HR: 14:10h
AN: T13E-03 INVITED     [Abstracts]
TI: Dynamic fault weakening by thermal pressurization: field and laboratory constraints
AU: * Wibberley, C A
EM: wibbs@geoazur.unice.fr
AF: Université de Nice - Sophia Antipolis, Géosciences Azur CNRS, 250 rue A. Einstein, Valbonne, 06560 France
AB: The long-standing idea that earthquake slip zones must dynamically weaken during slip to allow continued motion and dynamic runaway has lead to an equally long-running debate as to how such dynamic weakening may happen. This talk focuses on testing slip weakening mechanisms involving the role of pore water in the slip zone material. A combined field and laboratory study using an excellent exposure of the Median Tectonic Line fault zone in Japan has revealed a central principal displacement zone (a few cm wide) of low permeability clay gouge (k = 10^-20 to 10^-21 m² parallel to foliation at effective pressures of 80 - 180 MPa). This most recent slip zone is laterally continuous across most of the outcrop. Earlier oblique slip zones attest to a complexity in previous rupture behaviour. A range of adjacent fault rocks is affected by these oblique zones, such as high-permeability cemented cataclasites and coarse gouges. A model of slip weakening by thermal pressurization is tested whereby frictional heating of pore water in the slip zone gouge allows fluid pressure to either build-up (in a low permeability case) or dissipate by fluid escape (in a high permeability case). The modelling assumes a narrow slip zone of finite hydraulic diffusivity (constrained by the laboratory data), surrounded by a fracture-dilatant material of infinite hydraulic diffusivity. In this process, a trade-off exists between narrower slip zones causing higher rates of frictional shear heating than wider zones, (faster fluid pressure build-up), against the fluid pressure dissipation being faster in a narrower zone (shorter fluid escape path). Modelling this thermal pressurization process using data from the clay gouge slip zone suggests that fluid pressure may rise to lithostatic in about one second. Using the data from other fault rocks in the modelling, such as adjacent coarse gouges, only partial pressurization occurs, and only in relatively wide zones of 3 cm - 10 cm. The modelling predicts slip weakening distances similar to those interpreted from seismological data of large earthquakes, provided that the new rupture is confined to the low-permeability clay gouge.
DE: 7209 Earthquake dynamics (1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: Fall Meeting 2005