Introduction to Defects and Earthquakes Flashcards
Causes of defects(fractures)
- Tectonic forces: compression, tension and shear
- Uplift and/or erosion: pressure relief
- Cooling of lava: contraction
Two groups of defects
Joints and Faults
Wha are joints
- defects that exhibit little or no movement parallel to plane of fracture
- most common structure in rocks
How do joints form
1) Compressional, tensional, shear forces from tectonic processes - brittle deformation
2) Uplift and/or erosion of a rock mass - rocks out of equilibrium at surface; pressure around rock relieved which leads to relaxation and fracturing
3) Cooling of lava flows, lava lakes - cooling lava contracts forming network of joints leading to columnar jointing
What are joint sets
parallel joints within a rock mass
What are conjugate joint sets
rock masses that contain two or more joint sets at different attitudes
How are blocks formed
Intersection of joint sets - slabbing, wedge failure etc. and danger when excavating or tunnelling
Columnar Jointing
- lava cools from outside in
- forms polygonal joint network
- joints define columns
- column diameter dependant on cooling rate i.e. slow cooling -> wider columns
What are the important physical characteristics of joints
- orientation: strike and dip of joint plane
- spacing: defines the size of individual blocks
- aperture: distance between adjacent walls
- persistence: length, continuity into rock mass
- surface morphology: determines how blocks mover against each other -> increased roughness = increased shear strength
- lining: implies water flow
- infilling: can be weaker than rock; can enhance strength
Faults
- generally planar fractures that exhibit displacement parallel to fracture plane (vertical, horizontal, oblique)
- fault planes have orientation
- faults classified according to relative movement direction: dip-slip, strike-slip, oblique slip
Normal Faults (Dip-Slip)
- occur in response to extensional tectonic forces
- lengthening and extension
- vertical displacement
- hanging wall movement in dip direction of fault plane
- hanging wall moves down with respect to footwall
Four aspects of hanging wall block
- Footwall: wall you can walk on
- Hanging wall: overhead
- Throw: amount of vertical displacement
- Heave (ho): amount of horizontal displacement
Reverse Faults (Dip-Slip)
- occur in response to compressional tectonic forces
- shortening occurs
- vertical displacement
- hanging wall movement opposite dip direction of fault plane
- hanging wall moves up with respect to footwall
Thrust Faults (Dip-Slip)
Low angle (<15degrees) reverse faults
Strike-Slip Faults
- response to shearing tectonic forces
- horizontal offset only
- movement in strike direction of fault plane
- no vertical component to movement
- Classified on direction of lateral movement
What are the two types of strike-slip faults
1) Right-lateral (dextral): opposite block to that on which the observer is standing moves right
2) Left-lateral (sinistral): opposite block to that on which observer is standing moves left
Oblique-Slip Faults
- vertical and horizontal components to fault movement
- combinations of normal, reverse, dextral or sinistral movement
Geomorphological clues of faults
1) scarps, linear features, sag ponds (due to blocked drainage)
2) truncation/abrupt termination of landforms
3) faceted spurs - triangular shaped faces on spurs on fault scarp due to erosion
4) horst and graben
5) offset landforms
6) lineation - rupture zone delineated due to differential weathering of softer fault gorge
Earthquakes
Rapid ground movements cause by shock waves generated within the earth
What are the sudden energy releases that produce earthquakes
1) Stressed rocks deform elastically up to their elastic limit
2) When limit reached rocks break and rebound to undeformed shape
3) During rebound stored elastic strain is released as shock waves
What are the occurrences of earthquakes
- when faults form
- movement of existing faults
- at plate boundaries
How do new faults develop
- tectonics force act on rock mass
- initial elastic behaviour
- cracks develop (brittle behaviour)
- rock raptures as cracks join and fault forms (elastic rebound)
Difference between stick and slip
Stick is stress buildup over a long time period and slip is stress release over a very short time
What are the hypo and epicentres
- Focus (hypocentre): point on a fault where rocks begin to rupture
- Epicentre: point on surface directly above the focus
- seismic waves radiate outwards from the focus
P waves (body)
- compressional or longitudinal
- ground alternately compressed and dilated in direction of propagation
- travel through solids and fluids
- travel very fast
S waves (body)
- transverse
- ground displaced perpendicular to direction of propagation
- travel only through solids
- slower
Rayleigh waves (surface)
- roll along ground
- move up and down and side to side in direction of wave propagation
Love waves (surface)
- roll along ground
- move the ground from side to side causing horizontal shearing
- slightly faster than Rayleigh waves
Earthquake location
P waves and S waves move at different speeds (P faster) so the difference in arrival time between them is proportional to the distance from the epicentre
How is the Richter Magnitude Scale calculated
- measuring wave amplitudes
- 1 whole unit increase on scale is a 10-fold increase in wave amplitude and a 32-fold increase in energy released
Moment Magnitude equation
Seismic moment = shear modulus x area of fault plane ruptured x average displacement
Plate boundaries and earthquakes
- occur at shallow depths near convergent plate boundaries and get deeper further away from the boundary
- earthquake foci define a plane which dips landward
- earthquakes occur at shallow depths along transform (strike-slip) plate boundaries
How to date faults
- lowest (deepest) datable undisturbed layer above fault gives minimum age of last movement
- or highest datable layer distributed by fault gives max age for last movement
- use ages along with throw to calculate movement rate
Liquefaction, settlement and lateral spreading
- during earthquake shaking saturated, unconsolidated sediments are transformed into a substance that acts like a fluid
- water in spaces between sediment grains is squeezed and resulting water pressure causes sediment to lose grain to grain contact
- this means sediments can flow and are unable to support the weight of structures so they tilt or sink
- pressurised water forced up through cracks take sediments to the surface
Rockfalls and landslides
- ground shaking during earthquakes destabilises cliffs and steep slopes
- boulders or large soils masses move rapidly downslope
- hazard can exist long after initial earthquake passes
Tsunami
- displacement of land under or into water can cause significant displacement of large volumes of water onto lad via tsunami
Flood
- dams breaks
- leeves break (river floods surrounding land)
- landslides into dam reservoirs
- landslides can dam rivers
- water pressurised during liquefaction can flow to surface
- burst water mains, ruptured water tanks
