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000200010270575680_R1_CH08_p218-245.qxd 16/02/2011 05:00 PM CP 1 Next Generation Sunshine State Standards Chapter 8 LA.910.2.2.3. The student will organize information to show understanding or relationships among facts, ideas, and events (e.g., representing key points within text through charting, mapping, paraphrasing, summarizing, comparing, contrasting, or outlining). LA.910.4.2.2. The student will record information and ideas from primary and/or secondary sources accurately and coherently, noting the validity and reliability of these sources and attributing sources of information. MA.912.S.3.2. Collect, organize, and analyze data sets, determine the best format for the data and present visual summaries from the following: bar graphs line graphs cumulative frequency (ogive) graphs SC.912.E.6.1. Describe and differentiate the layers of Earth and the interactions among them. SC.912.N.1.1. Define a problem based on a specific body of knowledge, for example: biology, chemistry, physics, and earth/space science, and do the following: 1. 3. 4. 7. 8. 9. pose questions about the natural world, examine books and other sources of information to see what is already known, review what is known in light of empirical evidence, pose answers, explanations, or descriptions of events, generate explanations that explicate or describe natural phenomena (inferences), use appropriate evidence and reasoning to justify these explanations to others SC.912.N.1.4. Identify sources of information and assess their reliability according to the strict standards of scientific investigation. SC.912.N.2.2. Identify which questions can be answered through science and which questions are outside the boundaries of scientific investigation, such as questions addressed by other ways of knowing, such as art, philosophy, and religion. Florida Sunshine State Standards Chapter 8 000200010270575680_R1_CH08_p218-245.qxd 16/02/2011 05:00 PM CP 2 Overall Instructional Quality The major tool introduces and builds science concepts as a coherent whole. It provides opportunities to students to explore why a scientific idea is important and in which contexts that a science idea can be useful. In other words, the major tool helps students learn the science concepts in depth. Additionally, students are given opportunities to connect conceptual knowledge with procedural knowledge and factual knowledge. Overall, there is an appropriate balance of skill development and conceptual understanding. Tasks are engaging and interesting enough that students want to pursue them. Real world problems are realistic and relevant to students’ lives. Problem solving is encouraged by the tasks presented to students. Tasks require students to make decisions, determine strategies, and justify solutions. Students are given opportunities to create and use representations to organize, record, and communicate their thinking. Tasks promote use of multiple representations and translations among them. Students use a variety of tools to understand a single concept. The science connects to other disciplines such as reading, art, mathematics, and history. Tasks represent scientific ideas as interconnected and building upon each other. Benchmarks from the Nature of Science standard are both represented explicitly and integrated throughout the materials. Florida Sunshine State Standards Chapter 8 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 09:55 AM Page 218 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 09:55 AM Page 219 Earthquakes and Earth’s Interior C H A P T E R 8 Aftermath of a devastating tsunami at the coastal town of Banda Aceh on the Indonesian island of Sumatra, December 26, 2004. (Photo by Mark Pearson/Alamy) 219 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 220 O n October 17, 1989, at 5:04 P.M. Pacific daylight time, millions of television viewers around the world were settling in to watch the third game of the World Series. Instead, they saw their television sets go black as tremors hit San Francisco’s Candlestick Park. Although the earthquake was centered in a remote section of the Santa Cruz Mountains, 100 kilometers to the south, major damage occurred in the Marina District of San Francisco. The most tragic result of the violent shaking was the collapse of some double-decked sections of Interstate 880, also known as the Nimitz Freeway. The ground motions caused the upper deck to sway, shattering the concrete support columns along a mile-long section of the freeway. The upper deck then collapsed onto the lower roadway, flattening cars as if they were aluminum beverage cans. This earthquake, named the Loma Prieta quake for its point of origin, claimed 67 lives. In mid-January 1994, less than five years after the Loma Prieta earthquake devastated portions of the San Francisco Bay Area, a major earthquake struck the Northridge area of Los Angeles. Although not the fabled “Big One,” this moderate 6.7-magnitude earthquake left 57 dead, more than 5,000 injured, and tens of thousands of households without water and electricity. The damage exceeded $40 billion and was attributed to an apparently unknown fault that ruptured 18 kilometers (11 miles) beneath Northridge. The Northridge earthquake began at 4:31 A.M. and lasted roughly 40 seconds. During this brief period, the quake terrorized the entire Los Angeles area. In the three-story Northridge Meadows apartment complex, 16 people died when sections of the upper floors collapsed onto the first-floor units. Nearly 300 schools were seriously damaged, and a dozen major roadways buckled. Among these were two of California’s major arteries—the Golden State Freeway (Interstate 5), where an overpass collapsed completely and blocked the roadway, and the Santa Monica Freeway. Fortunately, these roadways had practically no traffic at this early morning hour. In nearby Granada Hills, broken gas lines were set ablaze while the streets flooded from broken water mains. Seventy homes burned in the Sylmar area. A 64-car freight train derailed, including some cars carrying hazardous cargo. But it is remarkable that the destruction was not greater. Unquestionably, the upgrading of structures to meet the requirements of building codes developed for this earthquake-prone area helped minimize what could have been a much greater human tragedy. Students Sometimes Ask . . . How often do earthquakes occur? All the time—in fact, there are literally thousands of earthquakes daily! Fortunately, the majority of them are too small to be felt by people, and many of them occur in remote regions. Their existence is known only because of sensitive seismographs. 220 What Is an Earthquake? Forces Within Earthquakes An earthquake is the vibration of Earth produced by the rapid release of energy (Figure 8.1). Most often earthquakes are caused by slippage along a fault in Earth’s crust. The energy released radiates in all directions from its source, the focus 1foci = a point2, in the form of waves. These waves are analogous to those produced when a stone is dropped into a calm pond (Figure 8.2). Just as the impact of the stone sets water waves in motion, an earthquake generates seismic waves that radiate throughout the Earth. Even though the 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 221 FIGURE 8.1 Destruction caused by a major earthquake that struck northwestern Turkey on August 17, 1999. More than 17,000 people perished. (Photo by Yann Arthus-Bertrand/Peter Arnold, Inc.) energy dissipates rapidly with increasing distance from the focus, sensitive instruments located throughout the world record the event. More than 30,000 earthquakes that are strong enough to be felt occur worldwide annually. Fortunately, most are minor tremors and do very little damage. Only about 75 significant earthquakes take place each year, and many of these occur in remote regions. Occasionally, however, a large earthquake occurs near a large population center. Under these conditions, an earthquake is among the most destructive natural forces on Earth. The shaking of the ground, coupled with the liquefaction of some soils, wreaks havoc on buildings and other structures. In addition, when a quake occurs in a populated area, power and gas lines are often ruptured, causing numerous fires. In the famous 1906 San Francisco earthquake, much of the damage was caused by fires (Figure 8.3), which quickly became uncontrollable when broken water mains left firefighters with only trickles of water. Earthquakes and Faults Epicenter Wave fronts Focus Fault FIGURE 8.2 The focus of an earthquake is located at depth. The surface location directly above it is called the epicenter. The tremendous energy released by atomic explosions or by volcanic eruptions can produce an earthquake, but these events are relatively weak and infrequent. What mechanism produces a destructive earthquake? Ample evidence exists that Earth is not a static planet. We know that Earth’s crust has been uplifted at times, because we have found numerous ancient wavecut benches many meters above the level of the highest tides. Other regions exhibit evidence of extensive subsidence. In addition to these vertical displacements, offsets in fence lines, roads, and other structures indicate that horizontal movement is common (Figure 8.4). These movements are associated with large fractures in Earth’s crust called faults. Typically, earthquakes occur along preexisting faults that formed in the distant past along zones of weakness in Earth’s 221 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 222 FIGURE 8.3 San Francisco in flames after the 1906 earthquake. (Reproduced from the collection of the Library of Congress) Inset photo shows fire triggered when a gas line ruptured during the Northridge earthquake in southern California in 1994. (AFP/Getty Images) crust. Some are very large and can generate major earthquakes. One example is the San Andreas Fault, which is a transform fault boundary that separates two great sections of Earth’s lithosphere: the North American plate and the Pacific plate. Other faults are small and produce only minor earthquakes. Most faults are not perfectly straight or continuous; instead, they consist of numerous branches and smaller fractures that display kinks and offsets. Such a pattern is displayed in Figure 10.A (p. 292), which shows that the San Andreas Fault is actually a system that consists of several large faults and innumerable small fractures (not shown). It is also clear that most faults are locked, except for brief, abrupt movements that accomFIGURE 8.4 Slippage along a fault produced an offset in this orange grove east of Calexico, California. (Photo by John S. Shelton) Inset photo shows a fence offset 2.5 pany an earthquake rupture. The primary meters (8.5 feet) during the 1906 San Francisco earthquake. (Photo by G. K. Gilbert, U.S. reason faults are locked is that the confining Geological Survey) pressure exerted by the overlying crust is enormous and essentially squeezes the fractures in the crust shut. Nevertheless, even faults that have been inactive for thousands of years can rupture again if the stresses acting on the region increase sufficiently. Discovering the Cause of Earthquakes 222 The actual mechanism of earthquake generation eluded geologists until H. F. Reid of Johns Hopkins University conducted a study following the great 1906 San Francisco earthquake. The earthquake was accompanied by horizontal surface displacements of several meters along the northern portion of the San Andreas Fault. Field investigations determined that during this single earthquake, the Pacific plate lurched as much as 4.7 meters (15 feet) northward relative to the adjacent North American plate. 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 223 What Is an Earthquake? 223 The mechanism for earthquake formation that Reid deduced from this information is illustrated in Figure 8.5. Part A of the figure shows an existing fault, or break in the rock. In part B, tectonic forces ever so slowly deform the crustal rocks on both sides of the fault, as demonstrated by the bent features. Under these conditions, rocks are bending and storing elastic energy, much like a wooden stick does if bent. Eventually, the frictional resistance holding the rocks in place is overcome. As slippage occurs at the weakest point (the focus), displacement will exert stress farther along the fault, where additional slippage will occur, releasing the built-up strain (Figure 8.5C). This slippage allows the deformed rock to “snap back.” The vibrations we know as an earthquake occur as the rock elastically returns to its original shape. The “springing back” of the rock was termed elastic rebound by Reid, because the rock behaves elastically, much like a stretched rubber band does when it is released. Most of the motion along faults can be satisfactorily explained by the plate tectonics theory, which states that large slabs of Earth’s lithosphere are in continual slow motion. These mobile plates interact with neighboring plates, straining and deforming the rocks at their margins. In fact, it is along faults associated with plate boundaries that most earthquakes occur. Furthermore, earthquakes are repetitive: As soon as one is over, the continuous motion of the plates adds strain to the rocks until they eventually fail again. In summary, most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great stress. Once the strength of the rock is exceeded, it suddenly ruptures, causing the vibrations of an earthquake. EarthFIGURE 8.5 Elastic rebound. As rock is deformed it bends, storing elastic energy. Once the rock is strained beyond its breaking point it ruptures, releasing the stored-up energy in the form of quakes most often occur along earthquake waves. existing faults whenever the frictional forces on the fault surfaces Deformation of rocks Deformation of a limber stick are overcome (see Box 8.1). Foreshocks and Aftershocks Stream Fault A. Original position A. Original position Fault B. Buildup of strain B. Buildup of strain C. Slippage (earthquake) C. Slippage (earthquake) Stream offset D. Strain released D. Strain released The intense vibrations of the 1906 San Francisco earthquake lasted about 40 seconds. Although most of the displacement along the fault occurred in this rather short period, additional movements along this and other nearby faults continued for several days following the main quake. The adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks. Although these aftershocks are usually much weaker than the main earthquake, they can sometimes destroy already badly weakened structures. This occurred, for example, during a 1988 earthquake in Armenia. A large aftershock of magnitude 5.8 collapsed many structures that had been weakened by the main tremor. In addition, small earthquakes called foreshocks often precede a major earthquake by days or, in some cases, by as much as several years. Monitoring of these foreshocks has been used as a means of predicting forthcoming major earthquakes, with mixed success. We consider the topic of earthquake prediction in a later section of this chapter. 000200010270575680_R1_CH08_p218-245.qxd 224 CHAPTER 8 03/30/11 07:20 PM Page 224 Earthquakes and Earth’s Interior BOX 8.1 PEOPLE AND THE ENVIRONMENT Damaging Earthquakes East of the Rockies When you think earthquake, you probably think of California and Japan. However, six major earthquakes have occurred in the central and eastern United States since colonial times. Three of these had estimated Richter magnitudes of 7.5, 7.3, and 7.8, and they were centered near the Mississippi River Valley in southeastern Missouri. Occurring on December 16, 1811, January 23, 1812, and February 7, 1812, these earthquakes, plus numerous smaller tremors, destroyed the town of New Madrid, Missouri, triggered massive landslides, and caused damage over a six-state area. The course of the Mississippi River was altered, and Tennessee’s Reelfoot Lake was enlarged. The distances over which these earthquakes were felt are truly remarkable. Chimneys were reported downed in Cincinnati, Ohio, and Richmond, Virginia, while Boston residents, located 1,770 kilometers (1,100 miles) to the northeast, felt the tremor. Despite the history of the New Madrid earthquake, Memphis, Tennessee, the largest population center in the area today, does not have adequate earthquake provisions in its building code. Furthermore, because Memphis is located on unconsolidated floodplain deposits, buildings are more susceptible to damage than similar structures built on bedrock. It has been estimated that if an earthquake the size of the 1811–1812 New Madrid event were to strike in the next decade, it would result in casualties in the thousands and damages in tens of billions of dollars. Damaging earthquakes that occurred in Aurora, Illinois (1909), and Valentine, Texas (1931), remind us that other areas in the central United States are vulnerable. The greatest historical earthquake in the eastern states occurred August 31, 1886, in Charleston, South Carolina. The event, which spanned 1 minute, caused 60 deaths, numerous injuries, and great economic loss FIGURE 8.A Damage to Charleston, South Carolina, caused by the August 31, 1886, earthquake. Damage ranged from toppled chimneys and broken plaster to total collapse. (Photo courtesy of U.S. Geological Survey) within a radius of 200 kilometers (120 miles) of Charleston. Within 8 minutes, effects were felt as far away as Chicago and St. Louis, where strong vibrations shook the upper floors of buildings, causing people to rush outdoors. In Charleston alone, over 100 buildings were destroyed, and 90 percent of the remaining structures were damaged (Figure 8.A). Numerous other strong earthquakes have been recorded in the eastern United States. New England and adjacent areas have experienced sizable shocks since colonial times. The first reported earthquake in the Northeast took place in Plymouth, Massachusetts, in 1683, and was followed in 1755 by the destructive Cambridge, Massachusetts, earthquake. Moreover, ever since records have been kept, New York State alone has experienced over 300 earthquakes large enough to be felt. San Andreas Fault: An Active Earthquake Zone The San Andreas is undoubtedly the most studied fault system in the world (Figure 8.6). Over the years, investigations have shown that displacement occurs along discrete segments that are 100 to 200 kilometers long. Furthermore, each fault segment behaves somewhat differently from the others. Some portions of the San Andreas exhibit a slow, gradual dis- Earthquakes in the central and eastern United States occur far less frequently than in California. Yet history indicates that the East is vulnerable. Furthermore, these shocks east of the Rockies have generally produced structural damage over a larger area than counterparts of similar magnitude in California. The reason is that the underlying bedrock in the central and eastern United States is older and more rigid. As a result, seismic waves are able to travel greater distances with less weakening than in the western United States. It is estimated that for earthquakes of similar magnitude, the region of maximum ground motion in the East may be up to 10 times larger than in the West. Consequently, the higher rate of earthquake occurrence in the western United States is balanced somewhat by the fact that central and eastern U.S. quakes can damage larger areas. placement known as fault creep, which occurs relatively smoothly and therefore with little noticable seismic activity. Other segments regularly slip, producing small earthquakes. Still other segments remain locked and store elastic energy for hundreds of years before rupturing in great earthquakes. The latter process is described as stick-slip motion, because the fault exhibits alternating periods of locked behavior followed by sudden slippage. It is estimated that great earthquakes should occur about every 50 to 200 years along those 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 225 Seismology: The Study of Earthquake Waves 225 still active. Currently, laser beams are used to measure the relative motion between the opposite sides of this fault. These measurements reveal a displacement of 2–5 centimeters (1–2 inches) per year. Although this seems slow, it produces substantial movement over millions of years. To illustrate, in 30 million years this rate of displacement would slide the western portion of California northward so that Los Angeles, on the Pacific plate, would be adjacent to San Francisco on the North American plate! More important in the short term, a displacement of just 2 centimeters each year produces 2 meters of offset every 100 years. Consequently, the 4 meters of displacement produced during the 1906 San Francisco earthquake should occur at least every 200 years along this segment of the fault zone. This fact lies behind California’s concern for making buildings earthquakeresistant in anticipation of the inevitable “Big One.” Seismology: The Study of Earthquake Waves Forces Within Earthquakes FIGURE 8.6 Trace of the San Andreas Fault, north of Landers California. (Photo by Roger Ressmeyer/CORBIS) sections of the San Andreas Fault that exhibit stick-slip motion. This knowledge is useful when assigning a potential earthquake risk to a given segment of the fault zone. The tectonic forces along the San Andreas Fault zone that were responsible for the 1906 San Francisco earthquake are Students Sometimes Ask . . . Do moderate earthquakes decrease the chances of a major quake in the same region? No. This is due to the vast increase in release of energy associated with higher-magnitude earthquakes. For instance, an earthquake with a magnitude of 8.5 releases millions of times more energy than the smallest earthquakes felt by humans. Similarly, thousands of moderate tremors would be needed to release the huge amount of energy equal to one “great” earthquake. The study of earthquake waves, seismology 1seismos = shake, ology = the study of2, dates back to attempts by the Chinese almost 2,000 years ago to determine the direction of the source of each earthquake. Modern seismographs 1seismos = shake, graph = write2 are instruments that record earthquake waves. Their principle is simple: A weight is freely suspended from a support that is attached to bedrock (Figure 8.7). When waves from an earthquake reach the instrument, the inertia of the weight keeps it stationary, while Earth and the support vibrate. The movement of Earth in relation to the stationary weight is recorded on a rotating drum. (Inertia is the tendency of a stationary object to remain still, or a moving object to stay in motion.) Modern seismographs amplify and record ground motion, producing a trace as shown in Figure 8.8. These records, called seismograms 1seismos = shake, gramma = what is written2, reveal that seismic waves are elastic energy. This energy radiates outward in all directions from the focus, as you saw in Figure 8.2. The transmission of this energy can be compared to the shaking of gelatin in a bowl that is jarred. Seismograms reveal that two main types of seismic waves are generated by the slippage of a rock mass. Some travel along Earth’s outer layer and are called surface waves. Others travel through Earth’s interior and are called body waves. Body waves are further divided into primary waves (P waves) and secondary waves (S waves). Body waves are divided into P and S waves by their mode of travel through intervening materials. P waves are push–pull waves—they push (compress) and pull (expand) rocks in the direction the wave is traveling (Figure 8.9A). Imagine holding someone by the shoulders and shaking them. This push–pull movement is how P waves move through the Earth. This wave motion is analogous to that generated by human vocal cords as they move air to create 000200010270575680_R1_CH08_p218-245.qxd 226 CHAPTER 8 03/30/11 07:20 PM Page 226 Earthquakes and Earth’s Interior Figure 8.9C. Unlike P waves, which temporarily change the volume of the intervening material by alternately compressing and expanding it, S waves temporarily change the shape Bedrock of the material that transmits them (Figure 8.9D). Because fluids (gases Weight hinged and liquids) do not respond elastically to allow movement to changes in shape, they will not transmit S waves. The motion of surface waves is somewhat more complex. As surface waves travel along the ground, they Support Mass does not moves with cause the ground and anything resting move with Earth B. ground motion upon it to move, much like ocean due to inertia Pen swells toss a ship. In addition to their up-and-down motion, surface waves have a side-to-side motion similar to an S wave oriented in a horizontal Rotating drum records motion plane. This latter motion is particularly Earth moves damaging to the foundations of structures. Bedrock By observing a typical seismic record, as shown in Figure 8.8, you can A. see a major difference among these FIGURE 8.7 Principle of the seismograph. The inertia of the suspended mass tends to keep it seismic waves: P waves arrive at the motionless, while the recording drum, which is anchored to bedrock, vibrates in response to seismic recording station first, then S waves, waves. Thus, the stationary mass provides a reference point from which to measure the amount of displacement occurring as the seismic waves pass through the ground below. and then surface waves. This is a con(Photo by Zephyr/Photo Rearchers, Inc.) sequence of their speeds. To illustrate, the velocity of P waves through gransound. Solids, liquids, and gases resist a change in volume ite within the crust is about 6 kilometers (4 miles) per second. when compressed and will elastically spring back once the Under the same conditions, S waves travel at 3.5 kilometers force is removed (Figure 8.9B). Therefore, P waves, which are (2 miles) per second. Differences in density and elastic propcompressional waves, can travel through all these materials. erties of the rock greatly influence the velocities of these In contrast, S waves shake the particles at right angles to waves. Generally, in any solid material, P waves travel about their direction of travel. This can be illustrated by fastening 1.7 times faster than S waves, and surface waves can be exone end of a rope and shaking the other end, as shown in pected to travel at 90 percent of the velocity of the S waves. As you will see, seismic waves allow us to determine the location and magnitude of earthquakes. In addition, seismic waves FIGURE 8.8 Typical seismic record. Note the time interval (about 5 provide us with an important tool for probing Earth’s interior. minutes) between the arrival of the first P wave and the arrival of the first S wave. Locating an Earthquake Surface waves First P wave First S wave 1 minute (Earlier) T I M E (Later) Forces Within A Earthquakes Recall that the focus is the place within Earth where earthquake waves originate. The epicenter 1epi = upon, center = a point2 is the location on the surface directly above the focus (see Figure 8.2). The difference in velocities of P and S waves provides a method for locating the epicenter. The principle used is analogous to a race between two autos, one faster than the other. The P wave always wins the race, arriving ahead of the S wave. But the greater the length of the race, the greater will be the difference in the arrival times at the finish line (the seismic station). Therefore, the greater the interval measured on 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 227 Locating an Earthquake 227 Slinky at rest Push slinky Compress Wave direction Expand Compress Wave direction Particle motion A. P waves generated using a slinky B. P waves traveling along the surface Rope at rest Shake rope Particle motion Wave direction Wave direction Particle motion C. S waves generated using a rope D. S waves traveling along the surface FIGURE 8.9 Types of seismic waves and their characteristic motion. (Note that during a strong earthquake, ground shaking consists of a combination of various kinds of seismic waves.) A. As illustrated by a slinky, P waves are compressional waves that alternately compress and expand the material through which they pass. B. The back-and-forth motion produced as compressional waves travel along the surface can cause the ground to buckle and fracture, and may cause power lines to break. C. S waves cause material to oscillate at right angles to the direction of wave motion. D. Because S waves can travel in any plane, they produce up-and-down and sideways shaking of the a seismogram between the arrival of the first P wave and the first S wave, the greater the distance to the earthquake source. A system for locating earthquake epicenters was developed by using seismograms from earthquakes whose epicenters could be easily pinpointed from physical evidence. From these seismograms, travel-time graphs were constructed (Figure 8.10B). The first travel-time graphs were greatly improved when seismograms became available from nuclear explosions, because the precise location and time of detonation were known. Using the sample seismogram in Figure 8.10A and the travel-time curve in Figure 8.10B, we can determine the distance separating the recording station from the earthquake in two steps: (1) using the seismogram, determine the time interval between the arrival of the first P wave and the first S wave, and (2) using the travel-time graph, find the P-S interval on the vertical axis and use that information to determine the distance to the epicenter on the horizontal axis. From this information, we can determine that this earthquake occurred 3800 kilometers (2350 miles) from the recording instrument. Now we know the distance, but what about the direction? The epicenter could be in any direction from the seismic sta- tion. As shown in Figure 8.11, the precise location of a quake can be found when the distance is known from three or more different seismic stations. On a globe, a circle is drawn around each station with each circle’s radius equal to the station’s distance from the epicenter. The point where the three circles intersect is the epicenter of the quake. This method is called triangulation. About 95 percent of the energy released by earthquakes originates in a few relatively narrow zones (Figure 8.12). The greatest energy is released along a path around the outer edge of the Pacific Ocean known as the circum-Pacific belt. Included in this zone are regions of great seismic activity, such as Japan, the Philippines, Chile, and numerous volcanic island chains, as exemplified by Alaska’s Aleutian Islands. Figure 8.12 reveals another continuous belt that extends for thousands of kilometers through the world’s oceans. This zone coincides with the oceanic ridge system, an area of frequent but low-intensity seismic activity. By comparing this figure with Figure 7.10 (pp. 196–197), you can see a close correlation between the location of earthquake epicenters and plate boundaries. 000200010270575680_R1_CH08_p218-245.qxd Page 228 Earthquakes and Earth’s Interior First P-wave First S-wave km 00 84 Time interval 5 minutes 0 1 2 3 4 5 Paris Montréal km CHAPTER 8 07:20 PM 670 0 228 03/30/11 6 7 8 9 10 11 12 13 14 15 Travel time in minutes A. Seismogram Epicenter Distance in miles 500 1000 1500 2000 2500 3000 15 0k 550 m São Paulo 14 13 Time in minutes 12 11 ve wa S 10 e rv cu 5 min. time interval 9 8 7 ve wa P- 6 5 rve cu 4 Intensity Scales 3 2 1 0 FIGURE 8.11 An earthquake epicenter is located using the distances obtained from three seismic stations. On a globe, a circle is drawn around each station, with each circle’s radius equal to the station’s distance from the earthquake’s epicenter. Today, computers using data from numerous seismic stations can rapidly pinpoint large earthquakes. 1000 2000 3000 Distance in kilometers 4000 B. Travel-time graph FIGURE 8.10 Using a seismogram and a travel-time graph to determine the distance to an earthquake’s epicenter. A. The time delay between the arrival of the first P- and S-waves on this seismogram is 5 minutes. B. Using the travel-time graph, it is determined that the epicenter is roughly 3,800 kilometers (2,350 miles) from the seismic station. Measuring the Size of Earthquakes Seismologists have employed a variety of methods to obtain two fundamentally different measures that describe the size of an earthquake: intensity and magnitude. The first of these to be used was intensity—a measure of the degree of earthquake shaking at a given locale based on the amount of damage. With the development of seismographs, it became clear that a quantitative measure of an earthquake based on seismic records rather than uncertain personal estimates of damage was desirable. The measurement that was developed, called magnitude, relies on calculations that use data provided by seismic records (and other techniques) to estimate the amount of energy released at the source of the earthquake. Until a little more than a century ago, historical records provided the only accounts of the severity of earthquake shaking and destruction. Using these descriptions—which were compiled without any established standards for reporting— made accurate comparisons of earthquake sizes difficult, at best. In order to standardize the study of earthquake severity, scientists developed various intensity scales that considered damage done to buildings, as well as individual descriptions of the event, and secondary effects such as landslides and the extent of ground rupture. By 1902, Guiseppe Mercalli had developed a relatively reliable intensity scale, which in a modified form is still used today. The Modified Mercalli Intensity Scale shown in Table 8.1 was developed using California buildings as its standard, but it is appropriate for use throughout most of the world to estimate the strength of an earthquake (Figure 8.13). For example, if some well-built wood structures and most masonry buildings are destroyed by an earthquake, a region would be assigned an intensity of X on the Mercalli scale (Table 8.1). Despite their usefulness in providing seismologists with a tool to compare earthquake severity, particularly in regions where there are no seismographs, intensity scales have severe drawbacks. In particular, intensity scales are based on effects (largely destruction) of earthquakes that depend not only on the severity of ground shaking but also on factors such as population density, building design, and the nature of surface materials. 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 229 FIGURE 8.12 Distribution of the 14,229 earthquakes with magnitudes equal to or greater than 5 for a 10-year period. Inset photo shows earthquake damage near Ismit, Turkey, in 1999. (Photo courtesy of CORBIS/SYGMA) Magnitude Scales In order to compare earthquakes across the globe, a measure was needed that did not rely on parameters that vary considerably from one part of the world to another, such as construction practices. As a consequence, a number of magnitude scales were developed. Richter Magnitude In 1935, Charles Richter of the California Institute of Technology developed the first magnitude scale using seismic records to estimate the relative sizes of earthquakes. As shown in Figure 8.14 (top), the Richter scale is based on the amplitude of the largest seismic wave (P, S, or surface wave) recorded on a seismogram. Because seismic waves weaken as the distance between the earthquake focus and the seismograph increases (in a manner similar to light), TABLE 8.1 I II III IV V VI VII VIII IX X XI XII Richter developed a method that accounted for the decrease in wave amplitude with increased distance. Theoretically, as long as the same, or equivalent, instruments were used, monitoring stations at various locations would obtain the same Richter magnitude for every recorded earthquake. (Richter selected the Wood-Anderson seismograph as the standard recording device.) Although the Richter scale has no upper limit, the largest magnitude recorded on a Wood-Anderson seismograph was 8.9. These great shocks release approximately 1026 ergs of energy—roughly equivalent to the detonation of 1 billion tons of TNT. Conversely, earthquakes with a Richter magnitude of less than 2.0 are not felt by humans. With the development of more sensitive instruments, tremors of a magnitude of minus 2 were recorded. Table 8.2 shows how Richter magnitudes and their effects are related. Modified Mercalli Intensity Scale Not felt except by a very few under especially favorable circumstances. Felt only by a few persons at rest, especially on upper floors of buildings. Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. During the day felt indoors by many, outdoors by few. Sensation like heavy truck striking building. Felt by nearly everyone, many awakened. Disturbances of trees, poles, and other tall objects sometimes noticed. Felt by all; many frightened and run outdoors. Some heavy furniture moved; few instances of fallen plaster or damaged chimneys. Damage slight. Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight-to-moderate in well-built ordinary structures; considerable in poorly built or badly designed structures. Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. (Fall of chimneys, factory stacks, columns, monuments, walls.) Damage considerable in specially designed structures. Buildings shifted off foundations. Ground cracked conspicuously. Some well-built wooden structures destroyed. Most masonry and frame structures destroyed. Ground badly cracked. Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Damage total. Waves seen on ground surfaces. Objects thrown upward into air. 229 000200010270575680_R1_CH08_p218-245.qxd 230 07:20 PM Page 230 Earthquakes and Earth’s Interior a N of Jap an CHAPTER 8 03/30/11 e A S Epicenter III IV V VI A J II P FIGURE 8.13 Zones of destruction associated with an earthquake that struck Japan in 1925. Intensity levels based on the Modified Mercalli Intensity Scale. FIGURE 8.14 Illustration showing how the Richter magnitude of an earthquake can be determined graphically using a seismograph record from a Wood-Anderson instrument. First, measure the height (amplitude) of the largest wave on the seismogram (23 mm) and then the distance to the focus using the time interval between S and P waves (24 seconds). Next, draw a line between the distance scale (left) and the wave amplitude scale (right). By doing this, you should obtain the Richter magnitude 1ML2 of 5. (Data from California Institute of Technology) Amplitude 23 mm 24 sec. P S 0 10 20 Distance, S-P, km sec. 500 50 400 40 300 30 200 100 60 40 20 0.5 20 10 Seismograph record Time sec. 30 mm Magnitude, ML Amplitude, mm 100 6 50 5 20 4 5 10 8 6 3 2 4 2 0.5 1 1.2 10 20 1 2 0.1 0 Earthquakes vary enormously in strength, and great earthquakes produce wave amplitudes that are thousands of times larger than those generated by weak tremors. To accommodate this wide variation, Richter used a logarithmic scale to express magnitude, where a tenfold increase in wave amplitude corresponds to an increase of 1 on the magnitude scale. Thus, the amount of ground shaking for a 5-magnitude earthquake is 10 times greater than that produced by an earthquake having a Richter magnitude of 4. In addition, each unit of Richter magnitude equates to roughly a 32-fold energy increase. Thus, an earthquake with a magnitude of 6.5 releases 32 times more energy than one with a magnitude of 5.5, and roughly 1,000 times more energy than a 4.5-magnitude quake. A major earthquake with a magnitude of 8.5 releases millions of times more energy than the smallest earthquakes felt by humans. Richter’s original goal was modest in that he only attempted to rank the earthquakes of southern California (shallow-focus earthquakes) into groups of large, medium, and small magnitude. Hence, Richter magnitude was designed to study nearby (or local) earthquakes and is denoted by the symbol 1ML2 where M is for magnitude and L is for local. The convenience of describing the size of an earthquake by a single number that could be calculated quickly from seismograms makes the Richter scale a powerful tool. Furthermore, unlike intensity scales that could only be applied to populated areas of the globe, Richter magnitudes could be assigned to earthquakes in more remote regions and even to events that occurred in the ocean basins. As a result, the method devised by Richter was adapted to a number of different seismographs located throughout the world. In time, seismologists modified Richter’s work and developed new magnitude scales. Moment Magnitude Seismologists have recently been employing a more precise measure called moment magnitude 1MW2, which can be calculated using several techniques. In one method, the moment magnitude is calculated from field studies using a combination of factors that include the average amount of displacement along the fault, the area of the rupture surface, and the shear strength of the faulted rock— a measure of how much energy a rock can store before it suddenly slips and releases this energy in the form of an earthquake (and heat). The moment magnitude can also be readily calculated from seismograms by examining very long period seismic waves. The values obtained have been calibrated so that small- and moderate-sized earthquakes have moment magnitudes that are roughly equivalent to Richter magnitudes. However, moment magnitudes are much better for describing very large earthquakes. For example, on the moment magnitude scale, the 1906 San Francisco earthquake, which had a Richter magnitude of 8.3, would be demoted to 7.9 on the moment magnitude scale, whereas the 1964 Alaskan earthquake with an 8.3 Richter magnitude would be increased to 9.2. The strongest earthquake on record is the 1960 Chilean earthquake with a moment magnitude of 9.5. 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 231 Destruction from Earthquakes TABLE 8.2 231 Earthquake Magnitudes and Expected World Incidence Richter Magnitudes 62.0 2.0–2.9 3.0–3.9 4.0–4.9 5.0–5.9 6.0–6.9 7.0–7.9 8.0 and above Effects Near Epicenter Generally not felt, but recorded Potentially perceptible Felt by some Felt by most Damaging shocks Destructive in populous regions Major earthquakes; inflict serious damage Great earthquakes; destroy communities near epicenter Estimated Number per Year 600,000 300,000 49,000 6,200 800 266 18 1.4 Source: Earthquake Information Bulletin and others. Moment magnitude has gained wide acceptance among seismologists and engineers because (1) it is the only magnitude scale that estimates adequately the size of very large earthquakes; (2) it is a measure that can be derived mathematically from the size of the rupture surface and the amount of displacement and it better reflects the total energy released during an earthquake; and (3) it can be verified by two independent methods—field studies that are based on measurements of fault displacement and seismographic methods using long-period waves. Destruction from Earthquakes Students Sometimes Ask . . . I’ve heard that the safest place to be in a house during an earthquake is in a doorframe. Is that really the best place? No! An enduring earthquake image of California is a collapsed adobe home with the door frame as the only standing part. From this came the belief that a doorway is the safest place to be during an earthquake. In modern homes, doorways are no stronger than any other part of the house and usually have doors that will swing and can injure you. If you’re inside, the best advice is to duck, cover, and hold. When you feel an earthquake, duck under a desk or sturdy table. Stay away from windows, bookcases, file cabinets, heavy mirrors, hanging plants, and other heavy objects that could fall. Stay under cover until the shaking stops. And, hold on to the desk or table: If it moves, move with it. The most violent earthquake to jar North America this century—the Good Friday Alaskan Earthquake—occurred in 1964. Felt throughout the state, the earthquake had a moment magnitude of 9.2 and reportedly lasted 3–4 minutes. This event left 131 people dead, thousands homeless, and the economy of the state badly disrupted because it occurred near major towns and seaports (Figure 8.15). Had the schools and business districts been open on this holiday, the toll surely would have been higher. Within 24 hours of the FIGURE 8.15 Region most affected by the Good Friday earthquake of 1964. Note the location of initial shock, 28 aftershocks the epicenter (red dot). Inset photo shows the collapse of a street in Anchorage, Alaska, caused by this earthquake. (Photo courtesy of U.S. Geological Survey) were recorded, 10 of which exceeded a Richter magnitude of 6. Damage from Seismic Vibrations The 1964 Alaskan earthquake provided geologists with new insights into the role of ground shaking as a destructive force. As the energy released by an earthquake travels along Earth’s surface, it causes the ground to vibrate in a complex manner by moving up and down as well as from side to side. The amount of structural Anchorage Whittier Seward Valdez Chenega Cook Inlet Gulf of Alaska Kodiak 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 232 Most large structures in Anchorage were damaged, even though they were built according to the earthquake provisions of the Uniform Building Code. Perhaps some of that destruction can be attributed to the unusually long duration of this earthquake. Most quakes consist of tremors that last less than a minute. The 1994 Northridge earthquake was felt for about 40 seconds, and the strong vibrations of the 1989 Loma Prieta earthquake lasted less than 15 seconds. But the Alaska quake reverberated for 3 to 4 minutes. FIGURE 8.16 Damage to the five-story J.C. Penney Co. building, Anchorage, Alaska. Very little structural damage was incurred by the adjacent building. (Courtesy of NOAA) damage attributable to the vibrations depends on several factors, including (1) the intensity and (2) duration of the vibrations, (3) the nature of the material upon which the structure rests, and (4) the design of the structure. All multistory structures in Anchorage were damaged by the vibrations, but the more flexible wood-frame residential buildings fared best. Figure 8.16 offers a striking example of how construction variations affect earthquake damage; you can see that the steel-frame building on the left withstood the vibrations, whereas the poorly designed J.C. Penney building was badly damaged. Engineers have learned that unreinforced masonry buildings are the most serious safety threat in earthquakes. Amplification of Seismic Waves Although the region within 20 to 50 kilometers (12 to 30 miles) of the epicenter will experience about the same intensity of ground shaking, the destruction varies considerably within this area. This difference is mainly attributable to the nature of the ground on which the structures are built. Soft sediments, for example, generally amplify the vibrations more than solid bedrock. Thus, the buildings located in Anchorage, which were situated on unconsolidated sediments, experienced heavy structural damage. By contrast, most of the town of Whittier, although much nearer the epicenter, rests on a firm foundation of granite and hence suffered much less damage. However, Whittier was damaged by a seismic sea wave (described in the next section). Liquefaction In areas where unconsolidated materials are saturated with water, earthquake vibrations can generate a phenomenon known as liquefaction 1liqueo = to be fluid, facio = to make2. Under these conditions, what had been a stable soil turns into a mobile fluid that is not capable of supporting buildings or other structures (Figure 8.17). As a result, underground objects such as storage tanks and sewer lines may literally float toward the surface of their newly liquefied environment. Buildings and other structures may settle and collapse. During the 1989 Loma Prieta earthquake, in FIGURE 8.17 Effects of liquefaction. This tilted building rests on unconsolidated sediment that San Francisco’s Marina District, imitated quicksand during the 1985 Mexican earthquake. (Photo by James L. Beck) foundations failed and geysers of sand and water shot from the ground, indicating that liquefaction had occurred (Figure 8.18). What Is a Tsunami? Large undersea earthquakes occasionally set in motion massive waves of water called seismic sea waves, or tsunami* 1tsu = harbor, nami = waves2. These destructive waves often are called “tidal waves” by the media. However, this name is *“Seismic sea waves were given the name tsunami by the Japanese, who have suffered a great deal from them. The term tsunami is now used worldwide. 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 233 Destruction from Earthquakes 233 inappropriate, because these waves are not created by the tidal effect of the Moon or Sun. Most tsunami result from vertical displacement along a fault located on the ocean floor, or from a large underwater landslide triggered by an earthquake (Figure 8.19). Once formed, a tsunami resembles the ripples created when a pebble is dropped into a pond. In contrast to ripples, a tsunami advances across the ocean at speeds between 500 and 950 kilometers (300 and 600 miles) per hour. Despite this, a tsunami in the open ocean can pass undetected because its height is usually less than 1 meter and the distance between wave crests is great, ranging from 100 to 700 kilometers (62 to 435 miles). Upon entering shallower coastal water, these destructive waves are slowed and the water begins to pile up to heights that occasionally exceed 30 meters (100 feet), as shown in Figure 8.19. As the crest of a tsunami approaches shore, it appears as a rapid rise in sea level with a turbulent and chaotic surface. The first warning of an approaching tsunami is usually a rapid withdrawal of water from beaches. Some residents living near the Pacific basin have learned to heed this warning and move to higher ground, because about 5 to 30 minutes later the retreat of water is followed by a surge capable of extending hundreds of meters inland. In a successive fashion, each surge is followed by a rapid oceanward retreat of the water. A. B. FIGURE 8.18 Liquefaction. A. These mud volcanoes were produced by the Loma Prieta earthquake in 1989. Mud volcanoes form when geysers of sand and water shoot from the ground, an indication that liquefaction has occurred. (Photo by Richard Hilton, courtesy of Dennis Fox) B. Students experiencing the “strength” of water-saturated soil, a hallmark of liquefaction. (Photo by M. Miller) Tsunami Damage from the 2004 Indonesian Earthquake A massive undersea earthquake of moment magnitude 9.0 occurred near the island of Sumatra on December 26, 2004, and sent waves of water racing across the Indian Ocean and Bay of Bengal (Figure 8.20A). This tsunami was one of the deadliest natural disasters of any kind in modern times, claiming more than 230,000 lives. As water surged several kilometers inland, cars and trucks were flung around like toys in a bathtub, and fishing boats were rammed into homes (see chapter opening photo). In some locations, the backwash of water dragged bodies and huge amounts of debris out to sea. FIGURE 8.19 Schematic drawing of a tsunami generated by displacement of the ocean floor. The speed of a wave moving across the ocean correlates with ocean depth. As shown, waves moving in deep water advance at speeds in excess of 800 kilometers per hour. Speed gradually slows to 50 kilometers per hour at depths of 20 meters. Decreasing depth slows the movement of the wave column. As waves slow in shallow water, they grow in height until they topple and rush onto shore with tremendous force. The size and spacing of these swells are not to scale. Tsunami speed: 835 km/hr Tsunami speed: 340 km/hr Tsunami speed: 50 km/hr Sea level Water depth: 5500 meters Subducti ng pla te Displacement Water depth: 900 meters Water depth: 20 meters 000200010270575680_R1_CH08_p218-245.qxd 234 CHAPTER 8 03/30/11 07:20 PM Page 234 Earthquakes and Earth’s Interior A. B. FIGURE 8.20 A massive earthquake 19.0 Mw2 off the Indonesian island of Sumatra sent a tsunami racing across the Indian Ocean and Bay of Bengal on December 26, 2004. A. Unsuspecting foreign tourists who walked seaward as the water receded, rush toward shore as the first of six tsunami rolled toward Hat Rai Lay Beach near Krabi in southern Thailand. (AFP/Getty Images. Inc.) B. Tsunami survivors walk among the debris from this earthquake-triggered event. (Photo by Kimmasa Mayama/Reuters/CORBIS) The destruction was indiscriminate, destroying luxury resorts and poor fishing hamlets on the Indian Ocean coast (Figure 8.20B). Devastation was most severe along the southeast coast of Sri Lanka, in the Indonesian province of Aceh, in the Indian state of Tamil Nadu, and on Thailand’s resort island of Phuket. Damages were reported as far away as the Somalia coast of Africa, 4,100 kilometers (2,500 miles) west of the earthquake epicenter. The killer waves generated by this massive quake achieved heights as great as 10 meters (33 feet) and struck many unprepared areas within three hours of the event. Although the Pacific basin was equipped with deep-sea buoys and tide gauges that can spot tsunami waves at sea, the Indian Ocean was not. (The deep-sea buoys have pressure sensors that detect changes in pressure as the earthquake’s energy travels through the ocean, and tide gauges measure the rise and fall 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 235 Destruction from Earthquakes in sea level.) The rarity of tsunami in the Indian Ocean also contributed to the lack of preparedness for such an event. It should come as no surprise that a tsunami warning system for the Indian Ocean has now been established. Tsunami Warning System In 1946, a large tsunami struck the Hawaiian Islands without warning. A wave more than 15 meters (50 feet) high left several coastal villages in shambles. This destruction motivated the U.S. Coast and Geodetic Survey to establish a tsunami warning system for coastal areas of the Pacific. From seismic observatories throughout the region, large earthquakes are reported to the Tsunami Warning Center in Honolulu. Scientists at the center use tidal gauges to determine whether a tsunami has formed. Within an hour, a warning is issued. Although tsunami travel very rapidly, there is sufficient time to evacuate all but the region nearest the epicenter. For example, a tsunami generated near the Aleutian Islands would take five hours to reach Hawaii, and one generated near the coast of Chile would travel 15 hours before reaching Hawaii (Figure 8.21). Students Sometimes Ask . . . What is the largest wave triggered by an earthquake? The largest wave ever recorded occurred in Lituya Bay, about 200 kilometers (125 miles) west of Juneau, Alaska’s capital. On July 9, 1958, an earthquake triggered an enormous rockslide that dumped 90 million tons of rock into the upper part of the bay. The rockslide created a huge splash wave (different than a tsunami, these waves are produced when an object splashes into water) that swept over the ridge facing the rockslide and uprooted or snapped off trees 1,740 feet above the bay. Even larger splash waves have occurred in prehistoric times, including an estimated 3,000-foot wave that is thought to have resulted from a meteorite impact in the Gulf of Mexico about 65 million years ago. Landslides and Ground Subsidence In the 1964 Alaskan earthquake, the greatest damage to structures was from landslides and ground subsidence triggered by the vibrations, not from the vibrations themselves. At the seaports of Valdez and Seward, the violent shaking caused water logged sediments to experience liquefaction; the subsequent slumping carried both waterfronts into the sea. Because the disaster could happen again, the entire town of Valdez was relocated about 7 kilometers (4 miles) away on more stable ground. In Valdez, 31 people on a dock died when it was jettisoned into the sea. Most of the damage in Anchorage was attributed to landslides caused by shaking and lurching ground. Many homes were destroyed in Turnagain Heights when a layer of clay lost its strength and more than 200 acres of land slid toward the ocean (Figure 8.22). A portion of this landslide has been left undisturbed to serve as a reminder of this destructive event. The site was named “Earthquake Park.” Downtown Anchorage was also disrupted as sections of the main business district dropped by as much as 3 meters (10 feet). FIGURE 8.22 Turnagain Heights slide caused by the 1964 Alaskan earthquake. A. Vibrations from the earthquake caused cracks to appear near the edge of the bluff. B. Within seconds blocks of land began to slide toward the sea on a weak layer of clay. In less than 5 minutes much of the Turnagain Heights bluff area had been destroyed. C. Photo of a small portion of the Turnagain Heights slide. (Photo courtesy of U.S. Geological Survey) Turnagain Heights FIGURE 8.21 Tsunami travel times to Honolulu, Hawaii, from selected locations throughout the Pacific. (Data from NOAA) 200 meters Original profile Asia Aleutian Is. North America 9 6 7 5 hr 4 hr 3 h hr 2h r 1 hr r 10 8 hr h hr r 3 hr hr B. 12 11 hr 10 hr hr 9 h 8 r hr 7 h 6 r hr 5h 4h r 3 r h 2 r h 1 r hr 2 hr 4 hr 5 hr Honolulu New Guinea Australia Layers of sand Bootlegger and gravel Cove clay Cracks developed A. 1 hr 235 South America C. 000200010270575680_R1_CH08_p218-245.qxd 236 CHAPTER 8 03/30/11 07:20 PM Page 236 Earthquakes and Earth’s Interior Fire The 1906 San Francisco earthquake reminds us of the formidable threat of fire. The central city contained mostly large, older wooden structures and brick-clad buildings that were mostly destroyed by fires that started when gas and electrical lines were severed. The fires raged uncontrolled for three days and devastated over 500 city blocks (see Figure 8.3). The problem was compounded by the initial ground shaking, which broke the city’s water lines into hundreds of unconnected pieces. The fire was finally contained when buildings were dynamited along a wide boulevard to create a fire break, the same strategy used in fighting a forest fire. Although only a few deaths were attributed to the fires, such is not always the case. A 1923 earthquake in Japan (their worst quake prior to the 1995 Kobe tremor) triggered an estimated 250 fires, which devastated the city of Yokohama and destroyed more than half the homes in Tokyo. More than 100,000 deaths were attributed to the fires, which were driven by unusually high winds. Can Earthquakes be Predicted? The vibrations that shook Northridge, California, in 1994 inflicted 57 deaths and about $40 billion in damage (Figure 8.23). This was from a brief earthquake (about 40 seconds) of moderate rating 1MW 6.72. Seismologists warn that earth- quakes of comparable or greater strength will occur along the San Andreas Fault, which cuts a 1,300-kilometer (800-mile) path through the state. The obvious question is, can earthquakes be predicted? Short-Range Predictions The goal of short-range earthquake prediction is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame. Substantial efforts to achieve this objective are being put forth in Japan, the United States, China, and Russia—countries where earthquake risks are high (Table 8.3). This research has concentrated on monitoring possible precursors—phenomena that precede and thus provide a warning of a forthcoming earthquake. In California, for example, some seismologists are measuring uplift, subsidence, and strain in the rocks near active faults. Some Japanese scientists are studying peculiar anomalous behavior that may precede a quake. One claim of a successful short-range prediction was made by Chinese seismologists after the February 4, 1975, earthquake in Liaoning Province. According to reports, very few people were killed, although more than 1 million lived near the epicenter, because the earthquake was predicted and the population was evacuated. Recently, some Western seismologists have questioned this claim and suggest instead that an FIGURE 8.23 Damage to Interstate 5 caused by the January 17, 1994, Northridge earthquake. (Photo by Tom McHugh/Photo Researchers, Inc.) 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 237 Can Earthquakes Be Predicted? TABLE 8.3 Some Notable Earthquakes Year 1556 1755 *1811–1812 *1886 *1906 1908 1923 1960 *1964 1970 *1971 1975 1976 1985 1988 *1989 1990 1993 *1994 1995 1999 1999 2001 2003 2004 2005 237 Location Shensi, China Lisbon, Portugal New Madrid, Missouri Charleston, South Carolina San Francisco, California Messina, Italy Tokyo, Japan Southern Chile Alaska Peru San Fernando, California Liaoning Province, China Tangshan, China Mexico City Armenia San Francisco Bay area Iran Latur, India Northridge, California Kobe, Japan Izmit, Turkey Chi-Chi, Taiwan Bhuj, India Bam, Iran Indian Ocean Pakistan/Kashmir Deaths(est.) Magnitude 830,000 70,000 Few 60 1,500 120,000 143,000 5,700 131 66,000 65 1,328 240,000 9,500 25,000 62 50,000 10,000 51 5,472 17,127 2,300 25,000+ 41,000+ 230,000 83,000 Unknown Unknown Unknown Unknown 8.3 Unknown 7.9 9.5 9.2 7.8 6.5 7.5 7.6 8.1 6.9 7.1 7.3 6.4 6.7 6.9 7.4 7.6 7.9 6.6 9.0 7.6 Comments Possibly the greatest natural disaster. Tsunami damage extensive. Three major earthquakes. Greatest historical earthquake in the eastern United States. Fires caused extensive damage. Fire caused extensive destruction. The largest-magnitude earthquake ever recorded. Greatest North American earthquake. Great rockslide. Damage exceeded $1 billion. First major earthquake to be predicted. Not predicted. Major damage occurred 400 km from epicenter. Poor construction practices. Damages exceeded $6 billion. Landslides and poor construction practices caused great damage. Located in stable continental interior. Damages in excess of $15 billion. Damages estimated to exceed $100 billion. Nearly 44,000 injured and more than 250,000 displaced. Severe destruction; 8,700 injuries. Millions homeless. Ancient city with poor construction. Devastating tsunami damage. Many landslides; 4 million homeless. *U.S. earthquakes. Source: U.S. National Oceanic and Atmospheric Administration intense swarm of foreshocks that began 24 hours before the main earthquake may have caused many people to evacuate spontaneously. Furthermore, an official Chinese government report issued 10 years later stated that 1,328 people died and 16,980 injuries resulted from this earthquake. One year after the Liaoning earthquake at least 240,000 people died in the Tangshan, China, earthquake, which was not predicted. The Chinese have also issued false alarms. In a province near Hong Kong, people reportedly left their dwellings for over a month, but no earthquake followed. Clearly, whatever method the Chinese employ for short-range predictions, it is not reliable. For a short-range prediction scheme to warrant general acceptance, it must be both accurate and reliable. Thus, it must have a small range of uncertainty as regards to location and timing, and it must produce few failures, or false alarms. Can you imagine the debate that would precede an order to evacuate a large city in the United States, such as Los Angeles or San Francisco? The cost of evacuating millions of people, arranging for living accommodations, and providing for their lost work time and wages would be staggering. Long-Range Forecasts In contrast to short-range predictions, which aim to predict earthquakes within a time frame of hours or at most days, long-range forecasts give the probability of a certain magni- tude earthquake occurring on a time scale of 30 to 100 years or more. Stated another way, these forecasts give statistical estimates of the expected intensity of ground motion for a given area over a specified time frame. Although long-range forecasts may not be as informative as we might like, the data are important for updating the Uniform Building Code, which contains nationwide standards for designing earthquake-resistant structures. Long-range forecasts are based on the premise that earthquakes are repetitive or cyclical, like the weather. In other words, as soon as one earthquake is over, the continuing motions of Earth’s plates begin to build strain in the rocks again, until they fail once more. This has led seismologists to study historical records of earthquakes to see if there are any discernible patterns so that the probability of recurrence might be established. One study conducted by the U.S. Geological Survey gives the probability of a rupture occurring along various segments of the San Andreas Fault for the 30 years between 1988 and 2018 (Figure 8.24). From this investigation, the Santa Cruz Mountains area was given a 30 percent probability of producing a 6.5-magnitude earthquake during this time period. In fact, it produced the Loma Prieta quake in 1989, of 7.1 magnitude. The region along the San Andreas Fault given the highest probability (90 percent) of generating a quake is the Parkfield section. This area has been called the “Old Faithful” of 000200010270575680_R1_CH08_p218-245.qxd 238 CHAPTER 8 03/30/11 07:20 PM Page 238 Earthquakes and Earth’s Interior North Coast San Francisco Peninsula Probability of a major earthquake from 1988 to 2018 on the San Andreas Fault South Santa Cruz Mountains 90% Less than 10% Parkfield Cholame Mojave Carrizo 20% 30% San Andreas Fault 40% 20% Segment of very low probability 10% 30% Epicenter of Oct. 17, 1989, earthquake Coachella Valley 30% San Francisco San Bernardino Mountains There are also variations in composition and temperature with depth that indicate the interior of our planet is very dynamic. The rocks of the mantle and crust are in constant motion, not only moving about through plate tectonics, but also continuously recycling between the surface and the deep interior. Furthermore, it is from Earth’s deep interior that the water and air of our oceans and atmosphere are replenished, allowing life to exist at the surface. Formation of Earth’s Layered Structure As material accumulated to form Earth (and for a short period afterward), the Los Angeles high-velocity impact of nebular debris and the decay of radioactive elements caused FIGURE 8.24 Probability of a major earthquake from 1988 to 2018 along the San Andreas the temperature of our planet to steadily Fault. increase. During this time of intense heating, Earth became hot enough that iron and nickel began to melt. Melting produced liquid blobs of heavy earthquake zones because activity here has been very regular metal that sank toward the center of the planet. This process since record keeping began in 1857. In late September 2004, a occurred rapidly on the scale of geologic time and produced magnitude 6.0 earthquake again struck this area. Although Earth’s dense iron-rich core. the event was more than a decade overdue, it did demonstrate The early period of heating resulted in another process of the potential usefulness of long-range forcasts. Another section chemical differentiation, whereby melting formed buoyant between Parkfield and the Santa Cruz Mountains is given a masses of molten rock that rose toward the surface, where very low probability of generating an earthquake. This area they solidified to produce a primitive crust. These rocky mahas experienced very little seismic activity in historical times; terials were rich in oxygen and “oxygen-seeking” elements, rather, it exhibits a slow, continual movement known as fault particularly silicon and aluminum, along with lesser amounts creep. Such movement is beneficial because it prevents strain of calcium, sodium, potassium, iron, and magnesium. In adfrom building to high levels in the rocks. dition, some heavy metals such as gold, lead, and uranium, In summary, it appears that the best prospects for making which have low melting points or were highly soluble in the useful earthquake predictions involve forecasting magnitudes ascending molten masses, were scavenged from Earth’s inand locations in time scales of years, or perhaps even decades. terior and concentrated in the developing crust. This early These forecasts are important because they provide inforperiod of chemical segregation established the three basic dimation used to develop the Uniform Building Code and asvisions of Earth’s interior—the iron-rich core; the thin primitive sist in land-use planning. crust; and Earth’s largest layer, called the mantle, which is located between the core and crust (Figure 8.25). Earth’s Interior Forces Within Earth’s Interior If you could slice Earth in half, the first thing you would notice is that it has three distinct layers. The heaviest materials (metals) would be in the center. Lighter solids (rocks) would be in the middle and liquids and gases would be on top. Within Earth we know these layers as the iron core, the rocky mantle and crust, the liquid ocean, and the gaseous atmosphere. More than 95 percent of the variations in composition and temperature within Earth are due to layering. However, this is not the end of the story. If it were, Earth would be a dead, lifeless cinder floating in space. Earth’s Internal Structure In addition to these three compositionally distinct layers, Earth can be divided into layers based on physical properties. The physical properties used to define such zones include whether the layer is solid or liquid and how weak or strong it is. Knowledge of both types of layers is essential to our understanding of basic geologic processes, such as volcanism, earthquakes, and mountain building (Figure 8.25). Earth’s Crust The crust, Earth’s relatively thin, rocky outer skin, is of two types: continental crust and oceanic crust. Both share the word crust, but the similarity ends there. The oceanic crust is roughly 7 kilometers (4 miles) thick and 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 239 Earth’s Interior Hydrosphere (liquid) Atmosphere (gas) Layering by Physical Properties Layering by Chemical Composition Oceanic crust Continental crust Crust (low density rock 7–70 km thick) Lithosphere Up pe rm an tl e (s Lithosphere (solid and brittle 100 km thick) Asthenosphere (solid, but mobile) id ol ) Lower mantle (solid) Mantle (high density rock) 0k m 90 28 Upper mantle m Inner core (solid) 515 0 660 km k 289 Outer core (liquid) km 410 km 660 km 239 Core (iron + nickel) 63 71 km FIGURE 8.25 Views of Earth’s layered structure. The right side of the large cross section shows that Earth’s interior is divided into three different layers based on compositional differences—the crust, mantle, and core. The left side of the large cross section depicts the five main layers of Earth’s interior based on physical properties and hence mechanical strength—the lithosphere, asthenosphere, lower mantle, outer core, and inner core. The block diagram to the left of the large cross section shows enlarged views of the upper portion of Earth’s interior. composed of the dark igneous rock basalt. By contrast, the continental crust averages 35–40 kilometers (22–25 miles) thick but may exceed 70 kilometers (40 miles) in some mountainous regions such as the Rockies and Himalayas. Unlike the oceanic crust, which has a relatively homogeneous chemical composition, the continental crust consists of many rock types. Although the upper crust has an average composition of a granitic rock called granodiorite, it varies considerably from place to place. Continental rocks have an average density of about 2.7 grams per cubic centimeter, and some are 4 billion years old. The rocks of the oceanic crust are younger (180 million years or less) and denser (about 3.0 grams per cubic centimeter) than continental rocks.* *Liquid water has a density of 1 gram per cubic centimeter; therefore, the density of basalt is three times that of water. Earth’s Mantle More than 82 percent of Earth’s volume is contained in the mantle, a solid, rocky shell that extends to a depth of about 2,900 kilometers (1,800 miles). The boundary between the crust and mantle represents a marked change in chemical composition. The dominant rock type in the uppermost mantle is peridotite, which is richer in the metals magnesium and iron than the minerals found in either the continental or oceanic crust. The upper mantle extends from the crust-mantle boundary down to a depth of about 660 kilometers (410 miles). The upper mantle can be divided into two different parts. The top portion of the upper mantle is part of the stiff lithosphere, and beneath that is the weaker asthenosphere. The lithosphere (sphere of rock) consists of the entire crust and uppermost mantle and forms Earth’s relatively cool, rigid outer shell. Averaging about 100 kilometers (62 miles) in thickness, the lithosphere is more than 250 kilometers (155 000200010270575680_R1_CH08_p218-245.qxd 240 CHAPTER 8 03/30/11 07:20 PM Page 240 Earthquakes and Earth’s Interior miles) thick below the oldest portions of the continents (Figure 8.25). Beneath this stiff layer to a depth of about 350 kilometers (217 miles) lies a soft, comparatively weak layer known as the asthenosphere (“weak sphere”). The top portion of the asthenosphere has a temperature/pressure regime that results in a small amount of melting. Within this very weak zone, the lithosphere is mechanically detached from the layer below. The result is that the lithosphere is able to move independently of the asthenosphere. It is important to emphasize that the strength of various Earth materials is a function of both their composition and the temperature and pressure of their environment. The entire lithosphere does not behave like a brittle solid similar to rocks found on the surface. Rather, the rocks of the lithosphere get progressively hotter and weaker (more easily deformed) with increasing depth. At the depth of the uppermost asthenosphere, the rocks are close enough to their melting temperature that they are very easily deformed, and some melting may actually occur. Thus, the uppermost asthenosphere is weak because it is near its melting point, just as hot wax is weaker than cold wax. From 660 kilometers (410 miles) deep to the top of the core, at a depth of 2,900 kilometers (1,800 miles), is the lower mantle. Because of an increase in pressure (caused by the weight of the rock above), the mantle gradually strengthens with depth. Despite their strength, however, the rocks within the lower mantle are very hot and capable of very gradual flow. Earth’s Core The composition of the core is thought to be an iron-nickel alloy with minor amounts of oxygen, silicon, and sulfur—elements that readily form compounds with iron. At the extreme pressure found in the core, this iron-rich material has an average density of nearly 11 grams per cubic centimeter and approaches 14 times the density of water at Earth’s center. The core is divided into two regions that exhibit very different mechanical strengths. The outer core is a liquid layer 2,270 kilometers (1,410 miles) thick. It is the movement of metallic iron within this zone that generates Earth’s magnetic field. The inner core is a sphere with a radius of 1,216 kilometers (754 miles). Despite its higher temperature, the iron in the inner core is solid due to the immense pressures that exist in the center of the planet. Surprisingly, meteorites—known as shooting stars—that collide with Earth provide evidence of Earth’s inner composition. Because meteorites are part of the solar system, they are assumed to be representative samples. Their composition ranges from metallic meteorites made of iron and nickel to stony meteorites composed of dense silicate minerals similar to those found in the rock peridotite. Because the crust contains a much smaller percentage of iron than do meteorites, geologists believe that most of Earth’s iron and other dense metals sank toward the center of the planet in its early history. The surrounding mantle is composed of rock similar to stony meteorites. Strong evidence for a molten iron outer core is provided by Earth’s magnetic field. The most widely accepted mechanism explaining Earth’s magnetic field requires that a large por- tion of the core be made of a material that conducts electricity, such as iron, and further that it is mobile enough to flow. Both of these conditions are met by our current model of Earth’s core—a solid inner core and mobile liquid outer core. Probing Earth’s Interior: “Seeing” Seismic Waves Discovering the structure and properties of Earth’s deep interior has not been easy (see Box 8.2). Light does not travel through rock, so we must find other ways to “see” into our planet. The best way to learn about Earth’s interior is to dig or drill a hole and examine it directly. Unfortunately, this is only possible at shallow depths. The deepest a drilling rig has ever penetrated is only 12.3 kilometers (8 miles), which is about 1/500 of the way to Earth’s center! Even this was an extraordinary accomplishment because temperature and pressure increase so rapidly with depth. Fortunately, many earthquakes are large enough that their seismic waves travel all the way through Earth and can be recorded on the other side (Figure 8.26). This means that the seismic waves act like medical x-rays used to take images of a person’s insides. There are about 100 to 200 earthquakes each year that are large enough (about M w 7 6) to be well recorded by seismographs all around the globe. These large earthquakes provide the means to “see” into our planet and have been the source of most of the data that allowed us to figure out the nature of Earth’s interior. FIGURE 8.26 Slice through Earth’s interior showing some of the ray paths that seismic waves from an earthquake would take. Notice that in the mantle, the rays follow curved (refracting) paths rather than straight paths because the seismic velocity of rocks increases with depth, a result of increasing pressure with depth. Earthquake Inner core Outer core Mantle 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 241 Probing Earth’s Interior: “Seeing” Seismic Waves 241 BOX 8.2 UNDERSTANDING EARTH Recreating the Deep Earth Seismology alone cannot determine what Earth is made of. Additional information must be obtained by some other means so that the seismic velocities can be interpreted in terms of rock type. This is done using mineral physics experiments performed in laboratories. By squeezing and heating minerals and rocks, physical properties like stiffness, compressibility, and density (and therefore seismic velocities) can be directly measured. This means that the conditions of the mantle and core can be simulated, and the results compared to seismic modeling. Most mineral physics experiments are done using giant presses involving very hard carbonized steel. The highest pressures, however, are obtained using diamond-anvil presses like the one shown in Figure 8.B. These take advantage of two important characteristics of diamonds—their hardness and transparency. The tips of two diamonds are cut off, and a small sample of mineral or rock is placed in between. Pressures as great as those in the interior of Jupiter have been obtained by squeezing the two diamonds together. High temperatures are achieved by firing a laser beam through the diamond and into the mineral sample. Besides measuring seismic velocities at the conditions of different depths within Earth, there are other important mineral FIGURE 8.B High-pressure experiments inside a diamond anvil cell (left photo) can recreate the conditions at the center of a planet. The whole apparatus is small and can fit on top of a table. High pressures are generated by cutting the tips off high-quality diamonds (right photo), putting a small sample of rock between, squeezing the diamonds together and heating the sample with a laser. (Left photo courtesy of Lawrence Livermore National Laboratory; right photo by Douglass L. Peck Photography) physics experiments. One experiment determines the temperature at which minerals will begin to melt under various pressures. Another experiment determines (at different temperatures) the pressures at which one mineral phase will become un- Interpreting the waves recorded on seismograms in order to identify the structure of Earth’s interior is complicated by the fact that seismic waves usually do not travel along straight paths. Instead, seismic waves are reflected, refracted, and diffracted as they pass through our planet. They reflect off boundaries between different layers, they refract (or bend) when passing from one layer to another layer, and they diffract around any obstacles they encounter (Figure 8.26). These different wave behaviors have been used to identify the boundaries that exist within Earth. One of the most noticeable behaviors of seismic waves is that they follow strongly curved paths (Figure 8.26). This occurs because the velocity of seismic waves generally increases with depth. In addition, seismic waves travel faster when rock is stiffer or less compressible. These properties of stiffness and compressibility are then used to interpret the composition and temperature of the rock. For instance, when rock is hotter, it becomes less stiff (imagine taking a frozen chocolate bar and then heating it up!), and waves travel more slowly. Waves also travel at different speeds through rocks of different compositions. Thus, the speed that seismic waves travel can help determine both the kind of rock that is inside Earth and how hot it is. stable and convert into a new high-pressure phase. Yet another involves making these same tests for slightly different mineral compositions. All of these experiments are needed because there are changes in composition and temperature within Earth. Discovering Boundaries: The Moho The boundary between the crust and mantle, called the Moho, was one of the first features of Earth’s interior discovered using seismic waves. Croatian seismologist Andrija Mohoroviĉiĉ discovered this boundary in 1909 and it is named in his honor. At the base of the continents P waves travel about 6 km/s but abruptly increase to 8 km/s at a slightly greater depth. Mohoroviĉiĉ cleverly used this jump in seismic velocity to identify the boundary. He noticed that there were two different sets of seismic waves that were recorded within a few hundred kilometers of an earthquake. One set of waves moved across the ground at about 6 km/s, and the other set of waves moved across the ground at about 8 km/s. From this data, Mohoroviĉiĉ correctly determined that the different waves were coming from two different layers, as shown in Figure 8.27. When a shallow earthquake occurs, there is a direct wave that moves straight through the crust and is recorded at nearby seismographs. Seismic waves also follow a path down through the crust and along the top of the mantle. These are called refracted waves because they are bent, or refracted, as 000200010270575680_R1_CH08_p218-245.qxd 242 CHAPTER 8 Earthquake Direct wave 03/30/11 07:20 PM Page 242 Earthquakes and Earth’s Interior Seismograph #1 Seismograph #2 Seismograph #3 Refracted wave Moho A. Earthquake #1 Direct wave #2 #3 Refracted wave Moho B. Earthquake #1 #2 Direct wave #3 Refracted wave Moho C. FIGURE 8.27 Diagram showing seismic waves from an earthquake arriving at three different seismographs. Over a short distance, such as at seismograph #1, the direct wave arrives first. For greater distances, such as at seismograph #3, the refracted wave arrives first. The distance at which both waves arrive at the same time can be used to determine the thickness of the crust. they enter the mantle. Refracted P waves travel across the ground at the speed of the waves in the mantle (8 km/s). At nearby locations the direct wave arrives first (Figure 8.27). However, at greater distances the refracted wave is the first to arrive. The distance at which both waves arrive at the same time can be used to determine the depth of the Moho. Thus, using just these two sets of waves and an array of seismographs, the thickness of the crust at any location can be determined. The difference in arrival times between direct and refracted waves is analogous to driving along local roads versus taking the interstate. For short distances, you will arrive faster if you take local roads. For greater distances, it takes less time if you first drive to a major highway and then travel along it. The point where both routes take the same amount of time is directly related to how far you are from a major highway. Or, when determining the depth of the Moho, how far the mantle (fast layer) is from the surface. Discovering Boundaries: The Core–Mantle Boundary Evidence that Earth has a distinct central core was uncovered in 1906 by a British geologist, Richard Dixon Oldham. (In 1914 Beno Gutenberg calculated the depth to the core bound- ary as 2900 kilometers, a value that has stood the test of time.) Oldham observed that at distances of more than about 100° from a large earthquake P and S waves were absent, or very weak. In other words, the central core produced a “shadow zone” for seismic waves as shown in Figure 8.28. As Oldham predicted, Earth’s core exhibits markedly different elastic properties from the mantle above, which causes considerable refraction of P waves—similar to how light is refracted (bent) as it passes from air to water. In addition, because the outer core is liquid iron, it blocks the transmission of S waves (recall S waves do not travel through liquids). Figure 8.28 shows the locations of the P and S wave shadow zones and how the paths of the waves are affected by the core. While there are still P and S waves that arrive in the shadow zone, they are all very different than would be expected for a planet without a core. Discovering Boundaries: The Inner Core–Outer Core Boundary The boundary between the solid inner core and liquid outer core was discovered in 1936 by Danish seismologist Inge Lehman. She could not tell whether the inner core was actually solid or not, but using basic trigonometry reasoned that some P waves were being strongly refracted by a sudden increase in seismic velocities at the inner core–outer core boundary. This is the opposite situation as Students Sometimes Ask . . . As compared to continental crust, oceanic crust is quite thin. Has there ever been an attempt to drill through it to obtain a sample of the mantle? Yes. Project Mohole was initiated in 1958 to retrieve a sample of material from Earth’s mantle by drilling a hole through Earth’s crust to the Mohoroviĉiĉ discontinuity, or Moho. The plan was to drill to the Moho to gain valuable information on Earth’s age, makeup, and internal processes. Despite a successful test phase, drilling was halted as control of the project was shifted from one governmental organization to another. Although Project Mohole failed in its intended purpose, it did show that deep-ocean drilling is a viable means of obtaining geological samples. 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 243 243 Chapter Summary Earthquake Earthquake Direct P waves Direct S waves Inner core Outer core Outer core Inner core Mantle 100° 100° 100° 100° Mantle P-wave shadow zone 140° 140° 180° P waves recorded A. P-wave shadow zone S-wave shadow zone (weak diffracted waves) B. S-wave shadow zone FIGURE 8.28 Two views of Earth’s interior showing the effects of the outer and inner cores on the ray paths of P and S waves. A. When P waves interact with the slow-velocity liquid iron of the outer core, their rays are refracted downward. This creates a shadow zone where no direct P waves are recorded (although diffracted P waves travel there). The P waves that travel through the core are called PKP waves. The “K” represents the path through the core, and comes from the German word for core, which is kern. The increase in seismic velocity at the top of the inner core can refract waves sharply so that some arrive within the shadow zone, which is shown here as a single ray. B. The core is an obstacle to S waves, because they cannot pass through liquids. Therefore, a large shadow zone exists for S waves. However, some S waves diffract around the core and are recorded on the other side of the planet. what occurs to produce the P-wave shadow zone. When seismic velocities suddenly decrease, such as at the mantle–outer core boundary, waves get bent so that there is a shadow zone where no direct waves arrive. When seismic waves suddenly increase, as they do at the outer core–inner core boundary, waves get bent so that several P waves can sometimes arrive at a single location. In the case of the inner core, these waves can even be refracted enough to arrive within the P-wave shadow zone. Both of these occurrences, shown in Figure 8.28A, are proof of a distinct inner core. In summary, although earthquakes can be very destructive, much of our knowledge about Earth’s interior comes from the study of these phenomena. As our knowledge of earthquakes and their causes improve, we learn more of our planet’s internal workings and possibly how to reduce the consequences of tremors. Chapter Summary Earthquakes are vibrations of Earth produced by the rapid release of energy from rocks that rupture because they have been subjected to stresses beyond their limit. This energy, which takes the form of waves, radiates in all directions from the earthquake’s source, called the focus. The movements that produce most earthquakes occur along large fractures, called faults, that are associated with plate boundaries. Two main groups of seismic waves are generated during an earthquake: (1) surface waves, which travel along the outer layer of Earth; and (2) body waves, which travel through Earth’s interior. Body waves are further divided into primary, or P, waves, which push (compress) and pull (expand) rocks in the direction the wave is traveling, and secondary, or S, waves, which shake the particles in rock at right angles to their direction of travel. P waves can travel through solids, liquids, and gases. Fluids (gases and liquids) will not transmit S waves. In any solid material, P waves travel about 1.7 times faster than do S waves. The location on Earth’s surface directly above the focus of an earthquake is the epicenter. An epicenter is determined using the difference in velocities of P and S waves. There is a close correlation between earthquake epicenters and plate boundaries. The principal earthquake epicenter zones are along the outer margin of the Pacific Ocean, known as the circum-Pacific belt and through the world’s oceans along the oceanic ridge system. Seismologists use two fundamentally different measures to describe the size of an earthquake: intensity and magnitude. Intensity is a measure of the degree of ground 000200010270575680_R1_CH08_p218-245.qxd 244 CHAPTER 8 03/30/11 07:20 PM Page 244 Earthquakes and Earth’s Interior shaking at a given locale based on the amount of damage. The Modified Mercalli Intensity Scale uses damages to buildings in California to estimate the intensity of ground shaking for a local earthquake. Magnitude is calculated from seismic records and estimates the amount of energy released at the source of an earthquake. Using the Richter scale, the magnitude of an earthquake is estimated by measuring the amplitude (maximum displacement) of the largest seismic wave recorded. A logarithmic scale is used to express magnitude, in which a tenfold increase in ground shaking corresponds to an increase of 1 on the magnitude scale. Moment magnitude is currently used to estimate the size of moderate and large earthquakes. It is calculated using the average displacement of the fault, the area of the fault surface, and the sheer strength of the faulted rock. The most obvious factors that determine the amount of destruction accompanying an earthquake are the magnitude of the earthquake and the proximity of the quake to a populated area. Structural damage attributable to earthquake vibrations depends on several factors, including (1) intensity, (2) duration of the vibrations, (3) nature of the material upon which the structure rests, and (4) the design of the structure. Secondary effects of earthquakes include tsunamis, landslides, ground subsidence, and fire. Substantial research to predict earthquakes is under way in Japan, the United States, China, and Russia—countries where earthquake risk is high. No consistent method of short-range prediction has yet been devised. Long-range forecasts are based on the premise that earthquakes are repetitive or cyclical. Seismologists study the history of earthquakes for patterns, so their occurrences might be predicted. Earth’s internal structure is divided into layers based on differences in chemical composition and on the basis of changes in physical properties. Compositionally, Earth is divided into a thin outer crust, a solid rocky mantle, and a dense core. Based on physical properties, the layers of Earth are (1) the lithosphere—the cool, rigid, outermost layer that averages about 100 kilometers thick; (2) the asthenosphere, a relatively weak layer located in the mantle beneath the lithosphere; (3) the more rigid lower mantle where rocks are very hot and capable of very gradual flow; (4) the liquid outer core, where Earth’s magentic field is generated; and (5) the solid inner core. The continental crust has an average composition of a granitic rock called granodiorite, whereas the oceanic crust has a basaltic composition. Rocks similar to peridotite make up the mantle. The core is made up mainly of iron and nickel. An iron core explains the high density of Earth’s interior as well as Earth’s magnetic field. Key Terms aftershock (p. 223) asthenosphere (p. 240) body wave (p. 225) core (p. 240) crust (p. 238) earthquake (p. 220) elastic rebound (p. 223) epicenter (p. 226) fault (p. 221) fault creep (p. 224) focus (p. 220) foreshock (p. 223) inner core (p. 240) intensity (p. 228) liquefaction (p. 232) lithosphere (p. 239) lower mantle (p. 240) magnitude (p. 228) mantle (p. 239) Modified Mercalli Intensity Scale (p. 228) Mohoroviĉiĉ discontinuity (Moho) (p. 241) moment magnitude (p. 230) outer core (p. 240) primary (P) wave (p. 225) Richter scale (p. 229) secondary (S) wave (p. 225) seismic sea wave (tsunami) (p. 232) seismogram (p. 225) seismograph (p. 225) seismology (p. 225) surface wave (p. 225) Review Questions 1. What is an earthquake? Under what circumstances do earthquakes occur? 2. How are faults, foci, and epicenters related? 3. Who was first to explain the actual mechanism by which earthquakes are generated? 4. Explain what is meant by elastic rebound. 5. Faults that are experiencing no active creep may be considered safe. Rebut or defend this statement. 6. Describe the principle of a seismograph. 7. Using Figure 8.10B, determine the distance between an earthquake and a seismic station if the first S wave arrives 3 minutes after the first P wave. 8. List the major differences between P and S waves. 9. Which type of seismic wave causes the greatest destruction to buildings? 10. Most strong earthquakes occur in a zone on the globe known as the _____. 11. In 1988 an earthquake in Armenia caused approximately 25,000 deaths. A year later an earthquake in the Bay Area of California, having the same Richter magnitude of 6.9, had only 62 deaths. Suggest a reason for the greater loss of life in the Armenian event. 12. An earthquake measuring 7 on the Richter scale releases about _____ times more energy than an earthquake with a magnitude of 6. 13. List three reasons why the moment magnitude scale has gained wide acceptance among seismologists. 000200010270575680_R1_CH08_p218-245.qxd 03/30/11 07:20 PM Page 245 GEODe: Earth Science 14. List four factors that affect the amount of destruction caused by seismic vibrations. 15. In addition to the destruction created directly by seismic vibrations, list three other types of destruction associated with earthquakes. 16. Distinguish between the Mercalli scale and the Richter scale. 17. What is a tsunami? In what two ways do earthquakes generate tsunami? 18. Cite some reasons why an earthquake with a moderate magnitude might cause more extensive damage than a quake with a high magnitude. 245 19. What evidence do we have that Earth’s outer core is molten? 20. Contrast the physical makeup of the asthenosphere and the lithosphere. 21. Why are meteorites considered important clues to the composition of Earth’s interior? 22. Describe the composition of the following: a. b. c. d. continental crust oceanic crust mantle core Examining the Earth System What potentially disastrous phenomenon often occurs when the energy of an earthquake is tranferred from the solid Earth to the hydrosphere (ocean) at their interface on the floor of the ocean? When the energy from this event is expended along a coast, how might coastal lands and the biosphere be altered? Online Study Guide The Earth Science Website uses the resources and flexibility of the Internet to aid in your study of the topics in this chapter. Written and developed by Earth science instructors, this site will help improve your understanding of Earth science. Visit http://www.prenhall.com/tarbuck and click on the cover of Earth Science 12e to find: • Online review quizzes • Critical thinking exercises • Links to chapter-specific Web resources • Internet-wide key term searches GEODe: Earth Science GEODe: Earth Science makes studying more effective by reinforcing key concepts using animation, video, narration, in- teractive exercises, and practice quizzes. A copy is included with every copy of Earth Science 12e