Earthquakes & Earth's Interior

Exploring Geology

Second Draft with Art and PhotosSeptember 2006

C H A P T E R

12 Earthquakes and Earth’s Interior

EARTHQUAKES CAN BE DEADLY. Ground shaking during an earthquake can topple buildings, liquefy normally solid ground, and unleash massive ocean waves that totally wipe out coastal cities. A single earthquakes can kill more than 100,000 people. What causes earthquakes, and how do we study them? In this chapter, we explore the important questions about earthquakes and the interior of the Earth.

The world’s strongest earthquake in 40 years struck Indonesia on December 26, 2004. The earthquake occurred beneath the ocean, pushing up a large region of sea floor and displacing sea water as a massive wave, called a tsunami. The tsu-nami spread outward across the Indian Ocean as a low wave, traveling at speeds approaching 800 km/hour (500 miles/hour)! The curved lines show the wave’s position by hour.

What causes earthquakes, and what hap-pens when an earthquake occurs under the sea as opposed to on land?

The tsunami increased in height as it crashed into the coasts of Indonesia, Thailand, Sri Lanka, India, east Africa, and various islands within the ocean. Low coastal areas were inundated by as much as 20 to 30 m of water (65 to 100 ft) in Indonesia and 12 m (40 ft) in Sri Lanka. Cities and villages were totally demol-ished along hundreds of kilometers of coastline, leaving more than 250,000 people dead or missing. The numbers below show casualties by location.

How does a tsunami form, how does it move through the sea, and what deter-mines how destructive it is?

The magnitude 9 earthquake was centered off the western coast of Su-matra, where the Indian plate is being subducted northeastward beneath the Asian plate. The earthquake was caused by thrusting along the plate boundary, and abruptly uplifted the overriding plate, triggering the tsunami. The red line on this map shows the length of the fault that ruptured during the earth-quake. Yellow dots nearby show the loca-tion of related earthquakes (aftershocks).

Where are earthquakes most likely to oc-cur, and what controls how powerful an earthquake will be?

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Topics in this Chapter • What Is an Earthquake? 12.1

• How Does Faulting Cause Earthquakes? 12.2

• Where Do Most Earthquakes Occur? 12.3

• How Do Earthquake Waves Travel Through the Earth? 12.4

• How Do We Determine the Location and Size of Earthquakes? 12.5

• How Do Earthquakes Cause Damage? 12.6

• What Were Some Significant North American Earthquakes? 12.7

• What Were Some Major World Earthquakes? 12.8

• How Does a Tsunami Form and Cause Destruction? 12.9

• How Do We Study Earthquakes In the Field? 12.10

• Can Earthquakes Be Predicted? 12.11

• What Is the Potential for Earthquakes Along the San Andreas Fault? 12.12

• How Do We Explore What Is Below

the Earth’s Surface? 12.13

• What Do Seismic Waves Indicate About Earth’s Interior? 12.14

• How Do We Investigate Deep Processes? 12.15

• Application: What Happened During the Great Alaskan Earthquake of 1964? 12.16

• Investigation: Where Did This Earthquake Occur, and What Damage Might It Cause? 12.17

E A R T H Q U A K E S A N D E A R T H ’ S I N T E R I O R 3

2004 Sumatran Earthquake and Indian Ocean Tsunami

The 2004 Sumatran earthquake struck on the morning of December 26, vio-lently shaking the region and trigger-

ing the massive Indian Ocean tsunami. It ranks as one of the two or three largest earth-quakes ever recorded. The magnitude of the earthquake is variably estimated at 9.0 to 9.3, depending on the approach used in the calcu-lations. Large aftershocks followed the main quake, including one that had a surprisingly large magnitude of 8.7. From the seismic records of the main quake and aftershocks, it is estimated that a fault surface 1,220 km in length slipped by as much as 10 m during the earthquake. The earthquake lasted over 8 minutes, an unusually long duration for an earthquake.

The earthquake occurred at a depth of 30 km and ruptured upward all the way to the sea floor. It pushed up the sea floor several meters, displacing tens of cubic kilometers of seawater that spread out in all directions. The tsunami rose to heights of more than 10 m when it came ashore, and in many places washed inland for more than a kilometer.

As a result of the earthquake, parts of the Andaman Islands, northwest of Sumatra, were changed forever. Coral reefs, which had been beneath the sea, were uplifted above sea level, and a light house that was origi-nally on land now lies surrounded by sea wa-ter one meter deep. The changes to the land seem insignificant compared to the massive loss of life in this event, one of the deadliest disasters in world history.

The satellite images below show Banda Aceh, before and after the tsunami came ashore. The buildings and vegetation on the “before” image (left) were stripped bare by the initial inward rush of the waters onto the land and the sub-sequent retreat of the deluge back to the sea. A slightly higher area to the north was largely untouched, retaining its forest.

What controls which areas along a coast are most at risk to a tsunami?

The destructive power of the tsu-nami is clear from this photograph of Banda Aceh, the regional capitol of Sumatra’s northernmost province. This city of 320,000 people was reduced to rubble, and nearly a third of its inhabitants were killed or are missing. The tsunami inflicted dam-age to low-lying coastlines around the Indian Ocean, including as far away as Somalia, along the eastern coast of Africa.


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What Is an Earthquake?

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How Do We Describe an Earthquake?

AN EARTHQUAKE OCCURS WHEN ENERGY stored in rocks is suddenly released. Most earthquakes are produced by slippage along faults. Similar kinds of energy releases are caused by volcanic eruptions, explosions, and even meteorite impacts. Earthquakes are definite hazard for those living in earthquake-prone areas, but the energy generated by earthquakes is invaluable for studying the Earth’s interior.

When an earthquake occurs, mechanical energy is released, some of which is transmitted through rocks as vibrations called seismic waves. These waves spread out from the site of the disturbance, travel through the interior or along the surface of the Earth, and can be recorded at seismic stations around the world. To begin to describe an earthquake, we want to know where and when it happened.

What Causes Most Earthquakes?

In a normal fault, the rocks above the fault (the hanging wall) move down with respect to rocks below the fault (the footwall). The crust is stretched horizontal-ly, so earthquakes related to normal faults are most common along divergent plate boundaries, both along oceanic spread-ing centers and in continental rifts.

Most earthquakes are generated by movement along faults. When rocks on opposite sides of a fault slip past one another abruptly, the movements generate seismic waves as materials near the fault are pushed, pulled, and sheared. Any type of fault can potentially generate an earthquake when the fault slips.

The world’s largest earthquakes are gen-erated along thrust faults, which are gen-tly dipping varieties of reverse faults. In thrust and reverse faults, the hanging wall moves up with respect to the footwall. Such faults are formed by compressional stresses, such as are associated with sub-duction zones and continental collisions.

In strike-slip faults, the two sides of the fault slip horizontally past each other, and this can generate large earthquakes. The largest strike-slip faults are transform plate boundaries, like the San Andreas fault in California and parts of the seismically dan-gerous Alpine fault, which cuts diagonally across the South Island of New Zealand.

3. Seismic waves, once generated, move out in all directions from the seismic event, as shown by the curved bands radiating out form the hypo-center. These waves can be measured some time later by seismic stations, such as at locations 1 and 2. Seismic stations closer to the hypocenter, such as station 1, will detect it sooner than those farther away, like station 2.

1. The place where the earthquake is generated is called the hypocenter or focus. For most earthquakes, this is at some depth within the Earth, from as shallow as several kilometers to as deep as 700 km in subduction zones. Most earthquakes occur at depths less than 100 km.

2. The epicenter is the point on the surface of the Earth directly above where the earthquake occurred. If the seismic event happens on Earth’s surface, such as from a surface explosion, then the epicenter and hypocenter are the same location.

Normal Faults Reverse and Thrust Faults Strike-Slip Faults


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12.1 E A R T H Q U A K E S A N D E A R T H ’ S I N T E R I O R 2

How can human activity cause earthquakes and other seismic waves? Besides blowing up ex-

plosives in the search for precious min-erals, we build reservoirs to store wa-ter. Reservoirs fill rapidly and load the Earth’s crust, which responds by flexing and by faulting. After Hoover Dam in Ne-vada was filled, hundreds of moderate earthquakes occurred under the reser-voir between 1934-1944. Similarly, very shallow (less than 3 km deep) earth-quakes occur near Monticello Reservoir in South Carolina. In China, there were

fears that the filling of the Three Gorges Dam, the world’s largest hydroelectric project, would trigger earthquakes in this seismically active area.

Humans also caused earthquakes by injecting waste waters underground in a deep well at the Rocky Mountain Ar-senal northwest of Denver. This caused more than a thousand small earthquakes and two magnitude 5 earthquakes that caused minor damage nearby. When the waste injection stopped and some waste was pumped back out of the ground, the number of earthquakes decreased.

Before You Leave This Page Be Able To:

✓ Explain what a hypocenter and epicenter each represent.

✓ Sketch and describe the types of faults that cause earthquakes.

✓ Describe some other ways earthquakes or seismic waves are formed, including volcanoes and ways that humans can cause earthquakes.

Earthquakes Caused By Humans

How Do Volcanoes and Magmas Cause Earthquakes?

Volcanoes generate seismic waves and cause ground shaking through several processes. An explosive volcanic eruption compresses the Earth and causes transmission of energy through waves and vibrations (shown here with yellow lines).

The ~100 km-wide Manicouagan ring lake in Canada is one of the Earth’s largest me-teorite impact sites. The impact occurred about 200 Ma, and would have released nearly 108 megatons of energy, resulting in ground shaking much larger than any recorded in history. This photograph of the impact site was taken from a space shuttle.

Mine blasts and nuclear explosions com-press Earth’s surface, producing a seismic wave with enough energy to be measured on seismic instruments far away. Moni-toring compliance with nuclear test-ban treaties is done in part using a worldwide array of seismic instruments, which record-ed a nuclear bomb exploded by India in 1998. Recorded seismic waves of a blast are distinct from a natural earthquake.

Catastrophic landslides, whether on land or beneath water, cause seismic waves. Lava flows forming new crust on the Big Island of Hawaii can become unstable and suddenly collapse into the ocean. Seismometers at nearby Hawaii Volcanoes National Park often record the seismic waves caused by such landslides.

Landslides

What Are Some Other Causes of Seismic Waves?

Volcanoes add tremendous weight to the crust, which can cause load-ing of the crust, leading to faulting and earthquakes. The fault here has dropped down the volcano relative to its surroundings.

Many volcanoes have steep, unstable slopes and rocks weakened by hot waters and other volcanic-related processes. The flanks of such volcanoes can fall apart catastrophically, shedding landslides that shake the Earth as they travel and come to rest.

As magma moves below the volcano, it can push rocks out of the way, causing a series of small and distinctive earthquakes. In some cases, the magma causes earthquakes as it opens space by inflating Earth’s surface.

Meteorite Impacts Explosions


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How Does Faulting Cause Earthquakes?

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How Do Faults Release Energy?

MOST EARTHQUAKES OCCUR ALONG FAULTS, and it takes huge amounts of energy to move large masses of rocks along a fault. Rocks respond to the strain that builds up across a fault in one of two ways — rocks can either flex and bend, or they can break. This latter response causes earthquakes.

Slippage along faults seldom happens in a smooth, continuous way, but rather in discrete jumps. As rocks are compressed and sheared, they can respond by changing shape slightly, but returning back to normal when the imposed stresses are released — this is called elastic behavior. The mechanism by which a fault slips involves elastic behavior leading up to the actual, sudden faulting event. Follow the process in this example.

Pre-Slip

Stress Increase and Elastic Strain

Slip and Earthquake

Post-Slip

An active strike-slip fault is not obvious on the surface, but has been offsetting a stream bed for thousands of years, causing it to bend to the left. The last fault movement occurred before people settled in the area. The straight section of the stream seemed a perfect place to put a wooden bridge to provide a crossing for a road.

Tectonic forces continues to push the rocks along the fault. This causes the rocks to flex, as shown by a warp in the side of the block, but the stresses are not enough to make the rocks break — yet! The wooden bridge starts looking a little crooked to people.

Finally the stress along the fault builds up enough so that the fault slips and the rocks on opposite sides of the fault move past each other. A large earthquake is gener-ated, causing seismic waves shown as brown circles of energy radiating from the fault. The bridge swivels off its foundations and falls into the stream as the fault moves.

After the earthquake, the stress begins to slowly build up again along the fault. A new steel bridge is installed over the stream and the road is realigned, at least until the next earthquake.

As stress builds in the rocks along the fault, the rocks deform elastically, slightly changing shape without breaking. If the rocks are strong enough and there is sufficient friction along the fault surface, the rocks and fault hold the added stress.

With the stresses relieved, the rocks next to the fault elasti-cally relax, returning to their original, unflexed shape. But the fault has slipped and the two sides have moved past each other, and this nonelas-tic deformation cannot be undone.

Through this sequence, rocks flex elastically before the earthquake, rupture during the earthquake, and then return to their original shape after an earthquake.

The strike-slip fault is present beneath the sur-face, but on the surface is largely covered with loose rocks, sand, and soil. The fault has little expression on the landscape, except for the offset of the stream.


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When a fault slips, it relieves some of the stress on the fault, caus-ing the stress levels to suddenly

drop. Gradually, over time, the stresses re-build until they exceed the strength of the rock or the ability for friction to keep the fault from slipping. An idealized model of how the amount of stress should change over time is shown below.

On this plot, the magnitude of the stress imposed on the fault builds up with time. When the amount of stress equals the strength of the fault, the fault slips, and the stress levels immediately decrease to the original levels. In this manner, the amount of stress on a fault plots in a zigzag pattern on the graph, increasing gradually (sloping line) and then decreasing abruptly (vertical line) during an earthquake. This process, where the fault slips and then al-lows the stress to build back up, is called the earthquake cycle, and is one model for why some faults may consistently produce earthquakes of a similar size.


✓ Describe or sketch how rocks can flex, fault, and then elastically recover.

✓ Describe or sketch how a rupture begins in a small area and grows over time, including how it ruptures the Earth’s surface.

✓ Describe some characteristics of a fault scarp and ruptures.

✓ Describe how stress changes through time along a fault according to the earthquake-cycle model of faulting.

Build Up and Release of Stress

How Do Earthquake Ruptures Grow?

Earthquake Ruptures in the Field

A rupture starts on a small patch below Earth’s surface and begins to expand in all directions along the pre-existing fault plane. Rock break adjacent to the fault, but most slip occurs on the actual fault surface.

As the edge of the rupture migrates outward, it may eventually reach the Earth’s surface, causing a break called a fault scarp. Seen from above, the rupture migrates in both directions, but may grow more in one direction than in the other.

The rupture continues to grow along the fault plane and the fault scarp lengthens. The faulting relieves some of the stress, and rupturing will stop when the remain-ing stresses can no longer overcome friction along the fault surface.

A geology graduate student examines small ruptures that gashed across a grassy field in his thesis area during a 2004 earthquake along the San Andreas fault near Parkfield, California.

The Landers earthquake in 1992 ruptured across the Mojave Desert of California, forming a fault scarp, here cutting granite. The fault had strike-slip movement, but also locally shifted the ground up or down.

The 1959 Hebgen Lake earthquake in southern Montana offset the land surface, forming this six-meter-high fault scarp. The earthquake and fault scarp were gen-erated by slip along a normal fault.

Most earthquakes occur by slip on a fault that already existed, but all parts of the fault do not begin to slip all at once during an earthquake. Instead, the earthquake rupture starts in a small area (the hypocenter) and becomes larger over time.


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Where Do Most Earthquakes Occur?

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Where Do Earthquakes Occur?

MOST EARTHQUAKES OCCUR ALONG PLATE BOUNDARIES or in regions near plate boundaries, but some also strike in the middle of a plate. Different sizes and depths of earthquakes characterize the dif-ferent tectonic settings, with some types of plate boundaries being much more dangerous than others.

As the newly created plate moves off the ridge, it bends and cools. The stresses associated with the bending forms steep faults, which are associ-ated with relatively small earthquakes.

Strike-slip earthquakes occur along the transform faults that link adjacent seg-ments of the spreading center. The typi-cally thin lithosphere keeps earthquakes along such faults from being very large.

Numerous, but quite-small, earthquakes occur due to the intrusion of magma that squeezes into fractures, such as in dikes.

Observe this map to note how earthquakes are distribut-ed, and how this distribution compares to other features, such as edges of continents, mid-ocean ridges, sites of subduction, continental collisions, etc.

Deep- and intermediate-depth earthquakes occur only near subduction zones, where there is a consistent pattern from shallow earthquakes close to the trench to progressively deeper earthquakes away from the trench. This pattern follows, and helps define, the depths of the subducted slab, which is inclined from the shallow to deep earthquakes.

How Are Earthquakes Related to Mid-Ocean Ridges?

Many earthquakes occur along the axis of a mid-ocean ridge, where spreading and slip along normal faults downdrops blocks along the narrow rift.

In mid-ocean ridges, sea-floor spreading forms new oceanic lithosphere. The oceanic crust and the entire lithosphere are very hot and thin, so stress levels, which increase downward in the Earth, never get very high before the rocks get too hot to fracture (they flow instead). As a re-sult, earthquakes along mid-ocean ridges are relatively small and shallow, with hypo-centers less than about 20 km deep.

Most earthquakes occur in narrow belts that coincide with plate boundaries. Mid-ocean ridges, such as this one south of Africa, only have shallow earthquakes.

This map shows the world distribution of earthquakes epicenters, col-ored according to depth. Yellow dots represent shallow earthquakes (0 to 70 km), green dots mark earthquakes with intermediate-depths (70 to 300 km), and red dots indicate earthquakes deeper than 300 km.


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✓ Explain why subduction zones have earthquakes at various depths, whereas mid-ocean ridges have only shallow earthquakes.

✓ Summarize how subduction and continental collisions cause earthquakes, identifying differences between these two settings.

✓ Describe how an earthquakes can occur within a continental plate.

How Are Earthquakes Related to Continental Collisions?

1. As the oceanic plate being subducted moves toward the trench, it is bent and stressed, caus-ing some earthquakes out from the trench.

How Are Earthquakes Generated Within Continents?

2. Larger earthquakes occur in thrust faults formed in the accretionary wedge as material is scraped off the downgoing plate.

3. Large earthquakes at subduction zones happen all along the contact between the subducting plate and the overridding plate. Huge thrust faults, called megathrusts, form here from compression.

4. As it is subducted into the mantle, the oceanic plate continues to produce earth-quakes from shearing along the boundary and downward-pulling forces on the sinking slab. Subduction zones are typically the only place in the world producing earthquakes deeper than 70 km. Below 700 km, the plate is too hot to behave brittlely and fault.

5. Earthquakes can also form from movement of magmas, eruption of volcanoes, and thrust faulting behind the magmatic arc.

During continental collisions, one continental plate underthrusts be-neath another one. Large thrust faults form in the overridding and underthrust plates, causing large but shallow earthquakes.

Thrust faults also form within the continental plates, causing moderately large earthquakes. The immense stresses associated with a collision can reactivate older faults within the interior of the continent and can cause strike-slip and normal faults as entire regions try to escape the wreckage of the collision zone.

Any oceanic plate material that was subducted prior to the collision is detached, so actual subduc-

tion stops, along with any deep earthquakes.

1. Transform faults, like the San Andreas fault, can cut through a continent, moving one piece of crust past another. The strike-slip motion causes earthquakes that mostly are shallow-er than 20-30 km, but can be quite large.

3. Intrusion of magma within the plate, such as from a hot spot, can cause rela-tively small earthquakes as the magma moves and makes space for itself.

Pre-existing faults in the crust can re-adjust and move as the continental plate ages and is subjected to new stresses. These

structures can produce large earth-

quakes, such as those in Missouri in 1811.

2. Continental rifts mostly cause normal-fault earthquakes, whether the rift is a plate boundary or is within a continental plate. Such earth-quakes are typically moderate in size.

How Are Earthquakes Related To Subduction Zones?Subduction zones, where an oceanic plate is underthrust beneath another plate, tend to be squeezed by compression and sheared along the plate boundary. They produce very large earthquakes in several settings.

6. A deep trench marks a subduction zone on the west side of South America.

7. In a side view, subduction-related earth-quakes are shallower to the west (near the trench) and get deeper to the east, record-ing the descent of the oceanic plate.

Large and deadly earthquakes are produced along the plate boundary, or megathrust.


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How Do Earthquake Waves Travel Through the Earth?

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What Kinds of Seismic Waves Can Earthquakes Generate?

EARTHQUAKES GENERATE VIBRATIONS that travel through rocks as physically distinguishable waves, called seismic waves. Geophysicists digitally record and process seismic waves to understand where and how the earthquake occurred. The word seismic comes from the Greek word for earthquake.

It wasn’t long after the development of the first seismometer in 1892 that scientists observed and described different types of seismic waves. Seismic waves that travel inside the Earth are called body waves and those that travel on the surface of the Earth are surface waves. Scientists who study earthquakes are seismologists.

Primary Body Wave Secondary Body Wave9. The secondary or S-wave shears the rock side to side or up and down, but in a direction perpendicular to the direction of travel. The wave below propagates to the right, but material shifts up and down. It could also shift side to side as long as this is perpendicular to the propagation of the wave.

4. When body waves generated during an earthquake reach Earth’s surface, some energy is transformed into new waves that only travel on the surface, that is, surface waves. People naturally can relate more easily to things on the surface of the Earth, than within it, so we begin with surface waves, of which there are two kinds.

6. The second type of surface wave is a horizontal surface wave. In these waves, material vibrates horizontally and shuffles material side to side, perpendicular to the direction in which the wave travels.

Surface Waves

5. The first type of surface wave is a vertical surface wave. This wave is similar to ocean waves, in that mate-rial vibrates up and down. In vertical surface waves, the waves propagate in the direction of the gray arrow, or per-pendicular to the crests of the waves.

3. An earthquake, depicted in the tan block below, generates seismic waves. Most earthquakes occur at depth and so first produce waves that travel through the Earth as body waves.

1. To think about seis-mic waves, we begin with what we mean by a wave, in general. Most waves are a se-ries of repeating crests and troughs.

2. Waves, whether moving through water in the ocean or through rocks in the Earth, can travel, or propa-

gate, for long distances, but the material within the wave barely moves. Sound waves travel through the air and thin apartment walls, but the wall does not move much. Think of a wave as a pulse of energy mov-ing through a nearly stationary material.

7. Body waves travel through the Earth, and come in two main varieties. The primary or P-wave is like a sound wave or an ocean wave — it compresses the rock in the direction it propagates, like a sound wave compresses the air through which it travels.

10. S-waves cannot travel through liquids because liquids have no rigidity (they

cannot be sheared). If an area of the Earth’s interior does not allow S-waves to pass, then it may be molten. S-waves are also slower than P-waves, travelling through rocks at about 3.6 km/sec.

8. P-waves can travel through solids and liquids, because these materi-als can be com-pressed and then released. The P-wave is the fast-est seismic wave and travels through rocks at between 6-14 km/sec.

Shape of Waves


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Seismic waves recorded on seismo-grams are characterized by how much the ground moves (wave

amplitude) and the time it takes for a complete wave to pass by, which is called the period. Both can be measured from a seismogram. Amplitude is criti-cal when estimating the magnitude and damage potential of an earthquake. The period can also be a critical component in assessing potential damage, consider-


✓ Describe the characteristics of P-waves, S-waves, and surface waves.

✓ Sketch or describe how seismic waves are recorded, and the order in which waves arrive at a seismometer.

✓ Describe why earthquake amplitude and period are important consider-ations for designing buildings.

How Are Seismic Waves Recorded?Today, sensitive digital instruments called seismometers are able to precisely detect a wide range of earth-quakes. The recorded seismic data are uploaded to computers that process signals from hundreds of instru-ments registering the earthquake. These computers calculate the hypocenter and magnitude, and produce digital maps showing magnitude of ground shaking.

1. Until the early 1990s, seismic waveforms were mostly represented as curves on a paper seismogram, which is a graphic plot of the recorded earthquakes waves. Seis-mologists developed this plot to better visualize ground shaking of earthquakes. Today, most seismic data is displayed on computer screens, rather than on paper.

4. After an earthquake, P-waves arrive first, marked by the larger squiggles record-ing the ground motion and the time of its arrival (2.5 minutes in this case).

5. The S-wave arrives later and is recorded on the seis-mograph. The delay between the P-wave and S-wave arrivals depends mostly on how far away the earthquake occurred. The longer the distance, the longer the delay.

6. Surface waves arrive last and cause intense ground shaking, as recorded by the seismograph.

Amplitude and Period

2. A large mass is suspended from a wire. It resists mov-ing during ground shaking.

3. The mass hangs from a frame that in turn is attached to the ground. When the ground shakes, the frame does too, but the suspended mass has inertia, and so resists moving. As the ground and frame move under the mass, a pen attached to the mass marks on a roll of

recording paper that slowly rotates. As a result, the pen records

(draws) the ground movement on the paper over time.

6. Seismologists place seismom-eters away from human noise and bury them to re-duce wind noise. Waves (in yellow) can come from any direction.

3. Background noise commonly looks like small, somewhat random squiggles on seismograms.

1. A seismometer detects and records the ground motion during earthquakes.

5, Modern seismic detectors contain 3 seismometers oriented 90° from each other to record three compo-nents of motion (N-S, E-W, and up-down). From these three components, seismologists can better investigate the direction and magnitude of the seismic signal.

How Are Seismic Records Viewed?2. This diagram (seismogram) shows the record of an earthquake as recorded by a seis-mometer. It plots vibrations versus time. On seismographs, time is marked at regular intervals so that the arrival of the first P- and S-wave can be determined.

ing that buildings vibrate when shaken by earthquakes. Every building has a natural period that can match, or reso-nate with, the earthquake wave. Reso-nance can cause intensified shaking and increased damage.

4. This device only records ground movement parallel to

the red arrows, and so records a single direction or component of motion.


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How Do We Determine the Location and Size of Earthquakes?

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How Do We Locate Earthquakes?

EARTHQUAKES OCCUR DAILY AROUND THE WORLD, and a network of seismic instruments records these events. Using the combined seismic data from several instruments, seismologists calculate where the earthquake started and how large it was. The principle measurement of size is called magnitude.

Seismologists maintain thousands of seismic stations that actively sense and record ground motions. When an earthquake occurs, this network can detect it depending on the energy released. Large earthquakes gen-erate seismic waves that can be detected around the world. Smaller earthquakes are detected only locally.

Seismometers in the U.S. National Seismic Network (shown below) represent a fraction of the installed seismometers.

On October 1, 2005, a moderate earthquake is felt in Colorado. Three stations (DUG, WUAZ and ISCO) record wave arrivals and are chosen to locate the epicenter.

5. Based on the S-P arrival intervals, ISCO is the closest station, followed by DUG and WUAZ based on S-P intervals.

2. P-waves travel faster than and arrive before S-waves. The farther away a station is from the earthquake, the longer will be the interval of time between the P-wave and S-wave arrivals.

The S-P interval is mathematically related to the distance between the earthquake epicenter and the seismic station that records the wave. The graphical representation of that relationship, shown below, is called a time-travel curve.

S-P intervals are measured off the seismograms from part 2 and then plotted on this graph, which then gives the distance for each station.

The intersection of the three (or more) circles is the epicenter of the earthquake.

We calculate the depth of the earth-quake’s hypocenter in a similar way, using the interval between the P-wave and another

compressional wave that is formed when the P-wave reflects off the Earth’s surface near the epicenter. Again, we use multiple stations.

The distance from each station to the epicenter is now known, but not the direction.

Station Distance (km)

WUAZ 670

DUG 540

ISCO 65

3. These names are abbreviations of the station locations (ISCO is the station near Idaho Springs, Colorado).

1. Records from at least three stations are normally compared when calculating an earthquake location.

1. Seismometer Network Senses a Quake 2. Select Earthquake Records

3. Estimate Station Distance from Epicenter 4. Triangulate the EpicenterThe distance from each station to the earthquake can be compared graphically to find the epicenter of the earthquake.

4. The three seismo-grams show differences in the span of time between arrival of the P-wave and S-waves, called the S-P Interval.

A circle is drawn around each station with a radius equal to the distance calculated from the S-P interval.


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A common measure of earthquake energy is moment magnitude or Mw, which is calculated from

the amount of slip (displacement) on the fault and the size of fault area that slipped. Moment magnitude works for very large to very small quakes. How do quakes compare to other energy releases we are familiar with? An average light-ning strike (Mw ~ 2) is miniscule com-pared to a small earthquake. However an average hurricane is larger than the

energy released by the largest historic earthquake, which struck Chile in 1960.


✓ Observe different seismic records of an earthquake and tell which one was closer to the epicenter.

✓ Describe using S-wave and P-wave arrival times to locate an epicenter.

✓ Explain or sketch how we use amplitude in calculating magnitude.

✓ Explain what a Mercalli intensity rating indicates.

Energy of Earthquakes

How Do We Measure the Size of Earthquakes?

What Can the Intensity of Ground Shaking Tell Us About an Earthquake?

The maximum height (am-plitude) of the S- wave is measured on the seismogram and is proportional to the earth-quake energy. This measure is used for shallow earthquakes.

The magnitude of an earthquake is a quantitative measurement of the released energy and is used to compare the size of earthquakes. There are several ways to calculate magnitude, depending on the earthquake’s depth. One commonly reported scale called the “Richter” or “Local” magnitude (Ml) is graphically illustrated here.

Some of the most damaging earthquakes occurred before any seismometers were in place to record it. Re-ports of damage and shaking intensity are another way that earthquakes are classified.

The Modified Mercalli Intensity Scale, ab-breviated as MMI, describes the effects of shaking in everyday terms. A value of “I” reflects a barely felt earthquake, while a value of “XII” means total destruction of buildings, with visible surface waves throwing objects into the air!

For each seismic station, a line is plotted connecting the dis-tance and amplitude.

A series of very large earthquakes in 1811 and 1812 shook Missouri, Arkansas, and surrounding areas. Shaking was felt over a wide region of the populated U.S. The magnitudes on this map, numbered from III to XI, indicate what you would feel if it happened today.

Seismographs are calibrated so that the measurements made by two different instruments are comparable.

Measuring Amplitude MagnitudeThis graph called a nomograph represents the mathematical relationship between distance, magnitude, and S-wave ampli-tude.

The earthquake’s magnitude is read where each line crosses the center column. These three lines for the 2005 Colorado earthquake all agree, and yield a 4.1 Ml Local magnitude.

III. Quake felt quite noticeably by persons in-doors, especially on upper floors of buildings.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects over-turned.

VI. Felt by all, many frightened. Some heavy furniture moved; some plaster cracks and falls. Damage slight.

XI. Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly. Much destruction.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.


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How Do Earthquakes Cause Damage?

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What Destruction Can Arise from Shaking Due to Seismic Waves?

MANY SEISMOLOGISTS HAVE SAID that “earthquakes don’t kill people, buildings do.” This is because most deaths from earthquakes are caused by buildings or other human structures falling down during an earthquake. Earthquakes in isolated areas far from human population rarely cause many deaths.

Direct damage from an earthquake results from ground shaking during the passage of seismic waves, espe-cially surface waves near the epicenter of the earthquake. Damage can also be due to secondary effects that are triggered by the earthquake, such as fires and flooding. The area below received mostly direct damage.

1. Mountainous regions that experience ground shaking from earth-quakes may release landslides, rock falls, and other earth move-ments.

7. In areas underlain by water-saturated, unconsolidated sediment, ground shaking causes the grains to lose grain-to-grain contact. When this happens, the material loses most its strength and begins to flow, a process called liquefaction. The building below collapsed due to liquefaction.

8. Fractured materials along fault scarps are prone to landslides, especially if faulting displaces the one side of the fault up relative to the other side, as occurred here. Structures built too close to the scarp may be damaged by ground ruptures and overrun by landslides. The building in the photograph below was damaged by ground rupturing.

5. A tsunami is a giant ocean wave

that can rapidly travel across oceans. An earth-

quake that occurs beneath sea level or along coastal

areas can generate a tsunami, which can cause damage thou-

sands of kilometers away on the other side of the ocean.

6. Aftershocks are earthquakes that occur after the main earthquake, but in

the same area; they are generally smaller than the main shock. Aftershocks occur because the main earthquake changes the stresses around the epicenter, and the crust adjusts to these changes by more faulting. Aftershocks are very danger-ous because they can collapse structures already damaged by the main shock. Aftershocks after a tsunami can cause widespread panic among people.

4. A concrete bridge farther down stream was too rigid and collapsed. Also, it was built upon delta sedi-

ments that did not provide a firm foundation

against shak-ing. In general,

loose un-consolidat-

ed sediment is subject to

more intense shaking than

solid bedrock.

3. Damage to structures from shaking depends on the type of construction. Concrete and ma-sonry structures, because they are rigid and do not flex easily, are more susceptible to damage than wood or steel structures, which are more flexible. In this area, a flexible, metal bridge in the center of the city survived the earthquake.

2. Ground ruptures, such as a fault scarp, form along parts of the fault that slips during an

earthquake. Any structures built across fault zones

will likely crack and fail.


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12.6

PHOTO OF FLOODING

PHOTO OF SUBSIDENCE

PHOTO OF BUILDING ENGINEERING

PHOTO OF REA L -TIME EA RTHQUAK E WA RNING

SYSTEM


There are things you can do during an earthquake that will reduce your chances of being hurt. If an earth-

quake strikes, you may be able to take cov-er under a heavy desk or table, and cover your head. You can also stand under door frames or next to inner walls, since these are the least likely to collapse. Being away from buildings, especially masonry ones, is always a good plan during an earthquake.

During the shaking, stay away from glass and heavy objects that could fall, such a bricks or other loose debris. Always keep a battery-operated flashlight handy, and avoid using candles, matches, or light-ers, since there could be gas leaks. Earth-quakes may interrupt electrical and water service, so keeping 72 hours worth of food and water in an easily carried backpack is a prudent plan, not just for earthquakes.


✓ Describe how earthquakes can cause destruction, both during and after the main earthquake.

✓ Describe some ways to limit earthquake risk.

✓ Discuss ways to reduce personal injury during an earthquake.

What To Do and Not Do During an Earthquake

What Destruction Can Happen Following an Earthquake?

How Can We Limit Risks from Earthquakes?

Fire is one of the main causes of destruc-tion after an earthquake. Natural gas lines may rupture, causing explosions and fires. The problem is compounded if water lines also break during the earthquake, limiting the amount of water that is available to extinguish fires. [Northridge, California]

Flooding may occur due to failure of hu-man-made dams as a result of ground rup-turing, subsidence, or liquefaction. Near Los Angeles in 1971, 80,000 people were evacuated because of earthquake damage to a nearby dam. The dam was later rebuilt using earthquake-proof technology.

Earthquakes may cause both uplift and subsidence of the land surface. Such changes in elevation can be dramatic, locally exceeding 10 m either up or down. Subsidence can cause areas, which had been part of the land before the earth-quake, to become inundated by the sea.

2. Earthquakes have different pe-riods, durations, and vertical and horizontal ground motion, making it difficult to design earthquake-proof buildings. Some buildings are on sturdy wheels or have shock absorb-ers that allow the building to shake less than the underlying ground.

3. Some utilities and hospitals have computerized warning systems that are notified by earthquake-monitor-ing equipment, in order to auto-matically shut down gas systems (to avoid fire) and turn on back-up generators to prevent an untimely loss of electrical power.

Some damage during an earthquake occurs from secondary effects that are triggered by the earthquake.

The probability that you will be affected by an earthquake depends on where you live and whether or not that area experiences tectonic activity. The potential risk for earthquake damage depends on the number of people living in the region, how well the buildings are constructed, and individual and civic preparedness.

1. Earthquake hazard maps show zones of potential earthquake damage. Near Salt Lake City, Utah, the risk is greatest (reds) near active normal faults of the Wasatch Front, the mountain front east of the city.


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What Were Some Significant North American Earthquakes?

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LARGE AND DAMAGING EARTHQUAKES have struck of North America since written and oral records have been maintained by people living there. We discuss six important earthquakes here.

This massive earthquake occurred when 470 km of the San Andreas fault ruptured in strike-slip motion. More than 3,000 were killed and large parts of the city burned afterward.

This magnitude Mw 6.7 earthquake was generated by a thrust fault beneath Los Angeles. The quake killed 57 people and caused $20 billion in damage.

This magnitude Ml 8.0 occurred on an un-derlying subduction zone to the west and killed at last 9,500 people. It damaged or destroyed many buildings in Mexico City, including this collapsed 21-story apart-ment building.

Within San Francisco, ground shaking brought down most of the brick and mortar buildings. Much of the city was destroyed by fires that broke out after the earthquake.

The earthquake was likely a mag-nitude Ml ~8 and ruptured the surface, leaving behind a series of cracks, open fissures, and new steps in the topography. Even be-fore the 1906 event, the fault was marked by linear valleys and other fault-related landscape features.

The thrust is not exposed on the sur-face but when it ruptured it lifted up a large section of land, as shown by the color-fringed area on this radar-pro-duced image.

Mexico City is built on lake sediments deposited in a bowl-shaped basin, which amplified the seismic waves, causing in-tensified and destructive ground shaking. Surface waves that caused the most dam-age traveled 200 km from their source!

A section of the Interstate-10 freeway buckled, crushing the steel-reinforced concrete slabs.

San Francisco, 1906

Northridge, 1994

Mexico City, 1985


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Why do large earthquakes occur in the middle of continents, in plac-es like New Madrid, Missouri? Al-

though the interior of North America is not near a plate boundary, the region is subject-ed to stresses generated along far-off plate boundaries, especially some compression from the Mid-Atlantic Ridge, called ridge push. These stresses are imposed on the crust and can reactivate ancient faults that

lie buried beneath the cover of sediments. In the case of New Madrid, there is seismic and other geophysical evidence to suggest that the area is underlain by an ancient rift basin that formed ~750 m.y. ago during the breakup of the supercontinent of Rodinia. Modern-day stresses, related to the cur-rent plate configuration, are interacting with the ancient faults, occasionally caus-ing them to slip and cause earthquakes.


✓ Describe some large North American earthquakes and how they were generated.

✓ Describe the various ways these earthquakes caused damage.

✓ Summarize why the eastern U.S. has earthquake risks.

Earthquakes in the Interior of Continents

New Madrid, Missouri experienced a series of large (Ml ~8) earth-quakes generated over a deep crustal weakness called the Reelfoot rift. The 1811-1812 earth-quake death toll was likely small because of the sparse population at the time.

This magnitude Ml 7.5 jolt was generated by slip along a normal fault northwest of Yellowstone National Park. Ground shaking set loose the massive Madison Canyon slide, which buried 26 campers and formed a new lake, aptly named Earth-quake Lake.

This quake accounts for the other high-risk area along the East Coast. It had an estimated magnitude Ml of 7.3, the largest ever record-ed in the Southeast. Build-ings received some dam-age, and 125 people died. The tectonic reasons for this quake are still debated among geologists.

Charleston, 1886

Highway 57 was instantly impassable as fractures caused by slope failure sliced apart the road bed.

Hebgen Lake, 1959

New Madrid, 1811-1812

This zone has a high earth-quake risk and is one of two locations east of the Rocky Mountains predicted to ex-perience strong earthquakes sometime in the future. Memphis lies in this zone, yet most of its buildings are not prepared to survive large earthquakes.


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What Were Some Major World Earthquakes?

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On December 23, 1972, a magnitude (Mw) 6.2 earthquake killed about 6,000 people in central America.

This huge, magnitude (Mw) 9.5 earthquake occurred offshore along a megathrust and triggered a destructive, Pacific-wide tsunami. At least 3,000 people died and $550 million in damages was done to infrastructure and buildings, such as in the city of Vadivia, Chile.

A magnitude (Mw) 9.2 quake, one of the largest two or three ever recorded, struck southern Alaska in 1964, killing 125 people, triggering landslides, and col-lapsing neighborhoods and the downtown area of a nearby city. This event was caused by thrusting along the subduction zone.

<-- On the Kenai Peninsula, this rail bridge was compressed and buckled.

<-- In the capi-tal city of Mana-gua, structures made of wood and adobe were leveled, while fractures opened in the street.

On November 1 (All Saints Day) in 1755, a large earthquake, estimated at magnitude (Mw) 8.5, shook Lisbon, Portugal. The earthquake destroyed the city and triggered destruc-tive tsunamis, which sank ships in Lisbon’s famous harbor. The event caused an upheaval in religious and scientific thought, as people began to explain such catastrophes as being due to natural causes.

Alaska, 1964

Chile, 1960 Lisbon, 1755

THE WORLD HAS ENDURED a number of large and tragic earthquakes. These earthquakes have struck a collection of geographically and culturally diverse places, causing deaths, damage, and disruptions.

Nicaragua, 1972

Most deaths and much damage were from a tsunami generated when a huge area of the sea floor was thrust upward.


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Mortality due to earthquakes aver-ages about 10,000 per year. Most earthquake-related deaths are due

to collapse of poorly built structures in cit-ies and villages. Earthquake-generated tsu-namis also account for a large part of the yearly average. The table to the right shows the most deadly earthquake events, with the highest death tolls occurring because of the deadly combination of high popula-tion densities, substandard construction practices, and being situated along sub-duction zones or other high-risk areas.


✓ Briefly describe some of the world’s most significant earthquakes and the tectonic settings in which these deadly earthquakes formed.

✓ Summarize the role that building collapse plays in earthquake deaths.

Deaths Due To Earthquakes

A magnitude (Mw) 7.2 thrust earth-quake killed 6,500 and left 300,000 homeless. Intense shaking partially collapsed this building’s lower floors.

This megathrust rupture generated a magnitude (Mw) 7.6 earthquake that was felt across Taiwan, killing 2,400 and displacing 600,000 people. The ShihKang Dam (below), only 50 km from the epicenter, was badly dam-aged, shutting off the local water supply.

In 1999, a large quake (Mw 7.4) generated along a transform fault zone through this region, killed more than 17,000 and se-verely impacted the economy. These multi-story apartment buildings were probably occu-pied when they collapsed.

Old masonry struc-tures were shaken apart in Leninakan, Armenia in December 1988. This transform-fault-generated earth-quake, killed 25,000.

Armenia, 1988

Kobe, Japan, 1995

Chi Chi, Taiwan, 1999

Turkey, 1999

Fatalities Mw Year Location830000 8 1556 Shaanxi, China11,000 6.9 1,857 Naples, Italy70,000 7.2 1908 Messina, Italy200,000 8.6 1920 Ninxia, China143,000 7.9 1923 Kanto, Japan200,000 7.9 1927 Tsinghai, China10,700 8.1 1934 Bihar, India32,700 7.9 1939 Erzincan, Turkey10,000 5.7 1960 Agadir, Morocco66,000 7.9 1970 Columbia23,000 7.5 1976 Guatemala31,000 6.6 2003 Iran

Table will be fixed by McGraw-Hill


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How Does a Tsunami Form and Cause Destruction?

How Are Tsunamis Generated?

AN EARTHQUAKE BENEATH THE OCEANS can cause a large ocean wave called a tsunami, which can wreak havoc on coastal communities. Most of Earth is covered by oceans and so many earthquakes, landslides, and volcanic eruptions occur beneath the sea, and each of these can generate a tsunami.

Tsunamis are waves that affect an entire body of water from top to bottom. They are generated by abrupt changes in water level in one area relative to another. This occurs when something is dropped into the water, like a landslide, or when the base of the water’s container is unevenly punched up or pulled down by some-thing, such as an earthquake fault.

4. A tsunami can wash many kilo-

meters inland, carry-ing rocks, sand, and large

chunks of coral. Once stranded on shore, these deposits may be the

only evidence of ancient tsunami activity.

Tsunami Triggered by Landslides Tsunami Caused by Eruptions

1. A tsunami can form when a sudden change in sea level accompanies fault movement. It is a wave, or series of waves, that radiates away from the disturbance. If the disturbance is linear, like a fault rup-ture, the wave forms two linear fronts that travel away from each other.

The 1883 eruption of Krakatau in Indonesia, and the collapse of its immense caldera, generated a series of tsunamis that killed 36,000 people.

A large mass of rock entering the water can catastrophically displace the water and generate tsunami waves that radiate away.

3. As the wave approaches shore, the energy is distributed over less ocean depth. The wave height increases while the velocity decreases. Tsunamis usually comprise a series of waves, eventually decreasing in intensity.

A single cata-strophic volca-nic explosion produced the loudest sound ever heard, and most of Krakatau Island was de-stroyed. Tsunami wave effects were recorded 7,000 km away!

After Krakatau was destroyed, eruptions starting in 1927 built this island called Anak Krakatau or “child of Krakatau” in the same location.

This occurred in the past, off the west side of Hawaii, where

huge landslide-debris depos-its (shown in green) sit on the ocean floor. The tsunami

generated by one of these slides deposited debris 6 km inland. The volume

of water displaced dur-ing these events likely produced a tsunami

that flattened coast-lines around the Pacific ~120,000 years ago.

1 1 2 . 9

2. A tsunami travels at speeds between 600-800 km/hr away from the source. In deep water, the wave may be barely noticeable, much smaller in height than can be shown here. The wave energy is distributed over the entire water depth, forming a wave only a meter or so high but ~700 km across.

5. Most tsunamis are more a step in the water, rather than a tall, but thin wave, as we think of for normal ocean waves.


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In an international effort to save lives, the U.S. National Oceanic and Atmo-spheric Administration (NOAA) main-

tains two tsunami warning centers for the Pacific Ocean. Twenty-six nations partici-pate in this effort. Informed by worldwide seismic networks, these centers broadcast warnings based on an earthquake’s poten-tial for generating a tsunami. Since the huge loss of life after the Sumatran earth-quake and accompanying tsunami in 2004, the United Nations has begun implement-ing a warning system in the Indian Ocean,

deploying warning buoys, like the one shown below, which can relay tsunami data by satellite. These buoys detect small changes in sea level as a tsunami passes underneath.


✓ Describe the different mechanisms by which tsunamis are generated.

✓ Summarize the kinds of damage tsunamis have caused.

✓ Briefly describe how tsunamis are monitored to provide an early-warning system.

Tsunami Warning System

What Kind of Destruction Can a Tsunami Cause?Tsunamis cause death and destruction along coastlines where human populations are concentrated. On May 22, 1960, the largest earthquake ever recorded on a seismograph (Mw= 9.5) occurred in the subduction zone (megathrust) off southern Chile. The tsunamis that followed flattened coastal settlements in Chile, and trav-eled across the Pacific to smash coastlines in Hawaii and Japan.

The tsunami rolled over Hilo, Hawaii dam-aging many buildings and causing $23 million in damage. Seven hours later, the tsunami killed 140 in Japan.

In 1993, a magnitude 7.8 earthquake oc-curred off the west coast of Hokkaido and within five min-utes a tsunami smashed the coastline. The tsunami killed at

least one hundred people and caused $600 million in property loss. This boat ended up on the landward side of a protective concrete barrier

The tsunami waves excavated a new lagoon behind the old shoreline, depositing and layer of sand and mud 650 m inland. This layer cov-ered vegetation and dwellings destroyed during the tsunami.

About 15 hours after the earthquake, the tsunamis hit Hawaii. Sixty-one people were killed by a wave 11 m high.

Tsunamis were generated parallel to the coast. One headed in toward the shore-line, quickly striking Chile and Peru. An-other set of tsunamis swept out across the Pacific Ocean at 670 km/hour! Each stripe equals one hour of travel time.

In Chile, the tsunami waves struck 15 min-utes after the earthquake. On Isla Chiloe, a 10-meter-tall wave swept over towns. The waves killed at least 2,000 people along the Peru-Chilean coast.

Chile, May 22, 1960

Papua New Guinea, 1998Hokkaido, Japan 1993

Hawaii, May 23, 1960

In 1998 a magnitude 7.1 earth-quake generated three tsu-nami waves that destroyed vil-lages along the country’s north coast, killing 2,200 people. The maximum wave height at the village of Arop was estimated at 10 m above the land. Before the tsunamis, several hundred houses would have been vis-ible in this photograph.


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How Do We Study Earthquakes in the Field?

1 1 2 . 1 0

How Do We Study Recent Earthquakes in the Field?

GEOLOGISTS USE A VARIETY of tools and techniques to study evidence left behind by recent and an-cient earthquakes. They examine and measure faults in natural exposures and in trenches dug across faults. Satellite and other tools allow faults to be studied in new and exciting ways.

Where a fault break through Earth’s surface, it can be scrutinized to better understand how it operates and moves during an earthquake. There are numerous features geologists investigate, such as these shown here.

4. Faulting is commonly ac-companied by changes in the topography of the land sur-face. Faulting can uplift linear ridges or form new hills, or can create ponds and other low areas by downdropping areas along the fault.

3. When a fault ruptures the surface, geologists take careful measurements of its location, dimen-sions, and orientation. Detailed drawings and

photographs are essential for noticing and documenting features along the fault.

The topography around a fault changes when the fault moves. Very small changes in elevation can be detected through laser surveying or by compar-ing satellite radar data sets before and after faulting. In this image, satellite radar scans the Earth’s surface to produce an elevation map.

2. When faults move, they can offset natu-ral and human-made features. Streams and gullies, as well as roads, fences, and tele-phone lines, provide pre-earth-quake datum to measure changes in orientation and how much and in what direction the fault has offset the features.

Comparing the degree of ground movement associ-ated with building failure to the underlying geology can help define fault characteristics and aid in planning for future earthquakes. The geologic map of San Francisco shows bay mud as pale gray.

After an earthquake, the same area is mapped again. The two maps are com-bined into an interferogram, which shows how the earth has deformed near the fault rupture. In this image, color bands or fringes indicate movement vertically up. The fault is cutting diago-nally through the view.

1. Faulting during an earthquake is commonly accompanied by smaller structures like cracks and smaller faults, The fault surface can often contain delicate textures that indicate the direction of fault movement.

Field Studies

Ground Displacement Geologic Control of Damage

How Do We Study Faults With Satellites and Geology-Based Models?

5. Rock layers and soils, whether exposed in natural exposures or in trenches dug to study

the fault, can preserve a history of motion and give clues to magnitudes and recurrence of past earthquakes.

The map below shows a geol-ogy-based model estimating the acceleration of the ground during the 1906 San Francisco earthquake. Dark reds indicate the most intense ground move-ment. Notice that areas underlain by mud would have experienced the highest level of acceleration.


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Geologists in California are engaged in a fantastic experiment. The San Andreas Fault Observatory

at Depth (SAFOD) sunk a deep drill hole through part of the San Andreas fault. The drill hole is equipped with a wide array of geophysical instruments that are providing data on this active fault system. The scien-tists hope to catch an earthquake as it hap-pens. In this figure, a drill hole crosses the fault at a 3.2 km depth.


✓ Summarize the kinds of field and remote measurements geologists use to investigate recent earthquakes.

✓ Summarize the methods of investi-gating prehistoric earthquakes on faults, including observations within trenches dug across a fault.

The San Andreas Experiment

How Do We Study Faults that Had Prehistoric Earthquakes?

Shallow trenches dug across the fault provide expo-sures of what is just below the surface. Most trenches are

dug several meters deep to allow geologists to examine the fault zone for clues about its earthquake history. In the trench

above, orange markers show locations of critical evidence that will help constrain the history of earthquakes on this section of fault.

To infer past fault movement, geologists observe modern fault-related features on the surface, such as stream beds that bend where they cross a fault, and ridges that are offset or that end abruptly along a fault.

1. In 1999, a magnitude (Mw) 7.4 earthquake ruptured over 100 km of the North Anatolian fault of Turkey. Soon after the earthquake, geologists conducted field studies to determine how much and how often the fault moved in the past. They used surveying equip-ment to precisely measure the height of fault scarps, to determine how much the fault moved in the most recent event. During this earthquake, one side of the fault moved up by 1.6 m during the earthquake, but much movement was actually horizontal.

4. From these careful studies, the geologists determined that a major earthquake occurs along this fault about every 200-300 years, and that previ-ous events were about the same size as the 1999 event. So such earthquakes may be characteristic of this fault.

Features on the Surface and in the Subsurface

Earthquakes Studies Along the North Anatolian Fault, Turkey

Geologists also evaluate the amount of movement on a fault by looking for distinctive rock units that have been cut and displaced by the fault. This pink-colored granite occurs on both sides of the fault but does not match up — it has been offset by an specific amount.

3. Samples of charcoal were dated by the Carbon-14 method, providing a time line for interpreting when the fault moved.

2. Several trenches dug along the fault revealed a wealth of information about the fault’s prior history. The geologists meticulously examined the walls of the trenches, carefully mapping how the fault offset the dif-ferent layers of sediment and soil. They documented that older layers were offset by several distinct earthquake events, whereas the young-est layers were only cut by the 1999 event.


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Can Earthquakes Be Predicted?

1 1 2 . 1 1

Can We Anticipate Which Areas are Most Likely to Have Earthquakes?

EARTHQUAKES CAN BE DEVASTATING, to places and people. For this reason, we have a great inter-est in finding ways to predict when and where earthquakes will occur. Although much is known about where earthquakes occur, there is no reliable way to predict exactly when one will strike.

The main way to predict which areas will likely have earthquakes is to understand the (1) frequency and size of historic earthquakes, (2) geologic record of prehistoric earthquakes, and (3) tectonic setting of the areas.

For the U.S., the risk for earthquakes is greatest in the most tectonically active areas, espe-cially areas near the plate margin in the western U.S. Here, the San Andreas fault, which forms the margin be-tween the Pa-cific plate and the North American plate, is respon-sible for about one magnitude 8 or greater earthquake per century.

U.S. Earthquake Hazard

In the western U.S., the Basin and Range Prov-ince is pulled by extensional stresses in the crust, creating many normal faults that are still active. Especially dangerous is the intermountain seismic belt, from Utah through the Yellowstone region.

Historically, large earth-quakes have occurred in New Madrid, Missouri, marked by the red area of high risk. Historical earth-quakes have also struck in Charleston, South Carolina, and along the St. Lawrence River near New York, so these at least moderate risk.

This seismic-hazard map shows the level of earthquake shaking expected on land. Red areas have the highest hazard, green areas have the lowest hazard, and yellow areas are considered to have a moderate seismic hazard.

The Middle East region is highly susceptible to earthquake haz-

ards, largely because the collision of the Arabian

plate is causing thrust and strike-slip faults

across the region.Note the pattern along convergent plate margins, such as the west coast of South America. The highest risk is from megathrust earthquakes along the coast (near the trench). Risk decreases into the continent as the distance from the con-vergent boundary increases and the subduction zone gets very deep.

Australia experienc-es few earthquake hazards, mostly be-

cause it is not along any type of plate

boundary. Islands to the north (New Guinea)

and southeast (New Zealand) straddle active plate boundaries and have higher hazards.

The patterns on this map largely reflect the locations of plate boundaries. What parts of the world have little risk from earthquakes, and what parts have high risk?

This map shows the most seismically active areas of the United States, including Hawaii and Alaska. What regions experience little damage from earthquakes, and what regions experience the most damage from earthquakes? Do some areas surprise you?

The upper Midwest and southeastern U.S. have few active faults and so very low earthquake hazards.

Southern Alaska experi-ences very large subduction earthquakes.

World Earthquake Hazard

Seismic hazard in Hawaii is higher to the southeast, toward the most active volcanism.


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12.11

PHOTO OF L ASER MONITORING



✓ Describe areas of the world that experience high risk from earthquake activity.

✓ Summarize why certain areas of the U.S. experience earthquakes, while others do not.

✓ Summarize ways geologists do long-range forecasting and short-range prediction.

How Do We Approach Long-Range Forecasting of Earthquakes?

How Successful Are Short-Term Predictions?

Long-term forecasting is based mainly on the knowledge of when and where earthquakes occurred in the past. Thus, geologists study present tectonic settings, historical records, and geological evidence of pre-his-toric events, with the aim of determining the location and recurrence intervals of past earthquakes.

2. In the top section, three segments of the fault have fewer earthquakes than other segments of the fault. These segments, called seismic gaps, are interpreted as being “locked” (not moving) and building up stress. The three seismic gaps were at San Francisco, Loma Prieta, and Parkfield.

2. Measurements taken near active faults sometimes show that, prior to an earthquake, the ground is uplift-ed or tilted as rocks swell under the strain building on the fault. The buildup in stress also may cause numer-ous small cracks, which slip and produce foreshocks, small earthquakes that happen before a main earth-quake. Foreshocks may advertise an upcoming quake.

Short-term prediction involves monitoring processes and activities along an earthquake-prone fault. These events are called precursor events, and can be gauged using sophisticated scientific equipment. The com-plexity in fault systems means that prediction technique is still developing, buts holds promise.

1. One approach to long-range forecasting is to examine patterns of seismic activity mea-sured along a fault. These two cross sections show seismicity along the San Andreas fault in northern California. The top shows earthquakes that occurred along the fault prior to October 17, 1989; the second shows seismicity after the Loma Prieta earthquake on October 17, 1989.

3. In 1989, a magnitude 7 earthquake struck the Loma Prieta gap. This earthquake and its aftershocks, shown in the lower section, filled in this gap. The Parkfield gap was filled by an earthquake in 2004. When will an earthquake fill the San Francisco gap?

1. Seismologists set up la-sers that shine across the fault in order to monitor small-scale movements that might be precursors to a larger earthquake, or even to record movement during the larger earth-quake.

3. The Parkfield segment of the San Andreas Fault, southeast of San Francisco, has had six magnitude (Mw) ~6 quakes since 1857. These occurred approxi-mately every 22 years and had similar characteristics. This situation provided an opportunity to study the short-term precursors of the next earthquake in the series, so seismologists set up a detailed array of seismic instruments to record the region’s many earth-quakes, shown here as red, black, and yellow symbols.

5. The next big quake was predicted to occur between 1988 and 1993. Scientists were kept waiting an extra 11 years, until the earthquake finally happened in 2004.

4. This graph shows when the six large historic Parkfield earthquakes actually occurred versus a blue line showing when they would have occurred if they were spaced exactly 22 years apart.

4. From various data, the USGS assigned probabilities of a magnitude 6.7 earth-quake on faults of the area before 2032.


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PHOTO OF SF EQ DESTRUCTION

PHOTO OF LOM A PRIETA EQ

DESTRUCTION

What Is the Potential for Earthquakes Along the San Andreas Fault?

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THE SAN ANDREAS FAULT is the world’s best known and most extensively studied fault. It slashes across California from the Mexican border to north of San Francisco and inflicts the region with de-structive earthquakes. What has happened along the fault in the recent past, and what does this history say about the fault’s current behavior and its likelihood of causing large earthquakes in the future?

The San Andreas fault has distinct segments that show different behaviors, as expressed by the size and inferred frequency of earth-quakes along each segment. As a result, the earthquake risk varies along the fault.

This map shows some of the major faults that have caused earth-quakes in California. These faults have accounted for the largest quakes, but there are many more recently active faults, some of which have caused damaging, moderate-sized earthquakes.

The northern San Andreas fault was responsible for the famous 1906 earthquake that destroyed much of San Francisco. The earthquake had a magnitude of 7.7 and ruptured 430 km of the fault, from south of the city all the way to the Mendecino triple junc-tion, where the fault ends. Damage (shown to the left) was caused by ground shaking, disastrous fires, and liquefaction of water-saturated soils in areas that had originally been part of San Francisco Bay.

The next segment to the south, shown in blue, is called the central creeping segment, because the two sides of the fault move past one another somewhat continuously and slowly (i.e. they creep), rather than storing up energy for a large earthquake. Creep continues to the north along the Hayward fault, also colored blue, through Oakland. The Hayward fault was the site of a damaging magnitude 7 earthquake in 1868.

South of the creeping segment is the Parkfield segment, a short part of the segment colored orange. The is characterized by moderate sized earthquakes that occur, on the average, every couple of decades. The Parkfield segment receives special scrutiny from geologists and seismologists because its pattern of fairly frequent earthquakes provides an opportunity to study the behavior of a fault before, during, and after an earthquake.

The southern part of this segment ruptured in 1989 in the magnitude 7.1 Loma Prieta earth-quake, famous for disrupting a World Series baseball game. Ground shaking collapsed parts of bridges and freeways.

The San Andreas continues to the southeast through a locked segment of the fault, the entire segment shown in orange, that last ruptured during the great Fort Tejon earthquake of 1857. This earthquake ruptured 300 km of the fault, from Parkfield all the way to east of Los Angeles. The earthquake was approximately a mag-nitude 8 quake, but damage was limited because the area was much less populated than it is now. This earth-quake is considered by many geologists to be the model for the “big one” along the San Andreas.

Recent Earthquake History of Different Segments of the San Andreas and Related Faults


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✓ Briefly summarize the main segments of the San Andreas fault and whether they have had major earthquakes.

✓ Summarize or sketch how you might recognize the fault from the air.

The San Andreas fault generally has a clear expression in the landscape, being marked by a number of fea-tures, whose names are not important here but illustrate the range of features that accompany the fault.

East of Los Angeles, the San Andreas branches southward into several faults, some of which expe-rienced several moderate-sized earthquakes in the 1900s, including some in near the important agricul-ture areas of the Imperial Valley.

In the middle of the southern locked (orange) segment, the San Andreas fault has a distinct curve or bend. The

bend in the San Andreas fault causes regional compres-sion and thrust faults, some of which are not exposed

at the surface. These thrust faults caused the 1994 magnitude (Mw) 6.7 Northridge earthquake in met-

ropolitan Los Angeles and have uplifted the large mountains north and northeast of the city.

North and east of the San Andreas is a series of faults, called the East California Shear Zone, which caused several >7 magni-tude earthquakes in the 1900s and the large 1872 Owen Valley earthquake. The zone continues from the eastern side of moun-tains Sierra Nevada southward through the Mojave Desert.

The aerial photo-graph to the right shows the same area of the San Andreas fault as de-picted in the figure above. Can you find and match some of these features, such as offset drainages and linear valleys and ridges?

Geologists have explored the fault to find localities that have a favorable set-ting for preserving a record of past faulting. Detailed studies of trenches dug across the fault help geolo-gists extend the historic record back hundreds or thousands of years.

Features Along the San Andreas Fault


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PHOTO OF CRUSTA L INCLUSIONS

PHOTO FROM DEEP MINE

IN SOUTH A FRICA

How Do We Explore What Is Below the Earth’s Surface?

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OUR VIEW OF GEOLOGY is typically limited to those rocks and structures that are actually exposed at the surface. In deep canyons we can glimpse what rocks and structures lie at depth — in the subsurface of the Earth. How else do we determine what is down there?

4. Mines provide a more detailed view of what is below the surface because the tunnels provide continuous exposures of rocks and structures. Some mines in South Africa are deeper than 5 km, as shown in the photo to the left.

3. A sense of what is below the surface can be gained by examining the types of rocks that are uplifted and exposed at the surface. Geologists study rocks under the microscope to constrain the temperature and pressure conditions under which the rocks formed and then infer the geologic pro-cesses that operated under these conditions.

2. As magma rises to the surface beneath volcanoes, it can extract pieces of rocks through which it passes. Ge-ologists study such pieces, called inclusions, to recon-struct the types of rocks that lie beneath the volcano.

1. The region shown here has a few hills of bedrock and a single volcano, but otherwise is covered by soil and vegetation. There are few clues as to what types of rocks and structures are at depth below the surficial cover. There are two general approaches for inves-tigating the subsurface geology: obtaining samples from rocks at depth or performing geophysical sur-veys that measure the subsurface magnetic, seismic, gravity, and electrical properties.

5. The geometry of rock units and geologic structures can be explored by sending seismic energy (sound waves) into the ground and measuring how the waves are reflected back to the surface off boundar-ies between rock types. This is accomplished by using large trucks that shake the ground in a controlled man-ner, as shown here. The sound waves bounce off rock layers, faults, and other rock boundaries and then are record-ed using seismic receivers, called geophones (shown on the next page), that are buried or stuck into the ground.

6. Seismic-reflection data, when processed using sophisticated computer programs, are plotted in cross section as a series of lines. These lines indicate the geometry of the units, but do not indicate what rocks are actually present.

7. The geometry of reflections, as expressed on the seismic profile, is integrated with information about the area’s rock sequence and structures to construct a geologic cross-section that represents an interpretation of the subsurface.

8. Geologists and others drill holes into the Earth in pur-suit of petroleum, minerals, groundwater, and scientific knowledge. Most drill holes are less than several hundred meters into the subsurface, but some reach depths of 5 km or more. Cylinder-shaped samples of rock, called drill core, can be retrieved during the drilling process to provide an intact sample of rocks at depth.


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12.13PHOTO OF TEA M DOING ELECTRICA L

SURVEY ING



✓ Summarize how volcanic inclusions, exposed geology, drill holes, and mines provide observations of the subsurface.

✓ Briefly summarize what is measured by the various types of geophysical surveys (seismic, magnetic, gravity, and electrical).

13. The strength of gravity varies slightly from one place to another on the Earth’s surface. This is because some rocks, such as basalt, are more dense and cause a stronger pull of gravity than less dense materials, such as sediments. The varia-tions in gravity can be measured using sensitive gravity meters.

16. Rocks also vary in how well they conduct an

electrical current. Some rocks, such as clays, conduct electrical

currents better than other rocks. Rocks containing groundwater conduct elec-trical current better than ones that are dry. Geologists and geophysicists use these principles to explore for mineral deposits and groundwater. An electri-cal transmitter runs current into the ground, and a receiver some distance away measures how much current has reached the surface.

15. From the gravity profile, computer pro-grams can model possible configurations of density that are consistent with the data.

14. In this area, the team of geophysicists mea-sured gravity across the buried stream channel. These data, when plotted on a profile relative to the average value of gravity for the area, show a gravity low caused by unconsolidated sediments within the buried channel.

17. Results of an electrical survey across the buried stream channel are plotted in cross section and contoured, with warmer colors showing rocks with higher conduc-tivity. Comparing these results with those for gravity helps identify interpretations that are consistent with both data sets.

10. Magnetic data are generally portrayed as a map with warmer colors (reds) representing more strongly magnetic rocks and cooler colors (blues) representing less mag-netic areas.

9. Instruments that mea-sure the intensity of the Earth’s magnetic field can be carried on foot or towed behind a plane to constrain the distribution of more magnetic versus less magnetic rocks in the subsurface. Earth scientists who measure and interpret magnetic data, seismic data, gravity data, and oth-er types of physical proper-ties are geophysicists. We refer to such data collection as a geophysical survey. Many geology graduates are involved with geophysi-cal surveys at some point in their careers.

11.Here, the dark lava flow and hills of gray granite are more magnetic than the sediments that cover the rest of the area.

12. A curving magnetic low, represented by the darker blue colors, coincides with a buried stream channel that forms a band of gray soil be-neath the feet of the two teams of geophysicists.


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What Do Seismic Waves Indicate About Earth’s Interior?

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EARTHQUAKES, EXPLOSIONS, AND OTHER SEISMIC EVENTS generate seismic waves that can be used to interpret Earth’s internal structure. The way seismic waves travel through Earth enables us to identify distinct layers and boundaries within the interior, including the crust, mantle, and core.

How Do Seismic Waves Travel Through the Earth’s Crust and Mantle?

1. Refraction causes seismic waves to take curved paths through the Earth. Steeply descending rays will first be refracted to shallower angles as they encounter seismically faster and faster material at depth. The waves will then be bent back toward the surface as they pass back through slower materi-als closer to the surface.

How Do Seismic Waves Travel Through Materials?An earthquake or other source of seismic energy generates seis-mic waves that radiate out from the source in all directions. The path that any part of the wave travels is a seismic ray.

Most seismic waves encounter boundaries between two ma-terials with different physical properties that cause waves to speed up or slow down.

If a descending seismic ray passes from a slow material to a faster one, it will be refracted to a shallower angle.

If a wave passes from seismically faster into slower materials, it will be refracted away from the interface at a steeper angle.

If a rising seismic ray passes from a fast material to slower one, it will be refracted upward toward the surface.

3. Close to the earthquake, waves that travel through the crust arrive sooner than those from the mantle because the distance they travel is shorter.

2. In the figures below, an earthquake sends seismic waves into the crust and mantle. Both waves are refracted back upward toward the surface. Waves in the mantle travel faster than those in the crust, resulting in an interesting, and useful, phenomenon.

4. Farther from the earthquake, waves that travel through the mantle arrive first because their faster velocities through the mantle let them overtake the crustal ones.

How Seismic Waves Refract Through Different Materials

5. Seismologists calculate the depth to the crust-mantle boundary by determining at what distance from the eipicenter mantle waves begin to arrive first. They then using simple computer models of the velocities, crustal thicknesses, and ray paths.

If the physical properties of the material do not change from

place to place, then a seismic ray travels in a straight line. In this case, a family of straight rays diverge outward from the source.

Other energy is bent as it passes between the two different materials; this process of bending is called refraction.

Some energy of the wave is reflected off the interface.


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The boundary between the crust and mantle is named the Mohorovicic Discontinuity after the last name of

the Croatian seismologist who discovered it. Most geologists simply call it the Moho.

Much effort is expended trying to de-termine the depth to the Moho, because this also tells us how thick the crust is. Geophysicists investigate this problem us-ing various approaches. Some observe the arrivals of seismic waves from naturally

occurring earthquakes, whereas others use mine blasts as the seismic source. The depth to the Moho can sometimes also be identified as reflections on some seismic reflection profiles. Since seismic waves travel through the crust at ~6 km per sec-ond, it takes 10 seconds for a wave to travel 30 km down to the Moho, bounce off, and travel 30 km back up. The calculation is:

60 km total travel /6 km per sec = 10 sec


✓ Sketch or describe reflection and refraction of seismic waves.

✓ Sketch and explain how seismic waves pass through the crust and mantle.

✓ Describe how seismic waves are used to identify the diameter of the core and that the outer core is molten.

How Are Seismic Waves Used to Examine Earth’s Deep Interior?

The Moho

Seismologists recognize distinct boundaries within the Earth, largely based on changes in seismic velocities. Such changes reflect the physical and chemical properties of the rock layers through which the seismic waves pass. Also, not all seismic waves make it through every part of the Earth, and observing where particular kinds of waves are blocked helps determine which parts of the Earth are molten.

1. As P-waves travel through the Earth, they speed up and slow down as they pass through different kinds of material. Their velocity depends upon three factors: (1) how easily the rocks are compressed; (2) how rigid the mate-rial is; and (3) the density of the material. When all of these somewhat interrelated factors are considered, seismolo-gists conclude that faster velocities mark rocks that are more dense.

2. The graph below plots P-wave velocities as a function of depth in the Earth. Overall, P-wave velocities increase with depth as the rocks become more rigid and dense.

3. As P-waves and S-waves travel through the Earth, many follow curved paths that return them to the surface.

5. There is a zone, called the P-wave shadow zone, that receives no direct P-waves.

This is because the P-waves are either refracted upward

before they reach here or are refracted inward through the core.

4. Along the core-mantle bound-ary, some P-waves are refracted inward because the outer core has slower velocities than the adja-cent mantle. These P-waves pass through the core and out toward the other side of the Earth.

6. On the opposite side of Earth from the seismic source, there is also an S-wave shad-ow zone, that receives no direct S-waves. This implies that S-waves traveling along these paths could not pass through the core. From this and other observations, seis-mologists conclude that the outer part of the core is molten and blocked the S-wave.

7. From the size and location of the P-wave and S-wave shadows, we can readily determine the diameter and depth of Earth’s core. Seismologists also study indirect waves, which are waves that have reflected off bound-aries or have changed from one type of wave to another as they passed from one material to another, such as from the mantle to the core.


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PHOTO OF SURFACE EXPOSURES

PHOTO OF L A B EXPERIMENTS

PHOTO OF COMPUTER SCREEN

A ND NUMERICA L MODEL

How Do We Investigate Deep Processes?

1 1 2 . 1 5

How Do We Investigate Deep Conditions?

ROCK PROPERTIES CHANGE THROUGHOUT the Earth in terms of density, temperature, pressure, and composition. Seismologists use observations of seismic-wave velocities to determine how rock proper-ties change with depth and how material moves in the Earth’s mantle and at the core-mantle boundary.

Much of what we know about Earth’s interior comes from our knowledge of seismic-wave velocities and how they vary within the deep interior of Earth.

How Does Seismic Tomography Help Us Explore the Earth?

One way to constrain the conditions that operate deep within the Earth is to find samples of rocks that have resided at such depths. Some metamorphic rocks in Norway and China contain high-pressure minerals, which indicate that they were at depths of 60 to 100 km. Documenting the minerals and the types of structures that formed under these conditions provides insight into what is going on down there.

In the laboratory, rocks can be subjected to high temperatures and pressures in order to determine the conditions under which they melt, solidify, flow in the solid state, and convert into minerals that are only stable under high pressures. The pressures under which these changes occur are then applied to depths within the Earth that should have comparable pressures.

Sophisticated numerical models can provide insight into how the mantle might flow upward, downward, or laterally if there are lateral variations in density, such as caused by differences in temperature and the type of minerals that are present. The image above shows a sophisticated computer-derived model of how the man-tle might convect. Red areas are moving up, whereas blue ones are descending.

2. In this diagram, the direction in which the seismic waves passed through the region are shown as a series of lines, called ray paths.

Seismologists examine the Earth using earthquakes in much the same way that medical doctors examine the internal parts of the body with CAT scans and the other types of new imaging technologies. The technique seismologists use is called seismic tomography, where “tomography” means an image of what is inside.

1. The approach used in seismic to-mography is to examine a number of earthquake waves that have passed through the same subsurface region, but from different directions.

3. Earthquakes coming from points A and B are recorded on a number of seismometers, shown as triangles.

4. If part of the crust or mantle has a higher velocity than other areas, then seismic waves passing through that area will arrive sooner than ex-pected. Those that travel via slow re-gions will arrive later than expected.

5. This figure models the velocities in the same region using seismic tomography. Red areas are slower than normal and are interpreted to represent areas that are perhaps hotter than normal.

6. Cooler colors (blues) identify areas that are faster than expected. Such areas might be abnormally cool or composed of stiff, dense rocks, com-pared to surrounding regions.

Seismic Observations

Seismic Interpretation


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✓ Describe four ways we can investigate or model the Earth’s interior.

✓ Summarize how the seismic tomography method identifies different regions within the Earth.

✓ Describe some interpretations that seismic tomography has led to about flow of material in the mantle.

What Processes Are Occurring in the Mantle?

1. This globe shows computed veloci-ties of seismic shear waves in the lower-most mantle, as modeled from seismic tomography. Red areas represent seismically slow materials, whereas blues represent materials that are seismically faster, than average. The outlines of the continents (centered on North and South America) are shown on the surface for reference.

Seismic wave velocities increase abruptly at the Moho, passing from the crust down into the mantle. They vary within the mantle due to major changes in mineralogy and density with depth and because of upward and downward flow of mostly solid mantle material.

2. The red areas in the model are slower than normal and are inter-preted to represent rising masses of hot, but mostly solid mantle ma-terial. Many, but not all, seismologists regard these rising masses as the source areas for mantle plumes and hot spots.

3. Cooler colors (blues) identify areas that are faster than expected and interpreted to be dense plates that

have been subducted into the lowermost mantle.

4. Spirals in the outer

core represent the flow of material

and electrical current to generate Earth’s magnetic field.

Seismic Velocities of the Lowermost Mantle

4. Recent advances in seismic instruments, computer processing, and numerical ap-proaches led to the discovery that there is a thin layer along the boundary between the core and lowermost mantle. This boundary layer, called D’’ (dee-dou-ble-prime)

is irregular in thickness

and has up-wellings acccord-

ing to this model.

A View of Flow Within the Earth

1. Seismologists and other geologists strive to develop models for the flow of materials within the entire Earth. This figure, from seismologist Ed Garnero, presents one view of the inner workings of Earth. There are many other views.

6. Mid-ocean ridges do not show prominently on this figure, because they are not interpreted to represent large-scale

convection currents in the mantle or upwelling from the lower mantle. Instead, when two oceanic plates spread apart,

the space is filled by local flow of the shallow asthe-nosphere, not by deep mantle flow. There may

be some exceptions, such as where a mid-ocean ridge coincides with a hot spot.

2. In this mod-el, cold, dense material from subducted slabs sinks deeply into the mantle. These slabs are expressed as the blue, fast velocities in Ed’s seismic tomography figure above.

5. This model shows large-scale upwelling of material from the core-mantle bound-ary, corresponding to the red areas in the tomography figure above. Material rising from the tops and edges of these upwell-ings are interpreted as the source areas for mantle plumes and hot spots.

3. The cold slabs are interpreted as locally traveling all the way down to the base of the mantle, where they pile up to form the D’’ layer, which is here greatly exaggerated in thickness.


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Application: What Happened During the Great Alaskan Earthquake of 1964?

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THE SOUTHERN COAST OF ALASKA experienced one of the world’s large earthquakes in 1964. The mag-nitude (Mw) 9.2 earthquake, which is the strongest to have ever struck North America, destroyed buildings, shook loose massive landslides, and unleashed a tsunami that caused damage and deaths from Alaska to California. The event provides an overview of the causes and various manifestations of an earthquake.

What Types of Damage Did the Earthquake Cause?

The epicenter of the earthquake was along the southern coast of Alaska, between the cities of Anchorage and Valdez. The earthquake began at depths of 20 to 30 km. Based on the wide distri-

bution of ~600 after-shocks, the earth-quake is interpreted to have ruptured a fault surface across an area that was over 900 km long and 250 km wide.

What Damage Did the Earthquake Cause on Land?

Other colored lines on the map mark the limits of different kinds of damage related

to the earthquake.

Parts of downtown An-chorage were completely destroyed, when shaking caused the underlying land to slip and collapse. Some buildings sunk so much that some second stories were level with the ground.

Ground shaking destroyed buildings and shook loose huge landslides of rock and soil, including this dark, rocky one that covered parts of the white Sherman Glacier.

Severe damage occurred in the Turnagain Heights area of Anchorage, where a layer of weak clay liquefied, taking shattered houses along for the ride.

The earthquake occurred along the southern coast, but was felt throughout Alaska, except for along the north coast. These yellow circles show distances out from the epicenter in kilometers.


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✓ Summarize the events associated with the Alaskan earthquake, including the effects on land and sea and how measuring uplift of beaches helped lead to the theory of plate tectonics.

What Happened in the Sea During the Earthquake?

How Did Geologists Study the Aftermath of the Earthquake?

1. The USGS team investigated the coastline, mea-suring amounts of uplift and subsidence in hundreds of sites. They plotted and contoured the measurements (in feet) on a detailed map. Numbers are positive for uplift and negative for subsidence.

The earthquake, because it occurred along the coast, was also expressed by faulting and uplift of the sea floor, by huge waves generated by landslides into water, and by a tsunami that struck the coasts of Alaska, British Columbia, Washington, Oregon, California, Hawaii, and Japan.

The main fault that caused the earthquake did not break the land surface, but two sub-sidiary faults did. One fault cut a notch into a mountain and uplifted the sea floor 4 to 5 m (15 ft). The white material on the uplifted (left) side of the fault consists of calcare-ous marine organism that were below sea level before the earthquake. The maxi-mum observed uplift was 11.5 m (38 ft)! Other areas subsided as much as 6 m (20 ft) during the earthquake, flooding docks, oil tanks, and buildings along the coast.

Faulting uplifted a large area of sea floor off the south coast of Alaska, sending a large tsunami out across the sea and up the many bays and inlets along the coast. The highest tsunami recorded was 67 m (220 ft), in a bay near Valdez. The photo above shows damage done to Kodiak by a wave “only” 6 m (20 ft) high. The tsunami killed 106 people in Alaska and 17 more in Oregon and California.

Immediately after the earthquake, the U.S. Geologic Survey dispatched a team of geologists to (1) survey the damage, (2) document the faults, landslides, and other manifestations of the earthquake, (3) understand what had actually happened, and (4) identify risky areas and devise plans to minimize loss from future earthquakes.

2. The detailed map was used to identify broad zones of subsidence and uplift, which to-gether affected an area of over 250,000 km2 (100,000 mi2)! The large size of the area af-fected reflects the huge area of the fault surface that ruptured. 3. USGS geologist George

Plafker constructed a cross section showing how the up-lift was explained by south-ward thrusting of the conti-nent over the oceanic crust. His paper in 1964 predated the idea of plate tectonics

and was a key step that led to development of the theory. This huge earthquake was along a megathrust, where oceanic crust is subducting beneath the continent.


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Investigation: Where Did This Earthquake Occur, and What Damage Might It Cause?

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THE REGION BELOW CONTAINS TWO FAULTS, an active volcano, and a steep-sided mountain prone to landslides. Any of these features could cause ground shaking. You will use seismic records from a recent earthquake to determine which feature caused the observed shaking. From this information, you will decide what potential hazards might affect each of the small towns in the area.

Goals of this Exercise

• Observe the land and sea below and read the text boxes describing the types of features that are present

• Use the seismograms to determine which feature is likely to have caused the earthquake

• Use your understanding of possible hazards from earthquakes to determine what dangers an earthquake and all its manifestations might pose for each small town in the area

• Decide which town you think is the safest from earthquake-related hazards and justify your decision with supporting evidence

The area has a number of small towns and three seismometers, each named after the town in which it is near. Seismograms recorded at each seismic station during a recent earth-quake are shown at the top of the next page. Use the available information to complete the following steps, entering your answers in the appropriate places on the worksheet.

1. Observe the features shown on the three-dimensional perspective. Read the text boxes associated with each location and think about what each statement implies about the significance of that setting with regards to earthquake hazards.

2. Inspect the seismograms for the three seismic stations to determine where in the area the earthquake likely occurred. You can get an idea just from comparing the time intervals between the arrival of P-waves and S-waves for each station.

3. Your instructor may have you to use the graph next to the seismograms to determine the distance from each station to the epicenter and to more precisely locate the epicenter. Detailed instructions for this procedure are listed in topic 12.5 earlier in this chapter.

4. From the general location of the earthquake, infer which geologic feature likely caused the earthquake.

5. Use the information about the topographic and geologic features of the landscape to interpret what types of hazards that recent earthquake posed for each small town. From these considerations, decide which three towns are the least safe and which two are the safest with regards to earthquakes. There is not necessarily one right answer, so explain and justify your logic on the worksheet.

Procedures Along one part of the coastline, there is a very thin steep beach that rises upward sharply to some nearby small mountains. The sea floor offshore is also fairly steep as it drops off to the trench.

Beneath the ocean, there is a deep trench along the edge of the continent. Ocean drilling encountered fault slices of oceanic sediments.

A nearby town, called Sand-point, is built upon land that was reclaimed from the sea by piling up loose rocks and beach sand until the area was above sea level.

Offshore is a coral reef that blocks larger waves, creating a quiet-water lagoon between the reef and the shore.


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Seismograms

One picturesque town, called Hill-side, lies inland of some small moun-tains. The town is build on a flat clear-ing, flanked by hills with fairly gentle slopes.

In the northern part of the area, there is a flat-topped mountain, called Red Mesa, surrounded by steep cliffs. A landslide lies along the south-ern flank of the mountain.

A volcano looms above the right half of the area. It has steep slopes and is surrounded by layers of volcanic ash that appear to have erupted quite re-cently. Every so often, the volcano releases steam and makes rumbling nois-es. The shaking causes landslides to come down the hill sides.

The Gray Cliffs form a nearly vertical step in the landscape. Streams pour over the cliffs in pleasant waterfalls, but then each takes a curious jog to the left after crossing the cliffs. Rocks along the cliffs are fractured and shat-tered.

A small village named Cliffside lies next to a gray cliff. It was built on a marshy area that was underlain by soft, un-consolidated sediments. Several streams drain into the area and no streams are able to leave because the area is lower than the surrounding landscape. As a result, the soil is commonly very soft and causes people to sink in as they walk.

A resort town, called White Sands, is along a white-sand beach. The white sand comes from the offshore coral reef. There is a seismic station, shown by a triangle symbol, in town with the same name as the town.

These seismograms, for the three seismic stations, represent the same time period, from just before the earthquake to 1.5 sec-onds after it. The first arrivals of P-waves and S-waves are labeled for each graph, along with the P-S delay times.

Riverton, a pic-turesque town, is built near to a river at the head of a sandy bay. The sea floor slopes out to the bay at a gentle angle. Muddy wa-ters from the river prevent reefs from growing offshore in front of the bay.

A seismograph station, shown by a triangle symbol, lies just to the east of the town and so is named the Hillside Seismic Station.

A small town and a seis-mic station, both called Mesaview, lie between the mesa and a high

volcano.

Use the graph to the right to determine the distance from each seismic station to the earthquake’s epicenter Find the appropriate time on the horizontal axis, project it upward to the line, and read off the correspond-ing distance on the vertical axis.


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Earthquakes & Earth's Interior