FLUID OVERPRESSURE AND FLOW IN FAULT ZONES: FIELD MEASUREMENTS AND MODELS

Abstract

Field studies of small normal faults (throws of metres to tens of metres) show 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 0–1m with a wavelength of 5–10m. The breccia acts mechanically as an inclusion; soft, ductile and sometimes creeping when the fault zone is active, but stiff and brittle when the fault zone is inactive. During interseismic periods, and when the fault has become inactive, the breccia behaves as a very dense, low-permeability material that is a barrier to transverse flow of groundwater. The breccia barrier thus collects water and channels it downdip or updip along the contact between the fault core and the damage zone. For a typical 1-m-thick interseismic breccia, the maximum transmissivity is estimated at Tp∼10−10m2s−1. The field data, however, indicate that during the high strain rates associated with faulting, seismogenic slip may occur either along the breccia, or along its contacts with the damage zone. The resulting fractures with apertures of ∼0.3cm may temporarily increase the transmissivity of the fault core by at least 8 orders of a magnitude, to as much as Tf∼10−2m2s−1. It is suggested that slip of faults of this type is commonly associated with the flow of overpressured water into the fault plane. High water pressure lowers the critical driving shear stress needed for fault slip and may greatly increase the aperture, hence the fluid transport, of the slipping fracture. Theoretical considerations indicate that, other things being equal, fluid flow along strike–slip faults is favoured over flow along dip–slip faults and that, generally, the steeper the dip of the fault, the more effective it is for fluid transport.