Defects & Earthquakes Flashcards
3 causes of defects in rock masses
- tectonic forces (compression, shear, tension)
- uplift and/or erosion
- cooling of lava = contraction
2 forms of defect
joints (no movement parallel to fracture surface) and faults (movement parallel to the fracture surface)
define a joint
defect that exhibits little or no movement parallel to the plane of fracture
joint sets
parallel joints within a rock mass
columnar jointing
as lava cools it contracts, cools from outside inwards forming a polygonal joint network. longer cooling = wider columns
joint spacing vs. aperture
joint spacing is the size of an individual block, whereas aperture is the distance between adjacent walls.
joint persisitance
length into the rock
joint lining
implies water flow
infilling of joints
can enhance strength e.g. quartz veining
how do joints affect permeability of rock?
increase permeability, movement of water controlled more by fractures than porosity
faults definition
generally planar fractures that exhibit displacement parallel to fracture plane
Normal faults (DIP-SLIP)
- occur in response to extensional tectonic forces
________ is a wall you can walk on. ________ wall is an overhead and ______ is the ammount of vertical displacement, with ______ being the horizontal displacement.
footwall is a wall you can walk on. _hanging _ wall is an overhead and throw is the ammount of vertical displacement, with heave being the horizontal displacement.
how does the hanging wall move in relation to the footwall in normal vs. reverse fault?
hanging wall moves down with respect to footwall in a normal fault. opposite in reverse fault
reverse faults (DIP-SLIP)
occur in response to compressional tectonic forces
heave vs heave ho
heave ho compressional and related to reverse fault
thrust faults
low angle (less than 15 degrees) reverse fault
strike-slip faults
response to shearing tectonic forces. No vertical movement e.g alpine fault in the south island
oblique-slip faults
vertical and horizontal movement at the fault
opposite block to which observer is standing moves right
right-lateral (dextral) [left is sinistral]
when do we want to know the recurrence period of a fault?
engineering design
faults and groundwater
can act as barriers or conduits
earthquake definition
rapid ground movements caused by shock waves generated within the earth when elastic energy is released
energy release causing earthquakes (elastic rebound theory)
- stressed rocks deform elastically up to their limit
- when these rocks break they rebound to undeformed shape
- during rebound all the elastic strain is released as shock waves
occurence of elastic rebound theory
- when faults form
- movement of existing faults
- at plate boundaries
strain accumulation (stick) vs rebound (slip)
years, decades, centuries vs. seconds
as strain accumulates, what happens to the rock mass?
cracks develop after the initial elastic behaviour
faults start below the surface. ______ is the point where rocks begin to rupture. ________ is the point on the surface directly above this point. _______ waves radiate outwards from here and cause damage when they reach the surface as ________ waves.
faults start below the surface. focus(hypocentre) is the point where rocks begin to rupture. epicentre is the point on the surface directly above this point. seismic waves radiate outwards from here and cause damage when they reach the surface as surface waves.
P waves (body waves)
“p” for primary or pressure.
travel very fast and movement mimics a sound wave
S waves (body)
"s" for secondary or shear. transverse wave. ground displaced perpendicular to direction of propagation. shape mimics light wabe. slower than p waves
Rayleigh waves (surface)
roll along the ground.
responsible for most of the shaking in an earthquake. move ground up and down and ide to side
love waves (surface)
roll along the ground and move the ground side to side (shear) slightly faster than Rayleigh wave
summary of 3 seismic wave types
- P
- S
- surface
P waves arrive before S waves. Their difference in arrival of P and S waves is proportional to…?
distance from the epicentre. therefore you can locate the epicentre using 3 seismometers
Richter magnitude (absolute) measures what? uses what scale? why?
measures wave amplitude. log scale because range of earthquakes is huge.
Modified Mercalli Intensity scale (qualitative)
based on the amount of vibration people feel and the amount of building damage.
degree of shaking on soft sediment vs hard rock
more shaking on soft sediment
seismic waves ____ when encountering less dense material. Amplitude of seismic waves ______ as they slow, thus shaking _______.
seismic waves slow when encountering less dense material. Amplitude of seismic waves increases as they slow, thus shaking intensifies.
earthquakes occur at _______ depths near convergent and transform plate boundaries
shallow
wadati-benioff zone
dipping plane of earthquake foci at the surface of a descending oceanic plate
earthquake periodicity
the return period of fault movement is important in categorising a fault as active or inactive
normal and reverse faults cause _______ displacement when they intersect the ground surface
vertical
liquefaction
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 grains to lose grain-grain contact and the sediments can now flow and cannot support weight.
lateral spreading
lateral displacement of gently sloping ground as a result of liquefaction in a subsurface layer
Tsunami
displacement of land under or into water can cause significant displacement of large volumes of water onto land via tsunami
wavelength of tsunami
long wavelength. can travel huge distances. when wave approaches land sea-bed shallows and height of wave is amplified. increased height + long wavelength = heeeeeeaaaaaappps of water
other things earthquakes can cause
- floods (damage dams, landslides, river banks break, burst water mains)
- fire and disease
