Week 9

Question 1

SFL pitch =

Answer 1

angle it makes on the fault plane with the strike

Question 2

Stereonet; “SFL pitching XX N/E/S/W”

Answer 2

Count XX in on fault plane FROM N/E/S/W direction

Question 3

Recap; how to read off stereonet

Answer 3

Line up point with horizontal

Count in = dip/plunge

Dash on side of circle in line with point

Rotate to a pole and read

Question 4

Andersonian faults stereonet; fault planes cross suggest

Answer 4

Normal fault and slightly inclined σ2 = slightly oblique (N.B. think about focal mechanism)

Question 5

Andersonian faults stereonet; scenario - one fault, know reverse/normal/strike-slip, given SLF

Answer 5

• go through normal steps
• assume acute angle 60’; σ1 = 30’ from SF on σ1σ3 plane (direction determined from fault type)
• theoretical conjugate fault goes through σ2
• σ3 90’ from σ1

Question 6

Is σn +ve or -ve for tensile strength?

Answer 6

-ve, where crosses negative x axis in Coulomb-Naiver failure criterion graph

Question 7

How to determine cohesion for Coulomb-Naiver criterion graph?

Answer 7

When σn = 0

y=intercept

Question 8

Typical values, irrespective of rock type for:

φ
μ
θ

Answer 8

φ = 30-40'
μ = 0.58-0.85
θ = 50-60'

Question 9

What are the two modes of frictional sliding at low confining P

Answer 9

1. Stable sliding (aseismic)

2. Stick-slip (seismogenic)

Question 10

What causes frictional resistance along fault surface?

Answer 10

Interlocking asperities

Question 11

Amonton’s Law

Answer 11

Ease due of sliding due to:

• orientation
• frictional properties
• confining pressure (σ3)

Increase depth = increase σn = forces asperities together = increases slip RESISTANCE

τf = μsσn

μs = coefficient of sliding friction

Question 12

Byerlee’s law

Answer 12

Shallow depths <10km τf = 0.85σn

Greater depths τf = 0.5 + 0.6σn

I.E. FAULTS ARE STRONGER WITH DEPTH

Wide range of values (not so much for water-rich clays)

Question 13

Stable sliding (aseismic)

Answer 13

Constant rate, doesn’t increase stress

In reality = STEADY SLIP HARDENING due to slip zone damage = increase stress

Most common in uppermost crust (<3km), σn lowest

+/ clay-rich fault zone gouges

Question 14

Stick-slip (seismiogenic)

Answer 14

Sudden slip events and periods of no slip

Release = earthquake

Magnitude due to size of stress drop

Dominant > 3km

Question 15

What does a fault with km-scale offsets indicate?

Answer 15

Very large scale number of earthquakes

Throw rates 1-10mm/yr

Different patches slip during different events = complex displacement accumulation

Question 16

What does it mean if a Mohr circle does not cross Byerlee’s line?

Answer 16

New fault fractures will keep being created, no frictional sliding will take place

Question 17

Determining what will happen at a fault; plotting failure envelopes for Coulomb-Naiver failure criterion and Byerlee’s law…

Answer 17

1. Plot σ3
2. Circle must intersect at failure (trial and error)
3. If doesn’t cross Byerlee’s = no frictional sliding i.e. new fault fractures created

Question 18

Cataclasites

Answer 18

Indicate faulting at great depths within brittle crust

Distributed brittle deformation
‘Ductile’

Question 19

What causes pore fluid pressure (PFP)?

Answer 19

Magma

Hydrocarbons

Water

Question 20

What is hydrostatic pressure at equilibrium conditions?

Answer 20

= (water density) x g x h

Question 21

What is lithostatic pressure (confining pressure) at equilibrium conditions?

Answer 21

= (rock density) x g x h

Question 22

What is PFP at equilibrium conditions?

Answer 22

0.4 x lithostatic pressure

• assuming free movement
• never greater than lithostatic pressure (WOULD MEAN LIQUEFACTION)

Question 23

What causes overpressure?

Answer 23

1. Restricted fluid movement e.g. compaction

2. Input of a new fluid e.g. diagenesis/metamorphism/hydrocarbon or magma migration

Question 24

Drained triaxial experiments

Answer 24

Pore fluid can escape

Increase confining pressure = increase σn

Fluids leak off = constant PFP

Ultimate strength rises with increasing depth

• same as dry clay

Question 25

Undrained triaxial experiments

Answer 25

Fluids can’t escape

Increase confining pressure = increase σn

PFP increases by EQUAL AND OPPOSITE AMOUNT

= ultimate strength is constant and μ = 0

Question 26

Rock mechanical behaviour is affected by…

Answer 26

FLUID PRESSURE, I.E. NOT JUST FLUID PRESENCE

Question 27

Effective stress, σ’n =

Answer 27

σn - PFP

(since PFP effectively counteracts σn)

Gives τf = S + μσ’n

Question 28

Axial stress =

Answer 28

σ1

Question 29

Ultimate strength =

Answer 29

Differential stress at failure = σ1-σ3

Question 30

What happens when pore fluid is introduced to a Mohr circle? (Without changing normal pressure)

Answer 30

Pore fluid pressure fracturing will occur when intersects with envelope of failure
= slide circle to left

N.B. σ’n = σn-pfp i.e. can calculate PFP

Differential stress remains constant
Mean stress decreased by amount due to PFP

Question 31

What is hydraulic fracturing of ‘fresh’ rock?

Answer 31

PFP counteracts normal stresses

Apparent reduction in normal stress = failure