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GEOG 235 > Earthquakes > Flashcards

Earthquakes Flashcards

1
Q

Earthquakes

A

Earthquakes are the shaking caused from the rupture and subsequent displacement of rocks (one body of rock moving with respect to another) beneath Earth’s surface

Earthquakes commonly occur on faults at or near plate boundaries; friction near these boundaries exerts strain or deformation

The principle of elastic deformation explains how and why earthquakes occur

EQs do not always occur at or near boundaries; hint at intraplate EQs.

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2
Q

Earthquake: Focus

A

At the point of rupture, seismic waves radiate outwards producing the shaking

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3
Q

Stress

A

Stress is a force applied over an area; when stress is applied to rock it results in deformation

If stress is not equal from all directions then the stress is a differential stress.

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4
Q

Tensional Stress

A

Tensional stress stretches rock (divergent plate boundaries)

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5
Q

Compressional Stress

A

Compressional stress pushes rock together (convergent plate boundaries)

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6
Q

Shear Stress

A

Shear stress results in slippage and translation

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7
Q

Confining Stress

A

Confining stress is a type of uniform stress resulting from the pressure due to the weight of overlying rocks

One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is where the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress and is referred to as confining stress.

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8
Q

Strain and Deformation

A

When a rock is subject to stress it changes its size, shape, or volume

The change in size, shape, or volume is referred to as strain

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9
Q

Elastic Deformation

A

Elastic deformation- strain is reversible

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10
Q

Permanent Deformation

A

Permanent deformation- strain is irreversible; two types: brittle vs ductile

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11
Q

Fracture

A

Fracture- irreversible strain wherein the material breaks

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12
Q

Brittle materials

A

have a small to large region of elastic behaviour and a small region of ductile behaviour before they fracture.

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13
Q

Ductile materials

A

have a small region of elastic behaviour and a large region of ductile behaviour before they fracture.

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14
Q

Permanent Deformation

A

High temperature and confining pressure result in ductile deformation; at shallow depths with low temperature and confining pressure brittle deformation predominates, leading to elastic deformation and earthquakes

Rocks that contain quartz, olivine, and feldspar (crust) are very brittle; rocks containing clay, micas, or calcite are more ductile

Water weakens the chemical bonds in rocks and increases slippage promoting ductile behaviour; dry rocks tend to behave in a brittle manner

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15
Q

Strain Rate

A

Strain rate refers to the rate at which deformation occurs; at high or variable strain rates materials tend to fracture and at low and gradual rates materials are ductile

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16
Q

Confining Pressure

A

At high confining pressure materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend to fracture sooner.

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17
Q

Temperature

A

At high temperature molecules and their bonds can stretch and move, thus materials will behave in more ductile manner. At low Temperature, materials are brittle.

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18
Q

Composition

A

Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas, and calcite are more ductile This is due to the chemical bond types that hold them together. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behavior of the rock. Another aspect is presence or absence of water. Water appears to weaken the chemical bonds and forms films around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner, while dry rocks tend to behave in brittle manner.

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19
Q

Measuring Earthquakes

Magnitude

A

estimates the amount of energy released from the earthquake

Local or Richter Magnitude (ML)

Body Wave Magnitude (MB)

Surface Wave Magnitude (MS)

Moment Magnitude (Mw or M)

The body wave and surface wave magnitudes are offshoots of the Richter magnitude using those particular waves only

The most popular and well known is the Richter magnitude which is also known as the local magnitude. The Richter or local magnitude is the worst of the four, which we will see in a minute.

The fourth and least well known is the best estimate of the size of an EQ and that is the moment magnitude

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20
Q

Measuring Earthquakes

Intensity

A

a measurement of damage that varies according to proximity to earthquake and depending on the composition of subsurface materials

Measured by the Modified Mercalli Intensity Scale

Shaking: typically measured as acceleration (g). Earthquake shaking is typically measured as acceleration; higher magnitude EQ cause more violent shaking, which in turn cause higher intensity

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21
Q

Local or Richter Magnitude

A

The local or Richter magnitude (ML) is the logarithm of the amplitude (measured in thousandths of millimetres or microns) of the largest seismic wave measured 100 km from the epicentre on a particular brand of seismometer.

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22
Q

Problems with the Richter Magnitude Scale

A

It is logarithmic, meaning, for each increase in the magnitude there is a ten-fold increase in the shaking (may cause public confusion)

It measures the largest seismic wave no matter what type it is: p, s, or surface

It is defined for a seismograph 100 km from the epicentre which is unlikely (thus error-prone calculations must be made)

The model of seismograph that Richter used is no longer in service

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23
Q

Moment Magnitude

A

The Mw or M scale is the most common magnitude scale in use by seismologists today

This scale is based on the seismic moment (Mo):
Mo = υAd

The seismic moment is determined by multiplying the amount of slip on the fault (d), the area of rupture on the fault plane (A), and the strength of the rock (υ)

The moment magnitude is also logarithmic, and in the same way may cause confusion

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24
Q

Faults

A

A fault is a break in the continuity of the rocks of Earth’s crust, resulting in displacement, or the movement of rocks along one side of the break relative to those along the other side

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25
Q

Inactive Faults

A

no movement during the past 2.6 million years (Pleistocene Epoch)

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26
Q

Potentially active faults

A

movement during the past 2.6 million years (Pleistocene Epoch)

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27
Q

Active Faults

A

movement during the past 11,600 years (Holocene Epoch)

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28
Q

Reactivated faults

A

are inactive faults along which earthquakes may occur to alleviate strain

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29
Q

Potentially active faults

A

movement during the past 2.6 million years (Pleistocene Epoch)

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30
Q

Inactive faults

A

no movement during the past 2.6 million years (determined by paleoseismicity)

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31
Q

Dip-slip faults

A

are inclined fractures where the blocks have mostly shifted vertically

Normal- occur in landscapes of tension (Catto, 2015)

Reverse/Thrust- occur in landscapes of compression

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32
Q

Strike-slip fault

A

are vertical are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally

Left-lateral
Right-lateral
(Occur in landscapes of lateral movement)

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33
Q

Oblique-slip fault

A

have significant components of different slip styles

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34
Q

Blind faults

A

do not extend the surface

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35
Q

Normal Fault

A

The hanging wall has moved downward relative to the footwall.

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36
Q

Reverse Fault

A

The hanging wall has moved up relative to the footwall.

If the fault plane angle is 45 degree or less, it is a thrust fault.

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37
Q

Fault plane

A

The fault plane is the planar (flat) surface along which there is slip during an earthquake.

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38
Q

Fault trace

A

The fault trace is the intersection of a fault with the ground surface; also, the line commonly plotted on geologic maps to represent a fault.

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39
Q

Fault Scarp

A

The fault scarp is the feature on the surface of the earth that looks like a step caused by slip on the fault. In other words, it is a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other. It is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults.

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40
Q

Fault Trace

A

The fault trace is the intersection of a fault with the ground surface; also, the line commonly plotted on geologic maps to represent a fault.

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41
Q

Strike-slip faults

Left-lateral

A

If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the left block moves toward you and the right block moves away

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42
Q

Strike-slip faults

right-lateral

A

If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the right block moves toward you and the left block moves away.

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43
Q

Offset

A

The distance of movement across the fault

44
Q

Surface Rupture length

A

the total length of the break; most faults break in short segments across the total fault length

45
Q

Movement Along Faults

Stick-slip

A

involves the sudden movement of faults after the accumulation of stress

friction resists movement on fault surfaces causing strain and resulting in the eventual and sudden movement of the fault

Friction is reduced when water trapped in the fault zone makes movement possible at low fault inclinations

Friction is affected by asperities which may change the rupture pattern or stresses on a fault surface

46
Q

Movement Along Faults

Tectonic Creep

A

occurs when movement along a fault is so gradual that earthquakes are not felt

May slowly damage infrastructure such as roads, sidewalks, and building foundations

47
Q

Asperities

A

Rough spots in the fault surface

May change in the rupture pattern or stresses on fault surface

48
Q

Tectonic Environment Of Faults

Strike Slip Faults

A

At transform boundaries

Queen Charlotte, Denali Faults

49
Q

Tectonic Environment Of Faults

Megathrust Faults

A

Subduction Zones

Cascadia Subduction Zone

50
Q

Thrust Fault

A

At continent to continent collision boundaries

51
Q

Normal Faults

A

at spreading zones (e.g. Juan de Fuca ridge)

52
Q

Fault Systems

A

isolated from plate boundaries (intraplate earthquakes)

53
Q

Human Activity

A

(e.g. ‘fracking’, liquid waste disposal)

54
Q

Transform Boundary

A

Queen Charlotte Fault to the north separates the Pacific and North American plates (MODERATE to HIGH RISK - up to M 8)

55
Q

Divergent Boundary

A

Juan de Fuca Ridge (offshore)

separates the diverging Juan de Fuca and Pacific plates (LOW RISK)

56
Q

Convergent Boundaries

A

Cascadia subduction zone (CSZ) separates the Juan de Fuca and North American plates (HIGHEST RISK - up to M 9). Farther north, the Explorer plate is also subducting under the North American plate

57
Q

Queen Charlotte Fault, off the west coast of B.C.

A

The Queen Charlotte fault is predominantly a right lateral strike-slip fault separating the Pacific and North American plates

Responsible for Canada’s largest historic earthquake in 1949 (M 8.1) as well as several others ranging from M 7.4 to 7.8 from 1970-2013

58
Q

Shallow Or Crustal EQs

A

(900 AD, 1872) within the North American plate

In Cascadia, crustal earthquakes (with depths down to 35 km) are caused by the rupture of faults within the North American plate

Crustal earthquakes occur where the crust is under stress (e.g. as a result of nearby subduction or extension occurring within the interior of a plate)

Shallow earthquakes are dangerous because seismic wave amplitudes are greater near the surface causing more intense shaking to occur

Inflicts damage beyond shaking

Aftershocks are common

59
Q

Deep EQs

A

(1949, 1965, 2001) within the subducting Juan de Fuca plate

Do not produce many or any aftershocks

Shaking at surface is weaker but felt at a wider area

60
Q

Megathrust EQs

A

(1700) occur along the interface between the subducting Juan de Fuca and North American plates

Entire subduction zone is one giant fault if it breaks all at once it could produce a massive earthquake (M9)

61
Q

Episodic tremor slip

A

involves repeated episodes of slow fault slip of a few cm over a period of a few weeks, accompanied by seismic tremors

ETS is a process of fault movement discovered in 2003 by Canadian scientists, is termed episodic tremor slip (ETS)

62
Q

Five zones of seismic activity in eastern Canada

A

Zone 1: West Quebec including EQ in 1732 (M5.8), 1935 (M6.2), 1944 (M5.8)
Zone 2: Charlevoix-Kamouraska (1663, 1791, 1860, 1870, 1925, 2005)
Zone 3: Lower St. Lawrence
Zone 4: Northern Appalachians
Zone 5: Laurentian Slope

63
Q

Intraplate Earthquake

A

Earthquakes in Eastern Canada are intraplate earthquakes occurring in the interior of a tectonic plate, away from the plate boundary

Intraplate EQs may occur on strike-slip, reverse, or normal faults

Most recent intraplate EQs occurred in Saguenay in 1988 (M 5.9), Val de Bois in 2010 (M5.0), and Shawville in 2013 (M 4.6)

64
Q

leading research suggests that reactivation of faults are due to:

A

Slow movement of the North American plate away from the Mid-Atlantic Ridge

Post-glacial rebound from the ice sheet that covered Canada 10,000 years ago

65
Q

Intraplate EQs are dangerous for several reasons

A

Recurrence intervals are usually much longer than earthquakes that occur at plate boundaries

Faults are usually blind due to surface erosion over time

People are generally unaware and not prepared for this type of earthquake

Strong coherent rocks (e.g. bedrock) in the continental interior transmit ground motion more efficiently over longer distances

66
Q

Charlevois-Kamouraska

A

Most active seismic region in eastern Canada

Five M 6 earthquakes have occurred in this region in the past 350 years (1663, 1791, 1860, 1870, 1925, and 2005)

Many of these earthquakes were felt over wide areas

Most damage resulted from unreinforced masonry buildings

67
Q

Laurentian Slope

A

This region is located offshore the east coast of Canada, ~ 250 km south of Newfoundland

A M 7.2 EQ in generated a tsunami that killed 29 people on the Burin Peninsula, Nfld

68
Q

Northern Canada EQs

A

M6.3 Ungava, Quebec on December 25, 1989 - first historical evidence for surface faulting in eastern Canada

M7.3 Baffin Bay, Nunavut on November 20, 1933 - largest earthquake along the passive margin of North America and north of the Arctic Circle

69
Q

Seismicity in Northern Canada may be associated with

A

Crustal stress in the Richardson and Mackenzie Mountains of the Yukon and Northwest Territories

Reactivation of Mesozoic rift faults in the Eastern Arctic margin

Transform faults associated with an extinct spreading ridge in the Labrador Sea

Postglacial rebound in Baffin Island including the Boothia and Ungava Peninsulas

70
Q

Central Canada

A

The fewest EQ occur within the stable craton (SK and MB)

Since 1968, 14 natural earthquakes have occurred in SK

Largest known earthquake in this region occurred May 15, 1909 near the US border (M5.5)

Earthquakes in this region are caused by small faults in the subsurface and by the dissolution of rock salt

71
Q

Earthquakes and Human Activities

A

The weight from water reservoirs may create new faults or lubricate old ones

Liquid waste disposals deep in the Earth can create pressure on faults (e.g. Rocky Mountain waste arsenal in Colorado; see also Figure 3.29 on page 74 in Keller, Blodgett, and Clague, 2015)

Pumping of oil and gas and hydraulic fracturing can all cause small earthquakes (e.g. Fox Creek, Alberta, 2016; see also Figure 3.30 on page 74 in Keller, Blodgett, and Clague, 2015)

Nuclear explosions can cause the release of stress along existing faults

72
Q

Earthquake Damage

A

Intensity can vary significantly across a region and is determined by a number of factors including:
Magnitude
Distance from epicenter
Focal Depth
Direction of fault rupture
Time of day and other socio-economic conditions
Nature of the local earth and building material
Engineering and construction practices

73
Q

Intensity

A

Measured by Modified Mercalli Intensity Scale

An integer scale (e.g. no logarithmic increase between IV and V)

Denoted by Roman numerals to differentiate it from magnitude scales (I to XII)

The scale is qualitative and based on damage to structures and people’s perceptions

As a result, there may be several estimates of intensity at any one location

The magnitude of an earthquake is useful in comparing one event to another; intensity is more useful in assessing damage

74
Q

Modified Mercalli Scale (abridged)

A

I Felt by very few people

II Felt by only a few people at rest

III Felt noticeably indoors, esp. on upper floors of bldgs.

IV Felt indoors by many, outdoors by few (day)

V Felt by nearly everyone

VI Felt by all; people frightened and run outdoors

VII Damage is negligible in bldgs of good design and considerable in poorly built or designed structures

VIII Damage is slight in specially designed structures; considerable in ordinary bldgs with partial collapse; great in poorly built structures

IX Damage is considerable even in specially designed structures; great in substantial buildings with partial collapse

X Some well-built wooden structures are destroyed, most masonry and frame foundations destroyed; ground badly cracked. Some landslides and liquefaction.

XI Few if any masonry structures left standing; bridges are destroyed. Large fissures in ground. Landslides common.

XII Damage is total. Waves are seen on ground surface. Objects thrown into the air.

75
Q

Shake Maps

A

EQ intensities are shown on shake maps, with isoseisms to display differences between MMI values

Shake maps are constructed from measurements of ground motion from seismographs

Community internet intensity maps are created using the internet

76
Q

Shaking

A

Shaking is measured in acceleration (g), which is the acceleration due to gravity on the surface of Earth

The peak acceleration value is important because in Canada it informs how engineers design buildings (National Building Code of Canada)

The duration of shaking during an earthquake also matters in addition to the peak acceleration value because even well-designed buildings (especially those constructed with steel) may become fatigued by shaking

77
Q

Body Waves

A

Seismic waves emanate outwards from the focus, the location within Earth where the EQ begins
There are two types of seismic waves: body and surface waves

78
Q

P-Waves

A

primary, or compressional waves (5-7 km/s in crust)

Move fast with a push/pull motion

Can move through solid, liquid, and gas

79
Q

S-Waves

A

Secondary or shear waves (3-4km/s in crust)

Move slower with an up/down motion

Can only travel through solids

80
Q

Surface Waves

A

Surface waves move along Earth’s surface up and down, and side to side

Travel more slowly than body waves (~2-4.4 km/s)

Are responsible for intense ground motion especially near epicenter

81
Q

Love Waves

A

Cause horizontal shaking (side to side)

82
Q

Rayleigh Waves

A

Rolling waves with elliptical motion

83
Q

Epicentre

A

The epicentre of an earthquake is determined by using the arrival times of P and S waves detected by seismographs using a process called triangulation

84
Q

Seismographs

A

Seismographs are the instruments used at seismic stations to record the location and magnitude of earthquakes

85
Q

Seismograms

A

Seismograms record the arrival of P waves and S waves which travel at different rates and thus arrive at seismic stations at different times

86
Q

Triangulation

A

There is a predictable distance between the arrival of a P wave and the slower S wave.

Using arrival times of the P and S waves epicentral distances are determined from 3 different stations using a time travel curve. We still do not know the direction in which the earthquake occurred.

The epicentral distance is used to plot three circles around the stations. The place where the circles intersect is the location of the earthquake.

87
Q

Direction of Rupture

A

Directivity is an effect of fault rupture whereby ground motion is more severe in the direction of rupture than in other directions from the earthquake source

The end effect is increased shaking at the surface in the direction of fault rupture

88
Q

Local Soil and Rock Conditions

A

Local geology strongly influences the amount of ground motion during an earthquake
More damage can occur in areas further away from the epicenter depending on local soil and rock conditions
`

89
Q

Amplification

A

Amplification occurs when energy is transferred from P waves and S waves to surface waves, increasing the amount of shaking

This happens when seismic waves encounter unconsolidated sediments, especially those with high water content

Dense rocks (e.g. bedrock, Canadian Shield) transmit earthquake energy quickly

Seismic waves slow down in heterogeneous rocks, unconsolidated sediment, and even further in sediment with high water content (muds, sands, gravels, landfill)

Local geologic structures such as synclines and fault-bounded sedimentary basins may also amplify shaking

90
Q

Liquefaction

A

Liquefaction occurs when intense seismic shaking causes water- saturated unconsolidated sediment (quick clay, muds, or sand) to change from a solid to a liquid

This process involves the elevation of pore pressures at shallow depths so that the water suspends the sediment particles, which causes the sediment to flow

When the pressure decreases, the sediment returns to normal thus resuming its solid form

Liquefaction may cause the land surface to settle irregularly, causing potential damage to building foundations and buried utilities including water and sewage lines

91
Q

Primary effects of earthquakes

A

Shaking
(Damage to buildings, infrastructure may be due to amplification or liquefaction or resonance)

Ground Rupture
(Displacement along the fault causes cracks in the surface and fault scarps)
92
Q

Structural Damage

A

Structural and building collapse are the main EQ hazards leading to loss of life

Strength of shaking, length of shaking, type of soil, type of building, national building code standards

93
Q

Factors to consider

A
  • concrete, brick, masonry susceptible to more damage
  • steel structures are more flexible
  • wood more susceptible to fire
  • diagonal braces protect building from horizontal motion
  • Soft or weak stories are stories that have substantially less resistance or stiffness than floors above and/or below
94
Q

Strategies for mitigating building damage

A

Reinforcement: Bracing, shear walls (best for soft stories)

Base isolation:
Flexible link between building and foundation

Vibration damping: Add devices to resist shaking

95
Q

Strategies for mitigating settlement/liquefaction damage:

A

Reinforce structure to mitigate small motion

Improve foundation: deep piles, flexible piles, mat foundation

Stabilize soil: dewater, grout, densify, buttress

96
Q

Resonance

A

Resonance refers to the relationship between the fundamental period of a building and the fundamental period of the material on which the building is constructed

Seismic shaking may create resonance if the building’s fundamental period is the same as the fundamental period for the surface materials on which it is built

97
Q

Fundamental Period

A

Buildings of differing heights and various types of surface materials will each naturally vibrate at a characteristic period

98
Q

Secondary Effects

A
  1. Land Level Changes
    EQs can raise or lower the land over large areas which may cause substantial damage to structures built along shorelines and streams

2 Landslides
Ground motions cause rock or sediment to fail and move downslope earthquake in China; landslide triggered by 2002 M 7.9 Denali earthquake in Alaska

  1. Fires
    Ground shaking and surface rupture can sever electrical power and gas lines, water pipes may also be disrupted during shaking limiting fire control
  2. Tsunami
    Earthquakes can cause the generation of a tsunami by displacing the seafloor or the floor of a large lake, or by triggering a large landslide that displaces a body of water
99
Q

Tertiary Effects

A
  1. Disease
    No water, gas, food, electricity, toilets, or privacy (e.g.) up to 1 month later 1, 270,000 homes still without water
    Spread of colds and influenza as well as post-traumatic stress disorder (PTSD)
    Dust pollution and removal of harmful waste
  2. Loss of Housing/Critical Infrastructure
    48,300 temporary houses built (mostly for elderly persons)
    Took 7 months for railways to recover (1 hour commute became 4-5 hours)
    Unequal distribution of aid supplies (social vulnerability)
  3. Prolonged Recession
    People moved away after the event prolonging an ongoing recession, decrease in tourism; years later an annual Kobe Luminarie event honors victims and attracts 4 million people to Kobe every year
100
Q

Forecast

A

specifies the probability of an earthquake occurring (some success; must be scientifically reviewed)

101
Q

Prediction

A

specifies when and where an earthquake will occur (difficult with some success)

102
Q

Precursors

A

Pattern and frequency of earthquakes (foreshocks, episodic tremor and slip events)
Land-level change
Seismic gaps along faults
Physical and chemical changes in Earth’s crust

103
Q

Perception

A

One community’s experience does not stimulate other communities to improve their preparedness (recall the study that examined perceptions of volcanic hazards near Mount Vesuvius )
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104
Q

Mitigation Strategies

A

Critical facilities must be located in earthquake safe locations

Requires detailed maps of ground response to seismic shaking

Buildings must be designed to withstand vibrations

Retrofitting old buildings may be necessary

People must be prepared through education
Insurance must be made available

105
Q

Before an Earthquake

A

Assess local conditions: trees, power lines, electrical wires

Make sure that your home is structurally sound

Secure large objects

Learn to turn off gas, water and electricity

Make a personal plan of how to react to a quake; prepare an emergency kit
Assess local ground conditions for hazards

106
Q

During EQ

A

Don’t panic, find a safe spot

107
Q

After EQ

A

Meet in predetermined area

Check for damages and injuries

Turn off gas, water, and electricity

Stay away from power lines

GEOG 235 (8 decks)

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