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Creep, compaction and the weak rheology of major faultsPowerPoint Presentation

Creep, compaction and the weak rheology of major faults

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### Creep, compaction and the weak rheology of major faults

Norman H. Sleep & Michael L. Blanpied

Ge 277 – February 19, 2010

The problem

- San Andreas Fault: low heat flow
=> Sliding causes little frictional heating

=> t < 20 Mpa

- Across the fault, s = 200 – 570 MPa
t = m(s - pf)

m = 0.7, pf=hydrostatic

=> t = 90 – 260 MPa

The suggestion

- Low m (0.2) ? No material would account for it…
- t = m(s - pf)
if we have s ~ pf then t can be low.

Need a mechanism to have high fluid pressure:

permanently ?

transiently ?

Permanently high fluid pressure

- Dehydration of minerals ? Subduction zone only.
- Regional high fluid pressure ? No, more favorably orientated planes in the country rock would also be weakened.
- Where would the water come from ? No big reservoir available.

Transiently high fluid pressure

Pore pressure cycle: Water trapped around the fault by seals.

Interseismic compaction of fault zone by ductile creep

=> porosity decreases

=> fluid pressure (pf) increases

Coseismic restoration of porosity (dilatancy)

=> fluid pressure (pf) back to initial

Role of frictional heating

- Increases pore pressure during earthquake once the slip has started (>1mm/s)
[Segall & Rice, 2006]

Constant pore volume => scale length of slip to increase Pf to lithostatic pressure = 0.24m. (low)

- Increase porosity
Constant pore pressure => variation of porosity = 0.04/m.

Temperature = 600oC

V = 8.66 x 10-5 mm/s

= 100 MPa

Fault with gouge

undrained

mapp = t/(s-pp)

Axial displacement (mm)

Blanpied, Lockner & Byerlee, Nature (1992)

Temperature = 600oC

Pp = 100 MPa

drained

mdry granite = 0.7

mapp = t/(s-pp)

Axial displacement (mm)

Blanpied, Lockner & Byerlee, Nature (1992)

- Water at high temperature:
lowers rock’s strength at low strain rates

- Pore fluid in fault may be isolated from surrounding rock by seals
- Shear + compaction in the fault zone
=> increase in pore pressure

=> sliding at low effective stress

Field evidences

- Low permeability seals exhumed from 2 to 5 km.
- Arrays of subsidiary faults in surrounding rocks
=> near-fault-normal compression

=> low sliding resistance

- Episodes of formation and healing of fractures
=> fluid pressure reached lithostatic level (hydrofracturation)

Velocity of the rock

Shear viscosity

Porosity

y

Bulk viscosity

x

Seals:

Deformation: linear viscous

Variable parameters

Earthquake cycle < th < time fault active

bc, fraction of the faulting energy that goes into creating cracks

MODEL 1

THIN FAULT WITH HIGH VISCOSITY

Time for pores to compact a

significant amount of their volume:

Analogous time for cracks

BROAD FAULT WITH LOW VISCOSITY:

CREEPING FAULT

Cracks close too rapidly to have

an effect on the earthquake cycle.

Viscosity low => Pf increases to near

lithostatic before much shear traction

builds up.

Porosity as a state variable

- Rate and state friction law:
Aging evolution of the state variable

Ductile compaction of cracks:

where F is the crack porosity

DP = (s - pf)

hm is the bulk viscosity

V is the sliding velocity

[Mc Kenzie, 1984]

- Crack production rate:
where V is the sliding velocity

bm fraction of the energy that goes into crack production

Fc critical porosity

DP not constant…

Doesn’t consider

the thermal effect

on porosity…

Accounts for the friction change in experiences

from Linker and Dieterich (1992).

Conclusions

- Small amount of ductile creep allows porous fault zone to compact
=> In partially sealed fault zone, increases fluid pressure

=> earthquake failure at low shear traction.

- Porosity restored during earthquake.
- Nucleation size: Rubin & Ampuero [2005]:
would be too large…

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