Earthquakes

Question 1

Earthquakes

Answer 1

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.

Question 2

Earthquake: Focus

Answer 2

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

Question 3

Stress

Answer 3

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.

Question 4

Tensional Stress

Answer 4

Tensional stress stretches rock (divergent plate boundaries)

Question 5

Compressional Stress

Answer 5

Compressional stress pushes rock together (convergent plate boundaries)

Question 6

Shear Stress

Answer 6

Shear stress results in slippage and translation

Question 7

Confining Stress

Answer 7

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.

Question 8

Strain and Deformation

Answer 8

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

Question 9

Elastic Deformation

Answer 9

Elastic deformation- strain is reversible

Question 10

Permanent Deformation

Answer 10

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

Question 11

Fracture

Answer 11

Fracture- irreversible strain wherein the material breaks

Question 12

Brittle materials

Answer 12

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

Question 13

Ductile materials

Answer 13

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

Question 14

Permanent Deformation

Answer 14

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

Question 15

Strain Rate

Answer 15

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

Question 16

Confining Pressure

Answer 16

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.

Question 17

Temperature

Answer 17

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.

Question 18

Composition

Answer 18

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.

Question 19

Measuring Earthquakes

Magnitude

Answer 19

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

Question 20

Measuring Earthquakes

Intensity

Answer 20

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

Question 21

Local or Richter Magnitude

Answer 21

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.

Question 22

Problems with the Richter Magnitude Scale

Answer 22

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

Question 23

Moment Magnitude

Answer 23

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

Question 24

Faults

Answer 24

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

Question 25

Inactive Faults

Answer 25

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

Question 26

Potentially active faults

Answer 26

movement during the past 2.6 million years (Pleistocene Epoch)

Question 27

Active Faults

Answer 27

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

Question 28

Reactivated faults

Answer 28

are inactive faults along which earthquakes may occur to alleviate strain

Question 29

Potentially active faults

Answer 29

movement during the past 2.6 million years (Pleistocene Epoch)

Question 30

Inactive faults

Answer 30

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

Question 31

Dip-slip faults

Answer 31

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

Question 32

Strike-slip fault

Answer 32

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)

Question 33

Oblique-slip fault

Answer 33

have significant components of different slip styles

Question 34

Blind faults

Answer 34

do not extend the surface

Question 35

Normal Fault

Answer 35

The hanging wall has moved downward relative to the footwall.

Question 36

Reverse Fault

Answer 36

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.

Question 37

Fault plane

Answer 37

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

Question 38

Fault trace

Answer 38

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.

Question 39

Fault Scarp

Answer 39

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.

Question 40

Fault Trace

Answer 40

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.

Question 41

Strike-slip faults | Left-lateral

Answer 41

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

Question 42

Strike-slip faults | right-lateral

Answer 42

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.

Question 43

Offset

Answer 43

The distance of movement across the fault

Question 44

Surface Rupture length

Answer 44

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

Question 45

Movement Along Faults | Stick-slip

Answer 45

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

Question 46

Movement Along Faults | Tectonic Creep

Answer 46

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

Question 47

Asperities

Answer 47

Rough spots in the fault surface May change in the rupture pattern or stresses on fault surface

Question 48

Tectonic Environment Of Faults Strike Slip Faults

Answer 48

At transform boundaries | Queen Charlotte, Denali Faults

Question 49

Tectonic Environment Of Faults Megathrust Faults

Answer 49

Subduction Zones Cascadia Subduction Zone

Question 50

Thrust Fault

Answer 50

At continent to continent collision boundaries

Question 51

Normal Faults

Answer 51

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

Question 52

Fault Systems

Answer 52

isolated from plate boundaries (intraplate earthquakes)

Question 53

Human Activity

Answer 53

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

Question 54

Transform Boundary

Answer 54

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

Question 55

Divergent Boundary

Answer 55

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

Question 56

Convergent Boundaries

Answer 56

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

Question 57

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

Answer 57

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

Question 58

Shallow Or Crustal EQs

Answer 58

(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

Question 59

Deep EQs

Answer 59

(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

Question 60

Megathrust EQs

Answer 60

(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)

Question 61

Episodic tremor slip

Answer 61

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)

Question 62

Five zones of seismic activity in eastern Canada

Answer 62

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

Question 63

Intraplate Earthquake

Answer 63

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)

Question 64

leading research suggests that reactivation of faults are due to:

Answer 64

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

Question 65

Intraplate EQs are dangerous for several reasons

Answer 65

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

Question 66

Charlevois-Kamouraska

Answer 66

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

Question 67

Laurentian Slope

Answer 67

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

Question 68

Northern Canada EQs

Answer 68

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

Question 69

Seismicity in Northern Canada may be associated with

Answer 69

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

Question 70

Central Canada

Answer 70

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

Question 71

Earthquakes and Human Activities

Answer 71

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

Question 72

Earthquake Damage

Answer 72

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

Question 73

Intensity

Answer 73

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

Question 74

Modified Mercalli Scale (abridged)

Answer 74

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.

Question 75

Shake Maps

Answer 75

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

Question 76

Shaking

Answer 76

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

Question 77

Body Waves

Answer 77

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

Question 78

P-Waves

Answer 78

primary, or compressional waves (5-7 km/s in crust) Move fast with a push/pull motion Can move through solid, liquid, and gas

Question 79

S-Waves

Answer 79

Secondary or shear waves (3-4km/s in crust) Move slower with an up/down motion Can only travel through solids

Question 80

Surface Waves

Answer 80

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

Question 81

Love Waves

Answer 81

Cause horizontal shaking (side to side)

Question 82

Rayleigh Waves

Answer 82

Rolling waves with elliptical motion

Question 83

Epicentre

Answer 83

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

Question 84

Seismographs

Answer 84

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

Question 85

Seismograms

Answer 85

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

Question 86

Triangulation

Answer 86

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.

Question 87

Direction of Rupture

Answer 87

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

Question 88

Local Soil and Rock Conditions

Answer 88

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 `

Question 89

Amplification

Answer 89

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

Question 90

Liquefaction

Answer 90

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

Question 91

Primary effects of earthquakes

Answer 91

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) ```

Question 92

Structural Damage

Answer 92

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

Question 93

Factors to consider

Answer 93

- 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

Question 94

Strategies for mitigating building damage

Answer 94

Reinforcement: Bracing, shear walls (best for soft stories) Base isolation: Flexible link between building and foundation Vibration damping: Add devices to resist shaking

Question 95

Strategies for mitigating settlement/liquefaction damage:

Answer 95

Reinforce structure to mitigate small motion Improve foundation: deep piles, flexible piles, mat foundation Stabilize soil: dewater, grout, densify, buttress

Question 96

Resonance

Answer 96

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

Question 97

Fundamental Period

Answer 97

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

Question 98

Secondary Effects

Answer 98

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 3. Fires Ground shaking and surface rupture can sever electrical power and gas lines, water pipes may also be disrupted during shaking limiting fire control 4. 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

Question 99

Tertiary Effects

Answer 99

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

Question 100

Forecast

Answer 100

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

Question 101

Prediction

Answer 101

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

Question 102

Precursors

Answer 102

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

Question 103

Perception

Answer 103

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 ) `

Question 104

Mitigation Strategies

Answer 104

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

Question 105

Before an Earthquake

Answer 105

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

Question 106

During EQ

Answer 106

Don't panic, find a safe spot

Question 107

After EQ

Answer 107

Meet in predetermined area Check for damages and injuries Turn off gas, water, and electricity Stay away from power lines