Introduction to Defects and Earthquakes

Question 1

Causes of defects(fractures)

Answer 1

• Tectonic forces: compression, tension and shear
• Uplift and/or erosion: pressure relief
• Cooling of lava: contraction

Question 2

Two groups of defects

Answer 2

Joints and Faults

Question 3

Wha are joints

Answer 3

• defects that exhibit little or no movement parallel to plane of fracture
• • most common structure in rocks

Question 4

How do joints form

Answer 4

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

Question 5

What are joint sets

Answer 5

parallel joints within a rock mass

Question 6

What are conjugate joint sets

Answer 6

rock masses that contain two or more joint sets at different attitudes

Question 7

How are blocks formed

Answer 7

Intersection of joint sets - slabbing, wedge failure etc. and danger when excavating or tunnelling

Question 8

Columnar Jointing

Answer 8

• lava cools from outside in
• forms polygonal joint network
• joints define columns
• column diameter dependant on cooling rate i.e. slow cooling -> wider columns

Question 9

What are the important physical characteristics of joints

Answer 9

• 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

Question 10

Faults

Answer 10

• 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

Question 11

Normal Faults (Dip-Slip)

Answer 11

• 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

Question 12

Four aspects of hanging wall block

Answer 12

• Footwall: wall you can walk on
• Hanging wall: overhead
• Throw: amount of vertical displacement
• Heave (ho): amount of horizontal displacement

Question 13

Reverse Faults (Dip-Slip)

Answer 13

• 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

Question 14

Thrust Faults (Dip-Slip)

Answer 14

Low angle (<15degrees) reverse faults

Question 15

Strike-Slip Faults

Answer 15

• 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

Question 16

What are the two types of strike-slip faults

Answer 16

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

Question 17

Oblique-Slip Faults

Answer 17

• vertical and horizontal components to fault movement

- combinations of normal, reverse, dextral or sinistral movement

Question 18

Geomorphological clues of faults

Answer 18

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

Question 19

Earthquakes

Answer 19

Rapid ground movements cause by shock waves generated within the earth

Question 20

What are the sudden energy releases that produce earthquakes

Answer 20

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

Question 21

What are the occurrences of earthquakes

Answer 21

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

Question 22

How do new faults develop

Answer 22

• tectonics force act on rock mass
• initial elastic behaviour
• cracks develop (brittle behaviour)
• rock raptures as cracks join and fault forms (elastic rebound)

Question 23

Difference between stick and slip

Answer 23

Stick is stress buildup over a long time period and slip is stress release over a very short time

Question 24

What are the hypo and epicentres

Answer 24

• 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

Question 25

P waves (body)

Answer 25

- compressional or longitudinal - ground alternately compressed and dilated in direction of propagation - travel through solids and fluids - travel very fast

Question 26

S waves (body)

Answer 26

- transverse - ground displaced perpendicular to direction of propagation - travel only through solids - slower

Question 27

Rayleigh waves (surface)

Answer 27

- roll along ground | - move up and down and side to side in direction of wave propagation

Question 28

Love waves (surface)

Answer 28

- roll along ground - move the ground from side to side causing horizontal shearing - slightly faster than Rayleigh waves

Question 29

Earthquake location

Answer 29

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

Question 30

How is the Richter Magnitude Scale calculated

Answer 30

- 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

Question 31

Moment Magnitude equation

Answer 31

Seismic moment = shear modulus x area of fault plane ruptured x average displacement

Question 32

Plate boundaries and earthquakes

Answer 32

- 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

Question 33

How to date faults

Answer 33

- 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

Question 34

Liquefaction, settlement and lateral spreading

Answer 34

- 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

Question 35

Rockfalls and landslides

Answer 35

- 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

Question 36

Tsunami

Answer 36

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

Question 37

Flood

Answer 37

- 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