HR: 1330h
AN: S42C-0182    [PDF]
TI: Mechanisms of hydrodynamic fault rock fluidization and liquification and their effects on fault strength evolution
AU: * Eichhubl, P
EM: eichhubl@stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences, Stanford, CA 94305-2115 United States
AB: Coseismic entrainment and transport of fragmented fault rock in moving fluid has previously been inferred to explain layering and sorting of cataclastic fault rocks and their injection into connected fractures. I distinguish two mechanisms of fault rock entrainment with different impacts on the local stress state in the fault zone and on fault rheology: 1.Fault rock fluidization results from the entrainment of wear particles or fault breccia in rapidly upward moving fluid following the coseismic rupture of a pressure seal. Fault rock fluidization requires that fluid contained within the fault, as well as in the surrounding host rock, is at super-hydrostatic pore fluid pressure prior to rupture. 2. Fault rock liquification results from the coseismic suspension of wear particles in fluid that is contained within fracture and pore space of the fault zone at nominally hydrostatic pressure. In this case, particles are suspended due to turbulent fluid flow induced by viscous drag along the slipping fault surfaces and by local fluid pressure gradients resulting from the creation and reduction of fracture space along the fault. Liquification will be aided by an overall coseismic fault zone dilatancy and subsequent transient influx of pore fluid into the fault zone. The coseismic pressure evolution within the fault zone will be different for both mechanisms: For fault rock fluidization, a rapid initial pressure drop will be followed by a slower pressure decline as fluid migrates from the fault into a sink at higher structural levels and as suspended material settles out of the fluid. For fault rock liquification, an initial drop due to coseismic fault dilatancy will be followed by a transient increase in fluid pressure due to the load of suspended material within the fluid column contained in the fault, reverting to the initial pressure as suspended material settles out. The transient increase in fault fluid pressure due to liquification would result in an equally transient reduction in effective fault normal stress potentially promoting seismic or aseismic afterslip. While the fluid pressure evolution is different for both mechanisms, they have similar effects on fault rock rheology. In both cases, entrainment of solid wear particles and breccia would increase the fluid volume contained in the fault, reducing frictional contact area along the fault surface, and thus resulting in coseismic slip weakening. In contrast to fault rock fluidization, slip weakening by fault rock liquification does not require super-hydrostatic fluid pressures prior to rupture enabling this mechanism in tectonic regimes that lack efficient mechanisms of interseismic pore pressure buildup.
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8045 Role of fluids
SC: Seismology [S]
MN: 2003 Fall Meeting