Fluid overpressure and flow in fault zones: field measurements and models

Field studies of small normal faults (throws of metres to tens of metres) s how that their fault cores consist of breccias that vary in thickness along the fault plane. Commonly, the down-dip variation in the breccia thickness is 0-1 in with a wavelength of 5-10 m. The breccia acts mechanically as an inclusion; soft, ductile and sometimes creeping when the fault zone is act ive, but stiff and brittle when the fault zone is inactive. During intersei smic periods, and when the fault has become inactive, the breccia behaves a s a very dense, low-permeability material that is a barrier to transverse f low of groundwater. The breccia barrier thus collects water and channels it downdip or updip along the contact between the fault core and the damage z one. For a typical I-m-thick interseismic breccia, the maximum transmissivi ty is estimated at T-p similar to 10(-10) m(2) s(-1). The field data, howev er, indicate that during the high strain rates associated with faulting, se ismogenic slip may occur either along the breccia, or along its contacts wi th the damage zone. The resulting fractures with apertures of similar to0.3 cm may temporarily increase the transmissivity of the fault core by at lea st 8 orders of a magnitude, to as much as T-f similar to 10(-2) m(2) s(-1). It is suggested that slip of faults of this type is commonly associated wi th the flow of overpressured water into the fault plane. High water pressur e lowers the critical driving shear stress needed for fault slip and may gr eatly increase the aperture, hence the fluid transport, of the slipping fra cture. Theoretical considerations indicate that, other things being equal, fluid flow along strike-slip faults is favoured over flow along dip-slip fa ults and that, generally, the steeper the dip of the fault, the more effect ive it is for fluid transport. (C) 2001 Elsevier Science B.V. All rights re served.