Defects & Earthquakes

Question 1

3 causes of defects in rock masses

Answer 1

1. tectonic forces (compression, shear, tension)
2. uplift and/or erosion
3. cooling of lava = contraction

Question 2

2 forms of defect

Answer 2

joints (no movement parallel to fracture surface) and faults (movement parallel to the fracture surface)

Question 3

define a joint

Answer 3

defect that exhibits little or no movement parallel to the plane of fracture

Question 4

joint sets

Answer 4

parallel joints within a rock mass

Question 5

columnar jointing

Answer 5

as lava cools it contracts, cools from outside inwards forming a polygonal joint network. longer cooling = wider columns

Question 6

joint spacing vs. aperture

Answer 6

joint spacing is the size of an individual block, whereas aperture is the distance between adjacent walls.

Question 7

joint persisitance

Answer 7

length into the rock

Question 8

joint lining

Answer 8

implies water flow

Question 9

infilling of joints

Answer 9

can enhance strength e.g. quartz veining

Question 10

how do joints affect permeability of rock?

Answer 10

increase permeability, movement of water controlled more by fractures than porosity

Question 11

faults definition

Answer 11

generally planar fractures that exhibit displacement parallel to fracture plane

Question 12

Normal faults (DIP-SLIP)

Answer 12

• occur in response to extensional tectonic forces

Question 13

________ is a wall you can walk on. ________ wall is an overhead and ______ is the ammount of vertical displacement, with ______ being the horizontal displacement.

Answer 13

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.

Question 14

how does the hanging wall move in relation to the footwall in normal vs. reverse fault?

Answer 14

hanging wall moves down with respect to footwall in a normal fault. opposite in reverse fault

Question 15

reverse faults (DIP-SLIP)

Answer 15

occur in response to compressional tectonic forces

Question 16

heave vs heave ho

Answer 16

heave ho compressional and related to reverse fault

Question 17

thrust faults

Answer 17

low angle (less than 15 degrees) reverse fault

Question 18

strike-slip faults

Answer 18

response to shearing tectonic forces. No vertical movement e.g alpine fault in the south island

Question 19

oblique-slip faults

Answer 19

vertical and horizontal movement at the fault

Question 20

opposite block to which observer is standing moves right

Answer 20

right-lateral (dextral) [left is sinistral]

Question 21

when do we want to know the recurrence period of a fault?

Answer 21

engineering design

Question 22

faults and groundwater

Answer 22

can act as barriers or conduits

Question 23

earthquake definition

Answer 23

rapid ground movements caused by shock waves generated within the earth when elastic energy is released

Question 24

energy release causing earthquakes (elastic rebound theory)

Answer 24

1. stressed rocks deform elastically up to their limit
2. when these rocks break they rebound to undeformed shape
3. during rebound all the elastic strain is released as shock waves

Question 25

occurence of elastic rebound theory

Answer 25

• when faults form
• movement of existing faults
• at plate boundaries

Question 26

strain accumulation (stick) vs rebound (slip)

Answer 26

years, decades, centuries vs. seconds

Question 27

as strain accumulates, what happens to the rock mass?

Answer 27

cracks develop after the initial elastic behaviour

Question 28

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.

Answer 28

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.

Question 29

P waves (body waves)

Answer 29

“p” for primary or pressure.

travel very fast and movement mimics a sound wave

Question 30

S waves (body)

Answer 30

"s" for secondary or shear.
transverse wave.
ground displaced perpendicular to direction of propagation.
shape mimics light wabe.
slower than p waves

Question 31

Rayleigh waves (surface)

Answer 31

roll along the ground.

responsible for most of the shaking in an earthquake. move ground up and down and ide to side

Question 32

love waves (surface)

Answer 32

roll along the ground and move the ground side to side (shear) slightly faster than Rayleigh wave

Question 33

summary of 3 seismic wave types

Answer 33

1. P
2. S
3. surface

Question 34

P waves arrive before S waves. Their difference in arrival of P and S waves is proportional to…?

Answer 34

distance from the epicentre. therefore you can locate the epicentre using 3 seismometers

Question 35

Richter magnitude (absolute) measures what? uses what scale? why?

Answer 35

measures wave amplitude. log scale because range of earthquakes is huge.

Question 36

Modified Mercalli Intensity scale (qualitative)

Answer 36

based on the amount of vibration people feel and the amount of building damage.

Question 37

degree of shaking on soft sediment vs hard rock

Answer 37

more shaking on soft sediment

Question 38

seismic waves ____ when encountering less dense material. Amplitude of seismic waves ______ as they slow, thus shaking _______.

Answer 38

seismic waves slow when encountering less dense material. Amplitude of seismic waves increases as they slow, thus shaking intensifies.

Question 39

earthquakes occur at _______ depths near convergent and transform plate boundaries

Answer 39

shallow

Question 40

wadati-benioff zone

Answer 40

dipping plane of earthquake foci at the surface of a descending oceanic plate

Question 41

earthquake periodicity

Answer 41

the return period of fault movement is important in categorising a fault as active or inactive

Question 42

normal and reverse faults cause _______ displacement when they intersect the ground surface

Answer 42

vertical

Question 43

liquefaction

Answer 43

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.

Question 44

lateral spreading

Answer 44

lateral displacement of gently sloping ground as a result of liquefaction in a subsurface layer

Question 45

Tsunami

Answer 45

displacement of land under or into water can cause significant displacement of large volumes of water onto land via tsunami

Question 46

wavelength of tsunami

Answer 46

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

Question 47

other things earthquakes can cause

Answer 47

• floods (damage dams, landslides, river banks break, burst water mains)
• fire and disease