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6-Understanding Planet Earth > Chapter 8: Earthquakes Slide Set 8 > Flashcards

Chapter 8: Earthquakes Slide Set 8 Flashcards

1
Q 
The point on the surface of the Earth that lies directly above the place where a slip on a fault occurs is the \_\_\_\_ .   
A. Hypocenter 
B. Focus 
C. Epicenter 
D. All of the above
A

C

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2
Q

On a _________ fault, the hanging-wall block slips down the surface of the fault, relative to the footwall. If the fault displaces the ground surface, a _________ develops.

A. Reverse/fault trace
B. Normal/fault scarp
C. Strike slip/fault trace
D. Reverse/fault scar

A

B

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3
Q

Explain the elastic-rebound theory:

A
  • Rocks bend elastically due to accumulated stresses.

- Rock snaps back after slip along fault releases stress.

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4
Q

major earthquake may be preceded by ____.

A

foreshocks

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5
Q

___ usually follow a large earthquake.

A

Aftershocks

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6
Q

How much does a fault slip during an earthquake?

A
  • Larger earthquakes have larger areas of slip
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7
Q

True/False

Displacement is greatest near the hypocenter

A

True

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8
Q

True/False

Fault slip is cumulative

A

True

Faults can offset rocks by hundreds of kilometers over geologic time.

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9
Q

What are seismic waves?

A

are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.

Can travel along Earths exterior = surface waves
Can travel along Earths interior = body waves

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10
Q

What are the types of seismic waves?

A

Body waves: P waves and S waves

Surface waves: L waves and R waves

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11
Q

What are P waves and its features?

A
  • primary or compressional waves).
  • Waves travel by compressing and expanding material.
  • Material moves back and forth parallel to wave direction. - P-waves are the fastest.
  • They travel through solids, liquids, and gases.
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12
Q

What are S waves and its features?

A
  • secondary or shear waves
  • Waves travel by moving material back and forth.
  • Material moves perpendicular to wave travel direction.
  • S-waves are slower than P-waves.
  • They travel only through solids, never liquids or gases.
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13
Q

True/False

S waves are faster than P waves

A

False

P waves are the fastest

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14
Q

True/False

Surface waves are the fastest and most destructive.

A

FALSE

Surface waves are the slowest and most destructive.

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15
Q

What are L waves and its features?

A

L-waves (Love waves)

  • S-waves that intersect the land surface.
  • Move the ground back and forth like a writhing snake.
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16
Q

What are R waves and its features?

A

R-waves (Rayleigh waves)

  • P-waves that intersect the land surface.
  • Cause the ground to ripple up and down like water.
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17
Q

How are the earthquakes measured?

A

Seismometer—instrument that records ground motion.

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18
Q

How does the seismometer work?

A
  • A weighted pen on a spring traces movement of the frame.
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19
Q

How does the horizontal motion seismometer work?

A

the paper cylinder is horizontal and the weight hangs from a wire.
Sideways back-and-forth movement of the cylinder and the frame relative to the pen causes the pen to trace out waves

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20
Q

How does the vertical motion seismometer work?

A

consists of a heavy weight (like a pendulum) suspended from a spring. The spring connects to a sturdy frame that has been bolted to the ground. A pen extends sideways from the weight and touches a vertical revolving cylinder of paper that has been connected to the seismometer frame.
When an earthquake wave arrives and causes the ground surface to move up and down, it makes the seismometer frame also move up and down. The weight, however, because of its inertia (the tendency of an object at rest to remain at rest), remains fixed in space. As a consequence, the revolving paper roll moves up and down under the
pen and the position of the pen moves relatively away from the reference line

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21
Q

What is a seismogram?

A

the data record by a seismometer. It depicts earthquake wave behavior, particularly the arrival times of the different waves, which are used to determine the distance to the epicenter.

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22
Q

How is the epicenter of an earthquake determined?

A
  • P-waves always arrive first; then S-waves.
  • P-wave and S-wave arrivals are separated in time.
  • Separation grows with distance from the epicenter.
  • The time delay is used to establish this distance.
  • Data from three or more stations pinpoints the epicenter. - The distance radius from each station is drawn on a map. - Circles around three or more stations will intersect at a point.
  • The point of intersection is the epicenter.
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23
Q

What does the difference between the arrival times of P and S waves tell us?

A

P-wave and S-wave arrival times can be graphed. A travel-time curve plots the increasing delay in arrivals. The time gap yields distance to the epicenter.

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24
Q

According to the elastic rebound theory of earthquake generation . . .

A. rock bends elastically before breaking; the sudden break and/or frictional sliding causes earthquakes.
B. rock slowly flows underground, like plastic. After it has flowed a certain distance, an earthquake occurs.
C. most earthquakes occur when bubbles underground burst, like rubber balloons.
D. None of the above

A

A

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25
Q

A mechanical seismograph consists of a pen attached to a suspended weight, and a rolling drum with paper attached. During an earthquake:

A. the weight stays fixed in space, while the drum and paper move.
B. the drum and paper stay fixed in position, while the weight bounces about.
C. both the drum and the weight bounce about.

A

A

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26
Q

How is the earthquake sizes determined?

A

Earthquake size is described by two measurements.

  • intensity
  • magnitude
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27
Q

What is the intensity of an earthquake?

A
  • The severity of damage (intensity) – a local measurement
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28
Q

What is the magnitude of an earthquake?

A
  • The total radiated seismic energy measured at distance via ground motion (magnitude)
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29
Q

What scale is used to measure the intensity?

A

Mercalli Intensity Scale – amount of shaking damage.

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30
Q

Explain how the Mercalli intensity scale works?

A

Roman numerals assigned to different levels of damage.
I = low
XII = high
- Damage occurs in zones.
- Damage diminishes in intensity with distance.

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31
Q

What are the scales that can be used to measure the magnitude of an earthquake?

A 
Richter scale (ML)
Moment magnitude scale (MW)
32
Q

Explain the Richter Scale (ML)

A

related to maximum amplitude at a distance of 100 km using a specific type of seimograph. Best for local measurements (near the epicenter).

33
Q

Explain the Moment magnitude scale (MW)

A

the best measure. Based on characteristics of different seismic waves and the area and displacement of fault slip

34
Q

Magnitude scales are_______ and it is related to _______

A

logarithmic

the energy released

35
Q

To locate the epicenter of an earthquake, what information do you need?

A. the amplitude of the P-waves
B. the velocity of P-waves recorded at a seismic station
C. the velocity of surface waves recorded at a seismic
station
D. the difference in the arrival times of the P- and S-waves,
as recorded at three different stations
E. the amplitude of the S-waves

A

D

36
Q

Which is the correct sequence of seismic waves that will be recorded at a distance from the epicenter?

A. S waves/surface waves/ P-waves 
B. Surface waves/P-waves/S-waves
C. P-waves/S-waves/surface waves 
D. Bogus question: all seismic waves travel at the same 
    velocity
A

C

37
Q

The “intensity” of an earthquake

A. is a measure of the amplitude of vibrations recorded by
a seismograph.
B. is defined by the amount of damage caused by the
earthquake.
C. is a measure of the elevation change of the ground
caused by the earthquake.
D. is a constant throughout a continent for a given
earthquake.

A

B

38
Q

A magnitude 4 earthquake results in ground motion that is ______ the ground motion that occurs during a magnitude 2 earthquake, and releases approximately _____ the amount of energy.

A. 2X/4X
B. 2X/2X
C. 100X/100X
D. 100X/1000X

A

D

39
Q

How are the earthquakes classified in terms of their depth and what plate boundaries are related to them?

A
  • Shallow—divergent and transform boundaries.

- Intermediate and deep—convergent boundaries.

40
Q

Explain the features of earthquakes that occur at the Mid-Ocean Ridges

A

Two types of faulting typify MORs;

  • normal faults at the spreading ridge axis and strike-slip
    faults along the transforms.
  • MOR earthquakes are shallow (<10 km deep) and low
    energy.
41
Q

How deep do the earthquakes happen at convergent boundaries?

A

Convergent plate boundaries have both shallow, intermediate, and deep earthquakes

42
Q

Shallow earthquakes occur on ______.

A

both plates

43
Q

Normal faults form where the downgoing slab bends and large thrust faults occur at the ______.

A

contact between plates.

44
Q

How do the Convergent plate boundary earthquakes occur?

A

The subducting slab bends the overriding plate downward (red circle). The overriding plate can snap back, creating a huge megathrust earthquake.

45
Q

Large megathrust earthquakes are linked to ______ and are deadly.

A

tsunamis

46
Q

What is the Wadati-Benioff zone?

A

Intermediate and deep earthquakes trace the path of the subducting slab (the Wadati-Benioff zone) (blue oval). .

47
Q

What is the deepest depth where an earthquake can occur?

A

Earthquakes are rare below 660 km as the mantle becomes too ductile

48
Q

__________ earthquakes occur in the shallow crust and are usually major disasters.

A

Large continental transform

49
Q

The San Andreas Fault cuts through western California where the ______ shears north and the ________ south.

A

Pacific plate

North American plate

50
Q

The San Andreas is a very active ______ fault seeing hundreds of earthquakes annually

A

strike-slip

51
Q

What are continental rifts and what is the differences and similarities between the earthquakes occur at mid-ocean ridges? Give examples

A

Continental rifts occur where tension and stretching creates normal faults. Rifting generates shallow earthquakes similar to those at the mid-ocean ridge, except these normal faults impact people.

Examples: Basin and Range Province (Nevada, Utah, and Arizona); Rio Grande Rift (New Mexico); East African Rift.

52
Q

What are collision zones and what occurs there?

A
  • Orogenic crustal compression.
  • Continental lithosphere compresses along thrust faults.
  • Earthquakes can be very large.
  • Orogenic uplift creates landslide hazards.
53
Q

What are intraplate earthquakes?

A
  • About 5% of earthquakes are not near plate boundaries.

- Remnant crustal weakness in former fault zones ancient plate boundaries

54
Q

Intraplate vs Interplate earthquakes

A

Intraplate refers to a variety of earthquake that occurs within the interior of a tectonic plate; this stands in contrast to an interplate earthquake, which occurs at the boundary of a tectonic plate

55
Q

How do an earthquake cause damage?

A
  • Ground shaking and displacement.
  • Earthquake waves arrive in a distinct sequence.
  • Different waves cause different motion.
56
Q

Which wave is stronger? S or P?

A

S

57
Q

P waves create ______ motion

A

up and down

58
Q

S waves create ______ motion

A

back and forth

59
Q

What is the motion of the Love waves?

A

L waves follow quickly behind the S-waves.

They cause the ground to writhe like a snake.

60
Q

What is the motion of the R waves?

A
  • The land surface undulates like ripples across a pond.
  • These waves usually last longer than the other kinds.
  • R-waves cause extensive damage.
61
Q

The severity of shaking and damage depends on:

A
  • The magnitude (energy) of the earthquake.
  • The distance from the focus.
  • The nature of the subsurface material.
  • The frequency of the earthquake waves.
62
Q

How does the nature of the subsurface material affect the severity of the damage?

A
  • Bedrock transmits seismic waves quickly = less damage.

- Sediments reflect and refract waves = amplified damage.

63
Q

Earthquakes cause damages such as:

A
  • Ground Shaking (Bridges, roads, buildings collapse)
  • Landslides and avalanches.
  • Liquefaction
  • Fire
  • Tsunami
64
Q

What are landslides and avalanches?

A
  • Shaking causes material on steep slopes to fail.
  • Hazardous slopes bear evidence of ancient slope
    failures.
  • Landslides frequently accompany earthquakes in
    uplands.
65
Q

What are Liquefaction?

A
  • Waves liquefy H2O-filled sediments
  • Groundwater forces grains apart reducing friction.
  • Liquefied sediments flow as a slurry.
  • Sand becomes quicksand: clay becomes quickclay.
  • Sand blows and sand volcanoes disrupt ground surface.
66
Q

What does liquefaction cause?

A
  • Liquefaction causes soil to lose strength.
  • Land, and the structures on it, will slump and flow.
  • Buildings may founder and topple over intact.
67
Q

What are Tsunamis?

A
  • Tsunamis result from displacement of the sea floor.
  • Earthquake, submarine landslide, or volcanic explosion.
  • Faulting displaces the entire volume of overlying water.
  • A giant mound (or trough) forms on the sea surface.
  • This feature may be enormous (up to a ten thousand square mile area).
  • Feature collapse creates waves that race rapidly away.
68
Q

Tsunamis race at jetliner speed across the open ocean and may be almost imperceptible due to _________ and _________

A 
low wave height (amplitude)
long wavelength (frequency)
69
Q

How is Tsunami waves different than Wind waves

A

Wind waves
- Influence the upper ~100 m.
- Have wavelengths of several tens to hundreds of meters. - Wave height and wavelength related to wind speed.
- Wave velocity maximum several tens of km per hour.
- Waves break in shallow water and expend all stored
energy.

Tsunami waves
- Influence the entire water depth.
- Have wavelengths of several tens to hundreds of
kilometers.
- Wave height and wavelength unrelated to wind speed.
- Wave velocity maximum several hundreds of km per
hour.
- Water arrives as a raised plateau that pours onto the land
with no dissipation.

70
Q

What are the signs of Tsunami

A
  • strong ground shaking from an earthquake: if you are on the coast and there is an earthquake, it may have caused a tsunami.
  • unusual sea-level fluctuations: a noticeable rapid rise or fall in coastal waters is a sign that there may be a tsunami approaching. If you see the water recede quickly and unexpectedly from a beach exposing the ocean floor (the so-called drawback).
  • abnormally huge wave: the first wave in a tsunami wave train is usually not the largest, so if you see an abnormally huge wave, even bigger waves could be coming soon.
  • loud ocean roar: if you hear a roaring sound offshore, similar to that of a train or jet aircraft, a tsunami may be approaching
71
Q

Can we predict earthquakes?

A

Yes and no

  • They CAN be predicted in the long term (tens to
    hundreds of years).
  • They CANNOT be predicted in the short term (hours or
    weeks)
72
Q

How is the Long-Term Earthquake Prediction done?

A

Probability of a certain magnitude earthquake occurring. Requires determination of seismic zones and recurrence intervals by:

  • paleoseismology
73
Q

What is paleoseismology?

A

Examining evidence of modern or ancient earthquakes
- Evidence of seismicity—fault scarps, sand volcanoes,
etc.
- Historical records.
- Geologic evidence.

74
Q

What are the precursors do earthquakes have?

A
  • Clustered foreshocks.
  • Crustal strain.
  • Stress triggering.
  • And, possibly:
    Level changes in wells.
    Gases (Rn, He) in wells.
    Unusual animal behavior.
75
Q

All else being equal, which building would more likely survive a large earthquake?

A. one built over a reclaimed swamp, so that the
foundation lies over layers of wet clay
B. one whose foundation is built on an exposure of granite
bedrock
C. one built over recent deposits of wet sand, which were
buried in turn by compacted clay

A

B

76
Q

Which statement about the distribution of earthquakes at convergent plate boundaries is correct?

A. Hypocenters occur in the Wadati-Benioff zone, down to
a depth of about 670 km below the surface.
B. Hypocenters occur only in the downgoing plate.
C. Epicenters occur only on the seaward side of the
trench.
D. Hypocenters occur only in the upper 15 km.

A

A

6-Understanding Planet Earth (18 decks)

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