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Geology 102 > Earthquakes > Flashcards

Earthquakes Flashcards

1
Q

What is the definition of a earthquake

A

the vibration generated by the
sudden release of energy associated
with the rapid movement of rock
along a fault

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2
Q

Define a fault

A

*A fracture (or series of fractures) in the Earth’s crust
*Can occur at locations in the crust where rocks are
under stress and break
*Can also describe the boundary between two plates

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3
Q

What are the features of faults

A

Epicenter, Fault surface, Focus, Seismic waves, Footwall, hanging wall

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4
Q

What is an epicenter

A

a location on
the Earth’s surface
directly above the focus.

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5
Q

What is a fault surface

A

the
fracture surface between
one block and another
along which movement
occurs

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6
Q

What is the focus

A

‘origin’ of the
earthquake. The point
within the Earth’s crust
where movement first
occurred

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7
Q

What are seismic waves

A

waves
of energy that travel
like shock waves from
the focus to the
surrounding area

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8
Q

What are the two most important kinds of seismic waves

A

P-waves and S-waves

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9
Q

Describe the motion of a P-wave

A

This is a type of compression wave. Imagine a slinky and your hand is motion it left and right, so the slinky becomes compressed. The wave travels in the same path of motion as your hand does.

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10
Q

Describe the motion of a S-wave

A

This is a shear wave. The hand motion travels up and down and the wave travels left with it looking like a sine wave.

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11
Q

What are the three main kinds of faults + one that is another variation of the reverse fault.

A

Strike-slip fault, Normal Fault, Reverse Fault
Thrust fault are low-angle reverse faults

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12
Q

Is the strike slip tension or compression?

A

Shearing tension - pulling in opposite directions along a plane

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13
Q

Is the Normal fault tension or compression and where would we expect to see it

A

Tension
occurs mainly at divergent plate boundaries

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14
Q

Is a thrust fault tension or compression and where would we expect to see it

A

Compression
Convergent plate boundary

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15
Q

Is a reverse fault tension or compression and where would we expect to see it

A

Compression
Convergent plate boundary

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16
Q

How do we measure earthquakes

A

Seismometers and seismographs

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17
Q

________ ________ are detected by an instrument called a seismometer and recorded as a paper or electronic trace known as a ____________

A

Seismic waves
Seismogram

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18
Q

Earthqaukes often occur in clusters.
What is the name to describe each of these definitions
1. Biggest earthquake in the cluster
2. occurs before the main shock
3. occurs after the main shock

A
  1. Main shock
  2. Foreshock
  3. Aftershock
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19
Q

Prior to the earthquake the _____________ foci are close to the focus.
After the earthquake the _____________ foci travel away from the focus

A

Foreshock
aftershock

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20
Q

What are the scales we use to measure earthquakes

A

Moment magnitude scale (Mw)
Logarithmic scale

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21
Q

What scale did the moment magnitude scale replace

A

Richter scale

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22
Q

Identify each of the scales as quantitative and qualitative
Richter scale
Moment magnitude
Modified Mercalli intensity scale
Peak ground acceleration

A

Richter Scale: the oldest and best known, quantitative

Moment Magnitude (Mw
): used by seismologists, quantitative

Modified Mercalli Intensity Scale: qualitative

Peak ground acceleration: not a scale technically, used by
engineers, quantitative, site specific

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23
Q

What is the Richter magnitude calculations based on

A

difference between P and S wave arrival times
maximum seismic wave amplitude on a seismograph

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24
Q

Limitations of Richter Magnitude

A
  • Not accurate for describing large earthquakes (8 or greater)
  • Not accurate for deep earthquakes or earthquakes a long way
    away from the seismograph station
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25
Q

The moment magnitude is based on _______ ________

A

energy released

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26
Q

The moment magnitude is similar values to ______ _______ for small to large earthquakes. However, the Moment magnitude is a more precise scale for measuring magnitude larger than __

A

Richter Magnitudes
8

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27
Q

What are the measurements used to determine the moment magnitude

A

based on seismograph measurements and measurements of fault displacement

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28
Q

How is peak ground acceleration measured and what do the measurements depend on

A

Using an accelerogram the maximum ground acceleration during an earthquake, in horizontal and vertical directions

measures ground shaking at a specific location - not magnitude of the earthquake

These measurements depend on the local geology (sand shakes more than solid rock)

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29
Q

What does PGA represent

A

Peak Ground Acceleration

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30
Q

Seismic risk maps present what?

A

Seismic risk maps present expected PGA values for a
region if an earthquake were to occur

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31
Q

What is the modified Mercalli intensity scale based on

A

This scale is based on
observations and
perceptions of people who
felt the earthquake,
therefore it is subjective
(based on personal
experience) and
qualitative.

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32
Q

How many degrees of intensity does the modified Mercalli intensity scale have? What do they depend on?

A

12
The degree of intensity will
vary depending on where
the earthquake occurs

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33
Q

For these descriptions, which is a 1 on the modified Mercalli intensity scale and which is a 12

Detected only by seismic instruments; causes no damage

Earthquake waves cause visible undulations of the ground surface; objects are thrown up off the ground; there is complete destruction of buildings and bridges of all types

A

The first definition would be a 1, the second description is a 12

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34
Q

What are two websites you can report your expirince of a earthquake to then build a MMI map

A

Canada - NRCan website
World - USGS website

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35
Q

What kind of sediments/rock shake more in an earthquake? Order these from Hard(least seismic wave amplification) to soft(most seismic wave amplification)
Stiff soil(mud), Hard rock(igneous rock), Very dense soil and soft rock(sandstone), soils requiring site-specific evaluation(peat, some clays), Soft soil(artificial fill), Rock(volcanic rock)

A

Hard Rock
Rock
very dense soil and soft rock
stiff soil
soft soil
soils requiring site-specific evluations

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36
Q

The softer the geological media (soil, rock, or sediment) under a building or
structure, the _______ likely there will be a failure in an earthquake

A

more

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37
Q

Worldwide, _______ earthquakes happen more than once per month. _________ earthquakes, such as magnitude 2 earthquakes, occur several hundred times a day. To create a mountain system might take several million _________ sized earthquakes over tens of millions of years.

A

Strong
Smaller
Medium

38
Q

What are natural causes for earthquakes

A
  • creation of a new fault by rock fractures along a fault plane
  • movement along an old fault
  • movement of magma below the surface or a volcanic explosion
  • huge landslide
39
Q

What are anthropogenic causes of earthquakes

A

*underground nuclear
explosive tests
*fracking - injections of
water and sand
*wastewater injection
water pressure increases
in inactive faults and
pushes them apart

btw, anthropogenic means human-made

40
Q

Where are earthquakes concentrated in Canada

A

Along the west coast - plate tectonic boundaries

several in Ontario and Quebec along old fault zones, a weakness in the crust

41
Q

Why would earthquakes occur in northern Canada

A

Northern Canada
may relate to retreat of
glaciers 1000s of years
ago - land slowly rising
(was depressed by the
glaciers)

42
Q

What are the magnitude of earthquakes in Estevan and Radville

A

M4-M5, Iv - V Mercalli

Due to rocks slowly
dissolving at depth,
shifting along faults.

43
Q

Describe elastic rebound theory

A
  • Energy is stored in the form of elastically deformed rock.
  • When strain exceeds rock strength, the rock fractures and energy is released
44
Q

Stages of elastic rebound theory occurring

A
  1. Curst at rest (no strain)
  2. Crust deformed, strain accumulates (strain build up)
  3. Ruptured crust causing earthquake (rupture with elastic rebound)
  4. Fractured crust, Fault zone (strain released)
45
Q

What is the elastic limit?

A

The breaking point

46
Q

What occurs when you exceed the breaking point during an earthquake

A

The material will fracture

47
Q

Earthquakes happen when the _______ _______ of the rock along a fault with accumulated stresses is __________

A

Elastic limit
exceeded

48
Q

The sudden release of energy, as
the rock ‘snaps’ or fails, generates
a series of ‘shock waves’ called
______ _______, that move through
the rock

A

Seismic waves

49
Q

Which of these statements are true:
Stress - Force/area - elastic bending
Strain - Force/area - elastic bending
rupture - Force/area - elastic bending
Stress - deformation - rupture formation
Strain - deformation - rupture formation
Rupture - deformation - rupture formation
Strain - slip and vibration
Stress - slip and vibration
Rupture - slip and vibration

A

Stress - Force/area - elastic bending
Strain - deformation - rupture formation
Rupture - slip and vibration

50
Q

Which type of waves travel through the earths interior and consist of P-waves and S-waves

A

Body waves

51
Q

Which type of wave travels along the surface of the lithosphere and consist of love waves and Rayleigh waves

A

Surface waves

52
Q

Seismic waves are detected by an
instrument called a ____________. They
contain several instruments that detect
motion in different directions.

A

seismometer

53
Q

The results are recorded as a paper
or electronic trace known as a
______________. _____ waves arrive first,
then ______ waves, then ______ waves.

A

seismogram
P
S
Surface

54
Q

How fast the
waves travel
depends on the
rock they are
travelling
through – ________
materials, faster
travel!

A

Harder

55
Q

_________ is used to determine the layers inside the earth such as the crust, Mantle, and core

A

Seismology

56
Q

The more consolidated / cemented the rock, or the
denser the minerals in the rock, the ______ the speed of
a wave

A

Faster

57
Q

What is rarefraction

A

“bending” of the seismic waves as they
move from one material to another of different density

58
Q

Why is wave refraction used

A

is used to identify a discontinuity (change) e.g. a
discontinuity in the rock density and/or composition

59
Q

What is wave reflection

A

waves “bounce” off surfaces

60
Q

If a wave travels from a less dense rock to a more dense rock, will the refracted wave be bent down or up?

A

Refracted wave bent up

61
Q

If a wave travels from a more dense rock to a less dense rock, will the refracted wave be bent down or up?

A

Refracted wave bent down

62
Q

Can S-waves and P-waves travel through solids and/or liquids?

A

S-waves travel through solids but not liquids
P-waves travel through solids and liquids

63
Q

Are s-waves or P-waves detected first at a seismic station?

A

P-waves

64
Q

Are P-waves or S-waves analogous to sound waves?

A

P-waves

65
Q

Are P-waves or S-waves faster?

A

P-waves

66
Q

How many degrees is the P-wave shadow zone?

A

150 - 103 = 47 degrees

67
Q

How many degrees is the S-waves shadow zone?

A

It begins at 103 degrees on both sides, waves cannot travel through the core of the earth.

68
Q

Do P-waves or S-waves cause the most damage?

A

S-waves

69
Q

_______ ________ confirm the existence of
discontinuities in Earth’s interior

A

Seismic data

70
Q

How can we tell there are discontinuities in the earths interior using seismic data

A

S waves are attenuated (stopped) by liquid
outer core layer
* P waves refract when they go from one
layer to another and there is a change in
composition
* By studying seismic data at seismograph
stations around the world, we can see the
evidence for the Earth’s layers!

71
Q

Describe the movement of love waves

A

cause horizontal shifting at the surface of the earth

72
Q

Describe the movement of Rayleigh waves

A

rotating waves along surfaces, create
vertical displacement (movement), like ripples on water

73
Q

Divergent Boundaries:
_______ earthquakes within ocean basins, foci are located:
Along _______ _______
Along ________ ______

A

Shallow
mid-ocean ridges
transform faults

74
Q

Transform-Fault boundaries:
Transform faults along plate boundaries have _______ earthquakes

_______ _______ mechanism due to shearing forces from tectonic plate movement.

Can produce ______, _________ earthquakes

A

many
strike-slip fault
large, destructive

75
Q

Convergent Boundaries:
Largest earthquakes occur at __________ zones.
-called __________ earthquakes
-Overriding plate thrust _______ relative to the subducting plate

The three _______ earthquakes ever measured by seismometers were of this type

A

Subduction
megathrust
upward
largest

76
Q

*Zone of seismicity along the plane of the subducting plate is
called

A

the Wadati-Benioff Zone

77
Q

The size or magnitude of an earthquake is related to

A

depth

78
Q

___% of earthquakes occur at a depth less than 100 km

A

90

79
Q

____ severe earthquakes are shallow earthquakes (focus is in the upper 70km)

A

Most

80
Q

Are the focus depths off the west coast of BC shallow or deep?

A

Shallow

81
Q

The subducting juan de fuca plate is at a very shallow angle. This creates a weak or strong earthquake?

A

Strong

82
Q

Intense __________ of the plates where they
collide, causes folding and faulting, thrust faults

A

compression

83
Q

Earthquakes at ________-_________ __________
plate boundaries are generally shallow, spread over
a wide area in the region compressed by the
collision

A

continent-continent convergent

84
Q

What is an example of a continent-continent convergence

A

Himalayas

85
Q

What are the ways earthquakes cause damage

A

Faulting and shaking – primary hazards
*Landslides
*Liquefaction
*Tsunamis
*Fires

86
Q

What does liquefaction mean

A

*Shaking causes water-saturated
sediment to temporarily behave like a
fluid
*Friction decreases as water moves
between particles
*Particles consolidate and pore waters are
expelled

87
Q

Fill in the blanks for these characteristics of tsunamis:
* Undersea megathrust earthquakes often generate __________
waves
* Thrust faulting leads to ______ seawater displacement, forms
water wave
* Waves travel ________ from epicenter, when reach shore slow
down and amplify vertically

A

destructive
vertical
outward

88
Q

A tsunami is only a few _________ high in the deep ocean but can _________ to _______ _______ high close to shore

A

centimeters
increase
many meters

89
Q

The more consolidated/cemented the rock, the ______ the speed of a wave

A

faster

90
Q

As speed decreases, wave amplitude _________ and
so does the potential for damage

A

increases

91
Q

Where is seismic risk higher

A

*Delta sediments
*Floodplain
sediments
*Mountain slopes
*Saturated
materials
*Thicker sediments

92
Q

anthropogenic causes of earthquakes

A

mining, injecting water, oilfield activity

Geology 102 (11 decks)

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