Earthquakes And Engineering Flashcards
Where do Earthquakes Happen?
Along plate boundaries, namely the ring of fire, alpide belt, circum-pacific belt, and the mid Atlantic ridge
What Causes Earthquakes
Stress in the rust caused by plate tectonics
What is a Seismograph?
Record the amplitude of ground movement as to record earthquake arrival
Fault types
Dip-slip faults
— normal
— reverse
— thrust
Strike-slip faults
— left thermal strike-slip
— right lateral strike slip
Normal fault
Extension, hanging wall goes down
Reverse fault
Compression, Hanging wall goes up, high angle
Thrust fault
Compression, Hanging wall goes up, at a low angle
Left lateral strike-slip fault
Shear, left plate goes down, has mountains
Right lateral strike-slip fault
Shear, Right plate goes down, has mountains on it
Why do faults occur?
Directed stress builds up in rocks by…
1. Compression
2. Tension
3. Shear (slipping along a plane parallel to the stress
Strain
An Accumulation of stress in a rock produces that deformation
Three stages:
1. Elastic deformation
2. Ductile defomation
3. Brittle deformation
Elastic deformation
Fully reversible, stretch and bounces back
Ductile defomation
Irreversible, can stretch but wont go back to its original shape
Brittle deformation
Fracture
Stretch and it breaks
Faults
Planar fractures in rocks where the rocks on either side have moved
Vary in size (can be mm)
Elastic rebound theory
When sufficient strain energy has accumulated in rocks,they may rupture rapidly (like how a rubber band breaks when it is stretched too far) and the stored energy is released as vibrations that radiation outward in all directions
1. Stress gets added
2. Rocks deform and store energy
3. Rocks break and release energy (earthquake)
4. Rocks rebound to undeformed shape
Rupture zone
The part of the fault that slips
Can’t predict how large/ when a rupture occurs
Focus/hypocentre?
Where pinpoint within the earth where the rupture starts
Epicentre
The location on the surface where the seismic waves first hit
Directly above focus
Where do the deepest earthquakes occur?
The ring of fire (circum pacific seismic belt)
Most earthquake foci occur at depths of <15km in the crust. Why don’t earthquakes occur in the mantle?
The deeper you go, the more ductile deformation you get. Without the brittle deformation, Faulkner cannot be generated.
Where can shallow, intermediate and deep earthquakes occur?
Subduction zones
Where can only shallow earthquakes occur?
Transform and divergent boundaries
Upper 15km of crust
body waves
Generated at the focus and travel through the interior of the earth
Two types:
1. P-wave
2. S-wave
P-waves
Primary waves
Compression-extension, longitudinal waves
Velocity increases with depth, fastest waves
Pass through solid and liquid
S-waves
Secondary
Transverse, shear waves
Produce ground motion perpendicular to direction of travel
Can only travel through solids (bc liquids and gases cannot be ruptured)
Slower
Surface waves
Arrival of body waves at the surface of the earth
—greatest amplitude in near-surface layers of sediment
— most destructive earthquake waves
Two types:
1. Love
2. Rayleigh
Love waves
Side to side motion on the surface
Third fastest speed
Rayleigh waves
Slowest wave,
up and down, forward and back motion, cyclical almost
Are deep or shallow earthquakes likely to cause more damage?
Shallow
What waves cannot travel through the outer core? Why?
S-waves cannot travel through the liquid outer core because they do not have any shear strength
Velocity of ___ change with ___
Velocity of waves change with density. Hence, refraction nearer to core
Modified mercantile scale
What you feel when in an earthquake
Weirdly specific
Richter magnitude scale
— amplitude of the largest wave is measured and the time interval between the P and S waves to determine the distance from the epicentre
— very imperfect, doesn’t accurately assign magnitude for large (duration), deep or far away quakes.
— for local magnitude
— doesn’t address litholgy
The moment magnitude (Mw)
accounts for the amount of displacement and area of rupture along a fault
Can we predict earthquakes?
We can predict where, but not when
— because earth is hard to model (dynamic)
—. And there are lots of small quakes (hard to measure)
What hazards are associated with earthquakes?
— rupture zone surfaces
— landslides, which lead to
— flooding
— fires
— liquefaction (loose sediment+water+ground shaking)
—tsunamis
How do we engineer for earthquakes?
— moment frames (strengthen joints)
— bracing systems (brace building, ductile)
— shear wall system (prevent building from shearing)
— dampers (reduce vibrations)
— seismic base isolations (wheels that take the weight of the base of the structure)
Rock mechanics
The study of the properties and mechanical behaviour of rock materials in response to the forces acting on them within their physical environment
(When will the rocks fail)
(Developed bc of excavating tunnels)
Mass wasting
The downslope movement of earth materials due to gravity
Classified by:
— type of movement (flow, slide, fall)
— type of material (rock or sediment or ice or snow)
— velocity
Falls
Free fall of earth materials
Rocks loosened by…
— root growth
— frost wedging
— heavy precipitation
Slides/landslides
—coherent masses of earth material slide down a slope along a failure surface called a slide plane within well-defined boundaries
— slump - curved failure surface
— Glide - planar failure surface
Slumps
Slow slope failures along a curved slide surface
—blocks rotate during failure
—. Usually occurs in homogenous substrate, as opposed to strongly stratified (layered) and lithified (stuck together) rock masses
Scarp
A steep scar on the undisturbed side of the failure. Zone of detachment
Block glides
Occur when coherent masses of rock or sediment move along planar sliding surfaces
Failure planes can be:
— sedimentary bedding planes
— metamorphic folioated planes
— faults
—fractures
Flows
Mass movements of unconsoildated material move overland
— fluid-like behaviour (sediments+water)
— caused by rainfall, steep slopes, lack of vegetation, presence of loose soil and debris
Creep
—imperceptibly slow downslope movement of rock sand soil particles near the ground surface
— appears o be continues, but is the result of numerous, minute, discrete downslope movements
— rate depends of steepness of slope, water content, type of sediment, and vegetation
Hills are sound bc of creep
Freeze0-thaw cycles or thermal expansion and concatenation cause creep
What are the effects of soil creep
Objects resting on top of the soul are carried by it as it deceaseds down the slope
Fast flows
—dense mixtures of sediment and water
— rock avalanches: rock fragments
— debris flows: course sediment
— mudflow: mud
Pore pressure
— in unsaturated sediments, water tension pulls grains toward each other (contraction)
— in starred sediments, pore pressure pushes grains apart (expansion)
—- pressure+tension = separated grains = fluid-like behaviour
Debris flows
— behave like a fluid and can be very fast
— most dangerous of all mass movements
— occur when heavy rainfall, snowmelt, or dam-burst water mixes with loose soil and rock on a sloping surface
— often gets funnelled into channel and posited on the valley floor
Rock mass properties
— A large body of rock
— Broken up by discontinuities that divide it into smaller blocks of intact rocks
— gives rocks discontinuous and anisotropic character
Anisotropic
Different properties in different directions
Types of discontinuities
- Bedding planes (sedimentary)
- Joints/cracks (breaks without displacement/movement)
- Faults (breaks with displacement/movement)
- Foliation (metamorphic rocks with layered formed perpendicular to stress)
How do we know when a rock will fail and the extent of the failure?
- How strong the rock mass is
- How stresses are redistributed due to interactions between the rock mass and the structure
How do we determine the strength of a rock mass?
- The strength of the intact rock and
- the strength of any discontinuities
— strain changes the shake and/or volume of a rock
How do we measure intact rock strength?
Unconfined compressive strength test (UCS)
— diving the maximum load at faerie bu the cross-sectional area of the sample (o=F/A)
What is rock strength
The maximum amount of stress you can apply to a rock before it fractures
What are the strongest rocks according to UCS
Igneous rocks are string than sedimentary rocks because they have an interlocking structure and less planes of weakness
More metamorphizized, the stronger the rock
EXTRUSTIVE, APHANETIC, IGNEOUS ROCKS. BASALT>GRANITE
What controls rock strength?
- Rock type (sedimentary, metamorphic, igneous) (texture, mineral assemblage, structure)
- Confining pressure (deeply buried, expressionist compression in all directions)
- Water (unsaturated vs saturated (porepressure))
- Amount and duration of stress (stress over a long time means more plastic like defomation. Like silly putty)
- Weathering (reduces rock strength)
Pore pressure
Under saturated conditions, water experiences confining stress and pushes back equally in all directions
How does weathering reduce rock strength
Increases pore space and permeability, increasing surface area exposed to wreathing
Weathering = more weathering
What controls the strength of discontinuities?
- Surface roughness (rough surfaces act to inc4ease friction)
- Joint width (hairline fractures are stronger than gapped fractures)
- Extent of weathering in fracture planes (weathered planes tent to be weaker)
- Water (wet fractures reduce friction) (flowing fractures elevate pore pressure and reduce stress)
- Continuity
- Spacing (closely spaced joints reduces rock strength)
- Orientation (the steeper the dip plane, the more likely the failure)
Daylighting
Fracture surface dips in the same direction as the slope of the outcrop
How do we classify rock mass strength?
RQD: Rock Quality Designation
‘RNR: Rock Mass Rating
RQD
Describes the mechanical quality of rock recovered why taking a core.
High RQD, high rock strength
RQD = (sum of lengths of core pieces over 10cm)/(total length of core) x 100
RMR
Semi-quantitative measure of the strength or stability of the rock mass
