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Geology 202 > Earthquakes And Engineering > Flashcards

Earthquakes And Engineering Flashcards

1
Q

Where do Earthquakes Happen?

A

Along plate boundaries, namely the ring of fire, alpide belt, circum-pacific belt, and the mid Atlantic ridge

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2
Q

What Causes Earthquakes

A

Stress in the rust caused by plate tectonics

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3
Q

What is a Seismograph?

A

Record the amplitude of ground movement as to record earthquake arrival

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4
Q

Fault types

A

Dip-slip faults
— normal
— reverse
— thrust
Strike-slip faults
— left thermal strike-slip
— right lateral strike slip

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5
Q

Normal fault

A

Extension, hanging wall goes down

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6
Q

Reverse fault

A

Compression, Hanging wall goes up, high angle

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7
Q

Thrust fault

A

Compression, Hanging wall goes up, at a low angle

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8
Q

Left lateral strike-slip fault

A

Shear, left plate goes down, has mountains

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9
Q

Right lateral strike-slip fault

A

Shear, Right plate goes down, has mountains on it

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10
Q

Why do faults occur?

A

Directed stress builds up in rocks by…
1. Compression
2. Tension
3. Shear (slipping along a plane parallel to the stress

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11
Q

Strain

A

An Accumulation of stress in a rock produces that deformation
Three stages:
1. Elastic deformation
2. Ductile defomation
3. Brittle deformation

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12
Q

Elastic deformation

A

Fully reversible, stretch and bounces back

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13
Q

Ductile defomation

A

Irreversible, can stretch but wont go back to its original shape

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14
Q

Brittle deformation

A

Fracture
Stretch and it breaks

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15
Q

Faults

A

Planar fractures in rocks where the rocks on either side have moved
Vary in size (can be mm)

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16
Q

Elastic rebound theory

A

When sufficient strain energy has accumulated in rocks,they may rupture rapidly (like how a rubber band breaks when it is stretched too far) and the stored energy is released as vibrations that radiation outward in all directions
1. Stress gets added
2. Rocks deform and store energy
3. Rocks break and release energy (earthquake)
4. Rocks rebound to undeformed shape

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17
Q

Rupture zone

A

The part of the fault that slips
Can’t predict how large/ when a rupture occurs

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18
Q

Focus/hypocentre?

A

Where pinpoint within the earth where the rupture starts

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19
Q

Epicentre

A

The location on the surface where the seismic waves first hit
Directly above focus

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20
Q

Where do the deepest earthquakes occur?

A

The ring of fire (circum pacific seismic belt)

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21
Q

Most earthquake foci occur at depths of <15km in the crust. Why don’t earthquakes occur in the mantle?

A

The deeper you go, the more ductile deformation you get. Without the brittle deformation, Faulkner cannot be generated.

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22
Q

Where can shallow, intermediate and deep earthquakes occur?

A

Subduction zones

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23
Q

Where can only shallow earthquakes occur?

A

Transform and divergent boundaries
Upper 15km of crust

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24
Q

body waves

A

Generated at the focus and travel through the interior of the earth
Two types:
1. P-wave
2. S-wave

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25
Q

P-waves

A

Primary waves
Compression-extension, longitudinal waves
Velocity increases with depth, fastest waves
Pass through solid and liquid

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26
Q

S-waves

A

Secondary
Transverse, shear waves
Produce ground motion perpendicular to direction of travel
Can only travel through solids (bc liquids and gases cannot be ruptured)
Slower

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27
Q

Surface waves

A

Arrival of body waves at the surface of the earth
—greatest amplitude in near-surface layers of sediment
— most destructive earthquake waves
Two types:
1. Love
2. Rayleigh

28
Q

Love waves

A

Side to side motion on the surface
Third fastest speed

29
Q

Rayleigh waves

A

Slowest wave,
up and down, forward and back motion, cyclical almost

30
Q

Are deep or shallow earthquakes likely to cause more damage?

A

Shallow

31
Q

What waves cannot travel through the outer core? Why?

A

S-waves cannot travel through the liquid outer core because they do not have any shear strength

32
Q

Velocity of ___ change with ___

A

Velocity of waves change with density. Hence, refraction nearer to core

33
Q

Modified mercantile scale

A

What you feel when in an earthquake
Weirdly specific

34
Q

Richter magnitude scale

A

— amplitude of the largest wave is measured and the time interval between the P and S waves to determine the distance from the epicentre

— very imperfect, doesn’t accurately assign magnitude for large (duration), deep or far away quakes.
— for local magnitude
— doesn’t address litholgy

35
Q

The moment magnitude (Mw)

A

accounts for the amount of displacement and area of rupture along a fault

36
Q

Can we predict earthquakes?

A

We can predict where, but not when
— because earth is hard to model (dynamic)
—. And there are lots of small quakes (hard to measure)

37
Q

What hazards are associated with earthquakes?

A

— rupture zone surfaces
— landslides, which lead to
— flooding
— fires
— liquefaction (loose sediment+water+ground shaking)
—tsunamis

38
Q

How do we engineer for earthquakes?

A

— moment frames (strengthen joints)
— bracing systems (brace building, ductile)
— shear wall system (prevent building from shearing)
— dampers (reduce vibrations)
— seismic base isolations (wheels that take the weight of the base of the structure)

39
Q

Rock mechanics

A

The study of the properties and mechanical behaviour of rock materials in response to the forces acting on them within their physical environment
(When will the rocks fail)
(Developed bc of excavating tunnels)

40
Q

Mass wasting

A

The downslope movement of earth materials due to gravity
Classified by:
— type of movement (flow, slide, fall)
— type of material (rock or sediment or ice or snow)
— velocity

41
Q

Falls

A

Free fall of earth materials
Rocks loosened by…
— root growth
— frost wedging
— heavy precipitation

42
Q

Slides/landslides

A

—coherent masses of earth material slide down a slope along a failure surface called a slide plane within well-defined boundaries
— slump - curved failure surface
— Glide - planar failure surface

43
Q

Slumps

A

Slow slope failures along a curved slide surface
—blocks rotate during failure
—. Usually occurs in homogenous substrate, as opposed to strongly stratified (layered) and lithified (stuck together) rock masses

44
Q

Scarp

A

A steep scar on the undisturbed side of the failure. Zone of detachment

45
Q

Block glides

A

Occur when coherent masses of rock or sediment move along planar sliding surfaces
Failure planes can be:
— sedimentary bedding planes
— metamorphic folioated planes
— faults
—fractures

46
Q

Flows

A

Mass movements of unconsoildated material move overland
— fluid-like behaviour (sediments+water)
— caused by rainfall, steep slopes, lack of vegetation, presence of loose soil and debris

47
Q

Creep

A

—imperceptibly slow downslope movement of rock sand soil particles near the ground surface
— appears o be continues, but is the result of numerous, minute, discrete downslope movements
— rate depends of steepness of slope, water content, type of sediment, and vegetation

Hills are sound bc of creep
Freeze0-thaw cycles or thermal expansion and concatenation cause creep

48
Q

What are the effects of soil creep

A

Objects resting on top of the soul are carried by it as it deceaseds down the slope

49
Q

Fast flows

A

—dense mixtures of sediment and water
— rock avalanches: rock fragments
— debris flows: course sediment
— mudflow: mud

50
Q

Pore pressure

A

— in unsaturated sediments, water tension pulls grains toward each other (contraction)
— in starred sediments, pore pressure pushes grains apart (expansion)
—- pressure+tension = separated grains = fluid-like behaviour

51
Q

Debris flows

A

— behave like a fluid and can be very fast
— most dangerous of all mass movements
— occur when heavy rainfall, snowmelt, or dam-burst water mixes with loose soil and rock on a sloping surface
— often gets funnelled into channel and posited on the valley floor

52
Q

Rock mass properties

A

— A large body of rock
— Broken up by discontinuities that divide it into smaller blocks of intact rocks
— gives rocks discontinuous and anisotropic character

53
Q

Anisotropic

A

Different properties in different directions

54
Q

Types of discontinuities

A
  1. Bedding planes (sedimentary)
  2. Joints/cracks (breaks without displacement/movement)
  3. Faults (breaks with displacement/movement)
  4. Foliation (metamorphic rocks with layered formed perpendicular to stress)
55
Q

How do we know when a rock will fail and the extent of the failure?

A
  1. How strong the rock mass is
  2. How stresses are redistributed due to interactions between the rock mass and the structure
56
Q

How do we determine the strength of a rock mass?

A
  1. The strength of the intact rock and
  2. the strength of any discontinuities
    — strain changes the shake and/or volume of a rock
57
Q

How do we measure intact rock strength?

A

Unconfined compressive strength test (UCS)
— diving the maximum load at faerie bu the cross-sectional area of the sample (o=F/A)

58
Q

What is rock strength

A

The maximum amount of stress you can apply to a rock before it fractures

59
Q

What are the strongest rocks according to UCS

A

Igneous rocks are string than sedimentary rocks because they have an interlocking structure and less planes of weakness
More metamorphizized, the stronger the rock
EXTRUSTIVE, APHANETIC, IGNEOUS ROCKS. BASALT>GRANITE

60
Q

What controls rock strength?

A
  1. Rock type (sedimentary, metamorphic, igneous) (texture, mineral assemblage, structure)
  2. Confining pressure (deeply buried, expressionist compression in all directions)
  3. Water (unsaturated vs saturated (porepressure))
  4. Amount and duration of stress (stress over a long time means more plastic like defomation. Like silly putty)
  5. Weathering (reduces rock strength)
61
Q

Pore pressure

A

Under saturated conditions, water experiences confining stress and pushes back equally in all directions

62
Q

How does weathering reduce rock strength

A

Increases pore space and permeability, increasing surface area exposed to wreathing
Weathering = more weathering

63
Q

What controls the strength of discontinuities?

A
  1. Surface roughness (rough surfaces act to inc4ease friction)
  2. Joint width (hairline fractures are stronger than gapped fractures)
  3. Extent of weathering in fracture planes (weathered planes tent to be weaker)
  4. Water (wet fractures reduce friction) (flowing fractures elevate pore pressure and reduce stress)
  5. Continuity
  6. Spacing (closely spaced joints reduces rock strength)
  7. Orientation (the steeper the dip plane, the more likely the failure)
64
Q

Daylighting

A

Fracture surface dips in the same direction as the slope of the outcrop

65
Q

How do we classify rock mass strength?

A

RQD: Rock Quality Designation
‘RNR: Rock Mass Rating

66
Q

RQD

A

Describes the mechanical quality of rock recovered why taking a core.
High RQD, high rock strength

RQD = (sum of lengths of core pieces over 10cm)/(total length of core) x 100

67
Q

RMR

A

Semi-quantitative measure of the strength or stability of the rock mass

Geology 202 (2 decks)

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