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Chapter 10: Earthquakes and Earth

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Title: 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.

4
Figure 10.1
5
Figure 10.2
6
Figure B10.01
7
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.

8
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.

9
Figure 10.3
10
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.

11
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.

12
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.

13
Figure 10.4
14
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.

15
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.

16
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.

17
Figure 10.5
18
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.

19
Figure 10.6
20
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.

21
Figure 10.7
22
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

23
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.

24
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.

25
Figure 10.8
26
Figure 10.9
27
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.

29
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).

30
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.

31
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).

32
Figure 10.10
33
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.

36
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.

37
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.

38
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.

39
Figure 10.15
40
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.

41
Figure 10.16
42
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.

43
Figure 10.17
44
Figure 10.18
45
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.

46
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.

47
Figure 10.19
48
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).

49
Figure 10.20
50
Figure B02
51
Figure 10.21
52
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.

53
Figure 10.22
54
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.

55
Figure 10.23
56
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.

57
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.

58
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.

59
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.

60
Figure 10.25
61
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.

62
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.

63
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.

64
Figure 10.26
65
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).

66
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.

67
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.

68
Figure 10.27
69
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.

70
Figure 10.28
71
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.

72
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.
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