Week 9 Flashcards
SFL pitch =
angle it makes on the fault plane with the strike
Stereonet; “SFL pitching XX N/E/S/W”
Count XX in on fault plane FROM N/E/S/W direction
Recap; how to read off stereonet
Line up point with horizontal
Count in = dip/plunge
Dash on side of circle in line with point
Rotate to a pole and read
Andersonian faults stereonet; fault planes cross suggest
Normal fault and slightly inclined σ2 = slightly oblique (N.B. think about focal mechanism)
Andersonian faults stereonet; scenario - one fault, know reverse/normal/strike-slip, given SLF
- 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
Is σn +ve or -ve for tensile strength?
-ve, where crosses negative x axis in Coulomb-Naiver failure criterion graph
How to determine cohesion for Coulomb-Naiver criterion graph?
When σn = 0
y=intercept
Typical values, irrespective of rock type for:
φ
μ
θ
φ = 30-40' μ = 0.58-0.85 θ = 50-60'
What are the two modes of frictional sliding at low confining P
- Stable sliding (aseismic)
2. Stick-slip (seismogenic)
What causes frictional resistance along fault surface?
Interlocking asperities
Amonton’s Law
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
Byerlee’s law
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)
Stable sliding (aseismic)
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
Stick-slip (seismiogenic)
Sudden slip events and periods of no slip
Release = earthquake
Magnitude due to size of stress drop
Dominant > 3km
What does a fault with km-scale offsets indicate?
Very large scale number of earthquakes
Throw rates 1-10mm/yr
Different patches slip during different events = complex displacement accumulation
What does it mean if a Mohr circle does not cross Byerlee’s line?
New fault fractures will keep being created, no frictional sliding will take place
Determining what will happen at a fault; plotting failure envelopes for Coulomb-Naiver failure criterion and Byerlee’s law…
- Plot σ3
- Circle must intersect at failure (trial and error)
- If doesn’t cross Byerlee’s = no frictional sliding i.e. new fault fractures created
Cataclasites
Indicate faulting at great depths within brittle crust
Distributed brittle deformation
‘Ductile’
What causes pore fluid pressure (PFP)?
Magma
Hydrocarbons
Water
What is hydrostatic pressure at equilibrium conditions?
= (water density) x g x h
What is lithostatic pressure (confining pressure) at equilibrium conditions?
= (rock density) x g x h
What is PFP at equilibrium conditions?
0.4 x lithostatic pressure
- assuming free movement
- never greater than lithostatic pressure (WOULD MEAN LIQUEFACTION)
What causes overpressure?
- Restricted fluid movement e.g. compaction
2. Input of a new fluid e.g. diagenesis/metamorphism/hydrocarbon or magma migration
Drained triaxial experiments
Pore fluid can escape
Increase confining pressure = increase σn
Fluids leak off = constant PFP
Ultimate strength rises with increasing depth
- same as dry clay
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
Rock mechanical behaviour is affected by…
FLUID PRESSURE, I.E. NOT JUST FLUID PRESENCE
Effective stress, σ’n =
σn - PFP
(since PFP effectively counteracts σn)
Gives τf = S + μσ’n
Axial stress =
σ1
Ultimate strength =
Differential stress at failure = σ1-σ3
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
What is hydraulic fracturing of ‘fresh’ rock?
PFP counteracts normal stresses
Apparent reduction in normal stress = failure
