Earthquakes Flashcards
Earthquakes
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.
Earthquake: Focus
At the point of rupture, seismic waves radiate outwards producing the shaking
Stress
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.
Tensional Stress
Tensional stress stretches rock (divergent plate boundaries)
Compressional Stress
Compressional stress pushes rock together (convergent plate boundaries)
Shear Stress
Shear stress results in slippage and translation
Confining Stress
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.
Strain and Deformation
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
Elastic Deformation
Elastic deformation- strain is reversible
Permanent Deformation
Permanent deformation- strain is irreversible; two types: brittle vs ductile
Fracture
Fracture- irreversible strain wherein the material breaks
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.
Permanent Deformation
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
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
Confining Pressure
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.
Temperature
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.
Composition
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.
Measuring Earthquakes
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)
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
Measuring Earthquakes
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). Earthquake shaking is typically measured as acceleration; higher magnitude EQ cause more violent shaking, which in turn cause higher intensity
Local or Richter Magnitude
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 the Richter Magnitude 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
Faults
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
Potentially active faults
movement during the past 2.6 million years (Pleistocene Epoch)
Inactive faults
no movement during the past 2.6 million years (determined by paleoseismicity)
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
Strike-slip fault
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)
Oblique-slip fault
have significant components of different slip styles
Blind faults
do not extend the surface
Normal Fault
The hanging wall has moved downward relative to the footwall.
Reverse Fault
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.
Fault plane
The fault plane is the planar (flat) surface along which there is slip during an earthquake.
Fault trace
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.
Fault Scarp
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.
Fault Trace
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.
Strike-slip faults
Left-lateral
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
Strike-slip faults
right-lateral
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.
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
Movement Along Faults
Stick-slip
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
Movement Along Faults
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
Asperities
Rough spots in the fault surface
May change in the rupture pattern or stresses on fault surface
Tectonic Environment Of Faults
Strike Slip Faults
At transform boundaries
Queen Charlotte, Denali Faults
Tectonic Environment Of Faults
Megathrust Faults
Subduction Zones
Cascadia Subduction Zone
Thrust Fault
At continent to 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)
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
Queen Charlotte Fault, off the west coast of B.C.
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
Shallow Or Crustal EQs
(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
Deep EQs
(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
Megathrust EQs
(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)
Episodic tremor slip
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)
Five zones of seismic activity in eastern Canada
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
Intraplate Earthquake
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)
leading research suggests that reactivation of faults are due to:
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
Intraplate EQs 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
Charlevois-Kamouraska
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
Laurentian Slope
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
Northern Canada EQs
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
Seismicity in Northern Canada may be associated with
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
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
Earthquakes and Human Activities
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
Earthquake Damage
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
Intensity
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
Modified Mercalli Scale (abridged)
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
Community internet intensity maps are created using the internet
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
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
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
S-Waves
Secondary or shear waves (3-4km/s in crust)
Move slower with an up/down motion
Can only travel through solids
Surface Waves
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
Love Waves
Cause horizontal shaking (side to side)
Rayleigh Waves
Rolling waves with elliptical motion
Epicentre
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
Seismographs
Seismographs are the instruments used at seismic stations to record the location and magnitude of earthquakes
Seismograms
Seismograms record the arrival of P waves and S waves which travel at different rates and thus arrive at seismic stations at different times
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.
Direction of Rupture
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
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
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Amplification
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
Liquefaction
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
Primary effects of earthquakes
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)
Structural Damage
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
Factors to consider
- 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
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
Resonance
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
Fundamental Period
Buildings of differing heights and various types of surface materials will each naturally vibrate at a characteristic period
Secondary Effects
- 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
- Fires
Ground shaking and surface rupture can sever electrical power and gas lines, water pipes may also be disrupted during shaking limiting fire control - 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
Tertiary Effects
- 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 - 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) - 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
Forecast
specifies the probability of an earthquake occurring (some success; must be scientifically reviewed)
Prediction
specifies when and where an earthquake will occur (difficult with some success)
Precursors
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
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 )
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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
Before an Earthquake
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
During EQ
Don’t panic, find a safe spot
After EQ
Meet in predetermined area
Check for damages and injuries
Turn off gas, water, and electricity
Stay away from power lines
