Geology 1

Can seismologists predict the next great earthquake, the proverbial "big one?

" The answer depends on the time frame of the prediction. With their present understanding of the distribution of seismic belts and the frequencies at which earthquakes occur, seismolo- gists can make long-term predictions (on a time scale of decades to centuries). For example, with some certainty, they can say that a major earthquake will probably rattle California during the next 100 years and that a major earthquake probably won't strike central Canada during the next 10 years. But despite extensive research, seismologists cannot make short-term predictions (on a time scale of ours to weeks or even years). They can-not say, for example, that an earthquake will happen in Montreal next month. ut new technologies may permit warnings to be sent seconds in advance of the arrival of seismic waves if an earthquake does happen.

The San Francisco earthquake of 1906 serves as an exam-ple of a continental transform-fault earthquake

(Fig. 10.20a) .In the wake of the gold rush, San Francisco was a booming city with broad streets and numerous large buildings. But it was built on the transform boundary along which the Pacific Plate moves north at an average rate of 6 cm per year relative to North America. Because of the stick-slip behavior of the fault, this movement tends to happen in sudden jerks, each of which causes an earthquake. At 5:12 a.m. on April 18, the fault near San Francisco slipped by as much as 7 m, and minutes later seismic waves struck the city. Witnesses watched in horror as the streets undulated, buildings swayed and banged together, laundry lines stretched and snapped, and weaker buildings collapsed. Fire followed soon after, consuming huge areas of the city (Fig. 10.20b) . Judging from the damage, seis-mologists estimate that the earthquake had an M W of 7.9.

Different kinds of earthquake waves cause different kinds of ground motion

(Fig. 10.26) . For example, P-waves are almost perpendicular to the ground surface when they arrive and cause the ground to buck up and down. S-waves also reach the surface at a steep angle, but these waves are more complicated and tend to cause back-and-forth motion parallel to the ground surface. L-waves, the first surface waves to arrive, cause a snake-like side-to-side undulation. R-waves, the last waves to arrive, result in a rolling motion as particles near the surface of the ground follow elliptical paths, in cross section.Interference among the different kinds of waves causes motion to be anything but regular.

what happens during an earthquake?

(a) Before deformation, rock layers in this example are not bent. (b) Before an earthquake, rock bends elastically, like a stick that you arch between your hands. The drawing exaggerates the amount of bending. (c) Eventually, the rock breaks, and sliding suddenly occurs on a fault. This break generates vibrations. You feel such vibrations when you break a stick.

Seismologists refer to studies leading to predictions as seismic-risk assessment

, or seismic-hazard assessment.

And in February 2011, sudden rupture on a fault near Christchurch, New Zealand

, toppled buildings and the walls of a stalwart stone cathedral.

Earthquakes do not take place everywhere on the globe

. By plotting the dis-tribution of earthquake epicenters on a map, seis-mologists find that most,but not all, earthquakes occur in elongate seismic belts, or localized seismic zones

The 1964 Good Friday earthquake in Alaska caused liq- uefaction beneath the Turnagain Heights neighborhood of Anchorage

. Contractors had built the neighborhood on a small terrace of uplifted sediment. A 20-m-high escarp- ment that dropped down to Cook Inlet, a bay of the Pacific Ocean, formed the edge of the terrace. Here, as the ground shaking began, a layer of wet clay beneath the neighborhood liquefied. In this case, liquefaction allowed the overlying terrace, along with the houses built on top of it, to slide seaward. In the process, the terrace broke into separate blocks that tilted, turning the landscape into a chaotic jum-ble (Fig. 10.30) . Liquefaction beneath Turnagain Heights happened because in wet clay, the clay flakes stick together.The flakes stick together because there is surface tension in the water between the flakes, caused by the attraction of the water molecules to one another. When still, wet clay can behave like a solid gel, but when shaken, the weak bonds between water molecules break, and the clay transforms into a viscous liquid. Clay that displays this behavior is called thixotropic clay, or quick clay.

Regardless of their cause, tsunamis differ greatly from familiar, wind-driven storm waves (Fig. 10.34)

. Large wind-driven waves can reach heights of 10 to 30 m in the open ocean or as they crash on beaches. But even such monster shave wavelengths (the distance between adjacent wave crests) of only tens of meters, so they contain a relatively small volume of water. In contrast, although a tsunami in deep water may cause a rise in the sea surface of, at most, only a few tens of centimeters—a ship crossing one wouldn't even notice it tsunamis have wavelengths of tens to hundreds of kilometers,and an individual crest can be several kilometers wide, as measured perpendicular to the wave front. Such waves contain an immense volume of water. In simpler terms, we can think of the width of a tsunami, in map view, as being more than 100 times the width of a wind-driven wave. Because of this difference, a storm wave and a tsunami have very different effectswhen they strike the shore

Seismologists have detected intraplate earthquakes on all continents

. Most are concentrated in specific seismic zones.

In contrast to other types of plate boundaries, where only shallow earthquakes occur, convergent boundaries also host intermediate and deep earthquakes

. These earthquakes occur in the downgoing slab as it sinks into the mantle, defining a sloping band of seismicity called a Wadati-Benioff zone, after the seismologists who first recognized it ( Fig. 10.21a ; see also Fig. 4.11c). Intermediate and deep earthquakes happen in response to stresses caused partly by shear between the downgoing plate and the mantle and partly by the pull of the sinking, deeper part of the plate on the shallower par

Geologists refer to such a wave, caused by the sudden displacement of a large volume of water, as a tsunami

. This Japanese word translates literally as harbor wave, an apt name because tsunamis can be particularly damaging to harbor towns. Though we most often hear of tsunamis generated by displacement due to an earthquake, they can also be triggered by submarine landslides (see Chapter 16) or by explosive eruption of island volcanoes (see Chapter 9). In older literature, tsunamis were called "tidal waves" because when one arrives on shore, as we will see, water rises as if a huge tide were coming in. But the waves have nothing to do with the Earth's daily tidal cycles, so the name is misleading. Though in popular media the word tsunami tends to be associated with giant or very high waves, in fact tsunamis come in all sizes— some may be just centimeters high and are barely noticeable, whereas others reach colossal heights, causing the sea surface to rise by as much as 30 m near the shore.

First, we define the point

. within the Earth at which rock starts to rupture and slip on a fault as the ypocenter, or focus, of an earthquake (Fig. 10.3a). Simplistically, we can picture the focus as the point on the fault fromwhich earthquake energy, in the form of vibrations, begins to propagate. (In reality, however, the area of a fault that slips uring an earthquake extends beyond the focus, so not all of the energy produce

Fortunately, most cause no damage or casualties, either because they are too small or they occur in underpopulated areas.

.But a few hundred earthquakes per year rattle the ground sufficiently to crack or topple buildings and injure their occupants, and every 5 to 20 years, on average, a great earthquake triggers a horrific calamity.

The intensity of an earthquake at a given location depends on four factors

: (1) the magnitude of the earthquake, because larger-magnitude events release more energy; (2) the dis-tance from the focus, because vibrations lose their energy as they pass through the Earth; (3) the nature of the substrate at the location (i.e., the character and thickness of materials beneath the ground surface), because earthquake waves tend to be amplified in weaker substrates; and (4) the frequency of the earthquake waves (where frequency equals the number of oscillations that pass a point in a specified interval of time)—high-frequency vibrations are not as dangerous as low-frequency vibrations because the former don't cause buildings to sway as much.

Geologists refer to the tsunami that struck Banda Aceh as a near-field tsunami, or local tsunami, because of its close proximity to the earthquake.

A far-field tsunami, or distant tsunami, has crossed an entire ocean. A far-field tsunami struck Sri Lanka and the eastern coast of India two hours after the 2004 earthquake, and then the coast of Africa, on the west side of the Indian Ocean, eight hours after the earthquake (Fig. 10.36) . Wherever it struck, coastal towns vanished, fishing fleets sank, and beach resorts collapsed into rubble By the end of that horrible day, more than 230,000 people had died.

But the catastrophe was not over.

A portion of the wave struck the Fukushima nuclear power plant. Although the plant had withstood the ground shaking of the earthquake and had automatically shut down, its radioactive reactor cores still needed to be cooled by water in order to remain safe.The tsunami not only destroyed power lines, cutting the plant off from the electrical grid, but also drowned the backup diesel generators, so the cooling pumps, now without power, stopped functioning. Eventually, the water surrounding the reactors cores boiled away. Some of the superheated water separated into hydrogen and oxygen gas, which then exploded, thereby breaching the integrity of three of the four reactor buildings and releasing radioactivity into the environment (Fig. 10.38b) .

The azure waters and palm-fringed islands of the Indian Ocean's eastern coast hide one of the most seismically active plate boundaries on the Earth—the Sunda Trench.

Along this convergent boundary, the Indian Ocean floor subducts at a rate of about 4 cm per year, leading to the accumulation of a large elastic strain during the "stick" phase of a stick-slip cycle. Just before 8:00 a.m. on December 26, 2004, the crust above a 1,300-km-long by 100-km-wide portion of one of these faults slipped and lurched westward by as much as 15 m The rupture started at the focus and then propagated north at 2.8 km per second, so that the rupturing process, overall,took about 9 minutes. This slip triggered a magnitude 9.3 earthquake—a great earthquake—and pushed the seafloor up by tens of centimeters (Fig. 10.32) . The rise of the sea- floor, in turn, shoved up overlying ocean water. Because the area that rose was so broad, it displaced an immense volume of seawater. As a consequence, tragedy of an unimaginable extent began to unfold. Gravity caused the water that had been pushed up to sink and then spread outward in a wave moving at speeds of about 800 km per hour—almost the speed of a jet plane (Fig. 10.33).

The shaking during an earthquake can tip over lamps, stoves, or candles with open flames, and it may break wires or topple power lines, generating sparks.

As a consequence, areas already turned to rubble, and even areas not so badly damaged, may be consumed by fire. Ruptured gas pipelines and oil tanks feed the flames, sending columns of fire erupting skyward (Fig. 10.31a) .Firefighters may not even be able to reach the fires if the doors to the firehouse won't open or rubble blocks the streets. Moreover, firefighters may find themselves without water, for ground shaking and landslides damage water lines.

The largest intraplate earthquakes to affect the United States took place in the early 19th century, near New Madrid, which lies near the Mississippi River in southernmost Missouri.

At the time, the region was inhabited by a small population of Native Americans and an even smaller population of European descent. During the winter of 1811-1812, three M W 7 to 7.4 earthquakes struck the region. The ground motion temporarily reversed the flow of the Mississippi River and toppled cabins (Fig. 10.25a) .The earthquakes resulted from slip on thrust and strike-slip faults that underlie the Mississippi Valley (Fig. 10.25b) . St. Louis, Missouri, and Memphis, Tennessee, lie close to the epicenter, so if large earth-quakes were to happen in the New Madrid region again, they could cause significant damage.

Unlike earlier examples, the tsunami that struck Japan soon after the 2011 M W 9.0 Tōhoku earthquake was capturedin high-definition video that was seen throughout the world, gener- ating a new level of international awareness.

Because the Tōhoku earthquake's epicenter was 130 km offshore, ground shaking on land during the event was not extremely intense. But, since tsunamis travel so fast, the first waves reached the shore only about 10 minutes after the earthquake struck, and there was not much time for residents of coastal towns to escape when the warning sirens went off. The 10-m-high seawalls that fringe the coast were not high enough to stop the advance of the wave that, in places, built to a height of 30 m when it reached shore. The rising sea picked up boats and ships in the harbor and flung them over the seawalls and, in some cases, onto the roofs of buildings. It crossed the beach at a speed of 30 km per hour, and once on land, it smashed through houses and tumbled cars as if they were peb- bles. As the churning wave picked up dirt and debris, it became a viscous slurry resembling a volcanic lahar, moving with such force that nothing could withstand its impact (see Fig. 10.2c). When the wave finally ran out of water, it receded to the sea, carrying debris and victims with it and leaving behind a wasteland (Fig. 10.38a)

If you're out in an open field during an earthquake, ground motion alone won't kill you, for your body flexes and bends.

Buildings and bridges aren't so lucky (Fig. 10.27) . When earthquake waves pass, they sway twist back and forth, or lurch up and down, depending on the type of wave motion. As a result, connectors between the frame and facade of a building may separate, so that the facade crashes to the ground. The flex- ing of walls shatters windows and wallboard and makes roofs collapse. Building floors or bridge decks may shift sideways and tip support columns over, or rise up and slam down on the support columns, thereby crushing them. Some buildings collapse with their floors piled on top of one another like pancakes in a stack, w

The 2004 Indian Ocean event remains etched in people's minds because of the immense death toll and the nonstop news coverage.

But it is not unique. Tsunamis generated by the magnitude 9.5 Chilean earthquake in 1960 destroyed coastal towns in South America and crossed the Pacific, causing a 10.7-m-high wall of water to strike Hawaii 15 hours later. Twenty-one hours after the earthquake, when the tsunami reached Japan, it flattened coastal villages and left 50,000 people homeless. A tsunami following the 1964 Good Friday earthquake in Alaska destroyed ports at Valdez and Kodiak (Fig. 10.37) . And a devastating tsunami struck Japan in 2011, as we have seen

At first glance, a fault may look simply like a break that cuts across rock or sediment.

But on closer examination, you may be able to see evidence of the sliding that occurred on a fault. For example, the rock adjacent to the fault may be broken up into angular fragments or may be pulverized into tiny grains, due to the crushing and grinding that can accompany slip, and the surface of a fault may be polished and grooved as if scratched by a rasp.

beneath the edge of Japan and sinks back into the mantle. Averaged over time, the relative movement across this boundary takes place at a rate of about 8 cm per yea

But the motion doesn't take place smoothly. Rather, for a while,rocks adjacent to the boundary quietly bend and distort to accommodate the motion, until suddenly, like a stick that snaps after you bend it too far, the rocks break and a large amount of slip on a fracture takes place in a matter of seconds to minute

Most earthquakes reflect geologic phenomena independent of human activity.

But the timing of some earthquakes relative to human-caused events suggests that in certain cases, people can indeed influence seismicity. Induced seismicity, meaning seismic activity caused by actions of people, generally occurs in response to changes in groundwater pressure. Simplistically, the pressure of ground water can slightly push apart the opposing surfaces of faults and, by doing so, effectively decrease the friction that resists slip on those faults. So when people increase groundwater pressure by pumping lots of water underground in a region con- taining an active fault, the fault may slip under regional stress conditions that might not have otherwise led to slip. Seismolo-gists observed such a relationship near Denver, Colorado, when engineers pumped wastewater from a military installation down a deep well—as soon as the pumping began, small earthquakes started in the region. A similar phenomenon has happened in Oklahoma, where seismicity increased markedly after 2009. The observed "earthquake swarm" lies near high-volume disposal wells into which wastewater com

To identify a seismic belt (or zone), seismologists produce a map showing the epicenters of earthquakes that have hap- pened during a set period of time (say, 30 years).

Clusters of epicenters define a seismic belt. The basic premise of long- term earthquake prediction can be stated as follows: a region in which there have been many earthquakes in the past will probably experience more earthquakes in the future. Seismic belts, therefore, are regions of greater seismic risk. This doesn't mean that disastrous earthquakes can't happen far from a seis- mic belt—they can and do—but the risk that an earthquake will happen in a given time window is less. Seismologists can produce epicenter maps only with data from only the past 60 years or so, because before that time, they did not have enough seismometers to locate epicenters accurately. Fortunately, geologists can help provide insight into seismic risk by examining landforms for evidence of recent faulting. For example, the presence of a distinct fault scarp in a landscape indicates that faulting has happened so recently that erosion has not yet had time to grind away the evidence (Fig. 10.39)

An example of a collision-zone earthquake took place in April 2015.

Compression resulting from the northward push of the Indian subcontinent into Asia caused an M W 7.8 earthquake in Nepal, a country that encompasses a portion of the Himalayas (Fig. 10.23c) . The event occurred when a 7,000 km 3 portion of a large thrust fault, 15 km below the surface, suddenly slipped by up to 3 m. Ground shaking destroyed whole towns. Tragically, thousands died and many more were injured

n 1964, an M W 7.5 earthquake struck Niigata, Japan. A portion of the city had been built on land underlain by wet sand.

During the ground shaking, foundations of over 15,000 buildings sank into their substrate, causing walls and roofs to crack. Several four-story buildings in a newly built apartment complex tipped over (Fig. 10.29a) . In 2011, an earthquake in Christchurch, New Zealand, not only caused sturdy stone buildings to crumble, but also caused sand to erupt and produce small, cone-shaped mounds, called sand volcanoes or sand blows, on the ground surface (Fig. 10.29b) . The transfer of sand from underground to the surface led to the formation of depressions large enough to swallow cars (

Once the ground shaking and fires have stopped, disease may still threaten lives in an earthquake-damaged region.

Earthquakes destroy housing, leaving victims exposed to the weather;sever water and sewer lines, there by contaminating clean-water supplies and exposing the public to bacteria; and cut transportation lines, preventing food and medicine from reaching the damaged area. The severity of such problems depends on the ability of emergency services to cope

Induced seismicity can also become a danger where people build dams and create large reservoirs in valleys overlying active faults.

Faulting generally breaks up rock, making it more erodible by rivers, so it's no surprise that deep river valleys form over large faults. When a reservoir fills the valley above a fault, water seeps down into the fault and, under the pressure caused by the water column above, might trigger earthquakes.

When making a prediction, we use the word probability because a prediction only gives the likelihood of an event.

For example, a seismologist may say, "The probability of a major earthquake occurring in the next 20 years in this state is 20%."This sentence implies that there's a one-in-five chance that an earthquake will happen during the 20-year period. Urban planners and civil engineers use long-term predictions to create building codes for a region—codes requiring stronger,more expensive buildings make sense for regions with greater seismic risk. Planners may also use predictions to determine whether to build vulnerable structures such as nuclear power plants, hospitals, or dams in potentially seismic areas. Seis- mologists base long-term earthquake predictions on two kinds of information: the identification of seismic belts and the recurrence interval (the average time between successive events) of earthquakes along a given fault.

Fault

Geologists refer to a fracture on which such sliding takes place as a

In fact, during the past two millennia

In fact, during the past two millennia, ground shaking, tsunamis, landslides, fires, and other phenomena caused by earthquakes have killed over 3.5 million people

A large earthquake at a location on the Earth's surface may last from a few seconds to several minutes, not including the after- shocks.

Its duration depends both on how long it took for slip on the earthquake-generating fault to take place, and on the distance between the focus and the surface location. The distance to the focus matters because different seismic waves travel at different velocities, so the difference between their arrival times increases as distance increases

Visitors to Lisbon in the months after the 1755 earthquake attested that an area ravaged by a major earthquake is a heart- breaking sight.

Let's consider the many factors that, sadly, can contribute to earthquake devastation. Understanding them, as we will see, may help prevent such devastation in the future.

These events serve as examples of sediment liquefaction.

Liquefaction in beds of wet sand or silt happens because ground shaking causes the sediment grains to try and settle together. But because water fills the spaces (pores) between grains, water pressure in the pores increases and pushes grains apart, and the wet silt or sand becomes a slurry called quicksand. As the material above the liquefied sediment settles downward, pressure squeezes the sand upward and out onto the ground surface—the result being sand volcanoes, as we've seen. The settling of sedimentary layers down into a liquefied layer can also disrupt bedding and can lead to formation of open fissures of the land surface

What geologic phenomena trigger earthquakes? Why do earthquakes take place where they do? How do they cause damage? Can we predict when earthquakes will happen or even prevent them from happening?

Many of these questions have been addressed by the hard work of seismologists(from the Greek word seismos, for shock or earthquake), geosci-entists who study earthquakes, during the past century. In this chapter, we present some of the answers that they have obtained

Once an earthquake-triggered fire starts to spread, it can become an unstoppable inferno.

Most of the destruction caused by the 1906 San Francisco earthquake resulted from fire. For 3 days, the blaze spread through the city until firefighters contained it by blowing up buildings to form a fire- break. By then, 500 blocks of structures had turned to ash, causing 20 times as much financial loss as the shaking itself. When a large earthquake hit Tokyo in September 1923, fires set by cooking stoves spread quickly through thewood-and- paper buildings, creating an inferno that heated the air above the city. The hot air rose like a balloon, and when cool air rushed in, wind gusts of over 100 mph stoked the blaze, which grew to engulf 120,000 people

What causes intraplate earthquakes?

Most seismologists favor the idea that stress applied to continental lithosphere triggers slip on pre-existing faults in the crust. Many of these faults may have first formed during Precambrian rifting events and represent long-lived weak "scars" in the crust. The source of the stress driving fault reactivation remains controversial. Some seismologists attribute the stress to the push acting on plate boundaries, whereas others suggest that it comes from the shear between the lithosphere and the underlying asthenosphere, or even from the unloading that happens when glaciers on the surface melt.

Earthquakes in southern Mexico, in southern Alaska, in eastern Japan, on the western coast of South America, on the coast of Oregon and Washington, and along island arcs in the western Pacific serve as examples of convergent-boundary earthquakes.

Notable examples include the 1960 M W 9.5 earthquake in Chile, the largest earthquake on record; the 1964M W 9.2 Good Friday earthquake near Anchorage, Alaska; the 1995 M W 6.9 earthquake in Kobe, Japan (Fig. 10.21b) ; the 2004M W 9.3 Sumatra earthquake, which triggered the giant Indian Ocean tsunami that killed 230,000 people; the 2010 M W 8.8 Chilean earthquake; the 2011 M W 9.0 Tōhoku earthquake; and the 2017 M W 7.1 earthquake southern Mexico.

Earthquake-triggered landslides that transport massive amounts of debris into reservoirs, lakes, or bays may cause huge waves.

One of the biggest known examples happened in 1958, when an M W 8.3 earthquake in southeastern Alaska triggered a landslide at the head of Lituya Bay. The splash from the dis- placed water washed the forest off the opposite wall of the bay up to an elevation of 516 m—this wall of water was 25% higher than the Empire State Building

The San Francisco earthquake has not been the only one to strike along the San Andreas and nearby related faults during historic time.

Over a dozen major earthquakes have hap- pened on these faults during the past two centuries, including the 1857 M W 7.7 Tejon Pass earthquake just east of LosAngeles and the 1989 M W 7.1 Loma Prieta earthquake, which occurred 100 km south of San Francisco, but nevertheless shut down a World Series game and caused the collapse of a double-decker freeway in that city (Fig. 10.20c) . Even deadlier earthquakes have happened on other transform faults

When the vibrations produced on March 11 reached Japan, the land surface lurched back and forth and bounced up and down for over a minute.

People became disoriented, panicked, and even seasick—some lost their balance and crouched or fell, and some heard a dull rumbling or thumping. Bottles and plates flew off shelves and crashed to the floor, buildings twisted and swayed, ceilings and facades fell in a shower of debris, dust rose from the ground to create a fog-like mist, power lines stretched and sparked, and weaker buildings collapsed (Fig. 10.2a). In addition, several natural gas tanks and pipes broke, sending flammable vapors into the air—locally, the gas ignited in billows of flame that set fire to damaged buildings.

At what depths do we find earthquake foci

Seismologists distinguish three classes of earthquakes based on depth: shallow earthquakes happen in the top 60 km of the Earth, intermediate earthquakes take place between 60 and 300 km, and deep earth- quakes occur down to a depth of about 660 km. Earthquake foci do not lie below a depth of about 660 km

Convergent-Boundary Seismicity

Such boundaries, where one plate subducts under another, tend to be geologically complicated regions at which several different kinds of earthquakes take place. Specifically, as the downgoing plate begins to subduct, it bends and scrapes along the base of the overriding plate. Large thrust faults define the con- tact between the downgoing and overrid- ing plates, and shear on these faults can produce disastrous, shallow earthquakes. Thrust faults in the ac- cretionary prism may also slip. Bending causes normal faults to develop in the downgoing plate, seaward of the trench. In some cases, shear between the downgoing plate and the over-riding plate also triggers shallow faulting in the overriding plate within and on both sides of the volcanic arc

herefore, two kinds of faults develop at divergent boundaries

T. Along spreading segments, stretching generates normal faults, whereas along transform faults, strike-slip displacement occurs (Fig. 10.19) . All earthquakes alongmid-ocean ridges have foci at depths of less than 25 km, so we classify them as shallow earthquakes. Since most ridges lie out in the ocean, far away from settled areas, mid-ocean ridge earthquakes don't cause damage. Only a few populated localities (such as Iceland) lie astride divergent boundaries.

Continental Rifts

The stretching of continental crust at continental rifts generates normal faults. Active rifts today include the East African Rift (Fig. 10.23a) , the Basin and Range Province (mostly in Nevada, Utah, and Arizona), and the Rio Grande Rift (in New Mexico). In all these places, shallow earthquakes, similar in nature to the earth-quakes at mid-ocean ridges, rattle the landscape. In contrast to seismic belts of mid-ocean ridges, belts in rifts occur on land and can be located under or near populated areas.

The case of the March 11 event, the fault delineates the boundary between Honshu and the Pacific Plate.

The sudden sliding and breaking of rock along a fault produces energy that propagates through the crust in the form of vibrations. In essence, these waves resemble the shock waves that travel through a breaking stick to your hands.

Occasionally, ground motion causes water in lakes, bays, reservoirs, and pools, in some cases far from the epicenter, to slosh back and forth.

The water's rhythmic movement, known as a seiche, can build up waves almost 10 m high and can last for hours. Seiches capsize small boats and flood shoreline homes. And if they occur in reservoirs, seiches may wash over and weaken retaining dams.

Some earthquakes affect the interiors of plates and are not associated with plate boundaries, active rifts, or collision zones (Fig. 10.24) .

These intraplate earthquakes, almost all of which have a focus that lies at a depth of less than 25 km, account for only about 5% of the earthquake energy released in a year. Most intraplate earthquakes happen in continents, and because some can occur under populated areas, large ones have the potential to cause significant damage

It was mid-afternoon of March 11, 2011, and in the many seaside towns along the eastern coast of Honshu, the north- ern island of Japan, fishing fleets unloaded their catch, fac- tories churned out goods, shoppers browsed the stores, and office workers tapped at their computers, unaware that their surroundings would suddenly be changing forever.

This coast s near the convergent-plate boundary at which the Pacific Plate slips beneath the edge of Japan and sinks back into the mantle. Averaged over time, the relative movement across this boundary takes place at a rate of about 8 cm per year.

The shaking of an earthquake can cause ground on steep slopes or ground underlain by weak sediment to give way.

This movement results in a landslide, the tumbling and flow of soil and rock downslope (see Chapter 16). Seismically triggered landslides occur all too frequently along the coast of California, for movement on faults has rapidly uplifted this coastline in the past few million years, resulting in the development of steep cliffs. When earthquakes take place, the cliffs collapse, often carrying expensive homes down to the beach below (Fig. 10.28a, b) . Such events lead to the misperception that "California will someday fall into the sea." Although small portions of the coastline do collapse, the state as a whole remains firmly attached to the continent, despite what Hollywood scriptwriters say.

Why do intermediate and deep earthquakes occur in a Wadati-Benioff zone? Shouldn't the rock of a subducted plate at these depths be too warm and soft to break brittlely?

To answer these questions, seismologists have studied the rate at which a subducting slab warms up as it sinks down through hot asthenosphere. They determined that because rock serves as an insulator, the interior of a downgoing plate actually remains cool enough to fracture seismically down to a depth of about 300 km. To explain deeper earthquakes, seismologists have studied the stability of minerals in the lithosphere. They found that at the extreme pressures developed in deeply subducted lithosphere, certain minerals collapse to form new, denser minerals. As such sudden phase changes (see Chapter 8) take place, the minerals abruptly decrease in volume, perhaps along a fault-like band. This process could generate an earthquake, but the exact process by which this occurs remains the subject of research. Some of a subducted plate may accumulate at 660km, whereas some sinks still deeper. But at depths greater than 660 km, processes that generate earthquakes in a sub- ducted plate can no longer take place

Traditional cultures commonly attributed it to the action of supernatural beasts

Traditional cultures commonly attributed it to the action of supernatural beasts. In Japanese folklore, for example, seismicity happened when a giant catfish, Namazu, which lived in the mud below the surface of the ground, started to thrash about. Similarly,in Indian folklore, seismicity happened when one of eight ele-phants holding up the Earth shook its head.

Collision Zones

Two continents collide when the oceanic lithosphere that once separated them has been completely subducted. Such collisions produce great mountain ranges such as the Alpine-Himalayan chain (Fig. 10.23b) . Although a variety of earthquakes happen in collision zones, the most common earthquakes result from movement on thrust faults

Second, we define the point on the surface of the Earth that lies directly above the focus as the earth-quake's epicenter

We can portray the position of an epicenter as a point on a map

To determine the recurrence interval for large earthquakes

at a location, geologists determine when large earthquakes happened at the location in the past. This type of research, called paleoseismology, uses the historical record to identify earthquakes that occurred during the past several centuries. To determine the ages of prehistoric earthquakes, they rely geologic evidence. For example, a trench cut into sedimentary strata near a fault may reveal layers of sand volcanoes and disrupted bedding in the stratigraphic record. Each such layer, whose age can be determined by using radiocarbon dating of plant fragments, records the time of an earthquake(Fig. 10.40) . By calculating the number of years between successive events and taking the average, seismologists obtain the recurrence interval. As an example, imagine that disrupted layers formed at 260, 820, 1,200, 2,100, and 2,300 years ago. We can say that the recurrence interval between events is about 510 years. Note again that a recurrence interval does not specify the exact number of years between events, only the average number. Since stress builds up over time on a fault, the probability that an earthquake will happen in a given year probably increases as time passes. Deter- mining a recurrence interval allows seismologists to refine seismic-hazard maps representing risk (

In many cases, the societal calamity due to an earthquake is a direct consequence of ground shaking,

because it causes buildings to collapse and crush their inhabitants and sends debris careening down slopes.

The majority of earthquakes happen on faults along plate boundaries

because the relative motion of plates triggers slip on faults.

The sudden displacement of the seafloor off Japan's coast

displaced a massive amount of ocean water and produced immense water waves known as tsunamis. When these reached land, they became a roaring jumble of water and debris that submerged low-lying and as far as 10 km in from the shoreline

Recently

geologists have recognized that in 1700, a huge earthquake, with a magnitude of 8.7-9.2, accompanied slip on the Cascadia subduction zone off the now densely populated coast of Oregon and Washington. An earthquake of similar size today would be disastrous to the region. GPS measurements have detected movement of the crust of the region, indicating the buildup of stress

All transform-fault earthquakes

have a shallow focus, so the larger ones on land can cause immense damage

In North America, for example,

intraplate earthquakes occur in the vicinities of New Madrid, Missouri; Charleston, South Carolina; eastern Tennessee; Montreal, Quebec; and the Adirondack Mountains in New York. An M W 7.3 earthquake occurred near Charleston in 1886, ringing church bells up and down the coast and vibrating buildings as far away as Chicago. In Charleston itself, over 90% of the buildings were damaged, and 60 people died. In 2011, an M W 5.9 earthquake rattled central Virginia, abruptly reminding residents of the eastern United States that the region is not immune to seismicity. The tremor was felt from the Carolinas to New England, and people evacuated buildings in Washington, D.C., where some damage occurred to the Washington Monument and other structures.

Earthquakes that happen away from plate boundaries are called

intraplate earthquakes; the prefix intra- means within(Fig. 10.18) . Eighty percent of the earthquake energy released on the Earth comes from the plate-boundary earthquakes in the seismic belts surrounding the Pacific Ocean

Seismologists have detected intraplate earthquakes

on all continents. Most are concentrated in specific seismic zone

What causes earthquake activity,

or seismicity

The majority of transform faults in the world link

seg- ments of mid-ocean ridges, as we've just discussed. But a few, such as the San Andreas fault of California, the Alpine fault of New Zealand, and the Anatolian fault in Turkey, cut across continental lithosphere.

But in Japan, a country struck by earthquakes fairly frequently, offi- cials have established stringent building cod

that successfully prevented widespread building collapse.Unfortunately, even strong buildings could not resist what happened minutes after the earthquake shock.

In great earthquakes,

the ground's movement can have an amplitude of as much as 1 m at the epi-center, but in moderate earthquakes, motions fall in the range of a few centimeters or less. Ground accelerations caused by moderate earthquakes lie in the range of 10%: Earthquakes to 20% of g (where g represents the acceleration due to gravity), and during a great earthquake,accelerations may approach 1 g, so ground movements can toss you into the air.

At divergent boundaries (mid-ocean ridges),

two oceanic plates form and move apart. Divergent boundaries consist of spreading segments linked by transform faults.

To make matters worse, the tsunamis destroyed power sources used to run the pumps that cooled reactors in the Fukushima nuclear power plant,

ultimately leading to the release of radioactivity into the environment. In the end, the immensity of the total devastation due to the Tōhoku earthquake was almost beyond comprehension.

in the earth

vibrations can race through the crust at an aver- age speed of 11,000 km (7,000 miles) per hour, 10 times the speed of sound in air.

the great Lisbon earthquake happened on All Saints' Day, November 1, 1755,

when a thrust fault suddenly accommo- dated some of the movement along the complex plate boundary between the Eurasian and African plates. It didn't take 10.6 How Do Earthquakes Cause Damage? 349long for seismic waves from the M W 8.5 to 9.0 earthquake to reach Lisbon, 440 km to the northeast. Lisbon, the capital of Portugal, was one of the great port cities and cultural centers of the day. The resulting ground shaking toppled 85% of the city's buildings, and fires set by overturned stoves then con- sumed much of the wreckage. Forty minutes later, a tsunami inundated the coast and washed away Lisbon's harbor. In the end more than 50,000 people lost their lives. Also, the library, which housed all the records of Portuguese exploration as well as countless Renaissance artworks, was destroyed. The event led philosophers such as Voltaire (1694-1778) to question the long-held belief that "this was the best of all possible worlds," and to suggest that bad things happen by chance, so that making a better world requires human effort. This concept set the stage for the Age of Enlightenment.

The majority of earth- quake-related deaths and inju- ries happen

when falling debris or collapsing structures crush people. Aftershocks worsen the problem because they may topple already weakened buildings, trapping rescuers. During earthquakes, roads, rail lines, and pipelines may also buckle and rupture.

At transform boundaries,

where one plate slides past another without the production or consumption of oceanic lithosphere, most faulting results in strike-slip motion.

Earthquakes are a fact of life on planet Earth

—almost 1 million detectable earthquakes happen every year. Most are a consequence of plate movement—they punctuate each step in the growth of mountains, the drift of continents, and the opening and closing of ocean basins

A major earthquakes

—an episode of ground shaking—had occurred. Geologists now refer to this event as the Tōhoku earthquake, named for the eastern province of Honshu

Scientific study instead associates seismicity with any of the following:

• the sudden formation of a new fault • sudden slip on an existing fault • a phase change, when atoms in minerals suddenly rearrange • movement of magma in, or explosion of, a volcano • a giant landslide • a meteorite impact • an underground nuclear-bomb test