GS ENVS 103 CH 3 Earthquake Geology and Seismology

Most of Rodinia was concentrated in the

Southern Hemisphere, but the majority of Pangaea was wrapped around the equator like a planetary belt. North America formed the core of Pangaea, called Laurentia, which dominated the South Polar Region[vi] .

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).

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 3.34). The bracing should be made of ductile materials that have the ability to deform without rupturing.

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.

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.

Weak buildings begin to suffer damage at horizontal accelerations of about

0.1 g. At accelerations between 0.1 to 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. 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.

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 wood frame 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. 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 3.28). 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. 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.

strike-slip fault

A type of fault in which rocks on either side move past each other sideways with little up or down motion. produces shear and causes most of the movement along a fault to be horizontal (parallel to the strike direction)

reverse fault

A type of fault where the hanging wall slides upward 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; The compressional motions of reverse faults are commonly found at areas of plate convergence where subduction or continental collision occurs.

Both a trombone and an earthquake have more higher-frequency waves if a shorter path is traveled

As the travel paths become longer for both trombone (extended) and earthquake (longer fault rupture), the number of low-frequency waves increases. 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.

The Lisbon Earthquake of 1755 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 worshipers. 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. 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). 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.

How can a house be built to resist seismic waves?

Bolt it. Bracket it. Brace it. Block it. Panel it.

The Lower Van Norman Reservoir held 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 3.31). 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.

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.

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 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.

Similarly, figure 3.14a depicts a right step along a right-lateral 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.

seismographs 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).

Machines that measure the times at which seismic waves arrive at different distances from an earthquake 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.

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 of P and S waves is determined by subtracting the P arrival time from the S time (S-P).

In Otago province in the southern part of the South Island, gold was discovered in 1861 in stream gravels.

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 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.

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. Because not all seismometers will be sitting 100 km from the epicenter, corrections are made for distance. 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).

Love Waves Love waves were recognized and first explained by the British mathematician A. E. H. Love. Their motion is similar to that of

S waves, except it is from side-to-side in a horizontal plane roughly parallel to Earth's surface. As with S waves, their shearing motion is at right angles to the 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.

TRANSFORM FAULTS Transform faults are a special type of horizontal-movement fault first recognized by the Canadian geologist J. Tuzo Wilson in 1965.

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 subducts. But what happens along the sides of the plates? They slide past each other at transform faults. In fact, transform faults must link spreading centers or connect spreading centers with subduction zones. However, passing both to the right and left of the spreading centers, notice that the two slabs are moving in the same direction and there they are called fracture zones there is no active offset across a fracture zone.

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.

Faults are not simple planar surfaces. Instead, faults are complex zones of breakage where rough and interlocking rocks are held together over an irregular surface that extends many miles below the ground.

Stress must build up over many years before enough potential energy is stored to allow a rupture on a fault. The initial break occurs at a weak point on the fault and then propagates rapidly along the fault surface. The point where the fault first ruptures is known as the hypocenter, or focus. The point on Earth's surface directly above the hypocenter is called the epicenter (figure 3.12).

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.

The bends along a fault have profound implications for the creation of topography. figure 3.13a is a sketch of a right-lateral fault with a bend (step) in it—a left-stepping bend.

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.

retrofitting

The process of reinforcing existing buildings to increase their resistance to seismic shaking 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.

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 Zealand.

The west (left) side has been moved 480 km (300 mi) toward the north.

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.

Rayleigh Waves Rayleigh waves were predicted to exist by Lord Rayleigh 20 years before they were actually recognized. Surface waves travel only through solid media.

They advance in a backward-rotating, elliptical motion (figure 3.18c) similar to the orbiting paths of water molecules in wind-blown 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 often heard 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.

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 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).

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. Surface waves travel only through solid media.

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 ruptured and moved horizontally as much as 6.5 m (20 ft) in 60 seconds. The great burst of energy generated by the fault movement was actually the release of elastic energy that had built up and been stored in the rocks for many decades.

Primary Waves (P-Waves) (body)

Travel the fastest through rock material by causing particles in the rock to move back and forth , or vibrate, in the same direction as the waves are moving. 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.

A fault is 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 sound waves. Both a fault rupture in the earth and your finger snap feature the same sudden slips that release energy in waves.

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.

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.

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.

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.

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.

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 be stronger at sites with softer sediments because seismic waves move more slowly but with greater amplitude.

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.

Law of Original Horizontality

layers of sediment are originally deposited horizontally under the action of gravity This is important because some older sedimentary rock layers are found at angles ranging from horizontal to vertical.

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.

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 ≈ 4 ∙ 1 km (0.62 mi) rupture ≈ 5 ∙ 10 km (6.2 mi) rupture ≈ 6 ∙ 40 km (25 mi) rupture ≈ 7 ∙ 400 km (250 mi) rupture ≈ 8 ∙ 1,000 km (620 mi) rupture ≈ 9 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).

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).

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.

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. These buildings are common casualties during earthquakes (figure 3.33).

Time is standardized in the United States by the

national clock in Boulder, Colorado.

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.

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.

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. 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.

Shear walls and cross bracing

provide strength and stiffness to resist future earthquakes designed to take horizontal forces from floors and roofs and transmit them to the ground must be strong themselves, as well as securely connected to each other and to roofs and floors.

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.

Extensional or normal-style faults are typical of the faults at

seafloor spreading centers and in regions of continents being pulled apart

body waves

seismic waves that travel through the Earth's interior 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.

Secondary (S) waves (body)

shaking motion at right angles; travels only through solids; slower velocity than P waves; slightly greater amplitude than P waves 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. 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.

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.

law of superposition

states that younger layers of rock are deposited on top of older layers 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).

The word earthquake is effectively a self-defining term—

the Earth quakes, the Earth shakes, and we feel the vibrations.

Earthquake Intensity— What We Feel During an Earthquake 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). Modified Mercalli Scale of Earthquake Intensity 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.

Moment Magnitude Scale Seismologists have moved on to other measures to more accurately determine earthquake size. The seismic moment (Mo) relies on

the amount of movement along the fault that generated the earthquake; that is, Mo 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: 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.

Strike-slip faults are further classified on the basis of the relative movement directions of the fault blocks. If you straddle a fault and

the block on your right-hand side has moved relatively toward you, then it is called a right-lateral, or dextral, fault (figure 3.11). Notice that this convention for naming the fault works no matter which way you are straddling the 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.

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.

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.

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.

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.

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.

Seismology

the study of earthquakes and seismic waves 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.

Hertz (Hz)

the unit of frequency, equal to one cycle per second. Note that period and frequency are inversely related: 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.

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.

The 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.

Epicenters can be located using seismograms from

three recording stations. If the distance from each station is plotted as the radius of a circle, the three circles will intersect at one unique point— 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.

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.

At any one location, the felt shaking in earthquakes above magnitude 6 does not increase 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 for a longer time will experience the intense shaking. A longer duration of shaking can greatly increase the amount of damage to buildings.

Seismic Waves

vibrations that travel through Earth carrying the energy released during an earthquake that pass through the whole body of the planet (body waves) and others that move near the surface only (surface waves).

FAULTS AND GEOLOGIC MAPPING The 19th-century recognition that fault movements cause earthquakes was a

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.

OTHER MEASURES OF EARTHQUAKE SIZE 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).

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.

The Supercontinents The prehistoric continents in the geologic past were all one massive landmass called "supercontinents", but did you know that there have been more than one?

• Vaalbara • Kenorland • Columbia • Rodinia • Pangaea

All these waves have the following similarities:

• amplitude, the height of the wave above the starting point (figure 3.17); • wavelength, the distance between successive waves; • period, the time between waves measured in seconds; and • frequency, the number of waves passing a given point during 1 second.

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.

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.

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 surface-wave scale (Ms) uses Rayleigh waves with 18- to 22-second periods. Thus, for great and major earthquakes, bodywave 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.

Dip-Slip faults are...

• normal and • reverse 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. This terminology is used to define the types of faults dominated by vertical movements

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.

MERCALLI SCALE VARIABLES 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.

In 2010, the worst flooding seen in

1000 years inundated the planet. Today, the remote Pacific island of Kiribati is slipping into the ocean, and evacuation plans have begun. We are entering a period of extreme volcanism.

Notice the distinctive "pyramidal" distribution of 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.

During the time between the disappearance of Rodinia and the reappearance of Pangaea, over

90 percent of all species on the Earth went extinct - a loss of life greater than the extinction of dinosaurs, 65 million years ago. The Earth was a very different world after this extinction; the planet's crust was much cooler, the atmosphere much thicker, there were fewer earthquakes and volcanoes, and the global climate patterns had dramatically changed. After the disappearance of Rodinia and before the reappearance of Pangaea, the Earth's angle changed its position facing the Sun, and this started a new life cycle that had never appeared before in Earth history. The Earth became a living paradise on land. Unit 3 Lecture - Unit 3 The Supercontinents The prehistoric continents in the geologic past were all one massive landmass called "supercontinents", but did you know that there have been more than one? Vaalbara The earliest known supercontinent was Vaalbara. Vaalbara is theorized to have formed about 3,600 million years ago (3.1 billion years ago [3.1 GA]). The basic structure of Vaalbara consisted of eastern South African rocks that match with the same rocks found in the northwest section of Western Australia. South Africa and Western Australia are two of the best-preserved existing continents on Earth today, and both have remarkably similar characteristics, beginning in the early Precambrian Period, 4.6 billion years ago[i] . Both South Africa and Western Australia have evidence of four large meteorite impacts from 3.2 to 3.5 billion years ago. The high temperatures created by the impacts' forces fused the continental sediments into small glassy spheres, which are the oldest-known terrestrial impact remains found on the Earth[ii] . Kenorland The supercontinent Kenorland was formed around 2.7 billion years ago. Kenorland formed what we know as today's North America, Greenland, Scandinavia, Western Australia, and the Baltic regions. Columbia The supercontinent Columbia formed around 2.0-1.8 billion years ago and broke apart about 1.5-1.3 billion years ago[iii]. Columbia is estimated to have only been about 12,900 kilometers (8,000 miles) from North to South, and about 4,800 km (3,000 miles) across its broadest stretch. Fossil records show that the east coast of India was attached to western North America during this time, and southern Australia was up against western Canada. Most of South America was positioned where the western edge of modern-day Brazil lined up with eastern North America, extending to the southern edge of Scandinavia. The Amazon region in South America first appeared on Columbia[iv]. Rodinia The supercontinent Rodinia formed about 1.1 billion years ago and broke up roughly 750 million years ago. This was a supercontinent that contained most of our present-day landmasses, but geologic records show these continents were located in an upside down world. North America, Alaska, and Scandinavia were located in the Southern Hemisphere close to the South Pole. Australia was located in the Northern Hemisphere where Europe is today, and Antarctica was at the equator. Rodinia, a Russian word meaning "homeland", was formed from the fragments of the breakup of the older supercontinent, Columbia. At the end of Rodinia's existence, the supercontinent broke into smaller continents just as Columbia, Kenorland, and Vaalbara had broken apart. Little is known about the history of Rodinia, other than it was very barren. It existed before life colonized on dry land, and it predated the formation of the ozone layer. With no ozone to protect the surface of the Earth, the land was exposed to high levels of ultraviolet sunlight. This prevented the survival of life on the continents, but marine life thrived[v] . Instability on the planet increased around 700 million years ago at a time when the Earth was heating up. The atmosphere thickened and the first atmospheric layers formed. The heat and humidity stimulated primitive life, and also triggered the breaking apart of Rodinia. Some 200 million years later, the broken pieces of Rodinia again reconnected as Pangaea. Most of Rodinia was concentrated in the Southern Hemisphere, but the majority of Pangaea was wrapped around the equator like a planetary belt. North America formed the core of Pangaea, called Laurentia, which dominated the South Polar Region[vi] . The southeastern United States was wedged between Africa and South America at the equator. Today's southern continents formed the largest landmass, called Gondwana. Siberia was located just south of the equator between Gondwana and Laurentia, and Scandinavia, Europe, European Russia, and much of what is today's Asia were in fragments along the north coast of Gondwana. But these subcontinents were neither north nor south. There was little to no ice on the Earth at this time, and none of the continents were spinning on frozen poles. Temperatures remained warm and mild. In fact, the global climate was warmer than it is today, and the seas covered most of the Earth. Pangaea Something was very different about Pangaea. This supercontinent appeared after a massive Earth shift, and it experienced more of the Earth's "growing pains" then the previous supercontinents that existed when the Earth was in its infantile stages. This time, the Earth shook with a much stronger and more violent force than ever before, and it was during this planetary change that the largest extinction on Earth took place. During the time between the disappearance of Rodinia and the reappearance of Pangaea, over 90 percent of all species on the Earth went extinct - a loss of life greater than the extinction of dinosaurs, 65 million years ago. The Earth was a very different world after this extinction; the planet's crust was much cooler, the atmosphere much thicker, there were fewer earthquakes and volcanoes, and the global climate patterns had dramatically changed. After the disappearance of Rodinia and before the reappearance of Pangaea, the Earth's angle changed its position facing the Sun, and this started a new life cycle that had never appeared before in Earth history. The Earth became a living paradise on land. Life climbed out of the oceans and onto the ground, and fresh water pooled on the land. The continents rose in elevation, and the air was cool at the higher altitudes. After 200 million years, Pangaea split into the northern and southern continents of Laurasia and Gondwana. After 200 million years, Pangaea split into the northern and southern continents of Laurasia and Gondwana.

Today's southern continents formed the largest landmass, called

Gondwana. Siberia was located just south of the equator between Gondwana and Laurentia, and Scandinavia, Europe, European Russia, and much of what is today's Asia were in fragments along the north coast of Gondwana. But these subcontinents were neither north nor south. There was little to no ice on the Earth at this time, and none of the continents were spinning on frozen poles. Temperatures remained warm and mild. In fact, the global climate was warmer than it is today, and the seas covered most of the Earth.

• Vaalbara

The earliest known supercontinent was Vaalbara. Vaalbara is theorized to have formed about 3,600 million years ago (3.1 billion years ago [3.1 GA]). Both South Africa and Western Australia have evidence of four large meteorite impacts from 3.2 to 3.5 billion years ago. The high temperatures created by the impacts' forces fused the continental sediments into small glassy spheres, which are the oldest-known terrestrial impact remains found on the Earth

Columbia

The supercontinent Columbia formed around 2.0-1.8 billion years ago and broke apart about 1.5-1.3 billion years ago[iii]. Fossil records show that the east coast of India was attached to western North America during this time, and southern Australia was up against western Canada. The Amazon region in South America first appeared on Columbia[iv].

Kenorland

The supercontinent Kenorland was formed around 2.7 billion years ago. Kenorland formed what we know as today's North America, Greenland, Scandinavia, Western Australia, and the Baltic regions.

Rodinia

The supercontinent Rodinia formed about 1.1 billion years ago and broke up roughly 750 million years ago. This was a supercontinent that contained most of our present-day landmasses, but geologic records show these continents were located in an upside down world. Rodinia, a Russian word meaning "homeland", was formed from the fragments of the breakup of the older supercontinent, Columbia Little is known about the history of Rodinia, other than it was very barren. It existed before life colonized on dry land, and it predated the formation of the ozone layer. With no ozone to protect the surface of the Earth, the land was exposed to high levels of ultraviolet sunlight. This prevented the survival of life on the continents, but marine life thrived[v] .

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.

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 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.

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.

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 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 3.39). 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. 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.

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, water saturated sand or mud and are distant from the epicenter, disaster may strike.

Strike is viewed in the 2-D horizontal view (map) as the

compass bearing of the rock layer where it pierces a horizontal plane.

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.

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.

seismometers

devices that measure the amount of ground motion caused by an earthquake; also called seismographs 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.

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.

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.

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). 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.

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).

Tornado cluster outbreaks are

increasing in intensity and ferocity. Our marine environments are heating up as countless aquatic animals wash dead upon the shore.

Pangaea Something was very different about Pangaea. This supercontinent appeared after a

massive Earth shift, and it experienced more of the Earth's "growing pains" then the previous supercontinents that existed when the Earth was in its infantile stages. This time, the Earth shook with a much stronger and more violent force than ever before, and it was during this planetary change that the largest extinction on Earth took place.

Instability on the planet increased around 700 million years ago at a time when the Earth was heating up. The atmosphere thickened and the first atmospheric layers formed. The heat and humidity stimulated

primitive life, and also triggered the breaking apart of Rodinia. Some 200 million years later, the broken pieces of Rodinia again reconnected as Pangaea.

Greater Tokyo, home to 35 million tightly packed people, has experienced a

three-fold increase in earthquake activity since the March 11, 2011 magnitude 9.0 Japan earthquake unleashed a killer tsunami.

The Earth's magnetic field is

weakening and moving towards a polarity reversal as we began the solar maximum in 2012 - 2013; today, we are now in the middle of the solar minimum. The planet's climate is becoming increasingly inhospitable.