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.
Confining pressure = 400 Mpa 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)
Confining pressure = 400 Mpa Temperature = 600oC Pp = 100 MPa drained mdry granite = 0.7 mapp = t/(s-pp) Axial displacement (mm) Blanpied, Lockner & Byerlee, Nature (1992)
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)
MODEL Velocity of the rock Shear viscosity Porosity y Bulk viscosity x Seals: Deformation: linear viscous Variable parameters
Models Parameters studied: W, fault width mi, intrinsic viscosity (i.e. shear and bulk viscosity)
Seal : Earthquake cycle < th < time fault active bc, fraction of the faulting energy that goes into creating cracks
least compressive stress MODEL 1 THIN FAULT WITH HIGH VISCOSITY Time for pores to compact a significant amount of their volume: Analogous time for cracks
MODEL 3 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
INTERSEIMIC REGIME 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 : would be too large…