Measuring Hydraulic Diffusivity of a Fault Damage Zone of the Blue Mountain Using Tidal Responses

Abstract:

Fluid diffusion along faults is thought to be a major factor in induced seismicity, yet direct measurement of the relevant hydrological parameters is rare. Inferences are often made from core-scale measurements by necessity even though fault zone damage creates fluid flow pathways that can only be captured at the field scale. Here we use fluctuations of water levels inside wells caused by Earth tides and barometric pressure to measure these properties in a geothermal field with known induced seismicity. We utilized three wells of a geothermal field in the Blue Mountain, Nevada to measure the in-situ properties by combining tidal and barometric responses. Our observation lasts for more than one year and shows negative phase response (water level lagging the solid Earth tide) for all the wells. We initially interpret the data using a homogeneous confined aquifer model. The range of the measured diffusivity is 10^-3-10^-1 m²/s under the model assumption, which is consistent with previous inferences on fault diffusivity. Such low diffusivity values are presented in studies of the Chi-Chi, Wenchuan and the San Andreas Fault (Doan et al, 2006; Xue et al, 2013, 2016). Perhaps the homogenous model is adequately capturing the structure in a region of multiple strands and copious secondary faulting creating a homogenized damage. The geological setting of the Blue Mountain makes us consider another model that contains a major fault-guided hydrogeological channel to interpret fault damage zones. In the more complex model, there is a trade-off between the fault and host rock diffusivity. The minimum bound of diffusivity for the damage zone is 0.1 m²/s and the host rock diffusivity can be between 10^-4 to 10^-3 m²/s. A combined inversion of wells that do and do not intersect the fault can resolve this ambiguity.