Chapter 10: Earthquakes and Earth

1
Chapter 10 Earthquakes and Earths Interior
2
Introduction

• When the Earth quakes, the energy stored in
  elastically strained rocks is suddenly released.
• The more energy released, the stronger the quake.
• Massive bodies of rock slip along fault surfaces
  deep underground.
• Earthquakes are key indicators of plate motion.

3
How Earthquakes Are Studied (1)

• Seismometers are used to record the shocks and
  vibrations caused by earthquakes.
• All seismometers make use of inertia, which is
  the resistance of a stationary mass to sudden
  movement.
• This is the principal used in inertial
  seismometers.
• The seismometer measures the electric current
  needed to make the mass and ground move together.

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Figure 10.1
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Figure 10.2
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Figure B10.01
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How Earthquakes Are Studied (2)

• Three inertial seismometers are commonly used in
  one instrument housing to measure up-down,
  east-west, north-south motions simultaneously.

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Earthquake Focus And Epicenter

• The earthquake focus is the point where
  earthquake starts to release the elastic strain
  of surrounding rock.
• The epicenter is the point on Earths surface
  that lies vertically above the focus of an
  earthquake.
• Fault slippage begins at the focus and spreads
  across a fault surface in a rupture front.
• The rupture front travels at roughly 3 kilometers
  per second for earthquakes in the crust.

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Figure 10.3
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Seismic Waves (1)

• Vibrational waves spread outward initially from
  the focus of an earthquake, and continue to
  radiate from elsewhere on the fault as rupture
  proceeds.

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Seismic Waves (2)

• There are two basic families of seismic waves.
• Body waves can transmit either
• Compressional motion (P waves), or
• Shear motion (S waves).
• Surface waves are vibrations that are trapped
  near Earths surface. There are two types of
  surface waves
• Love waves, or
• Rayleigh waves.

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Body Waves (1)

• Body waves travel outward in all directions from
  their point of origin.
• The first kind of body waves, a compressional
  wave, deforms rocks largely by change of volume
  and consists of alternating pulses of contraction
  and expansion acting in the direction of wave
  travel.
• Compressional waves are the first waves to be
  recorded by a seismometer, so they are called P
  (for primary) waves.

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Figure 10.4
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Body Waves (2)

• The second kind of body waves is a shear wave.
• Shear waves deform materials by change of shape,
• Because shear waves are slower than P waves and
  reach a seismometer some time after P waves
  arrives, they are called S (for secondary)
  waves.

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Body Waves (3)

• Compressional (P) waves can pass through solids,
  liquid, or gases.
• P waves move more rapidly than other seismic
  waves
• 6 km/s is typical for the crust.
• 8 km/s is typical for the uppermost mantle.

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Body Waves (4)

• Shear (S) waves consist of an alternating series
  of side-wise movements.
• Shear waves can travel only within solid matter.
• A typical speed for a shear wave in the crust is
  3.5 km/s, 5 km/s in the uppermost mantle.
• Seismic body waves, like light waves and sound
  waves, can be reflected and refracted by change
  in material properties.
• When change in material properties results in a
  change in wave speed, refraction bends the
  direction of wave travel.

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Figure 10.5
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Body Waves (5)

• For seismic waves within Earth, the changes in
  wave speed and wave direction can be either
  gradual or abrupt, depending on changes in
  chemical composition, pressure, and mineralogy.
• If Earth had a homogeneous composition and
  mineralogy, rock density and wave speed would
  increase steadily with depth as a result of
  increasing pressure (gradual refraction).
• Measurements reveal that the seismic waves are
  refracted and reflected by several abrupt changes
  in wave speed.

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Figure 10.6
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Surface Waves (1)

• Surface waves travel more slowly than P waves and
  S waves, but are often the largest vibrational
  signals in a seismogram.
• Love waves consist entirely of shear wave
  vibrations in the horizontal plane, analogous to
  an S wave that travels horizontally.
• Rayleigh waves combine shear and compressional
  vibration types, and involve motion in both the
  vertical and horizontal directions.

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Figure 10.7
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Surface Waves (2)

• The longer the wave length of a surface wave, the
  deeper the wave motion penetrates Earth. Surface
  waves of different wave lengths develop different
  velocities. This Behavior is called Dispersion

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Determining The Epicenter (1)

• An earthquakes epicenter can be calculated from
  the arrival times of the P and S waves at a
  seismometer.
• The farther a seismometer is away from an
  epicenter, the greatest the time difference
  between the arrival of the P and S waves.

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Determining The Epicenter (2)

• The epicenter can be determined when data from
  three or more seismometers are available.
• It lies where the circles intersect (radius
  calculated distance to the epicenter).
• The depth of an earthquake focus below an
  epicenter can also be determined, using P-S time
  intervals.

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Figure 10.8
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Figure 10.9
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Earthquake Magnitude

• The Richter magnitude scale is divided into steps
  called magnitudes with numerical values M.
• Each step in the Richter scale, for instance,
  from magnitude M 2 to magnitude M 3,
  represents approximately a thirty fold increase
  in earthquake energy.

28
Earthquake Frequency (1)

• Each year there are roughly 200 earthquakes
  worldwide with magnitude M 6.0 or higher.
• Each year on average, there are 20 earthquakes
  with M 7.0 or larger.
• Each year on average, there is one great
  earthquake with M 8.0 or larger.

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Earthquake Frequency (2)

• Four earthquakes in the twentieth century met or
  exceeded magnitude 9.0.
• 1952 in Kamchatka (M 9.0).
• 1957 in the Aleutian Island (M 9.1).
• 1964 in Alaska (M 9.2).
• 1960 in Chile (M 9.5).

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Earthquake Frequency (3)

• The nuclear bomb dropped in 1945 on the Japanese
  city of Hiroshima was equal to an earthquake of
  magnitude M 5.3.
• The most destructive man-made devices are small
  in comparison with the largest earthquakes.

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Earthquake Hazard

• Seismic events are most common along plate
  boundaries.
• Earthquakes associated with hot spot volcanism
  pose a hazard to Hawaii.
• Earthquakes are common in much of the
  intermontane western United States (Nevada, Utah,
  and Idaho).
• Several large earthquakes jolted central and
  eastern North America in the nineteenth century
  (New Madrid, Missouri, 1811 and 1812).

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Figure 10.10
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Earthquake Disasters (1)

• In Western nations, urban areas that are known to
  be earthquake-prone have special building codes
  that require structures to resist earthquake
  damage.
• However, building codes are absent or ignored in
  many developing nations.
• In the 1976 Tang Shan earthquake in China,
  240,000 people lost their lives.

34
Earthquake Disasters (2)

• Eighteen earthquakes are known to have caused
  50,000 or more deaths apiece.
• The most disastrous earthquake on record occurred
  in 1556, in Shaanxi province, China, where in
  estimated 830,000 people died.

35
Earthquake Damage (1)

• Earthquakes have six kinds of destructive
  effects.
• Primary effects
• Ground motion results from the movement of
  seismic waves.
• Where a fault breaks the ground surface itself,
  buildings can be split or roads disrupted.

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Earthquake Damage (2)

• Secondary effects
• Ground movement displaces stoves, breaks gas
  lines, and loosens electrical wires, thereby
  starting fires.
• In regions of steep slopes, earthquake vibrations
  may cause regolith to slip and cliffs to
  collapse.
• The sudden shaking and disturbance of
  water-saturated sediment and regolith can turn
  seemingly solid ground to a liquid mass similar
  to quicksand (liquefaction).
• Earthquakes generate seismic sea waves, called
  tsunami, which have been particularly destructive
  in the Pacific Ocean.

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Modified Mercalli Scale

• This scale is based on the amount of vibration
  people feel during low-magnitude quakes, and the
  extent of building damage during high-magnitude
  quakes.
• There are 12 degrees of intensity in the modified
  Mercalli scale.

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World Distribution of Earthquakes

• Subduction zones have the largest quakes.
• The circum-Pacific belt, where about 80 percent
  of all recorded earthquakes originate, follows
  the subduction zones of the Pacific Ocean.
• The Mediterranean-Himalayan belt is responsible
  for 15 percent of all earthquakes.

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Figure 10.15
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Depth of Earthquake Foci

• Most foci are no deeper than 100 km. down in the
  Benioff zone, that extends from the surface to as
  deep as 700 km.
• No earthquakes have been detected at depths below
  700 km. Two hypotheses may explain this.
• Sinking lithosphere warms sufficiently to become
  entirely ductile at 700 km depth.
• The slab undergoes a mineral phase change near
  670 km depth and loses its tendency to fracture.

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Figure 10.16
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First-Motion Studies Of The Earthquake Source

• If the first motion of the arriving P wave pushes
  the seismometer upward, then fault motion at the
  earthquake focus is toward the seismometer.
• If the first motion of the P wave is downward,
  the fault motion must be away from the
  seismometer.
• S-waves and surface waves also carry the
  signature of earthquake slip and fault
  orientation and can provide independent estimates
  of motion at the earthquake focus.

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Figure 10.17
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Figure 10.18
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Earthquake Forecasting And Prediction (1)

• Forecasting identifies both earthquake-prone
  areas and man-made structures that are especially
  vulnerable to damage from shaking.
• Earthquake prediction refers to attempts to
  estimate precisely when the next earthquake on a
  particular fault is likely to occur.

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Earthquake Forecasting And Prediction (2)

• Earthquake forecasting is based largely on
  elastic rebound theory and plate tectonics.
• The elastic rebound theory suggests that if fault
  surfaces do not slip easily past one another,
  energy will be stored in elastically deformed
  rock, just as in a steel spring that is
  compressed.
• Currently, seismologists use plate tectonic
  motions and Global positioning System (GPS)
  measurements to monitor the accumulation of
  strain in rocks near active faults.

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Figure 10.19
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Earthquake Forecasting And Prediction (3)

• Earthquake prediction has had few successes.
• Earthquake precursors
• Suspicious animal behavior.
• Unusual electrical signals.
• Many large earthquakes are preceded by small
  earthquakes called foreshocks (Chinese
  authorities used an ominous series of foreshocks
  to anticipate (the Haicheng earthquake in 1975).

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Figure 10.20
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Figure B02
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Figure 10.21
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Using Seismic Waves As Earth Probes (1)

• Seismic waves are the most sensitive probes we
  have to measure the properties of the unseen
  parts of the crust, mantle, and core.
• Distinct boundaries (or discontinuities) can be
  readily detected by refraction and reflection of
  body waves deep within Earth.

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Figure 10.22
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Using Seismic Waves As Earth Probes (2)

• Early in the twentieth century, the boundary
  between Earths crust and mantle was demonstrated
  by a Croatian scientist named Mohorovicic.
• A distinct compositional boundary separated the
  crust from this underlying zone of different
  composition (the Mohorovicic discontinuity).
• Seismic wave speeds can be measured for different
  rock types in both the laboratory and the field.

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Figure 10.23
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Using Seismic Waves As Earth Probes (3)

• The thickness and composition of continental
  crust vary greatly from place to place.
• Thickness ranges from 20 to nearly 70 km and
  tends to be thickest beneath major continental
  collision zones, such as Tibet.
• P-wave speeds in the crust range between 6 and 7
  km/s. Beneath the Moho, speeds are greater than 8
  km/s.

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Using Seismic Waves As Earth Probes (4)

• Laboratory tests show that rocks common in the
  crust, such as granite, gabbro, and basalt, all
  have P-wave speeds of 6 to 7 km/s.
• Rocks that are rich in dense minerals, such as
  olivine, pyroxene, and garnet, have speeds
  greater than 8 km/s.
• Therefore, the most common such rock, called
  peridotite, must be among the principal materials
  of the mantle.

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Using Seismic Waves As Earth Probes (5)

• Some evidence can be obtained from rare samples
  of mantle rocks found in kimberlite pipesnarrow
  pipe-like masses of intrusive igneous rock,
  sometimes containing diamonds, that intrude the
  crust but originate deep in the mantle.

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Using Seismic Waves As Earth Probes (6)

• Both P and S waves are strongly influenced by a
  pronounced boundary at a depth of 2900 km.
• Geologists infer that it is the boundary between
  the mantle and the core.
• Seismic-wave speeds calculated from travel times
  indicate that rock density increases from about
  3.3 g/cm3 at the top of the mantle to about 5.5
  g/cm3 at the base of the mantle.

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Figure 10.25
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Using Seismic Waves As Earth Probes (7)

• To balance the less dense crust and the mantle,
  the core must be composed of material with a
  density of at least 10 to 13 g/cm3.
• The only common substance that comes close to
  fitting this requirement is iron.

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Using Seismic Waves As Earth Probes (8)

• Iron meteorites are samples of material believed
  to have come from the core of ancient, tiny
  planets, now disintegrated.
• All iron meteorites contain a little nickel
  thus, Earths core presumably does too.
• P-wave reflections indicate the presence of a
  solid inner core enclosed within the molten outer
  core.

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Layers of Different Physical Properties in the
Mantle

• The P-wave velocity at the top of the mantle is
  about 8 km/s and it increases to 14 km/s at the
  core-mantle boundary.
• The low-velocity zone can be seen as a small blip
  in both the P-wave and S-wave velocity curves.
• An integral part of the theory of plate tectonics
  is the idea that stiff plates of lithosphere
  slide over a weaker zone in the mantle called the
  asthenosphere.
• In the low velocity zone rocks are closer to
  their melting point than the rock above or below
  it.

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Figure 10.26
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The 400-km Seismic Discontinuity

• From the P-and S-wave curves, velocities of both
  P and S waves increase in a small jump at about
  400 km.
• When olivine is squeezed at a pressure equal to
  that at a depth of 400 km, the atoms rearrange
  themselves into a denser polymorph (polymorphic
  transition).

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The 670-km Seismic Discontinuity

• An increase in seismic-wave velocities occurs at
  a depth of 670 km.
• The 670-km discontinuity may correspond to a
  polymorphic change affecting all silicate
  minerals present.

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Seismic Waves and Heat (1)

• Seismic wave speed is affected by temperature.
• Seismologists translate travel-time discrepancies
  into maps of fast and slow regions of Earths
  interior using seismic tomography.

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Figure 10.27
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Seismic Waves and Heat (2)

• Researchers hypothesize that these slow
  regions are the hot source rocks of most mantle
  plumes.
• Near active volcanoes, seismologists have
  interpreted travel-time discrepancies to
  reconstruct the location of hot and partially
  molten rock that supplies lava for eruptions.

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Figure 10.28
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Earthquakes Influence Geochemical Cycles (1)

• Earthquakes play an important role in the
  transport of volatiles through Earths solid
  interior.
• Earthquakes facilitate the concentration of many
  important metals into ore deposits.
• In the mantle, the carbon and hydrologic cycles
  are fed when the subducting slab releases water,
  CO2, and other volatiles at roughly 100-km depth
  beneath the overriding plate.

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Earthquakes Influence Geochemical Cycles (2)

• Some seismologists speculate that water released
  from the slab helps cause brittle fracture in the
  slab itself, and that water may be necessary for
  deep earthquakes to occur in the Benioff zone.