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 =>  < 20 Mpa Across the fault,  = 200 – 570 MPa  - p f   p f =hydrostatic =>  = 90 – 260 MPa

The suggestion Low  (0.2) ? No material would account for it…  - p f  if we have  p f then  can be low. Need a mechanism to have high fluid pressure: permanently ? transiently ?

Role of fluid pressure in Rock mechanics

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 (p f ) increases Coseismic restoration of porosity (dilatancy) => fluid pressure (p f ) 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 P f to lithostatic pressure = 0.24m. (low) Increase porosity Constant pore pressure => variation of porosity = 0.04/m.

Blanpied, Lockner & Byerlee, Nature (1992) = 100 MPa Confining pressure = 400 Mpa Temperature = 600 o C V = 8.66 x mm/s Axial displacement (mm)  app  -p p  undrained Fault with gouge

Blanpied, Lockner & Byerlee, Nature (1992) Confining pressure = 400 Mpa Temperature = 600 o C Axial displacement (mm)  app  -p p  P p = 100 MPa drained  dry granite = 0.7

Results from the experiment: – 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)

Deformation: linear viscous x y Velocity of the rock Shear viscosity Bulk viscosity Porosity MODEL Seals: Variable parameters

Models Parameters studied: W, fault width  i, intrinsic viscosity (i.e. shear and bulk viscosity)

Seal :  c, fraction of the faulting energy that goes into creating cracks Earthquake cycle < t h < time fault active

Time for pores to compact a significant amount of their volume: Analogous time for cracks MODEL 1 THIN FAULT WITH HIGH VISCOSITY least compressive stress

MODEL 3 BROAD FAULT WITH LOW VISCOSITY: CREEPING FAULT Cracks close too rapidly to have an effect on the earthquake cycle. Viscosity low => P f 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

INTERSEIMIC REGIME Ductile compaction of cracks: where  is the crack porosity  P =  - p f   m is the bulk viscosity V is the sliding velocity [Mc Kenzie, 1984]

Crack production rate: whereV is the sliding velocity  m fraction of the energy that goes into crack production  c critical porosity

Accounts for the friction change in experiences from Linker and Dieterich (1992).  P not constant… Doesn’t consider the thermal effect on porosity…

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…