Could an earthquake breach or rupture the aquifer, allowing saltwater to penetrate the groundwater supply?
No. During a strong earthquake, the aquifer would shake, but it is not a brittle container that is vulnerable to breaching or rupture. The Westside Basin aquifer is an underground reservoir composed of a sequence of sediment layers. The sediment layers are mostly particles of sand, with lesser amounts of clay, silt and gravel. The pore spaces (voids) between the particles of sediment are saturated with groundwater. The groundwater in the aquifer moves slowly from the Sunset District westward to the Pacific Ocean. This groundwater movement from the land area toward the ocean is what prevents the seawater from penetrating the groundwater supply. The seaward movement of groundwater would not be changed by the earthquake shaking.
