infiltrate the shallow subsurface, with potential adverse effects on groundwater chemistry, the vadose zone, and ambient air quality above ground. Large-scale storage failure could return CO 2 to the global atmosphere. Understanding the fate of these gases and fluids on timescales of thousands of years and longer is crucial to decisions on the wise use of carbon-based fuels. Approaches for geological sequestration of carbon dioxide. SOURCE: Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC). Used with permission.

OCR for page 95
 HAZARDS AND RESOURCES regions of thick vegetation, plants recycle much of the deep crust is likely to have influenced the planet’s water back to the atmosphere. In arid regions there is evolution. both little rainfall and little vegetation, which in some ways simplifies the analysis, but because little rainfall can landscapes Be managed to sustain human infiltrates in these regions, it is difficult to estimate Populations and ecosystems? the amount accurately. In addition, water in the form of rain is a weathering agent, which in combination Water flowing at the surface in rivers and streams with microbial processes and dust deposition produces transports dissolved ions, sediment, and organic mate- soil. Because soil is removed or modified by land use rial and constitutes a longstanding focus of geological changes, the rates of soil formation are critical in study. Surface water, through erosion and sediment predicting the future character of the land available redistribution, is the primary sculptor of Earth’s land- for agriculture, home sites, and industry. Finally, the scapes—or was until the rapid population growth over potential effects of global warming on groundwater the past century. We are living on a planet that we have availability and quality may alter the living patterns of “engineered” over the millennia. Humans have caused human populations in the future, but by how much is massive changes in the shape of landscapes as well as virtually unknown (IPCC, 2007b). the distribution of plants, animals, water, sediments, Some of the ways in which water influences gross and chemicals. These changes have been caused by planetary structure and evolution are equally obscure. resource extraction, as well as attempts to ensure wa- We know that the presence of water in the subsurface, ter availability, promote agriculture, build roads, and in particular its retention in soils and unconsolidated decrease the risk of floods and landslides. Recently rocks, tends to lower internal friction and promote we have learned that these changes generate new risks landslides. Water also plays a key role in determining in the longer term. If we are to protect and sustain the strength of faults and the deformability of rocks the planetary systems that provide us with essential deep in the crust and mantle (Question 9), but the services, we must base our future “engineering” deci- details are too complex to facilitate the prediction of sions on a thorough understanding of the fundamental earthquakes. Little is yet known about how fluids, pri- processes that govern how Earth works. marily water and carbon dioxide, are distributed in and A sustainable landscape is one that supports the move through the continental crust at depths greater continued use of resources while maintaining critical than a few kilometers. Water also promotes melting natural processes and ecosystem functions. Humans in planetary interiors (Question 4), and at moderate will always need to extract resources, but minimizing pressure and temperature inside Earth, water and sili- damage to the environment will require a more effec- cates may be completely miscible. The behavior of such tive capability to link specific actions to quantifiable high-temperature hydrosilicate fluids is poorly known consequences. In a single watershed, for example, but is likely to be important for understanding both the actions such as timber harvesting, plowing, and road distribution of water within planets and the origin of construction are known to cause downstream changes magmatism. Magmas constitute a class of fluids whose in sediment transport, water flow, and nutrient avail- flow and thermal, mechanical, and reactive behavior are ability. But models of watershed processes, linking only crudely understood. physical, biological, and geochemical processes, are Beyond Earth, water and other liquids may be poorly developed. New technologies are emerging important for understanding geological history and that should improve those models, including high- the present structure of other planets and moons in the resolution topographic data from airborne laser swath Solar System. A compelling example is Mars, where mapping, which can be used to measure even small the past and present distributions of water are guid- changes in landscape morphology; new sediment trac- ing our search for other life forms. Subsurface water is ers and dating methods; inexpensive wireless sensors also likely to have been critical to Martian landforms, that enable spatially distributed, intensive monitor- and the amount of water in the Martian mantle and ing; and more powerful computational capabilities

OCR for page 95
0 ORIGIN AND EVOLUTION OF EARTH that allow the integration of these diverse data into and climate forecasting, the availability of digital to- mathematical models. pography, and improved understanding of processes, The desire to restore landscapes and ecosystems hazard prediction is becoming spatially explicit, up to to their “predisturbance” states has led to an emerging date, and much more useful for mitigation efforts. With field of restoration geomorphology. A key question today’s 10-day forecasts of weather, flood forecasts is whether it is possible to help a dynamic landscape are becoming commonplace as well, although not yet persist through human-induced changes and retain achieving good spatial extent and accuracy. Scientists its most important and desirable attributes. A good are also beginning to forecast landslides in response example is stream restoration (Bernhardt et al., 2005), to predicted rainfalls, but they still lack the ability to which presents a surprisingly complicated set of objec- predict landslide size, location, travel distance, or speed. tives. In a typical situation the desired state might be Sea-level rise, changes in storminess, and reductions in a laterally migrating, self-maintaining stream channel sediment due to dams may influence the effects of large that passes the sediment it receives, rather than allow- storms on lowland river, delta, and coastal systems. ing it to accumulate in undesirable places; maintains However, we cannot yet predict how sea-level rise will habitat for plants and animals; and maintains its dis- affect levees or the flood heights on lowland rivers or solved load and nutrient content at appropriate levels. determine whether artificial levees could be removed Although we are learning how to address some of these while still retaining flood protection. Answering these objectives, we lack mechanistic models for river chan- and many other such questions will require a body nels that represent their morphology, sediment load, of field studies, experiments, theory, and numerical and interaction with vegetation. And even a good de- modeling sufficient to build the predictive science of sign for current conditions might not be useful through watershed resiliency and hazard mitigation. flood-drought cycles and longer term climate changes. Another example of landscape change is dam building. summary While this kind of change is completely human caused and initially local, it is now recognized to have effects Our ability to manage natural resources, safely dispose that are global in scale (Syvitski et al., 2003, 2005; see of wastes, and sustain the environment depends on our Box 4.4). understanding of fluids, both at the surface and below Given the inevitability of environmental change, ground. In particular, we need a better grasp of how whether natural or human induced, stream systems fluids flow, how they transport materials and heat, and need to be managed for the desirable ecosystem how they interact with and modify their surroundings. characteristics even if, for example, sea level rises, pre- The list of significant fluids begins with water, the cipitation changes, or mountain glaciers disappear. For most abundant and important Earth fluid, and includes example, global warming brings permafrost melting in steam, hydrocarbons, liquid and gaseous carbon dioxide, polar regions, along with a range of hydrological, eco- other organic liquids, and multiphase fluids (gas plus logical, and geochemical changes (Chapin et al., 2006). liquid, immiscible liquids, and gas plus immiscible liq- Because warming will continue well into the future no uids). For subsurface processes we need to understand matter how we attempt to manage greenhouse gases how these fluids are distributed in heterogeneous rock today, human societies need the capability to predict the and soil formations, how fast they flow, and how they consequences and take actions that preserve functions are affected by chemical and thermal exchange with the and resources (e.g., see Box 4.3). host formations. At Earth’s surface we are concerned Hazards from surface processes include landsliding, with the flow of water in rivers and streams, how stream flooding, and coastal retreat. Hazard mitigation has tra- erosion and sediment transport change landscapes, and ditionally relied on the use of maps that delineate some how human activities and climate change affect the aspect of risk, but such maps tend to rely on the intui- evolution of streams and landscapes. tive skill of the mapmaker and are typically based on Decades of research have brought substantial a fixed environmental state. This means that the maps knowledge about the flow and transport of fluids, but rapidly become inaccurate. With advances in weather application of this knowledge is strained by increased

OCR for page 95
 HAZARDS AND RESOURCES BOX 4.4 Global Impact of Dams Humans have constructed more than 45,000 dams above 14 m in height, which together are capable of holding back about 15 percent of the total global annual river runoff (Vörösmarty et al., 2003). Dams have reduced the total amount of sediment carried to the ocean by about 20 to 30 percent, even though human activities have increased the total sediment production by 30 percent. And dam building continues; between 160 and 320 new dams are built annually, especially in Asia. Dams cause major changes to local, and ultimately worldwide, physical, chemical, and ecological systems and in many cases simply terminate river ecosystem functions. Despite the negative impacts of dams, demand for power, flood control, and water supply means that many will remain and more will be built. Nonetheless, much can be done to preserve the desired physical, chemical, and ecological characteristics of affected watersheds. For example, the release of water from reservoirs could be designed to mimic important natural functions, such as sediment transport, recruitment of riparian vegetation, and fish reproduction. As we learn more about the long-term consequences of sediment depletion in downstream rivers and coastal environments, we can take action to compensate. Box 4.4 figure top.eps Basinwide Trapping Efficiency (%) 0 - 20 20 - 40 40 - 60 60 - 80 80 - 100 Summary of the impacts of dams on major global river systems. (Top) Geographical distribution of 633 large reservoirs (i.e., those with a storage capacity of 0.5 km3 or greater). (Bottom) Efficiency of basins in trapping suspended sediment. In some basins sediment load is severely restricted by Box 4.4 figure bottom.eps dams along the river course; in some cases virtually all sediment is trapped. SOURCE: Vörösmarty et al. (2003). Copyright 2003 by Elsevier Science and Technology Journals. Reproduced with permission.

OCR for page 95
 ORIGIN AND EVOLUTION OF EARTH population, resource demands, and the environmental airborne and spaceborne sensors that offer an unprec- consequences of our own success as a species. Meet- edented view of how water and other fluids are shaping ing these planetwide challenges will require a major our planet. The ultimate objective of this research is advance in our ability to understand fluids in and on robust mathematical models that can simulate natural Earth, manipulate them, and monitor their where- fluid-bearing systems and predict far into the future abouts and effects. These challenges are being met how they will behave and change. Only by building by new experimental tools that can illuminate what and skillfully using such models will we be able to make happens at the microscopic scale on mineral surfaces, informed decisions about the land and resources that new geochemical and geophysical field techniques, and support humankind and all life on Earth.