Earthquake Geology and Seismology Diseased nature oftentimes breaks forth In strange eruptions: oft the teeming earth Is with a kind of colic pinch’d and vex’d By the imprisoning of unruly wind Within her womb; which, for enlargement striving, Shakes the old beldam earth, and topples down Steeples, and moss-grown towers. —WILLIAM SHAKESPEARE, 1598, KING HENRY IV LEARNING OUTCOMES Earthquakes are shaking most commonly caused by earth movements along faults. Energy from movements is carried long distances by seismic waves. After studying this chapter you should • be able to describe the types of faults. • know the types of seismic waves. • understand the different ways of calculating earthquake magnitude. • be familiar with the variables that determine earthquake intensity, as in the Mercalli intensity scale. • comprehend the relationships between periods and frequencies of seismic waves, buildings, and geologic foundations. • recognize the types of buildings and building materials that fail during earthquakes. • understand how to construct buildings that do not fail during earthquakes. OUTLINE • Understanding Earthquakes • Types of Faults • Development of Seismology • Seismic Waves • Locating the Source of an Earthquake • Magnitude of Earthquakes • Ground Motion During Earthquakes • Earthquake Intensity—What We Feel During an Earthquake • A Case History of Mercalli Variables: The San Fernando Houses built on vertical posts in Bosques de las Lomas, Mexico City, have precious little shear strength to respond to seismic waves. Photo by Pat Abbott Valley, California, Earthquake of 1971 • Building in Earthquake Country Internal Energy CHAPTER 3 A Classic Disaster The Lisbon Earthquake of 1755 Portugal in the 18th century, and especially its capital city of Lisbon, was rich with the wealth its explorers brought from the New World. Portugal’s decline probably began with a set of earthquakes. On the morning of 1 November 1755—All Saints Day—Lisbon rocked under the force of closely spaced earthquakes originating offshore under the Atlantic Ocean. On this day of religious observance, the churches were full of worshippers. About 9:40 a.m., a thunderous underground sound began, followed by violent ground shaking. The severe ground movement lasted two to three minutes, causing widespread damage to the buildings in this city of more than 250,000 people. Most of Lisbon’s churches were built of masonry; they collapsed into the narrow streets, killing thousands of trapped and fleeing people. Tapestries fell onto candles and lamps—all lit on this holy day—and started fires that burned unchecked for six days. Before an hour had passed, crippled Lisbon was rocked by a second earthquake, more violent but shorter-lived than the first. In the panic, many of the frightened survivors of the first earthquake had rushed to the shore for safety, only to be swept away by quake-caused sea waves up to 10 m (33 ft) high. These walls of water spilled onto the land, carrying boats and cargo more than 0.5 km inland. As the seawater withdrew, it dragged people and debris from the earthquake-shattered structures back to the ocean. The two earthquakes killed almost 70,000 people and destroyed or seriously damaged about 90% of the buildings in Lisbon (figure 3.1). At the time, the city was rich in bullion, jewels, T he earth beneath our feet moves, releasing energy that shifts the ground and sometimes topples cities. Some earthquakes are so immense that their energy is equivalent to thousands of atomic bombs exploded simultaneously. The power of earthquakes to destroy human works, to kill vast numbers of people, and to alter the very shape of our land has left an indelible mark on many civilizations. Earthquake unpredictability instills an uneasy respect and fear in humankind that, through the millennia, have helped shape thought about life and our place in it. Ancient accounts of earthquakes tend to be quite incomplete. Instead of providing rigorous descriptions of Earth behavior, they emphasize interpretations. For more than 2,000 years, based on Aristotle’s ideas, many explanations of earthquakes were based on winds rushing beneath Earth’s surface. Even Leonardo da Vinci wrote in his Notebooks, about 1500 ce, that: When mountains fall headlong over hollow places they shut in the air within their caverns, and this air, in order to escape, breaks through the Earth, and so produces earthquakes. 48    Chapter 3   Earthquake Geology and Seismology Figure 3.1 The Lisbon earthquake. ©Science History Images/Alamy Stock Photo and merchandise, and it had great commercial and cultural importance. The destruction of this famous city by earthquakes and their resulting sea waves and fires was a shock to Western civilization. Not only were the losses of lives and buildings staggering, but the fires also incinerated irreplaceable libraries, maps and charts of the Portuguese voyages of discovery, and paintings by such masters as Titian, Correggio, and Rubens. The Lisbon earthquakes did more than devastate a city; they changed the prevailing philosophies of the era. All was not well in the world after all. Despite the profound effects that earthquakes have had on civilizations for so many centuries, scientific observations did not begin until the early 19th century, when good descriptions were made of earthquake effects on the land. Today, less than two centuries later, our knowledge of earthquakes has increased enormously. We have a fairly comprehensive understanding of what earthquakes are, why and where they happen, and how big and how often they occur at a given site. Our scientific data and theories allow us to understand phenomena that even the greatest minds of the past could not have glimpsed. Such are the rewards from the pyramidal building of knowledge we call science. Understanding Earthquakes The word earthquake is effectively a self-defining term—the Earth quakes, the Earth shakes, and we feel the vibrations. Earthquakes, or seisms, may be created by volcanic activity, meteorite impacts, undersea landslides, explosions of nuclear bombs, and more; but most commonly, they are caused by sudden earth movements along faults. A fault is a fracture surface in the Earth across which the two sides move past each other (figure 3.2). Stresses build up in rocks, but friction along fault surfaces holds the rocks together. When stress builds high enough, the rocks along the fault snap and move suddenly, releasing energy in waves we feel as the shaking of an earthquake. To visualize this fault movement, snap your fingers. As you prepare your finger snap, you push your thumb and finger together and sideways, but friction resists their moving past each other. When stress builds high enough, your thumb and finger slip rapidly, releasing energy as Figure 3.2 Offset of tilled farmland by 1979 movement of the Imperial fault, southernmost California. View is to the east; the west side of the fault (closest to you) has moved northward (to your left). ©Kerry Sieh sound waves. Both a fault rupture in the earth and your finger snap feature the same sudden slips that release energy in waves. FAULTS AND GEOLOGIC MAPPING The 19th-century recognition that fault movements cause earthquakes was a fundamental advance that triggered a whole new wave of understanding. With this relationship in mind, geologists go into the field to map active faults, which in turn identifies earthquake-hazard belts. Because a fault moves formerly continuous rock layers apart, the careful mapping of different rock masses can define sharp lines that separate offset segments of single rock masses. Fault surfaces can be vertical, horizontal, or at any angle to Earth’s surface. Some faults rupture the ground; some do not. The principles that help us understand faults begin with some of the earliest recognized relationships about rocks, which are still useful today. In 1669, the Danish physician Niels Steensen, working in Italy and known by his Latinized name of Steno, set forth several laws that are fundamental in interpreting geologic history. His law of original ­horizontality explains that sediments (sands, gravels, and muds) are originally deposited or settled out of water in horizontal layers. This is important because some older sedimentary rock layers are found at angles ranging from horizontal to vertical. But since we know they started out as horizontal layers (figure 3.3), their postdepositional history of deformation can be unraveled by mentally returning their orientations back to horizontal (figure 3.4). In the law of superposition, Steno stated that in an undeformed sequence of sedimentary rock layers, each Figure 3.3 North wall of the upper Grand Canyon. At the canyon bottom, the once horizontal sedimentary rock layers have been tilted to the east. Their uptilted ends have been eroded and buried by horizontal younger rock layers. ©University of Washington Libraries, Special Collections, John Shelton Collection, Shelton 1081 Understanding Earthquakes    49 Figure 3.4 These sedimentary rocks were deposited in horizontal layers, but have since been compressed into contorted layers by movements of the San Andreas fault. ©University of Washington Libraries, Special Collections, John Shelton Collection, Shelton KC13887 successive layer is deposited on top of a previously formed, and hence older, layer. Thus, each sedimentary rock layer is younger than the bed beneath it but older than the bed above it (figures 3.3 and 3.4). Steno’s law of original continuity states that sediment layers are continuous, ending only by butting up against a topographic high, such as a hill or a cliff, by pinching out due to lack of sediment, or by gradational change from one sediment type to another. This relationship allows us to appreciate the incongruity of a sedimentary rock layer that abruptly terminates. Something must have happened to terminate it. For example, a stream may have eroded through it, or a fault may have truncated it. Geologists spend a lot of time locating and identifying offsets of formerly continuous rock layers. In this way, we can determine the lengths of faults and estimate the magnitude of earthquakes they produce. Longer lengths of fault rupture create bigger earthquakes. On a much broader scale, we can find large offsets on long-acting, major faults. Figure 3.5 shows a pronounced line cutting across the land in a northeast-southwest trend; this is the Alpine fault on the South Island of New Z ­ ealand. The west (left) side has been moved 480 km (300 mi) toward the north. In Otago province in the southern part of the South Island, gold was discovered in 1861 in stream gravels. This set off a gold rush that brought in prospectors and miners from all over the world. The gold fever that had attracted so many fortune seekers to California in 1849 now moved to New Zealand. Prospectors panned the streams and worked their way upstream into bedrock hills to find the source of the gold. Yet much of the wealth lay 480 km to the northeast in Nelson province, where the same gold-bearing rock Figure 3.5 Aerial photo of part of South Island, New Zealand (see figure 3.6 for location). The Alpine fault cuts a prominent slash from near the lower left (southwest) corner of the photo to the top center (northeast). Arrowheads line up with the fault. Photo by Pat Abbott 50    Chapter 3   Earthquake Geology and Seismology Nelson block Nelson e Alpin fault N Queenstown Area covered by photo in figure 3.5 ine Alp lt fau Fiordland block Otago schists 0 100 200 300 km (a) Figure 3.6 Generalized geologic map of South Island, New Strik e Zealand. Each map color records a different type of rock. Locate the Alpine fault, and then match up the rock patterns across the fault. The gold-bearing rocks near Queenstown have been offset 480 km (300 mi) to near Nelson. Dip Hor izon inte rsec tal line of tion masses had been offset along the Alpine fault by more than 23 million years of fault movements (figure 3.6). As this example shows, fault studies also can have tremendous implications for locating mineral wealth. Water surface tion irec d Dip Dip angle Types of Faults As tectonic plates move, mountains are elevated and basins are warped downward. The brittle rocks of the lithosphere respond by fracturing (also called jointing or cracking). When regional forces create a large enough stress differential in rocks on either side of a fracture, then movement occurs and the fracture becomes a fault. Accumulated movements of rocks along faults range from millimeters to hundreds of kilometers. These movements can cause originally horizontal sedimentary rock layers to be tilted and folded into a wide variety of orientations (figure 3.7a). To describe the location in three-dimensional (3-D) space of a deformed rock layer, a fault surface, or any other planar feature, geologists make measurements known as dip and strike. Dip is seen in the two-dimensional (2-D) vertical view (cross-­section) as the angle of inclination from the horizontal of the tilted rock layer (figure 3.7b). It is also important to note the compass direction of the dip in the horizontal plane—for example, toward the northeast. Strike is viewed in the 2-D horizontal view (map) as the compass bearing of the rock layer where it pierces a horizontal plane. DIP-SLIP FAULTS The classification of faults uses some terminology of early miners. Many ore veins were formed in ancient fault zones. Thus, many mines consist of adits (passages) dug along old, (b) Figure 3.7 (a) A 75-million-year-old sandstone layer at La Jolla Bay, California, exposed at a moderately high tide. The sea surface forms a horizontal plane against the inclined sandstone bed. (b) The strike of a rock layer is the compass bearing of the “shoreline.” The dip angle is the number of degrees below horizontal that the rock layer is inclined. (a) Photo by Pat Abbott inactive faults. Ores are common along faults because when one block of rocks moves past another in a fault zone, the tremendous friction tends to shatter and pulverize the rocks in the fault zone. The broken rock creates an avenue of permeability through which water can flow. If the underground water carries a concentration of dissolved metals, they may precipitate as valuable elements or minerals within the fault zone. Early miners working in excavated fault zones called the floor beneath their feet the footwall and the rocks above their heads the hangingwall (figure 3.8). This terminology is used to define the types of faults dominated by vertical movements, called dip-slip faults. Faults with the major amounts of their offset in the dip or vertical direction are caused by either a pulling (tension) or a pushing (compression) force. Types of Faults    51 Figure 3.8 Schematic cross- Mine section of miners excavating ore that precipitated in broken rock within an old fault zone. Notice that the rock layers in the footwall and hangingwall are no longer continuous; this gives evidence of the movements that occurred along the fault in the past. Ground surface Hangingwall Footwall Figure 3.9 Schematic cross- section of a normal fault; that is, the hangingwall has moved downward (in a relative sense). Extensional forces are documented by the zone of omission, where the originally continuous rock layers are missing. The small arrows indicate movement; the larger arrows show force. Zone of omission There are two major types of dip-slip faults: normal faults and reverse faults. A normal fault occurs when the hangingwall moves down relative to the footwall. The dominant force is extensional, as recognized by the separation of the pulled-apart rock layers in a zone of omission (figure 3.9). The word normal as a name for this type of fault is unfortunate because it carries a connotation of normalcy, as if this were the standard or regular mode of fault movement; such is not the case. Extensional or normal-style faults are typical of the faults at seafloor spreading centers and in regions of continents being pulled apart. If the dominant force that creates a fault movement is compressional, then the rock layers are pushed together, or repeated, when viewed in cross-section (figure 3.10). With compressional forces, the hangingwall moves upward relative to the footwall; this type of fault is referred to as a 52    Chapter 3   Earthquake Geology and Seismology reverse fault. The compressional motions of reverse faults are commonly found at areas of plate convergence where subduction or continental collision occurs. The extensional versus compressional origins of movement can have enormous economic implications. Look again at figures 3.9 and 3.10. Visualize the emphasized (dotted) rock layer in each figure as being an oil reservoir. Now imagine yourself to be the landowner above either the zone of omission or the zone of repetition. In one case, it could mean poverty; in the other, great wealth. STRIKE-SLIP FAULTS When stress produces shear and causes most of the movement along a fault to be horizontal (parallel to the strike direction), the fault is referred to as a strike-slip fault. fault; try it facing both directions with figure 3.11. Similarly, if features on the left-hand side of the fault have moved closer to you, then it is a left-lateral, or sinistral, fault. We have looked at a large strikeslip fault in New Zealand, the Alpine fault, but the most famous strike-slip fault in the world is the San Andreas in California. This right-lateral fault is more than 1,300 km (800 mi) long. On 18 April 1906, a 430 km (265 mi) long segment of the San Andreas fault Zone ruptured and moved horizontally as of much as 6.5 m (20 ft) in 60 seconds. repetition The great burst of energy generated by the fault movement was actually Figure 3.10 Schematic cross-section of a reverse fault; that is, the hangingwall has moved upward (in a relative sense). Compressional forces are documented by the zone of repetition, where the the release of elastic energy that originally continuous rock layers have been split, shoved together, and stacked above each other. had built up and been stored in the rocks for many decades. These fault offsets are seen in map view as though from a Faults are not simple planar surfaces that glide readily balloon or airplane looking down on Earth’s surface. Strikewhen subjected to stress. Instead, faults are complex zones slip faults are further classified on the basis of the relative of breakage where rough and interlocking rocks are held movement directions of the fault blocks. If you straddle a together over an irregular surface that extends many miles fault and the block on your right-hand side has moved relabelow the ground. Stress must build up over many years tively toward you, then it is called a right-lateral, or dextral, before enough potential energy is stored to allow a rupture fault (figure 3.11). Notice that this convention for naming on a fault. The initial break occurs at a weak point on the the fault works no matter which way you are straddling the fault and then propagates rapidly along the fault surface. Much of the energy stored in the rocks is released as radiating seismic waves that humans call an earthquake. The point where the fault first ruptures is known as the hypocenter, or focus. The point on Earth’s surface directly Right-lateral fault above the hypocenter is called the epicenter (figure 3.12). Straddle the fault; right-hand A fault rupture is not a simple, one-time movement that side moves toward you. produces “the earthquake.” In fact, we never have just one earthquake. The stresses that build up in the rocks in an area are released by a series of movements along the fault, or several faults, that continue for weeks to months to years. Each fault movement generates an earthquake. Steps in Strike-Slip Faults Figure 3.11 Map of a right-lateral, strike-slip fault. As the man straddles the fault, the right-hand side of the fault has moved relatively closer to him. If he turns around, will the right-hand side of the fault still have moved closer to him? Strike-slip faults do not simply split the surface of Earth along perfectly straight lines. The rupturing fault tears apart the rocks along its path in numerous subparallel breaks that stop and start, bend left, and bend right. For analogy, visualize a sheet cake or pan of moist mud. Put your right hand on the upper right corner and your left hand on the lower left corner. Now pull toward you with your right hand and push away with your left. Do you visualize the cake ripping along one straight line? Or along several breaks that stop and start, bend left and bend right? So it is with Earth when it ruptures during an earthquake-generating fault movement. Normal and reverse faults also have bends; we just don’t see them as easily on the surface. The bends along a fault have profound implications for the creation of topography. Figure 3.13a is a sketch of a Types of Faults    53 Fault line on surface Surface rupture Epicenter Rupture area Slip Hypocenter Fault surface Figure 3.12 Block diagram of a fault surface. The hypocenter (focus) is the point on the fault surface where the rupture began; the epicenter is the point on Earth’s surface directly above the hypocenter. Notice that because the fault surface is inclined (it dips), the epicenter does not plot on the trace of the fault at the surface. Source: J. Ziony, ed., “Earthquakes in the Los Angeles Region.” US Geological Survey. right-lateral fault with a bend (step) in it—a left-stepping bend. Stand to either side of the fault and look at the region of the bend. Note that the fault segment left of the bend is closest to you; hence, this is a left-stepping, right-lateral fault. Notice what occurs at the bend in the fault when the two sides slide past each other—compression, pushing together, collision, constraint. The photo in figure 3.13b shows a left step in the right-lateral Superstition Hills fault west of Brawley, California, which was created on 16 November 1987. Notice how the compression at the bend produced a little hill. What size could this hill attain if movements at this left step were to occur for millions of years? It could grow into a mountain. Similarly, figure 3.14a depicts a right step along a rightlateral fault. Visualize what happens at the bend in the fault. In this case, the two sides pull apart from each other, extend, diverge, release. The photo in figure 3.14b is from the same earthquake, along a different length of the same fault, as in figure 3.13b. At this right step, the two sides pulled apart and created a down-dropped area—a wide crack or a little basin. TRANSFORM FAULTS Transform faults are a special type of horizontal-­ movement fault first recognized by the Canadian geologist J. Tuzo Wilson in 1965. Figure 3.15 depicts how a transform fault forms. Seafloor crust forms at oceanic volcanic ridges and is pulled apart by gravity and slab pull of subducting plates. When plates collide, the denser plate Left-stepping, right-lateral fault Stand to the side, look at bend in fault; left-hand side steps toward you. er eth sh Pu g To (a) Figure 3.13 (a) Left step in right-lateral fault. Notice that the land is pushed together at the fault bend whenever the fault moves. Movements will create a hill, which could grow to a mountain if the fault remains active for a long enough time. (b) Land offset along the Superstition Hills right-lateral fault during its 16 November 1987 earthquake. See the left step and the uplift at the bend. (Black arrows indicate directions of land movement.) (b) Photo by Pat Abbott 54    Chapter 3   Earthquake Geology and Seismology (b) (b) Right-stepping, right-lateral fault Stand to the side, look at bend in fault; right-hand side steps toward you. ll Pu art Ap (a) Figure 3.14 (a) Right step in right-lateral fault. Notice that the land is pulled apart at the fault bend whenever the fault moves. Movements will create a hole, which could become a basin if the fault stays active for a geologically long time. (b) Land offset along the Superstition Hills right-lateral fault during its 1987 rupture. See the right step and the pull apart at the bend. (Black arrows indicate directions of land movement.) (b) t nen Sub Sp duc t ion re ce adin nte g r zon Transform fault Fracture zone Deep-ocean trench e Spreading center Con ti (b) Photo by Pat Abbott Lithosphere Magma Asthenosphere Figure 3.15 Plate-tectonic model of a transform fault. Notice that the transform fault connects the two separated spreading centers; the seafloor moves in opposite directions here. Beyond the spreading centers, the two plates move in the same direction and are separated by a fracture zone; there is no transform fault here. Types of Faults    55 subducts. But what happens along the sides of the plates? They slide past each other at transform faults. Visualize this process in three dimensions. The spreading plates are rigid slabs of oceanic rock, tens of kilometers thick, that are being wrapped around a nearspherical Earth. How does a rigid plate move about a curved surface? The plates must fracture, and these fractures are transform faults. In fact, transform faults must link spreading centers or connect spreading centers with subduction zones. In figure 3.15, notice that in the region between the two spreading centers, the relative motions of the two plates are in opposite directions in typical strike-slip fault fashion. However, passing both to the right and left of the spreading centers, notice that the two slabs are moving in the same direction. There they are called fracture zones: there is no active offset across a fracture zone. Wire vibrates moves Framework Heavy weight does not move Seismogram Pen Concrete base moves Earth moves Figure 3.16 A basic seismograph. Earth moves, the seismograph framework Development of Seismology moves, and the hanging wire vibrates, but the suspended heavy mass and pen beneath it remain relatively steady. Ideally, the pen holds still while Earth moves beneath the pen to produce an inked line. Three seismometers sensing vibrations in orthogonal directions of ground shaking are required to record the full 3-D shaking at a point. The study of earthquakes is known as seismology (after seism, meaning “earthquake”). The earliest earthquake-indicating device known was invented in China in 132 ce by Chang Heng. The modern era of seismologic instrumentation began about 1880. Instrumentation continues to evolve through many different styles, but a basic need is to record the 3-D movement of earthquake waves. This is achieved by having instruments detect Earth motions (seismometers) and record them (seismographs) as north-south horizontal movements, east-west horizontal movements, and vertical movements. To accurately record the passage of seismic waves, a seismometer must have a part that remains as stationary as possible while the whole Earth beneath it vibrates. One way to accomplish this is by building a frame that suspends a heavy mass (figure 3.16). The support frame rests on Earth and moves as Earth does, but the mass suspended by a wire must have its inertia overcome before it moves. The principle of inertia explains that a stationary object—for example, the suspended mass—tends to remain stationary. The differences between motions of the frame and the hanging mass are recorded on paper by pen and ink or, increasingly, as digital data. Visualize the process this way: hold an ink pen steady in your hand and then vibrate the entire Earth beneath your pen to make an inked line. Other important pieces of information to record include the arrival times and the durations of the various seismic waves. This is accomplished by having time embedded in the seismographic record either as inked tick marks on the paper graph 56    Chapter 3   Earthquake Geology and Seismology or within the digital data. Time is standardized in the United States by the national clock in Boulder, Colorado. First-order analysis of the seismic records allows seismologists to identify the different kinds of seismic waves generated by the fault movement, to estimate the amount of energy released (magnitude), and to locate the ­epicenter/ hypocenter (where the rock hit the water, so to speak). WAVES Throw a rock into a pond, play a musical instrument, or experience a fault movement, and the water, the air, or the Earth will transmit waves of energy that travel away from the initial disturbance. All these waves have the following similarities: amplitude, the height of the wave above the starting point (figure 3.17); wavelength, the distance B Wavelength A Amplitude Travel direction Figure 3.17 Wave motion. Amplitude is the height of the wave above the starting point. Wavelength is the distance between wave crests B and A. Period is the amount of time in seconds for wave crest B to travel to site A. between successive waves; period, the time between waves measured in seconds; and frequency, the number of waves passing a given point during 1 second. Frequencies are measured in hertz (Hz), where 1 Hz equals one cycle per second. Note that period and frequency are inversely related: Period = 1 frequency (in hertz) For example, if five waves passed a given point in 1 second, then the frequency is 5 Hz and the period of time between each wave is 0.2 second. Seismic Waves When a fault slips, or an explosion occurs, it releases energy in seismic waves that pass through the whole body of the planet (body waves) and others that move near the surface only (surface waves). BODY WAVES Body waves are the fastest and are referred to as either primary or secondary waves. Body waves ranging from about 0.02 Hz to tens of Hz produce measurable ground shaking. These high-frequency, short-period waves are most energetic for short distances close to the hypocenter/epicenter. Primary Waves The primary (P) wave is the fastest and thus the first to reach a recording station. P waves move in a push-pull fashion, alternating pulses of compression (push) and extension (pull); this motion is probably best visualized using a Slinky toy (figure 3.18a). P waves radiate outward from their source in an ever-expanding sphere, like a rapidly inflating balloon. They travel through any material, be it solid, liquid, or gas. Their speed depends on the density and compressibility of the materials through which they pass. The greater the resistance to compression, the greater the speed of the seismic waves passing through packed atomic lattices. Representative velocities for P waves in hard rocks (e.g., granite) are about 5.1 to 5.5 km/sec (about 11,400 to 12,300 mph). P waves in water slow to 1.4 km/sec (about 3,100 mph). Because P waves and sound waves are both compressional waves, they can travel through air. P waves may emerge from the ground, and if you are near the epicenter, you may be able to hear those P waves pulsing at around 15 cycles per second as low, thunderous noises. The arrival of P waves at your home or office is similar to a sonic boom, including the rattling of windows. Secondary Waves The secondary (S) wave is the second wave to reach a recording station. S waves are transverse waves that propagate by shearing or shaking particles in their path at right angles to the direction of advance. This motion is probably most easily visualized by considering how a jump rope moves when you shake one end up and down (figure 3.18b). S waves travel only through solids. S waves do not propagate through fluids. On reaching fluid or gas, the S wave energy is reflected back into rock or is converted to another form. The velocity of an S wave depends on the density and resistance to shearing of materials. Fluids and gases do not have shear strength and thus cannot transmit S waves. Representative velocities for S waves in dense rocks (e.g., granite) are about 3 km/sec (about 6,700 mph). With their up-and-down and side-to-side motions, S waves shake the ground surface and can do severe damage to buildings. SEISMIC WAVES AND EARTH’S INTERIOR Large earthquakes generate body waves energetic enough to be recorded on seismographs all around the world. These P waves and S waves do not follow simple paths as they pass through Earth; they speed up, slow down, and change direction, and S waves even disappear. Analysis of the travel paths of the seismic waves gives us our models of Earth’s interior (figure 3.19). Earth is not homogeneous. Following the paths of P and S waves from Earth’s surface inward, there is an initial increase in velocity, but then a marked slowing occurs at about 100 km (62 mi) depth; this is the top of the asthenosphere. Passing farther down through the mantle, the velocities vary but generally increase until about 2,900 km (1,800 mi) depth; there, the P waves slow markedly and the S waves disappear. This is the mantle-core boundary zone. The disappearance of S waves at the mantle-core boundary, due to their reflection or conversion to P waves, indicates that the outer core is mostly liquid. Moving into the core, P wave velocities gradually increase until a jump is reached at about 5,150 km (3,200 mi) depth, suggesting that the inner core is solid. SURFACE WAVES Surface waves are created by body waves disturbing the surface. They are of two main types—Love waves and Rayleigh waves. Both Love and Rayleigh waves are referred to as L waves (long waves) because they take longer periods of time to complete one cycle of motion and are the slowest moving. The frequencies of surface waves are low—less than one cycle per second. The low-frequency, long-period waves carry significant amounts of energy for much greater distances away from the epicenter. Seismic Waves    57 direction of advance; to understand this, visualize the jump rope in figure 3.18b lying on the ground. Love waves generally travel faster than Rayleigh waves. Like S waves, they do not move through water or air. (a) P wave Rayleigh Waves (b) Rayleigh waves were ­predicted to exist by Lord Rayleigh 20 years before they were actually recognized. They advance in a backward-­ rotating, elliptical motion (figure 3.18c) similar to the orbiting paths of water molecules in windblown waves of water, except that waves in water are forward-rotating (figure 3.18d). The shaking produced by Rayleigh waves causes both vertical and horizontal movement. The shallower the hypocenter, the more P and S wave energy will hit the surface, thus putting more energy into Rayleigh waves. The rolling waves pass through both ground and water. The oftenheard report that an earthquake feels like being rocked in a boat at sea well describes the passage of Rayleigh waves. These waves have long periods, and once started, they go a long way. S wave (c) Direction of Rayleigh wave motion (d) Wind Ocea n su Ocean waves beach SOUND WAVES AND SEISMIC WAVES Waves are fundamental to both music and seismology. M ­ usicians use instruments to produce the sound waves we hear as music. For example, a trombone player controls the amount of sound with his breath, and changes the Figure 3.18 Types of seismic waves. (a) P waves exhibit the push-pull motion of a Slinky toy. frequencies of the sound waves (b) S waves move up and down perpendicular to the direction of advance, like a shaken jump rope. (c) Rayleigh waves advance in a backward-rotating motion, as opposed to (d) wind-blown ocean by extending and retracting the waves, which cause water to move in forward-rotating circles. slide on the trombone. Earthquakes generate body and surface waves; seismologists record Love Waves and analyze the seismic wave frequencies to understand the Love waves were recognized and first explained by the Britearthquake. ish mathematician A. E. H. Love. Their motion is similar Music is a common part of our lives and we are familto that of S waves, except it is from side-to-side in a horiiar with hearing sound waves. Sound waves and seismic zontal plane roughly parallel to Earth’s surface. As with waves can be presented in the same visual form. WaveS waves, their shearing motion is at right angles to the forms for a trombone and a moderate-size earthquake are rface 58    Chapter 3   Earthquake Geology and Seismology Hydrosphere (liquid) Wave velocity (km/sec) 6 8 10 12 14 0 Lithosphere (solid) 1,000 Asthenosphere (“soft plastic”) 2,000 P wave S wave Mantle 3,000 Depth (km) Atmosphere 4 4,000 Outer core 5,000 P wave Inner core (solid) 6,000 Figure 3.19 Varying velocities of P waves and S waves help define the internal structure of Earth. Magnitude 5.1 Earthquake at Two Distances Station 10 km from earthquake Ground velocity Air pressure One Note or Source on the Trombone but Varying the Path Slide retracted — short path Slide extended — long path Station 120 km from earthquake Relative magnification 36x Time 0.01 second Time 2 seconds Figure 3.20 Comparison of wave patterns for a trombone and an earthquake for short and long-distance travel paths. Source: A. Michael, S. Ross, and D. Schaff, “The Music of Earthquakes; Waveforms of Sound and Seismology” originally presented at Sigma Xi conference on Science and Art. USGS. shown in figure 3.20. Both a trombone and an earthquake have more higher-frequency waves if a shorter path is traveled—that is, the trombone is retracted and has a short length, and the fault-rupture length is short. As the travel paths become longer for both trombone (extended) and earthquake (longer fault rupture), the number of low-frequency waves increases. Musically, as the path through the trombone lengthens, the vibrations per second decrease, the frequencies are lower, and the tone is lower. Seismically, a rupturing fault sends off high-frequency seismic waves, but as the fault rupture grows longer, more low-frequency seismic waves are generated. The ranges of some common frequencies are listed in table 3.1. TABLE 3.1 Some Common Frequencies (in hertz) Sound Waves 30,000 Hz—heard by dogs 15–20 Hz to 15,000–20,000 Hz—range of human hearing 15–20 Hz—P waves in air heard by humans near epicenter Seismic Waves 0.02–30 Hz—body waves 0.002–0.1 Hz—surface waves Seismic Waves    59 In Greater Depth Seismic Waves from Nuclear Bomb Blasts Versus Earthquakes North Koreans buried an atom bomb and detonated it on 12 February 2013, releasing energy equivalent to a magnitude 5.1 earthquake. The seismic wave pattern recorded at the IRIS/USGS Global Seismic Network Station in Mudanjiang, China (figure 3.21a) shows an explosion of compressional energy yielding an abundance of P waves, with lesser shearing, S wave energy. Compare the bomb-blast seismic record with that of a magnitude 5.0 earthquake recorded at the same seismic station in China (figure 3.21b). The natural earthquake has less compressional energy as shown by lesser P waves. The earthquake has much greater shear wave energy as shown by the prominent S wave development. The different P and S wave patterns are useful for distinguishing between human-caused and natural events. 60 seconds (a) Bomb P waves (b) Earthquake Figure 3.21 Seismic records from Mudanjiang, China. (a) Recording of magnitude 5.1 bomb blast set off in North Korea on 12 February 2013. Note the prominent development of the early arriving P waves. (b) Recording of magnitude 5.0 earthquake. Note the lesser P waves and prominent development of later arriving S waves. of P and S waves is determined by subtracting the P arrival time from the S time (S–P). Inspection of the seismogram in figure 3.23 shows that S waves arrived 11 minutes after P waves. Figure 3.22 indicates that an S–P arrival time difference of 11 minutes corresponds to an earthquake about 8,800 km (5,400 mi) away. But in what direction? Epicenters can be located using seismograms from three recording stations. As an example, S–P wave arrival time differences yield distances to the epicenter of 164 km (102 mi) from University of Memphis in Tennessee, 236 km (146 mi) Locating the Source of an Earthquake Using the lengths of time the various seismic waves take to reach a seismograph, the locations of the epicenter and hypocenter can be determined. P waves travel about 1.7 times faster than S waves. Thus, the farther away from the earthquake origin, the greater is the difference in arrival times between P and S waves (figure 3.22). When a seismograph records an earthquake, the difference in arrival times e av 11 min s Sw wa ve 15 P rfa ce Su Minutes after start of earthquake earthquake for seismic waves. Note that the arrival time difference for P and S waves of 11 minutes in figure 3.23 corresponds to a distance of about 8,800 km (5,400 mi). L 20 10 Figure 3.22 Plot of travel time versus distance from S 25 ave Pw 5 0 2,000 S waves 4,000 6,000 8,000 10,000 Distance from earthquake in kilometers 60    Chapter 3   Earthquake Geology and Seismology P S 11 minutes Figure 3.23 Seismogram recorded in Finland of the Sumatran earthquake on 26 December 2004. Notice that the difference in arrival times of P and S waves is 11 minutes. See figure 3.22 to read the distance traveled by the seismic waves. from St. Louis University in Missouri, and 664 km (412 mi) from Ohio State University in Columbus. If the distance from each station is plotted as the radius of a circle, the three circles will intersect at one unique point—an epicenter at New Madrid, Missouri (figure 3.24). Computers usually make the calculations to determine epicenter locations; however, a better mental picture of the process is gained via the hand-drawn circles. The difference in arrival times of P and S waves (S–P) actually measures the distance from the recording station to the hypocenter (or focus) of the earthquake, the site of initial fault movement (see figure 3.12). If the hypocenter is on Earth’s surface, then the hypocenter and epicenter are the same. However, if the hypocenter is deep below the surface, it will affect the arrival time of surface (L) waves because L waves do not begin until P waves strike the Earth’s surface. The depth to a hypocenter is best determined where an array of seismometers is nearby, thus allowing careful analysis of P wave arrival times. Magnitude of Earthquakes Magnitude is an estimate of the relative size or energy release of an earthquake. The magnitude is proportional to the area of the fault surface that moves or slips and how much it slips. It is commonly measured from the seismic wave traces on a seismogram. RICHTER SCALE In 1935, Charles Richter of the California Institute of Technology devised a quantitative scheme to describe the magnitude of California earthquakes, specifically events with shallow hypocenters located near (less than 300 mi from) the seismometers. Richter based his scale on the idea that the bigger the earthquake, the greater the shaking of Earth and thus the greater the amplitude (swing) of the lines made on the seismogram. To standardize this relationship, he defined magnitude as: the logarithm to the base ten of the maximum seismic wave amplitude (in thousandths of a millimeter) recorded on a standard seismograph at a distance of 100 kilometers from the earthquake center. Columbus St.Louis New Madrid Memphis Figure 3.24 Location of an earthquake epicenter. S–P arrival time difference calculations gave a radius of 164 km from Memphis, 236 km from St. Louis, and 664 km from Columbus. The circles plotted with these values intersect uniquely at New Madrid, Missouri—the epicenter. Because not all seismometers will be sitting 100 km from the epicenter, corrections are made for distance. Richter assigned simple, whole numbers to describe magnitudes; for every 10-fold increase in the amplitude of the recorded seismic wave, the Richter magnitude increases one ­number—for example, from 4 to 5. The energy released by earthquakes increases even more rapidly than the 10-fold increase in amplitude of the seismic wave trace. For example, if the amplitude of the seismic waves increased 10,000 times (10 × 10 × 10 × 10), the Richter magnitude would move up from a 4 to an 8. However, the energy release from 4 to 8 increases by 2,800,000 times (table 3.2). What does this increase mean in everyday terms? If you feel a magnitude 4 earthquake while sitting at your dinner table, and then a magnitude 8 comes along while you are still at the table, would you really be shaken 2,800,000 times as hard? No. The greater energy of the magnitude 8 earthquake would be spread out over a much larger area, and over a time interval about 20 times longer (e.g., 60 seconds as opposed to 3 seconds). At any one location, the felt shaking in earthquakes above magnitude 6 does not increase Magnitude of Earthquakes    61 for a longer time will experience the intense shaking. A longer duration of shaking can greatly increase the amount of damage to buildings. Computing a Richter magnitude for an earthquake is quickly done, and this is one of the reasons for its great popularity with the deadline-conscious print and electronic media. Upon learning of an earthquake, usually by phone calls from reporters, one can rapidly measure (1) the amplitude of the seismic waves and (2) the difference in arrival times of P and S waves. Figure 3.25 has reduced Richter’s equation to a nomograph, which allows easy determination of magnitude. Take a couple of minutes to figure out the magnitude of the earthquake whose seismogram is printed above the nomograph. Each year, Earth is shaken by millions of quakes that are recorded on seismometers. Most are too small to be felt by humans. Notice the distinctive “pyramidal” distribution of TABLE 3.2 Energy of Richter Scale Earthquakes Richter Magnitude Energy Increase Energy Compared to Magnitude 4 4 1 5 = 48 Mag 4 EQs 48 6 = 43 Mag 5 EQs 2,050 7 = 39 Mag 6 EQs 80,500 8 = 35 Mag 7 EQs 2,800,000 30 S P 20 10 millimeters very much more (maybe three times more for each step up in magnitude); it certainly does not increase as much as the values in table 3.2 might lead us to think. In effect, the bigger earthquake means that more people in a larger area and Amplitude (peak height) 0 10 20 seconds S–P (arrival time difference) 500 400 50 40 100 6 300 30 200 100 60 40 20 20 10 20 4 5 10 8 6 3 2 4 2 1 1 2 0–5 Distance (km) 5 50 0.5 0.2 0.1 0 S– P (sec) Magnitude 62    Chapter 3   Earthquake Geology and Seismology Amplitude (mm) Figure 3.25 Nomograph of the Richter scale allowing earthquake magnitudes to be estimated. On the seismogram, read the difference in arrival times of P and S waves in seconds and plot the value on the left column of the nomograph. Next read the amplitude of the peak height of the S wave and plot this value on the far right column. Draw a line between the two marked values, and it will pass through the earthquake magnitude on the center column. Check your answer in Questions for Review at the end of the chapter. TABLE 3.3 Earthquakes in the World Each Year Magnitude Number of Quakes per Year 8.5 and up 0.3 8–8.4 1 7.5–7.9 3 7–7.4 15 6.6–6.9 56 6–6.5 210 Strong (destructive) 5–5.9 800 Moderate (damaging) 4–4.9 6,200 Light 3–3.9 49,000 Minor 2–2.9 350,000 Very minor 0–1.9 3,000,000 Description Great Major earthquakes by size—the smaller the earthquake magnitude, the greater their numbers (table 3.3). Yet the fewer than 20 major and great earthquakes (magnitudes of 7 and higher) each year account for more than 90% of the energy released by earthquakes. At the upper end of the magnitude scale, the energy increases are so great that more energy is released going from magnitude 8.9 to 9 than from magnitude 1 to 8. These facts underscore the logarithmic nature of the Richter scale; each step up the scale has major significance. OTHER MEASURES OF EARTHQUAKE SIZE An earthquake is a complex event, and more than one number is needed to assess its magnitude. Although the Richter scale is useful for assessing moderate-size earthquakes that occur nearby, the 0.1- to 2-second-period waves it uses do not work well for distant or truly large earthquakes. The short-period waves do not become more intense as an earthquake becomes larger. For example, the Richter scale assesses both the 1906 San Francisco earthquake and the 1964 Alaska earthquake as magnitude 8.3. However, using other scales, the San Francisco earthquake is a magnitude 7.8 and the Alaska seism is a 9.2. The Alaska earthquake was at least 100 times bigger in terms of energy. The Richter scale is now restricted to measuring only local earthquakes with moderate magnitudes (noted as ML). Because earthquakes generate both body waves that travel through Earth and surface waves that follow Earth’s uppermost layers, two other magnitude scales have long been used: mb and Ms. The body-wave (mb) scale uses amplitudes of P waves with 1- to 10-second periods, whereas the surfacewave scale (Ms) uses Rayleigh waves with 18- to 22-second periods. Early on, all magnitude scales were considered equivalent, but now we know that earthquakes generate different proportions of energy at different periods. For example, larger earthquakes with their larger fault-­rupture surfaces radiate more of their energy in longer-period seismic waves. Thus, for great and major earthquakes, body-wave magnitudes (mb) will significantly underestimate the actual size of the earthquake. Even a composite of these three methods of determining earthquake magnitude (ML, mb, and Ms) does not necessarily yield the true size of an earthquake. Moment Magnitude Scale Seismologists have moved on to other measures to more accurately determine earthquake size. The seismic moment (M0) relies on the amount of movement along the fault that generated the earthquake; that is, M0 equals the shear strength of the rocks times the rupture area of the fault times the average displacement (slip) on the fault. Moment is the most reliable measure of earthquake size; it measures the amount of strain energy released by the movement along the whole rupture surface. Seismic moment has been incorporated into a new earthquake magnitude scale by Thomas Hanks and Hiroo Kanamori, the moment magnitude scale (Mw), where: Mw = 2/3 log10(M0) − 10.7 The moment magnitude scale is used for big earthquakes. It is more accurate because it is tied directly to physical parameters such as fault-rupture area, fault slip, and energy release. For great earthquakes, it commonly takes weeks or months to determine Mw because time is required for the aftershocks to define the area of the rupture zone. Some of the largest moment magnitudes calculated to date are the 1960 Chile earthquake (Ms of 8.5; Mw of 9.5), the 1964 Alaska earthquake (Ms of 8.3; Mw of 9.2), the 2004 Sumatra event (Mw of 9.1), and the 2011 Japan seism (Mw of 9.0). These gigantic earthquakes occurred at subduction zones. A variety of energetic events are placed on a logarithmic scale for comparison in figure 3.26. Each step or increment up the scale is a 10-fold increase in magnitude. FORESHOCKS, MAINSHOCK, AND AFTERSHOCKS Large earthquakes do not occur alone; they are part of a series of movements on a fault that can go on for years. The biggest earthquake in a series is the mainshock. Smaller earthquakes that precede the mainshock are foreshocks, and those that follow are aftershocks. Realistically, there are no differences between these earthquakes other than size; they are all part of the same series of stress release on the fault. A large-scale fault movement increases the stress on adjacent sections of a fault, helping trigger the additional fault movements that we feel as aftershocks. The danger Magnitude of Earthquakes    63 Meteorite impact (10 km diameter, 20 km/sec velocity) 1030 TABLE 3.4 Rupture Length and Duration Magnitude Rupture Length (km) Duration (seconds) 1964 Alaska 9.2 1,000 420 1906 San Francisco, CA 7.8 400 110 1992 Landers, CA 7.3 70 24 1983 Borah Peak, ID 7.0 34 9 2001 Nisqually, WA 6.8 20 6 1933 Long Beach, CA 6.4 15 5 2001 Yountville, CA 5.2 4 2 Earth's daily receipt of solar energy 1028 Earth's annual internal heat flow U.S. annual energy consumption Energy (Ergs) (logarithmic scale) 1026 Chile 1960 earthquake (M9.5) Alaska 1964 and Sumatra 2004 earthquakes (M9.2) Average annual seismic energy release on Earth World's largest nuclear explosion 1024 Hurricane (kinetic energy) 1022 Average hurricane (10-day lifetime) Average annual seismicity in continent interiors Mount St. Helens eruption New Madrid 1812 earthquake (M7.5) 1 megaton nuclear explosion ∙∙ 10 km (6.2 mi) rupture ≈ magnitude 6 ∙∙ 40 km (25 mi) rupture ≈ magnitude 7 ∙∙ 400 km (250 mi) rupture ≈ magnitude 8 ∙∙ 1,000 km (620 mi) rupture ≈ magnitude 9 Hiroshima 1945 atomic bomb 1020 Electrical energy of typical thunderstorm Average tornado (kinetic energy) 1018 1016 2.0 Lightning bolt 4.0 6.0 8.0 10.0 Equivalent moment magnitude (M) (unitless numbers) 12.0 Figure 3.26 Equivalent moment magnitude of a variety of seismic (green dots), human-made (yellow squares), and other phenomena (red triangles). Source: A. C. Johnston, “An earthquake strength scale for the media and the public” in Earthquakes and Volcanoes 22 (no. 5): 214–16. US Geological Survey. of large aftershocks is greatest in the three days following the mainshock. Sometimes a big earthquake is followed by an even bigger earthquake, and then the first earthquake is reclassified as a foreshock. MAGNITUDE, FAULT-RUPTURE LENGTH, AND SEISMIC-WAVE FREQUENCIES Fault-rupture length greatly influences earthquake magnitude. As approximations, these fault-rupture lengths yield the following earthquake magnitudes: ∙∙ 100 m (328 ft) rupture ≈ magnitude 4 ∙∙ 1 km (0.62 mi) rupture ≈ magnitude 5 64    Chapter 3   Earthquake Geology and Seismology A rupture along a fault during an earthquake typically moves 2 to 4 km/sec. A lengthier rupture gives a lengthier duration of movement (table 3.4). Fault-rupture lengths and durations in seconds also affect the frequencies of seismic waves produced during earthquakes. Faults that move for short distances and short amounts of time generate mostly high-frequency seismic waves. Faults that rupture for longer distances and longer times produce increasingly greater amounts of low-­ frequency seismic waves. Seismic waves die off with distance traveled. High-­ frequency seismic waves die out first—at shorter distances from the hypocenter. Low-frequency seismic waves carry significant amounts of energy farther—through longer distances. High-frequency seismic waves cause much damage at short distances from the epicenter. But at longer distances, it is the low-frequency seismic waves that do most of the damage. Ground Motion During Earthquakes Seismic waves radiate outward from a fault movement. The interactions among the various seismic waves move the ground both vertically and horizontally. Buildings usually are designed to handle the large vertical forces caused by the weight of the building and its contents. They are designed with such large factors of safety that the additional vertical forces imparted by earthquakes are typically In Greater Depth F = ma Newton’s second law of motion explains that force (F) is equal to mass (m) times acceleration (a). When a force is applied to a mass, it produces a proportional acceleration. Mass is measured in kilograms (kg). Acceleration is in meters per second per second (m/s/s). Force is measured in newtons (1 newton = 1 kg m/s2). A force of 1 newton will give a 1-kg mass an acceleration of 1 meter per second per second. The sudden slip on a fault is a localized Figure 3.27 This inadequately braced house failed due to horizontal acceleration during the 1971 San Fernando earthquake. Courtesy Rosemary Boost. Photo by Al Boost. not a problem. Usually, the biggest concern in designing buildings to withstand large earthquakes is the sideways push from the horizontal components of movement (figure 3.27). ACCELERATION Building design in earthquake areas must account for ­acceleration. As seismic waves move the ground and buildings up and down, and back and forth, the rate of change of velocity is measured as acceleration. As an analogy, when your car is moving at a velocity of 25 mph on a smooth road, you feel no force on your body. But if you stomp on the car’s accelerator and rapidly speed up to 55 mph, you feel a force pushing you back against the car’s seat. Following the same thought, if you hit the brakes and decelerate rapidly, you feel yourself being thrown forward. This same type of accelerative force is imparted to buildings when the ground beneath them moves during an earthquake. source of energy that creates forces on rock, which result in accelerations that shake us. For an analogy, hold your arm upright in front of you and wave it back and forth. You create rapid acceleration and high velocity, but no damage is done because the mass of your arm is small and the inertial forces are low. However, if a large mass, such as a building weighing thousands of tons, is subjected to the same acceleration, the motion produces large inertial forces that are difficult for the building to withstand. If these forces last long enough, the building may fail. The usual measure of acceleration is that of a free-falling body pulled by gravity; it is the same for all objects, regardless of their weight. The acceleration due to gravity is 9.8 m/sec2 (32 ft/sec2), which is referred to as 1.0 g and is used as a comparative unit of measure. Weak buildings begin to suffer damage at horizontal accelerations of about 0.1 g. At accelerations between 0.1 and 0.2 g, people have trouble keeping their footing, similar to being in the corridor of a fast-moving train or on a small boat in high seas. A problem for building designers is that earthquake accelerations have locally been in excess of 1 g. For example, in the hills above Tarzana, California, the 1994 Northridge earthquake generated phenomenal accelerations—1.2 g vertically and 1.8 g horizontally. PERIODS OF BUILDINGS AND RESPONSES OF FOUNDATIONS The concepts of period and frequency also apply to buildings. Visualize the shaking or vibration of a 1-story house and a 30-story office building. Do they take the same amount of time to complete one cycle of movement, to shake back and forth one time? No. Typical periods of swaying for buildings are about 0.1 second per story of height. The 1-story house shakes back and forth quickly at about 0.1 second per cycle. The 30-story building sways much slower, with a period of about 3 seconds per cycle. The periods of buildings are also affected by their construction materials. A building of a given height and design will have a longer period if it is made of flexible materials such as wood or steel; its period will be shorter if it is built with stiff materials such as brick or concrete. The velocity of a seismic wave depends on the type of rock the wave is traveling through. Seismic waves move faster through hard rocks and slower through softer rocks and loose sediments. Seismic waves are modified by the rocks they pass through; they become distorted. When seismic waves pass from harder rocks into softer rocks, they slow down and thus must increase their amplitude to carry the same amount of energy. Shaking tends to Ground Motion During Earthquakes     65 In Greater Depth What to Do Before and During an Earthquake BEFORE We have seen that earthquakes don’t kill us—it is our own buildings and belongings that fall during the shaking and harm us. What should you do to be prepared for an earthquake? First, walk into each room of your house, assume that strong shaking has begun, and carefully visualize (virtual reality) what might fall—for example, ceiling fan, chandelier, mirror, china cabinet, gas water heater. Now reduce the risk. Nail them. Brace them. Tie them. Velcro them. Lower them. Remove them. Second, walk outside, assume strong shaking, and visualize what might fall—for example, trees, power lines, brick chimney. Now reduce the risk. Trim them. Chop them. Replace them. be stronger at sites with softer sediments because seismic waves move more slowly but with greater amplitude (figure 3.28). When seismic waves of a certain period carry a lot of energy and their period matches the period of a building, the shaking is amplified and resonance can occur. The resonance created by shared periods for seismic waves and buildings is a common cause of the catastrophic failure of buildings during earthquakes. Understanding the concept of shared periods and resonance may be advanced by visualizing a tall flagpole with a heavy metal eagle on top. First, if you shake this pole, you will quickly learn that the pole has a strong tendency to move back and forth only at a certain rate or period. If the flagpole swings a complete cycle in 2 seconds, it has a period of 2 seconds. Second, if seismic waves of a 2-second period begin to shake the ground, the amount of movement of the flagpole starts to increase. The pole is now resonating, the forces it must withstand have increased, and the greater forces created by the combined periods may cause destruction. Seismic-wave travel In hard rocks Higher velocity Lower amplitude In soft sediments Lower velocity Higher amplitude Figure 3.28 Seismic wave velocity and amplitude are modified by the types of rock they pass through. 66    Chapter 3   Earthquake Geology and Seismology Third, repeat the visits inside and outside your home. This time, locate safe spots where protection exists—for example, under a heavy table, beneath a strong desk, under a bed. Remember these safe spots so you can occupy them quickly when shaking begins. Drop, cover, and hold on. DURING After preparing your home, program yourself to stay composed during the shaking. Remember that the severe shaking probably will last only 5 to 60 seconds. So, be calm and protect yourself for 1 minute. In most places, if you are inside, you should stay inside; if you are outside, stay outside. This advice was underscored in the San Simeon earthquake in California on 22 December 2003. Both fatalities occurred to women running out the door of a 19th-century building; they were killed by debris falling off the building front. The people who stayed inside the building were unharmed. Earthquake Intensity—What We Feel During an Earthquake During the tens of seconds that a large earthquake lasts, we feel ourselves rocked up and down and shaken from side to side. It is an emotional experience, and the drama of our personal accounts varies according to our location during the shaking and our personalities. But for personal narratives to have meaning that can be passed on to succeeding generations, common threads are needed to bind the accounts together. In the late 1800s, descriptive schemes appeared that were based on the intensity of effects experienced by people and buildings. The most widely used scale came from the Italian professor Giuseppi Mercalli in 1902; it was modified by Charles Richter in 1956. The Mercalli Intensity Scale has 12 divisions of increasing intensity labeled by Roman numerals (table 3.5). Earthquake magnitude scales are used to assess the energy released during an earthquake; earthquake intensity scales assess the effects on people and buildings (table 3.6). The difference between magnitude and intensity can be illuminated by comparison to a lightbulb. The wattage of a lightbulb is analogous to the magnitude of an earthquake. Wattage is a measure of the power of a lightbulb, and magnitude is a measure of the energy released during an earthquake. A lightbulb shining in the corner of a room provides high-intensity light nearby, but the intensity of light decreases toward the far side of the room. The intensity of shaking caused by a fault movement is great near the epicenter, but in general, it decreases with distance from the epicenter. (This generalization is offset to varying degrees by variations in geologic foundations and building styles.) TABLE 3.5 Modified Mercalli Scale of Earthquake Intensity or poorly built buildings, adobe houses, and old walls. Numerous windows and some chimneys break. Small landslides and caving of sand and gravel banks occur. Waves appear on ponds; water becomes turbid. VIII. Fright is general and alarm approaches panic. Disturbs drivers of automobiles. Heavy furniture overturns. Damage slight in specially designed structures; considerable in ordinary substantial buildings, including partial collapses. Frame houses move off foundations if not bolted down. Most walls, chimneys, towers, and monuments fall. Spring flow and well-water levels change. Cracks appear in wet ground and on slopes. IX. General panic. Damage considerable in masonry structures, even those built to withstand earthquakes. Wellbuilt frame houses thrown out of plumb. Ground cracks conspicuously. Underground pipes break. In soft sediment areas, sand and mud are ejected from ground in fountains and leave craters. X. Most masonry structures are destroyed. Some well-built wooden structures and bridges fail. Ground cracks badly with serious damage to dams and embankments. Large landslides occur on river banks and steep slopes. Railroad tracks bend slightly. XI. Few, if any, masonry structures remain standing. Great damage to dams and embankments, commonly over great distances. Supporting piers of large bridges fail. Broad fissures, earth slumps, and slips occur in soft and wet ground. Underground pipelines completely out of service. Railroad tracks bend greatly. XII. Damage nearly total. Ground surfaces seen to move in waves. Lines of sight and level distort. Objects thrown up in air. I. Not felt except by a very few people under especially favorable circumstances. II. Felt by only a few people at rest, especially those on upper floors of buildings or those with a very sensitive nature. Delicately suspended objects may swing. III. Felt quite noticeably indoors, especially on upper floors, but many people do not recognize it as an earthquake. Vibrations are like those from the passing of light trucks. Standing automobiles may rock slightly. Duration of shaking may be estimated. IV. Felt indoors during the day by many people, outdoors by few. Light sleepers may be awakened. Vibrations are like those from a passing heavy truck or a heavy object striking a building. Standing automobiles rock. Windows, dishes, and doors rattle; glassware and crockery clink and clash. In the upper range of IV, wooden walls and frames creak. V. Felt indoors by nearly everyone, outdoors by many or most. Awakens many. Frightens many; some run outdoors. Some broken dishes, glassware, and windows. Minor cracking of plaster. Moves small objects, spills liquids, rings small bells, and sways tall objects. Pendulum clocks misbehave. VI. Felt by all; many frightened and run outdoors. Excitement is general. Dishes, glassware, and windows break in considerable quantities. Knickknacks, books, and pictures fall. Furniture moves or overturns. Weak plaster walls and some brick walls crack. Damage is slight. VII. Frightens all; difficult to stand. Noticed by drivers of automobiles. Large bells ring. Damage negligible in buildings of good design and construction, slight to moderate in wellbuilt ordinary buildings, considerable in badly designed TABLE 3.6 Comparison of Magnitude, Intensity, and Acceleration Magnitude Mercalli Intensity Acceleration (% g) 2 and less I–II Usually not felt by people Less than 0.1–0.19 3 III Felt indoors by some people 0.2–0.49 4 IV–V Felt by most people 0.5–1.9 5 VI–VII Felt by all; building damage 2–9.9 6 VII–VIII People scared; moderate damage 10–19.9 7 IX–X Major damage 20–99.9 8 and up XI–XII Damage nearly total More than 100 = more than 1 g Earthquake Intensity—What We Feel During an Earthquake    67 Mercalli intensities also are crucial for assessing magnitudes of historical events before there were instrumented records, thus allowing us to assess recurrence intervals between major earthquakes. TABLE 3.7 Magnitude versus Duration of Shaking Richter Magnitude Duration of Strong Ground Shaking in Seconds MERCALLI SCALE VARIABLES 8–8.9 30 to 180 The Mercalli intensity value at a given location for an ­earthquake depends on several variables: (1) earthquake magnitude; (2) distance from the hypocenter/epicenter; (3) type of rock or sediment making up the ground surface; (4) building style—design, kind of building materials, height; and (5) duration of the shaking. These factors must be considered in assessing the earthquake threat to any region and even to each specific building. 7–7.9 20 to 130 6–6.9 10 to 30 5–5.9 2 to 15 4–4.9 0 to 5 1. Earthquake Magnitude: The relation between magnitude and intensity is obvious—the bigger the earthquake (the more energy released), the higher the odds are for death and damage. 2. Distance from Hypocenter/Epicenter: The relation between distance and damage also seems obvious; the closer to the hypocenter/epicenter, the greater the damage. But this is not always the case, as will be seen in chapter 4 with the 1989 World Series (Loma Prieta) and 1985 Mexico City earthquakes. 3. Foundation Materials: The types of rock or sediment foundation are important. For example, hard rock foundations can vibrate at high frequencies and be excited by energetic P and S waves near an epicenter; the shaking of soft or water-saturated sediments can be amplified by surface (L) waves from distant earthquakes; and steep slopes often fail as landslides when severely shaken. 4. Building Style: Building style is of vital importance. What causes the deaths during earthquakes? Not the shaking of the earth, but the buildings, bridges, and other structures that collapse and fall on us. Earthquakes don’t kill; buildings do. Buildings have frequencies of vibration in the same ranges as seismic waves. The vibrations of high-frequency P and S waves are amplified by (1) rigid construction materials, such as brick or stone, and (2) short buildings. If this type of building is near the epicenter, beware! The movements of low-frequency surface waves are increased in tall buildings with low frequencies of vibration. If these tall buildings also lie on soft, watersaturated sand or mud and are distant from the epicenter, disaster may strike. 5. Duration of the Shaking: The duration of the shaking is underappreciated as a significant factor in damages suffered and lives lost. Consider the ranges of shaking times in table 3.7. For example, if a magnitude 7 earthquake shakes vigorously for 50 seconds, rather than 20, the increase in damages and lives lost can be enormous. 68    Chapter 3   Earthquake Geology and Seismology A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971 The San Fernando Valley (Sylmar) earthquake of 9 February 1971 occurred within the northwestern part of the Los Angeles megalopolis at 6:01 a.m., causing 67 deaths (including nine heart attacks). One of the most critical factors in determining life loss from earthquakes is the time of day of the event. In California, the best time for an earthquake for most people is when they are at home; their typical one- and two-story woodframe houses are usually the safest buildings to occupy. 1. Earthquake Magnitude: The magnitude was 6.6, with 35 aftershocks of magnitude 4.0 or higher occurring in the first 7 minutes after the main shock. This is a lot of energy to release within an urban area. 2. Distance from Epicenter: The distance from the epicenter was a fairly consistent variable in this event. A rather regular bull’s-eye pattern resulted from contouring the damages reported in Mercalli numerals (figure 3.29). 3. Foundation Materials: The types of foundation materials were not a major factor in this event. 4. Building Style: Poorly designed buildings, bridges, and dams were the major problem. Three people died at the Olive View Hospital with the collapse of its “soft” first story featuring large plate-glass windows. “Soft” first-story buildings support the heavy weight of upper floors without adequate shear walls or braced frames to withstand horizontal accelerations (figure 3.30a). Many of these buildings still exist, despite their known high odds of failure during earthquakes (figure 3.30b). Another hospital failure was responsible for 47 deaths. Some of the pre-1933 buildings at the Veterans Administration Hospital used hollow, clay-tile bricks to build walls designed to carry only a vertical load. Many of the hollow-core clay bricks shattered under horizontal accelerations that measured up to 1.25 times gravity. 0 50 100 mi 0 80 160 km Yosemite Nat'l Park Nevada Limit of felt area Fresno Las Vegas California Parkfield Bakersfield I–IV V t Pa cif ic n Sa Oc ea n a aB ara rb VI VII VIII–XI ngeles sA Lo Santa Catalina Island San Fernando Pasadena Santa Ana Big Bear City Palm Springs Salton City VI V Freeway bridges collapsed and took three lives. A freeway bridge is a heavy horizontal mass (roadbed) suspended high atop vertical columns. Swaying of these top-heavy masses, which have poor connections between their horizontal and vertical elements, resulted in collapse as support columns moved out from under elevated roadbeds. The lessons learned from these 1971 failures had not been acted upon by 1989, when the Interstate 880 elevated roadway collapsed, killing 42 people in Oakland during the World Series earthquake. Failure happened again in Los Angeles in 1994 during the Northridge earthquake (figure 3.31). 5. Duration of Shaking: The strong ground shaking lasted 12 seconds. Earthquakes in the magnitude 6 range typically shake from 10 to 30 seconds (see table 3.7). The significance of the relatively short time of strong shaking in the San Fernando Valley earthquake is enormous. The Lower Van Norman Reservoir held San Diego I–IV Mexico Figure 3.29 Contour map of Mercalli intensities from the San Fernando earthquake of 9 February 1971 shows an overall decrease in intensity away from the epicenter. (a) (b) Figure 3.30 Buildings with “soft” first stories. (a) Bracing is inadequate on the first floor, and there are no shear walls to transmit seismic loads to the ground. Thus, seismic stresses are concentrated at the join between the first and second floors. When the ground accelerates to the right, the building lags behind and the first story flattens. (b) This eight-story medical-office building atop a “soft” first story is located in a California city near active faults. Sources: (a) “Improving Seismic Safety of New Buildings,” 1986, Federal Emergency Management Agency; (b) Photo by Pat Abbott A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971     69 (a) Figure 3.31 This freeway collapsed in Los Angeles during the 1994 Northridge earthquake. Vertical supports and horizontal roadbeds move at different periods. If not bound together securely, they separate and fall when shaken. Before 1971 earthquake Source: NOAA/NGDC, Mehmet Celebi, USGS 11,000 acre-feet of water at the time of the quake. Its dam was begun in 1912 as a hydraulic-fill structure where sediment and water were poured into a frame to create a large mass; this is not the way to build a strong dam. During the earthquake, the dam began failing by landsliding and had lost 30 ft of its height (800,000 cubic yards of its mass) and stood only 4 ft above the water level when the shaking stopped (figure 3.32). If the strong shaking had lasted another 5 seconds, the dam would have failed and released the water onto a 12-square-mile area below the dam where 80,000 people were at home. LEARNING FROM THE PAST The 1971 San Fernando Valley earthquake unequivocally demonstrated the hazard in this region. It has been eloquently stated that “past is prologue” and that “those who do not learn the lessons of history are doomed to 70    Chapter 3   Earthquake Geology and Seismology Dam crest Water level Earthen dam Alluvium Bedrock After 1971 earthquake Water level Thin dirt wall (b) Figure 3.32 Failure of the Lower Van Norman Dam. (a) A few more seconds of strong shaking would have unleashed the deadly force of 11,000 acre-feet of water on San Fernando Valley residents below the dam. (b) Landsliding lowered the dam by 30 feet. Source: (a) E.V. Leyendecker/USGS/NOAA repeat them.” How well were the lessons of 1971 learned? Another test was painfully administered on 17 January 1994, when the magnitude 6.7 Northridge earthquake struck the immediately adjacent area. This time, 57 people died and damages escalated to $30 billion. The same types of buildings again failed, and freeway bridges again fell down. Not all the lessons from 1971 were learned. Building in Earthquake Country One of the problems in designing buildings for earthquake country is the need to eliminate the occurrence of resonance. This can be done in several ways: (1) Change the height of the building; (2) move most of the weight to the lower floors; (3) change the shape of the building; (4) change the type of building materials; and (5) change the degree of attachment of the building to its foundation. For example, if the earth foundation is hard rock that efficiently transmits short-period (high-frequency) vibrations, then build a flexible, taller building. Or if the earth foundation is a thick mass of soft sediment with long-period shaking (low frequency), then build a stiffer, shorter building. For building materials, wood is flexible and lightweight, has small mass, and is able to handle large accelerations. Concrete has great compressional strength but suffers brittle failure all too easily under tensional stress. Steel has ductility and great tensional strength, but steel columns fail under compressive stress. Ground motion during an earthquake is horizontal, vertical, and diagonal—all at the same time. The building components that must handle ground motion are basic. In the horizontal plane are floors and roofs. In the vertical plane are walls and frames. An important component in building resistance is how securely the floors and roofs are tied or fastened to the walls so they do not separate and fail. SHEAR WALLS AND BRACING Walls designed to take horizontal forces from floors and roofs and transmit them to the ground are called shear walls. In a building, shear walls must be strong themselves, as well as securely connected to each other and to roofs and floors. In a simple building, seismic energy moves the ground, producing inertial forces that move the roofs and floors. These movements are resisted by the shear walls, and the forces are transmitted back to the ground. Even a “house of cards” is a shear-wall structure, although each “wall” does not have much strength. The walls must be at right angles and preferably in a simple pattern (figure 3.33). The house of cards is made enormously stronger if horizontal and vertical elements are all securely ­fastened—for example, by taping them together. A structure commonly built with insufficient shear walls is the multistory parking garage. Builders do not want the added expense of more walls, which eliminate parking spaces and block the view of traffic inside the structure. Stronger Figure 3.33 A “house of cards” is a structure with walls and floors but no strength. Earthquake resistance is greatly increased by tying the walls and floors together with tape. Source: “Improving Seismic Safety of New Buildings: 1986, Federal Emergency Management Agency These buildings are common casualties during earthquakes (figure 3.34). Bracing is another way to impart seismic resistance to a structure. Bracing gives strength to a building and offers resistance to the up, down, and sideways movements of the ground (figure 3.35). The bracing should be made of ductile materials that have the ability to deform without rupturing. Figure 3.34 This three-story parking structure for automobiles at the Northridge Fashion Center collapsed during the 17 January 1994 earthquake. Source: E.V. Leyendecker/USGS/NOAA Building in Earthquake Country    71 (a) Brace it. (d) Buttress it. (b) Infill it. (c) Frame it. (e) Isolate it. Figure 3.36 How to strengthen buildings. (a) Add braces. (b) Infill walls. (c) Add frames to exterior or interior. (d) Add buttresses. (e) Isolate building from the ground. Figure 3.35 A six-story building with a braced frame Source: After AIA/ACSA Council on Architectural Research. Photo by Pat Abbott Highway bridges and elevated roadways commonly collapse during major earthquakes. Part of the problem comes from the different frequencies of movement of vertical supports and horizontal roadbeds, but part comes from the behaviors of different construction materials. Bridge builders combine steel (for its ductility) with concrete (for its strength). During the 1994 Northridge earthquake, ­support-column failures occurred as concrete cracked and steel deformed (figure 3.37a). The rebuilding process incorporated in its design. RETROFIT BUILDINGS, BRIDGES, AND HOUSE CONSTRUCTION The process of reinforcing existing buildings to increase their resistance to seismic shaking is known as retrofitting. Figure 3.36 shows how some common designs in building retrofits give seismic strength to a building. (a) (a) (b) Figure 3.37 Support columns on Freeway 118 in Simi Valley, California. (a) Problem: This column failed during the 1994 earthquake when (b) brittle concrete cracked and ductile steel rebar buckled. (b) Solution: New columns have vertical steel rebar wrapped by circular rebar, and both are encased in concrete. In addition, columns are confined by bolted steel jackets that will be encased in concrete. ©Sandra L. Jewett. Photo by Peter W. Weigand, CSU Northridge 72    Chapter 3   Earthquake Geology and Seismology Double top Blocking Plywood panel shear wall o ag Di l na br ac e Studs Metal brackets Foundation Bolts (a) (b) Figure 3.38 How can a house be built to resist seismic waves? (a) Bolt it. Bracket it. Brace it. Block it. Panel it. (b) Change in building code for California houses. House built in 1971 had 2-inch-thick, 4-inch-wide, wood base bolted into concrete slab (see lower right) with a 3/8-inchdiameter bolt (yellow arrow). Decorative remodel at house in 2017 triggered seismic retrofit additions. (1) Hole drilled through wood base into concrete slab has 5/8-inch-diameter, 1-foot-long bolt inserted and held in place with strong epoxy (red arrow). (2) Heavy galvanized-iron “hold down” is secured with a huge bolt into the base and numerous bolts into the vertical wood supports (red arrow). Now horizontal and vertical elements of house are securely fastened together. (b) Photo by Pat Abbott employs additional alternating layers of concrete and steel to avoid future failures (figure 3.37b). Modern one- and two-story woodframe houses perform well during seismic shaking. Houses must be able to move up, down, and sideways without failing. The ability to withstand earth movements is given by building shear walls and by using bracing and other elements that tie the walls, foundation, and roof together (figure 3.38). For retrofitting, older houses must have these same resisting elements added to the foundation walls that hold the house above the ground. Additionally, much of the damage, injury, and even death during an earthquake occurs inside homes as personal items are thrown about— items such as unsecured water heaters, ceiling fans, cabinets, bookshelves, and electronic equipment. Bolt down or secure with Velcro your personal items so they don’t become airborne missiles inside your home during an earthquake. earthquakes can knock them down. For an example, see the failure of the massive support column in the 6.9 MW event in Kobe, Japan (figure 3.39). If buildings cannot stand up against the most powerful seismic waves, then we need to learn to roll with them. Modern designs employ BASE ISOLATION When the earth shakes, the energy is transferred to buildings. How can buildings be saved from this destructive energy? One approach is to build structures so huge and strong that an earthquake cannot knock them down. But Figure 3.39 Despite their huge size, the stiff and massive beams (note car for scale) supporting the elevated expressway in Kobe, Japan, failed in the 17 January 1995 earthquake of 6.9 Mw. Source: Dr. Richard Hutchison/NOAA Building in Earthquake Country    73 Lead core Rubber layers Steel layers (b) Figure 3.40 (a) The Office of Disaster Preparedness in San Diego County is housed in a two-story, 7,000 ft2 building sitting on top of 20 lead-impregnated rubber supports (base isolators) that each weigh 1 ton. (b) An example of a base isolator. Cutaway view into a 1 m wide by 1 m tall sandwich shows alternating layers of rubber (each 15 mm thick) and steel (each 3 mm thick) with a central core of lead. During an earthquake, the rubber and steel flex and the lead absorbs energy. (a) base ­isolation whereby devices are placed on the ground or within the structure to absorb part of the earthquake energy. For example, visualize yourself standing on Rollerblades during an earthquake. Would you move as much as the earth? Base isolation uses wheels, ball bearings, shock absorbers, “rubber doughnuts,” rubber and steel sandwiches, and other creative designs to isolate a building from the worst of the ground shaking (figure 3.40). Photo by Pat Abbott The goal is to make the building react to shaking much like your body adjusts to accelerations and decelerations when you are standing in a moving train or bus. This concept has recently been used in building San Francisco’s new airport terminal. The 115-million-pound building rests on 267 stainless steel sliders that rest in big concave dishes. When the earth shakes, the terminal will roll up to 20 inches in any direction. Summary Earthquakes are shaking ground caused most often by sudden movements along cracks in the Earth called faults. Some major faults acting for millions of years have offset rock layers by hundreds of kilometers. Sedimentary rock layers originally are continuous, horizontal, and in superpositional order (oldest on bottom, youngest on top); however, fault movements cut rocks into discontinuous masses, and in places, fault deformation has tilted rock layers and even 74    Chapter 3   Earthquake Geology and Seismology overturned the superpositional sequence. Geologists measure the 3-D orientation of rock layers via dip (angle and direction of inclination) and strike (compass bearing of rock cutting a horizontal plane). Dip-slip fault types have dominantly vertical movements. Normal faults are due to extensional (pull-apart) forces. Reverse faults are due to compressional (pushtogether) forces. Strike-slip fault types have dominantly horizontal (shear) offsets. Straddling the fault, if the righthand side moves toward you, it is a right-lateral fault; if the left-hand side moves toward you, it is a left-lateral fault. Bends (steps) in strike-slip faults cause the land to either uplift or downdrop. Another type of fault, called a transform fault, connects offset spreading-center segments. Earthquakes, also called seisms, disperse their energy in seismic waves that radiate away from the hypocenter or point of fault rupture. The point on the surface above the fault rupture is the epicenter. Some seismic waves pass through the body of Earth; these are the P waves (primary waves with a push-pull motion) and the S waves (secondary waves with a shearing motion). Other seismic waves travel along the surface (Love and Rayleigh waves). Earthquake energy is assessed by its magnitude. Different estimates of magnitude are derived from different methods, based on local shaking (Richter scale), body waves (mb), surface waves (Ms), or seismic moment (Mw). Earth has more than a million earthquakes each year, but more than 90% of the energy is released by the 12 to 18 largest events. Seismic waves have different periods (time between cycles) and frequencies (number of cycles per second): Period = 1 frequency P waves commonly have from 1 to 20 cycles per s­ econd; surface waves commonly have 1 cycle every 1 to 20 seconds. Where the frequencies of seismic waves match the vibration frequencies of foundations and buildings, destruction may be great. Earthquake effects on structures and people are assessed via the Mercalli Intensity Scale. Its variables are earthquake magnitude, distance from the hypocenter/epicenter, type of rock or sediment foundation, building style, and duration of shaking. Mercalli intensities are of more than just scientific interest because earthquakes don’t kill; buildings do. Building components that must stand up to seismic shaking are horizontal (floors, roofs) and vertical (walls, frames). But horizontal and vertical components move at different frequencies. For buildings to stand up to earthquakes, the horizontal and vertical components must be securely tied together using bolts, brackets, braces, and such. New designs of large buildings utilize energy-­ absorbing base isolation devices placed between the building and the ground. Terms to Remember acceleration 65 aftershock 63 amplitude 56 base isolation 74 body waves 57 compression 51 cross-section 51 dip 51 dip-slip fault 51 fault 49, 53 footwall 51 foreshock 63 fracture 49 frequency 57 friction 49 granite 57 hangingwall 51 hertz (Hz) 57 hypocenter 53 inertia 56 joint 51 law of original continuity 50 law of original horizontality 49 law of superposition 49 left-lateral fault 53 magnitude 61 mainshock 63 map 51 normal fault 52 period 57 permeability 51 primary (P) wave 57 resonance 66 retrofit 72 reverse fault 52 right-lateral fault 53 secondary (S) wave 57 seism 48 seismic moment 63 seismic wave 57 seismogram 60 seismograph 56 seismology 56 seismometer 56 shear 52 slip 61 stress 49 strike 51 strike-slip fault 52 surface waves 57 tension 51 transform fault 54 wavelength 56 Questions for Review Ans. In figure 3.25, the earthquake magnitude is close to 5. 1. Draw a cross-section of a sequence of sedimentary rock layers. Label and explain the laws of original horizontality, superposition, and original continuity. 2. Draw cross-sections of a normal fault and a reverse fault. What are the differing forces that determine which one forms? Which one involves tension? Compression? 3. Draw a map of a left-stepping, right-lateral fault. Explain what happens to the land at the step (bend) in the fault. 4. Draw a cross-section showing an inclined fault with a hypocenter at 15 km (9 mi) depth. Does the epicenter plot on the surface trace of the fault? 5. Sketch a map of a strike-slip and a transform fault. Explain their similarities and differences. 6. What do P and S seismic waves tell us about the nature of Earth’s interior? 7. How can arrival times of P and S waves be used to determine distance to the epicenter? 8. How are foreshocks distinguished from aftershocks? 9. What are typical P wave velocities in hard rock? Water? Air? 10. What are typical S wave velocities in hard rock? 11. How damaging to buildings are P waves? S waves? Rayleigh waves? 12. What is the frequency of a seismic wave with a period of 1 second? 1╱4 second? 1╱10 second? 13. What are typical frequencies for 1-story buildings? 10-story? 30-story? 14. Will a tall building be affected more by high- or lowfrequency seismic waves? Why? 15. Is resonance more likely for a 20-story building when shaken by P waves or Rayleigh waves? 16. Building designers must account for acceleration. What does this statement mean? Questions for Review    75 17. What are the differences between earthquake magnitude and earthquake intensity? 18. List five main variables affecting Mercalli intensities. 19. How does the Richter magnitude scale for earthquakes differ from moment magnitude? 20. Explain how base isolation systems can reduce the shaking of buildings during an earthquake. 21. How do recorded P and S wave patterns differ from a bomb blast versus an earthquake? Questions for Further Thought 1. Immediately after the start of a big earthquake, how can the greater velocity of P waves be utilized to provide some protection for hospitals, computer systems, and trains? 2. What is the quake potential of the Moon (moonquakes)? Does the Moon have similar numbers and magnitudes of quakes as Earth? Why? 3. If you are in an airplane over the epicenter of a great earthquake, what will you experience? 4. How earthquake safe is your home or office? What are the nearest faults? What kind of earth materials is your home 76    Chapter 3   Earthquake Geology and Seismology or office built upon? How will your building size, shape, and materials react to shaking? What nearby features could affect your home? What hazards exist inside your home? 5. Make a list of the similarities between snapping your fingers and the movement of a fault. Disaster Simulation Game Your challenge is to protect a city from earthquake disaster by constructing new buildings and retrofitting old ones. You are given a budget. Then you have real choices to make. The city you must protect has a specified population of people. You are provided with a map of the town and charged with protecting as many people, buildings, and livelihoods as possible. You must build a hospital and two schools plus retrofit 10 old buildings. Are you ready for the challenge? Go to http://www .stopdisastersgame.org. Click on Play Game. On the next page, click on Play Game again. Select the Earthquake scenario; then choose your preferred difficulty level: Easy (small map); Medium (medium-size map); or Large (large map). Good luck! Save as many people as you can. Criteria Excellent 58 - 60 points Timeliness Submits one initial response early in the session, and one or more thoughtful peer responses in the middle of the session, and one or more peer responses closer to the end of the session. (At least 3 responses per week) Posted early and continued to Timeliness makes contributions throughout each week. Quantity of Post Posted more than three times Posts each week. Post(s) attempt to engage the students and motivate the group discussion in a respectful manner. Post(s) also elicit responses and reflections from other learners and responses build upon and integrate multiple views from other learners to take the discussion deeper. Post(s) offer support for arguments, and take into consideration the ideas already offered by others. Post(s) help others feel safe about participating. Quality of Posts All original posts were directly related to the question, were thoughtful, and included references to the course readings. All response posts engaged classmates in further dialogue on the topic. Help others feel safe about participating; Show curiosity and willingness to experiment; Make or raise issues that are relevant to the current focus of the class; Offer support for arguments; and Take into consideration the ideas already offered by others. Post(s) display an excellent understanding of the required readings and underlying concepts including correct use of terminology. Postings integrate an only INTERNAL resource COURSE MATERIAL AND VIDEO(Chemistry of Explosive Volcanoes! ) Provided to support important points and extend the learning of the group. Overall Score Level 10 58 and Above 
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