GHC Ch 3: Earthquake Geology and Seismology Flashcards Preview

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Flashcards in GHC Ch 3: Earthquake Geology and Seismology Deck (106):
1

What are earthquakes most commonly caused by?

Caused most commonly by movement of the Earth across a fault.

2

Fault

Complex zones of breakage in the Earth across which the two sides move relative to each other. Has irregular surfaces, and is many miles wide and long.

3

How does an earthquake occur?

Stresses build up until they are enough to cause rocks to fracture and shift, sending off waves of seismic energy, felt as earthquake.

4

Understanding faults begins with understanding what rock relationships, formalized by Steno?

Law of original horizontality and Law of superposition

5

Law of original horizontality

sediments are originally deposited in horizontal layers

6

Law of superposition

in un-deformed sequence of sedimentary rock layers, each layer is younger than the layer beneath it and older than the layer above it

7

Law of original continuity

sediment layers are continuous

8

If a sedimentary layer ends abruptly, it may have been...?

eroded by water action or truncated by fault passing through layer

9

Identifying truncated sedimentary layers and recognizing their offset continuation allows determination of...?

fault length

10

Length of fault determines size of...?

earthquake possible on fault

11

Longer fault ruptures create bigger or smaller earthquakes?

bigger earthquakes

12

Where does 'jointing' occur?

where brittle lithospheric rocks fracture and crack

13

Large stress differential on either side of a fracture results in movement, and then...?

fracture becomes a fault

14

Movement, ranging from millimeters to hundreds of kilometers, results in what?

tilting and folding of layers

15

Use strike and dip to describe what?

describe location in 3D space of deformed rock layer

16

Dip

angle of inclination from horizontal of tilted layer

17

Strike

compass bearing of horizontal line in tilted layer

18

Dip-slip faults are dominated by what movement?

vertical

19

Miners refer to the block beneath them as the...?

footwall (block beneath the fault)

20

Miners refer to the block above them as...?

the hangingwall (block above the fault)

21

what are dip-slip faults caused by?

pushing or pulling force

22

Strike-slip faults are dominated by what movement?

horizontal

23

right-lateral strike-slip fault

When straddling a fault, if right-hand side moved towards you

24

left-lateral strike-slip fault

When straddling a fault, if the left-hand side has moved towards you

25

How does an earthquake occur at a fault?

Stress produces strain , which builds up until rupture occurs at weak point and propagates along fault surface

26

hypocenter or focus

Point where rupture first occurs

27

epicenter

Point directly above hypocenter on surface

28

Fault rupture

series of events over weeks to months to years, with largest event referred to as ‘the earthquake’

29

foreshock

Smaller events preceding ‘the earthquake’

30

Can you identify a foreshock before the earthquake has occurred?

no

31

aftershocks

Smaller events after ‘the earthquake’

32

Left step in right-lateral fault or right step in left-lateral fault

Compression, uplift, hills and mountains

33

Right step in right-lateral fault or left step in left-lateral fault

Extension, down-dropping, basins and valleys

34

transform faults

faults link spreading centers, or connect spreading center to subduction zone. transform fault motion is same as strike-slip fault

35

Seismology

study of earthquakes

36

seismometers

Instruments to detect earthquake waves

37

seismographs

Instruments to record earthquake wave

38

How does a seismograph work?

One part stays as stationary as possible while Earth vibrates: heavy mass fixed by inertia in frame that moves with the Earth, and differences between position of the frame and the mass are recorded digitally

39

Amplitude of a wave indicates

displacement

40

Wavelength indicates

distance between successive waves

41

Period

time between waves (= 1/frequency)

42

Frequency

number of waves in one second (hertz)

43

Seismic waves come in what two categories?

those that can pass through entire Earth (body waves) and those that move near surface only (surface waves)

44

Are body waves faster or slower than surface waves?

faster

45

Where are body waves most energetic?

near earthquake hypocenter

46

Do body waves have short or long periods?

short

47

What are the two types of body waves?

P and S

48

Which is the fastest of all waves?

P waves

49

Are the P or S waves the first to reach a recording station?

P waves

50

How does a P wave move?

as push-pull – alternating pulses of compression and extension, like wave through Slinky toy

51

Where can a P wave travel?

Through solid, liquid, and gas.

52

What kind of motion does an S wave exhibit?

transverse motion – shearing or shaking particles at right angles to the wave’s path (like shaking one end of a rope)

53

Where can an S wave travel?

Only through solids

54

What is interesting about the recording of large earthquakes?

Waves from large earthquakes can pass through the entire Earth and be recorded all around the world

55

Where do surface waves travel?

near the Earth’s surface

56

What creates surface waves?

body waves disturbing the surface

57

Do surface waves have longer or shorter periods than body waves? What does this mean?

Longer period than body waves. Means surface waves carry energy farther.

58

What are love waves?

Similar motion to S waves, but side-to-side in horizontal plane

59

Are love or Rayleigh waves faster?

Love

60

Where do love waves not move?

through air or water

61

How do rayleigh waves move?

Backward-rotating, elliptical motion produces horizontal and vertical shaking, which feels like rolling, boat at sea.

62

When is more energy released as Rayleigh waves?

when earthquake hypocenter is close to the surface

63

How far can a rayleigh wave travel?

Great distances

64

You need the distance of an earthquake from how many stations to pinpoint location of earthquake?

three

65

How does the Richter scale work?

Bigger earthquake means greater shaking, which produces greater amplitude of seismogram lines

66

What does an increase by one mean on the Richter scale in regards to magnitude?

10 fold increase in recorded amplitude

67

What does an increase by one mean on the Richter scale in regards to energy?

the energy release increases by about 45 times, but energy is also spread out over much larger area and over longer time

68

Bigger earthquake means more people will experience shaking and for longer or shorter time?Does this increase or decrease damage to buildings?

longer. increase.

69

What is one problem with the Richter scale?

Does not work well for distant or large earthquakes

70

Are all magnitude scales equivalent? Why?

no. Larger earthquakes radiate more energy at longer periods which are not measured by Richter scale or body wave scale, so large or distant earthquake magnitudes are underestimated

71

What does a Seismic moment (Mo) measure?

Measures amount of strain energy released by movement along whole rupture surface

72

Is the Moment Magnitude Scale more accurate for large or small earthquakes?

large

73

How is a Seismic moment (Mo) calculated?

Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault

74

What is the equation for a seismic moment?

Mw = 2/3 logsub10 (Mo) – 6

75

Large earthquakes are not just single events but...?

part of series of earthquakes over years

76

mainshock

Largest event in series of earthquakes.

77

foreshocks

Smaller events preceding mainshock

78

aftershocks

Smaller events following mainshock

79

Fault-rupture length greatly influences what?

magnitude

80

Fault-rupture length and duration influence

seismic wave frequency

81

Short fault rupture and duration mean

high frequency seismic waves

82

Long fault rupture and duration mean

low frequency seismic waves

83

Seismic wave frequency influences

damage

84

What do high frequency seismic waves do?

cause much damage at epicenter but die out quickly with distance from epicenter

85

Where do low frequency seismic waves do the most damage?

farther away

86

Where do low frequency seismic waves travel?

great distance from epicenter

87

What direction of forces are buildings designed to handle? How does this affect building damage in an earthquake?

Vertical. Horizontal shaking during earthquakes can do massive damage to buildings.

88

How is movement measured in an earthquake?

Measure in terms of acceleration due to gravity

89

Weak buildings suffer damage from horizontal accelerations of more than

0.1g

90

Periods of swaying are about how many seconds per story?

0.1

91

What affects building periods?

building materials

92

What induces longer periods of shaking?

Flexible materials (wood, steel)

93

What induces shorter periods of shaking?

Stiff materials (brick, concrete)

94

Velocity of seismic wave depends on what?

on material through which it is moving

95

When waves pass from harder to softer rocks, they slow down and must therefore increase their what in order to carry same amount of energy?

amplitude

96

resonance

If the period of the wave matches the period of the building, shaking is amplified and resonance results

97

Mercalli scale

developed to quantify what people feel during an earthquake

98

What should one do before an earthquake?

Inside and outside your home, visualize what might fall during strong shaking, and anchor those objects by nailing, bracing, tying, etc.
Inside and outside your home, locate safe spots with protection – under heavy table, strong desk, bed, etc.

99

What should one do during an earthquake?

Duck, cover and hold
Stay calm
If inside, stay inside
If outside, stay outside

100

What are the variables of the Mercalli scale?

Earthquake magnitude
Distance from hypocenter
Type of rock or sediment making up ground surface

101

How can one eliminate resonance?

Change height of building
Move weight to lower floors
Change shape of building
Change building materials
Change attachment of building to foundation
Hard foundation (high-frequency vibrations)  build tall, flexible building
Soft foundation (low-frequency vibrations)  build short, stiff building

102

Shear Walls

Designed to receive horizontal forces from floors, roofs and trusses and transmit to ground

103

Braced Frames

Bracing with ductile materials offers resistance

104

Base Isolation

Devices on ground or within structure to absorb part of earthquake energy

105

What kind of houses houses perform well in earthquakes?

Modern 1-, 2-story wood-frame houses

106

What are 5 ways a building can be made to withstand earthquakes?

(1) brace it (2) Infill it (3) frame it (4) buttress it (5) isolate it