Estimation of permeability, porosity, and grain-size distributions across the San Andreas Fault Zone in northwest Coachella Valley, California (Riverside County)

Abstract

The groundwater aquifer system in the northwestern region of Coachella Valley, California serves as a major natural resource for agricultural and municipal uses. In this region, the aquifer system is partitioned into four groundwater sub-basins due to the presence of the San Andreas fault zone. Previous investigation involving land surface deformation, seismic data, and groundwater data indicate there are at least three main strands of the San Andreas Fault- Mission Creek Strand, Banning Strand, and Garnet Hill Strand. For years, these faults have been characterized as simple barriers to fluid flow due to measureable offsets in the water table across the fault strands and hydrochemistry variations. The ability for a fault to act as a barrier to flow in an aquifer system is the result of significant development of gouge causing lateral variations in fault zone permeability but the mechanisms for gouge development in the Coachella Valley in unconsolidated-to-weakly consolidated sediments is unclear. Another explanation for variations in water chemistry, temperature, and groundwater levels is due to displacement of impermeable bedrock or relative offset of water bearing units. This study proposes to document variations of permeability and porosity across the San Andreas Fault zone. Field mapping, sampling and descriptions of fault zone, damage zone, and gouge width were recorded at four fault outcrop locations in the region. Analysis of porosity, hydraulic conductivity, intrinsic permeability, and grain size distribution across the fault zone associated with each fault strand indicate that different regions of the fault zone can xiii act to impede and/or enhance fluid flow across the faults. This data analysis of permeability and porosity variations across fault zones will help develop a better understanding of fault and aquifer interactions for future groundwater models, recharge activities, fault displacement, and development of gouge in unconsolidated sediments.