THE CHARACTERIZATION OF AQUIFER DEFORMATION IN REGIONS OF ACTIVE TECTONICS USING INSAR AND WELL LEVEL DATA: A CASE STUDY FOR SAN BERNARDINO BASIN, CALIFORNIA

Using InSAR and hydrogeologic data, we characterize the surface deformation in the San Bernardino basin, California, a complex major aquifer located at the junction of the San Bernardino segment of the San Andreas fault and the younger San Jacinto fault zone. InSAR is used to establish a time series of regional surface displacement from 1995-2000. The time series demonstrates that the surface deformation is dominated by hydrologic effects. ERS interferometric pairs with temporal baselines from one month to four years, spanning 1992 to 2000, show large phase gradients across a complex set of known, concealed, and inferred local fault splays, indicating that the local fault system is an active control on groundwater migration and that InSAR can be employed in the precise location of these features. We have developed a method for decomposing the InSAR time series into the hydrologic and tectonic components of vertical deformation, as some amount of vertical tectonic deformation is expected at strike-slip fault junctures. Our method involves a least-squares inversion using both InSAR and well level time series, and indicates that ~75% of the total observed deformation for this region is the elastic response to the natural and anthropogenic cycles of aquifer recharge. We invert for the poroelastic Skempton's coefficient and the linear vertical tectonic displacement rate at over 300 well sites in the San Bernardino basin. Preferred well sites for this technique access the majority of the aquifer column and are monitored continuously. We characterize three major sub-basins, the Chino basin, the Rialto-Colton (RC) basin, and the San Bernardino Valley (SBV) basin. In particular the RC and SBV basins appear to be complex systems of fault-bounded blocks deforming vertically at different rates. The directionality (ie. subsidence versus uplift) of inferred tectonic deformation is consistent with mapped faults, exposed bedrock, and basement structure. We normalize the estimated Skempton's coefficients by the basin depth and compare the results to other aquifer studies, finding storitivity values consistent with other alluvial aquifers.