The Hydraulic Diffusivity of Faults

Abstract:

Fluid pressure in faults is thought to play a major role in both earthquake initiation and co-seismic rupture. Pressure changes diffuse through a porous media, and thus it is essential to estimate the hydraulic diffusivity of active fault zones in order to accurately predict fault hydrologic behavior.

Few hydraulic diffusivity measurements of active fault zones exist as such a measurement requires a field experiment sensitive to 10’s-100’s of meter scale structure. Tidal responses provide some constraints. Existing tidal measurements on the Wenchuan Fault in China, San Andreas Fault in the USA and Blue Mountain Geothermal Field in the USA all suggest averaged hydraulic diffusivities ~10^-2 to 10^-1 m²/s in the near-fault damaged zone. In the Chelungpu Fault, a cross-hole experiment that crossed the fault core resulted in a much lower diffusivity of 7 x 10^-5 m²/s; similarly low values of diffusivity have been measured in laboratory experiments on samples of the fault core of the San Andreas. Interestingly, the San Andreas observations show that the damage zone of high permeability is narrower than the zone of high diffusivity. The covariance of storage and permeability results in a relatively uniform diffusivity over a wide swath of the damage zone. It is possible that damaged faults evolve to a preferred value, which means that the hydraulic diffusivities in the damage zone could potentially be generalized to other fault zones.