Module 3-Make sure to go back in lecture are read up on P & S waves Flashcards
eARTHQUAKES DEFINED
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
focus
At the point of rupture (termed the focus), seismic waves radiate outwards producing the shaking
T OR F
EQs do not always occur at or near plate boundaries;
hint at intraplate Eqs
Describe a faultplane
surface rupture: forms a fault scarp, ammount of slip o fault
Epicentre: point on surface above focus
focus:where rupture on fault plane started
Describe a faultplane
surface rupture: forms a fault scarp, ammount of slip o fault
Epicentre: point on surface above focus
focus:where rupture on fault plane started, where earthquake originates
Define stress and give 4 types
Stress is a force applied over an area; when stress is applied to rock it results in deformation
Tensional stress stretches rock (divergent plate boundaries)
Compressional stress pushes rock together (convergent plate boundaries)
Shear stress results in slippage and translation
Confining stress is a type of uniform stress resulting from the pressure due to the weight of overlying rocks
deformation of rock is caused by _____
stress
Define strain
define deformation and give three types
what is the relationship between strain and deformation?
strain causes deformation, the more strain, the less ability the rock has to return to normal!
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
When stress is applied to rock, it passes through various stages of deformation:
Elastic deformation- strain is reversible
Permanent deformation- strain is irreversible; two types: brittle vs ductile
Fracture- irreversible strain wherein the material breaks
-fault created from too much strain
Brittle materials
ductile materials
Brittle materials have a small to large region of elastic behaviour and a small region of ductile behaviour before they fracture.
Ductile materials have a small region of elastic behaviour and a large region of ductile behaviour before they fracture.
How material behaves in terms of deformation depends on 3 things: ,
confining pressure, strain rate, and rock composition:
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
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
Water weakens the chemical bonds in rocks and increases slippage promoting ductile behaviour; dry rocks tend to behave in a brittle manner
strain rate
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
Quartz, feldspar are most likely _______(oceanic/continental) rock
continental
4 stages in earthquake cycle along a strike-slip fault
- long period of inactivity along the fault
- no strain no displacement, stress building - Accumulated elastic strain produces small earthquakes as stress begins to release
- elastic strain starts building up
3.Occurs days to hours before the main quake; may be characterized by foreshocks, small to moderate size EQs that precede the main quake (this stage does not always occur)
- The main EQ and its aftershocks occur. Aftershocks are the small EQs that occur a few minutes to a year or more after the main EQ
- earthquake occurs and elastic strain is released
=results in displacement
may happen over 100s of thousands of years
aftershocks
Aftershocks are the small EQs that occur a few minutes to a year or more after the main EQ
2 methods for measuring earthquakes
There are two methods for measuring earthquakes:
1.Magnitude –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)
2.Intensity –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)
Describe Magnitude vs Intensity:
Magnitude measures energy released
intensity measures of damage that vary with proximity to earthquake and composition of subsurface materials
Earthquake ______ is typically measured as acceleration; higher magnitude EQ cause more violent _____(same), which in turn cause higher intensity
Earthquake shaking is typically measured as acceleration; higher magnitude EQ cause more violent shaking, which in turn cause higher intensity
-links intensity and magnitude together
The most popular and well known is the _____ magnitude
The fourth and least well known is the best estimate of the size of an EQ and that is the ____ magnitude
Richter
-worst one
moment
-best one
Richter Scale
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.
Problems with this scale:
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
Moment magnitude
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
Why would an earthquake have a higher magnitude for richter scale but lower on the moment magnitude scale?
Although the Chilean EQ released more energy, the energy was transmitted more efficiently through the Earth for the Alaska EQ causing more shaking at 100 km distance
Define Fault
4 general fault types based on activity in a certain time period
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
Inactive faults - no movement during the past 2.6 million years (Pleistocene Epoch)
Potentially active faults – movement during the past 2.6 million years (Pleistocene Epoch)
Active faults – movement during the past 11,600 years (Holocene Epoch)
Reactivated faults are inactive faults along which earthquakes may occur to alleviate strain
4 types of faults
- Dip-slip faults 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 (Catto, 2015) - Strike-slip faults are vertical (or nearly vertical) -fractures where the blocks have mostly moved horizontally
Left-lateral
Right-lateral - Oblique-slip faults have significant components of different slip styles
- Blind faults do not extend to the surface
Normal and reverse faults fit under what type of fault?
Dip-slip because they move vertically
strike slip faults move..
horizontally
Fault scarp
fault trace
fault plane
Fault scarp: planar flat surface along which there is slip during an earthquake
Fault trace: intersection of the fault with the ground surface
Fault plane: offset on the ground surface where one side of the fault has moved vertically with respect to the other
Offset
surface rupture length
stick-slip
Offset: the distance of movement across the fault
Surface rupture length: the total length of the break; most faults break in short segments across the total fault length
Stick-slip: involves the sudden movement of faults after the accumulation of stress
Movement along faults occurs in two ways
Stick-slip- 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
Tectonic creep- 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
Name where these generally occur? Strike slip faults Megathrust faults Thrust faults Normal faults Fault systems Human activity
- Strike slip faults at transform boundaries (e.g. Queen Charlotte, Denali faults)
- Megathrust faults at subduction zones (e.g. Cascadia subduction zone)
- Thrust faults at continent-continent collision boundaries
- Normal faults at spreading zones (e.g. Juan de Fuca ridge)
- Fault systems isolated from plate boundaries (intraplate earthquakes)
- Human activity (e.g. ‘fracking’, liquid waste disposal)
Why aren’t there many earthquakes over the canadian shield
Not much earthquakes over Canadian shield because of the compression strength of the rock
Western Canada Tectonic Setting
Transform boundary
Queen Charlotte Fault to the north separates the Pacific and North American plates (MODERATE to HIGH RISK- up to M 8)
Divergent boundary
Juan de Fuca Ridge (offshore)
separates the diverging Juan de Fuca and Pacific plates (LOW RISK)
Convergent boundaries
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
what type of boundary is responsible for Canadas largest hisotric earthquake ?
The Queen Charlotte fault(tRANSFORM bOUNDARY) 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
Megathrust earthquakes occur…where?
Deep earthquakes?
Shallow/crustal Earthquakes?
in between the subducting plate and the overlying plates
-Essentially the entire subduction zone is one giant fault- if it breaks all at once it could produce a massive earthquake (M9)
Aftershocks would be frequent and large enough to cause further damage
Examples of megathrust EQ’s along the west coast of North America: 1700 Cascadia EQ and 1964 Anchorage, Alaska EQ
-every 500 years
on subducted plate
- felt over larger distances then shallow
- 10 to 30 years
on overlying plate/crust
-felt more fiercly then deep
-after shocks most common in shallow
-occur everyday
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)
Intraplate Earthquakes
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
Sources of moderate to large intraplate EQ in this region are not well understood; leading research suggests that reactivation of faults are due to:
Slow movement of the North American plate from the Mid-Atlantic Ridge
Post-glacial rebound from the ice sheet that covered Canada 10,000 years ago
Why are intraplate earthquakes so dangerous?
Intraplate earthquakes are dangerous for several reasons:
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
Laurentian Slope
This region is located offshore the east coast of Canada, ~ 250 km south of Newfoundland
A M7.2 EQ in generated a tsunami that killed 29 people on the Burin Peninsula, Nfld
Seismicity in Northern Canada may be associated with:4
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
Central Canada earthquakes mostly caused by
Earthquakes in this region are caused by small faults in the subsurface and by the dissolution of rock salt
The consequences of an earthquake depend on its:6
magnitude, depth, direction of fault rupture, distance from populated areas, the nature of local earth materials, engineering and construction practices.
The consequences of an earthquake depend on its:7
Magnitude
Distance from epicenter
Focal depth (shallow, intermediate, deep)
Direction of fault rupture
Time of day and other socio-economic conditions
Nature of the local earth and building materials (may cause amplification, liquefaction, resonance)
Engineering and construction practices
Describe Modified Mercalli Intensity Scale (MMI)
Intensity is measured by the Modified Mercalli Intensity Scale (MMI)
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
-12 levels from felt by not many to total damage
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.
shake maps
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
Shaking
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
Body Waves: 2 types
Surface Waves: 2 types
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
Body waves- named so because they travel through the interior of the earth, and include two types:
1.P-Waves, primary, or compressional waves (5-7 km/s in crust)
Move fast with a push/pull motion
Can move through solid, liquid, and gas
2.S-Waves, secondary or shear waves (3-4 km/s in crust)
Move slower with an up/down motion
Can only travel through solids
-like snapping a rope
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
Two types-
1.Love waves-cause horizontal shaking (side to side)
2.Rayleigh waves-rolling waves with elliptical motion like sea waves
Seismographs vs. Seismograms
Seismographs are the instruments used at seismic stations to record the location and magnitude of earthquakes
Seismograms record the arrival of P waves and S waves which travel at different rates and thus arrive at seismic stations at different times
-What Does this Seismogram Tell Us?
The difference of 50 seconds in the first arrivals of P and S waves would allow a seismologist to determine how far away (but not in what direction) the epicentre is located using a time travel curve.
Describe triangulation in locating epicenters
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
- 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.
The greater the _______, the _______ the ground shaking.
amplitude, stronger
Depths of Earthquakes:
Shallow or crustal:___-___km
Intermediate:___-___km
Deep: ___-___km
Earthquakes typically occur at the following depth ranges:
Shallow or crustal: 0-70 km deep
Intermediate: 70-300 km deep
Deep: 300-700 km deep
Directivity
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
Local Soil aND ROCK conditions affect shaking:
also give 2 effects that increase ground motion:
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
We’ll look at two effects that increase ground motion during an earthquake in unconsolidated sediments: amplification and liquefaction
-Dense granitic and metamorphic rock of the Canadian Shield transmits earthquake energy very efficiently so even moderate EQ can cause damage over large areas
Seismic waves move more slowly in heterogeneous crust and unconsolidated sediments especially those with a high water content
1.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
-As P and S waves slow, some of their forward-directed energy is transferred to surface waves
-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
2.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
resonance
why would seismic shaking cause resonance?
resonance is when the vibration period of buildings matches the seismic waves, causing amplification of the seismic waves)
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
Buildings of differing heights and various types of surface materials will each naturally vibrate at a characteristic period, known as the fundamental period
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
Earthquake effects:
Primary effects
Primary Effects: occur as a result of the process itself and include:
Shaking
Causes damage to buildings, bridges, dams, tunnels, pipelines, etc (structural damage)
Effects may be intensified due to amplification or liquefaction, or resonance
Ground Rupture
Displacement along the fault causes cracks in the surface and fault scarps
1 reason for death in natural disasters?
building collapse is 77%
Damage to infrastructure depends on: 5
Strength of shaking
Length of shaking (depends on how the fault breaks)
Type of soil (increased in soft, thick, wet soils)
Type of building (not resistant to horizontal motion)
National building code standards/lack of standards (upkeep, age, quality of building materials
3 strategies for mitigating building damage
3 strategies for mitigating settlement/liquefaction dmage
Strategies for mitigating building damage:
Reinforcement: Bracing, shear walls (best for soft stories)
Base isolation: Flexible link between building and foundation
Vibration damping: Add devices to resist shaking
Strategies for mitigating settlement/liquefaction damage:
Reinforce structure to mitigate small motion
Improve foundation: deep piles, flexible piles, mat foundation
Stabilize soil: dewater, grout, densify, buttress
Earthquake hazards: secondary effects 4
Secondary Effects: occur only because a primary effect caused them
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 (e.g. 1700 Cascadia earthquake)
2.Landslides
Ground motions cause rock or sediment to fail and move downslope (e.g. In 1970, the towns of Yungay and Ranrahirca, Peru were buried by a landslide; multiple landslides were produced during the 2008 Wenchuan earthquake in China; also see landslide triggered by 2002 M 7.9 Denali earthquake in Alaska-Fig 3.29 in Keller, Blodgett, and Clague, p.71)
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 (e.g. 1906 San Francisco)
4.Tsunami (e.g. 2004 Sumatra, 1908 Messina Italy)
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
Earthquake Hazards: Tertiary Effects
Tertiary Effects: are long-term effects that are set off by the primary event (e.g. Kobe, 1995, ~6,000 died, cost ~$100 billion)
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
4 precursors to an earthquake
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
Mitigating earthquake effects: 7
Perception:
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 )
Mitigation Strategies:
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
