Deciphering vertical deformation and poroelastic parameters in a tectonically active fault-bound aquifer using InSAR and well level data, San Bernardino basin, California

Abstract

SUMMARY Using InSAR and hydrogeologic time-series spanning 1995 to 2000, we characterize the elastic storage and surface deformation in the vicinity of the San Bernardino basin, California. The region encompasses a complex major aquifer located at the junction of the San Andreas and San Jacinto faults that supplies groundwater to over 600,000 people. We remove the elastic vertical surface displacement associated with changes in groundwater levels from the InSAR time-series by a least squares inversion. Our method estimates a poroelastic ratio at 60 well sites, which we normalize by basin depth, allowing for comparison of the elastic response of the aquifer skeleton from site to site. Our method also estimates residual vertical displacement rates at each well site, surface displacement not explained by observed trends in groundwater levels. Residual vertical displacement rates reveal patterns of subsidence and uplift across the basin over the 5-yr period from 1995 to 2000. In a narrow zone of observed residual subsidence near the San Jacinto fault, where many normalized poroelastic ratios trend higher than expected, we find permanent compaction of clay-rich strata to be the most likely explanation for 0.5–2.0 mm yr –1 of residual subsidence. This permanent compaction is likely a result of delayed compaction due to previous overdraft conditions and/or a result of the installation of deep production wells during the span of the InSAR time-series. Observations of localized subsidence within stepovers of the San Jacinto fault zone and relative uplift at the range fronts are consistent with current basin development models; however, interseismic strain modelling of the regional faults does not reproduce the surface displacement pattern or magnitude of these observations.