Week 9 Flashcards

1
Q

SFL pitch =

A

angle it makes on the fault plane with the strike

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2
Q

Stereonet; “SFL pitching XX N/E/S/W”

A

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

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3
Q

Recap; how to read off stereonet

A

Line up point with horizontal

Count in = dip/plunge

Dash on side of circle in line with point

Rotate to a pole and read

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4
Q

Andersonian faults stereonet; fault planes cross suggest

A

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

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5
Q

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

A
  • 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
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6
Q

Is σn +ve or -ve for tensile strength?

A

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

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7
Q

How to determine cohesion for Coulomb-Naiver criterion graph?

A

When σn = 0

y=intercept

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8
Q

Typical values, irrespective of rock type for:

φ
μ
θ

A
φ = 30-40'
μ = 0.58-0.85
θ = 50-60'
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9
Q

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

A
  1. Stable sliding (aseismic)

2. Stick-slip (seismogenic)

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10
Q

What causes frictional resistance along fault surface?

A

Interlocking asperities

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11
Q

Amonton’s Law

A

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

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12
Q

Byerlee’s law

A

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)

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13
Q

Stable sliding (aseismic)

A

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

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14
Q

Stick-slip (seismiogenic)

A

Sudden slip events and periods of no slip

Release = earthquake

Magnitude due to size of stress drop

Dominant > 3km

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15
Q

What does a fault with km-scale offsets indicate?

A

Very large scale number of earthquakes

Throw rates 1-10mm/yr

Different patches slip during different events = complex displacement accumulation

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16
Q

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

A

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

17
Q

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

A
  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
18
Q

Cataclasites

A

Indicate faulting at great depths within brittle crust

Distributed brittle deformation
‘Ductile’

19
Q

What causes pore fluid pressure (PFP)?

A

Magma

Hydrocarbons

Water

20
Q

What is hydrostatic pressure at equilibrium conditions?

A

= (water density) x g x h

21
Q

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

A

= (rock density) x g x h

22
Q

What is PFP at equilibrium conditions?

A

0.4 x lithostatic pressure

  • assuming free movement
  • never greater than lithostatic pressure (WOULD MEAN LIQUEFACTION)
23
Q

What causes overpressure?

A
  1. Restricted fluid movement e.g. compaction

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

24
Q

Drained triaxial experiments

A

Pore fluid can escape

Increase confining pressure = increase σn

Fluids leak off = constant PFP

Ultimate strength rises with increasing depth

  • same as dry clay
25
Undrained triaxial experiments
Fluids can't escape Increase confining pressure = increase σn PFP increases by EQUAL AND OPPOSITE AMOUNT = ultimate strength is constant and μ = 0
26
Rock mechanical behaviour is affected by...
FLUID PRESSURE, I.E. NOT JUST FLUID PRESENCE
27
Effective stress, σ'n =
σn - PFP (since PFP effectively counteracts σn) Gives τf = S + μσ'n
28
Axial stress =
σ1
29
Ultimate strength =
Differential stress at failure = σ1-σ3
30
What happens when pore fluid is introduced to a Mohr circle? (Without changing normal pressure)
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
31
What is hydraulic fracturing of 'fresh' rock?
PFP counteracts normal stresses Apparent reduction in normal stress = failure