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Structural > Week 10 > Flashcards

Week 10 Flashcards

1
Q

In order to initiate a tensile fracture, the PFP needs to overcome…

A

1) ROCK TENSILE STRENGTH
- varies for different minerals (due to microdefects/flaws)

2) σ3

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2
Q

σ3’

value of 2θ

A

= -ve = tensile stress

= σ3 - PFP

= -T

2θ = 90’

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3
Q

PFP required for tensile failure =

A

σ3 + T

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4
Q

Paper experiment results

A

Flaws cause stress concentrations

  • greatest when flaw perpendicular to applied tensile stress
  • i.e. 45’ to slit
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5
Q

σr =

A

applied remote tensile stress

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6
Q

σt =

A

crack tip stress

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7
Q

Influence of crack shape on σr and σt

A

Circular; σt = 3σr

Elliptical; 3:1 ; σt = 7σr

Realistic; 100:1 ; σt > 200σr

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8
Q

Stress intensity factor Nic =

A

fracture toughness/critical stress intensity factor

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9
Q

Griffith’s Crack Theory

A

Influence of crack orientation relative to principle stresses (θ)

Cracks are elliptical in cross section (a»c) and generate large tensile stress in crack tip (σt)

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10
Q

Griffith’s Crack Theory; in a tensile stress field

A

Open cracks

θ = 90
σt // to σ3
σt = σtmax

Unstable tensile (mode I) fractures propagate rapidly

I.E. MATERIALS ARE WEAK IN TENSION

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11
Q

Griffith’s Crack Theory; in a compressional stress field

A

Closed cracks

Cracks experience shear stress

Tensile ‘wing cracks’ when θ>45’

Propagate slowly due to compressional σ3’

I.E. MATERIALS ARE STRONGER IN COMPRESSION

Link and grow = through-going shear fractures i.e. FAULTS

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12
Q

Microcracks in elliptical process zones…

A

Leave behind damage zone surrounding fracture

Associated with widespread dilatancy

= fracturing/fluid flow processes interactions (+IMPLICATIONS)

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13
Q

Using Griffith’s Crack Theory as a failure criterion

A

Not a straight line

S = 2T
-t

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14
Q

Griffith’s vs Coulomb-Naiver

A

GRIFFITHS

  • good for low and -ve σn’
  • slope too shallow in compression

COULOMB-NAVIER
- good for shear failure

= can form a COMPOSITE ENVELOPE

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15
Q

Strain partitioning

A

Hybrid between shear and tensile

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16
Q

Shear failure (dry/drained rocks) values for:

  1. σ3-σ1
  2. σ3’
  3. θ
A
  1. > 8T
  2. > 0
  3. 60’
17
Q

Shear hydraulic failure values for:

  1. σ3-σ1
  2. σ3’
  3. θ
A
  1. 8T
  2. > =0
  3. 60’
18
Q

Hybrid hydraulic failure values for:

  1. σ3-σ1
  2. σ3’
  3. θ
A
  1. 4T < X < 8T
  2. -T < X < 0
  3. 60-90’
19
Q

Tensile hydraulic failure values for:

  1. σ3-σ1
  2. σ3’
  3. θ
A
  1. =<4T
  2. -T
  3. 90’
20
Q

What happens to θ as you +fluid and +open?

A

Gets higher

21
Q

Fault refraction varies due to…

A

Different rock types

e.g. more steep in competent carbonate and sandstone than weak shales

(preferential mineralisation)

22
Q

Fluid transport properties vary in…

A

TIME and SPACE

23
Q

Wing cracks

A

Focus fluid flow and mineralisation

24
Q

Dilation logs

A

Form when pre-existing structures can be re-activated by normal faults

25
Q

Termination and interaction features

A

Splays

Horsetail splays

Antithetic splays

Jogs and relays

26
Q

What kind of major hydrological changes follow modern earthquakes?

A

Liquefaction

Formation of new springs

Increased stream discharge

Change in groundwater levels

N.B. Mineralisation is widely associated with ancient fault zones

27
Q

Is fluid flow driven by active faulting or are faults driven by fluid pressures?

A

Or both?

28
Q

Earthquakes and cyclicity

A

Stick-slip behaviour = cyclic

Therefore PFP and fluid flow events also likely to be cyclic and correlated in time/space

29
Q

Fault valve model; how to periodic build ups in pressure trigger earthquakes?

A

REQUIRES:

  • high PFP gradients (>10MPa/km)
  • focussed fluid source
  • local/regional impermeable barrier
  • once breached fault must be an effective fluid channelway

Fall in PFP leads to resealing

30
Q

Example of fault valve seismicity

A

1997 Colfiorito (Umbria Marche) earthquake sequence, Apennines

Normal fault sequence lasting 30 days after two Mw5-6 earthquakes (several small and also thousands of aftershocks)

Regions on high CO2 flux

Deep evaporite seal

Modlled rupture event and fluid migration = matched to aftershock sequence observed

Complex evolution with multiple over pressuring events

31
Q

Fault pump model

A

Brittle fracture = dilatancy = suction pumping

Only likely significant near surface (0-2km) where regional fluid pressures close to hydrostatic and known to be significant dilatancy associated with fracture systems

32
Q

Importance of tectonic regime during seismogenic loading cycles

A

REVERSE FAULTING

  • mean normal stress σm gets bigger
  • influences evolution of τf = stronger?

NORMAL FAULTING

  • mean normal stress σm gets smaller
  • influences evolution of τf = weaker?
33
Q

Load strengthening vs load weakening faults; influence on fluid transport

A

Normal faults = long suck, short blow

Reverse faults = long blow, short suck

(Strike-slip can show both)

Most likely significant in near surface where much more dilatancy, especially in crystalline rocks

34
Q

Near surface normal faulting (0-2km depth)

A

Especially if competent host rocks e.g. crystalline (basement/granite/basalt/limestone)

e.g. Iceland/N Africa

In ancient settings e.g. Devon = common below unconformities, often filled with sedimentary material +/ hydrothermal minerals

35
Q

How do fissures develop?

A

Decreasing differential stress with depth and development of near surface tensile stresses

Preservation favoured in strong host rocks - often see collapse features/infills e.g. Zechstein, NE England

36
Q

Value of θ for tensile failure?

A

90’

i.e. on Mohr diagram 2θ = 180’

37
Q

Why are features like dykes and joints geometrically quite simple?

A

Rapid propagation

Long and planar mode I fractures

Structural (17 decks)

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