Figure 1: TPCA results for 1992-2011 InSAR SBAS vertical ground velocity in the Santa Clara Valley. a) Scree plot of the percentage of variance explained by each PC. b) Eigenvectors of PC1 (top) and PC2 and 3 (bottom) showing the temporal variability of the PCs deformation patterns. The red double arrows highlight PC2 and 3 time lag. Grey bars show monthly precipitations. c) Scores maps showing the spatial patterns of the deformation of PC1, 2 and 3 (left to right). The score maps are converted to deformation by multiplying them by the eigenvector at a given time.
The Santa Clara Valley, also known as Silicon Valley, is a shallow basin located between the San Andreas and Hayward-Calaveras fault zones in the San Francisco Bay Area. Land uplift and subsidence in the valley are due to the recharge and withdrawal of groundwater, actively monitored by the Santa Clara Valley Water District (SCVWD) using wells measurements, which are expensive and spatially limited. Here, we demonstrate the benefits of space-derived ground deformation measurements for characterization of aquifer properties and groundwater levels.
We use InSAR time-series analysis of ERS, Envisat, and ALOS data to resolve 1992-2011 vertical ground deformation in the Valley and T-mode Principal Component Analysis to isolate temporally variable deformation patterns embedded in the multi-decadal time series. The first Principal Component (PC) corresponds to the longer-term deformation. Positive scores (red on Figure 1c), are observed in the Evergreen and Santa Clara basins. The eigenvector time series (Figure 1b, top) shows an increase (corresponding to uplift in positive-score areas) between 1992-2000 and remains nearly constant during 2000-2011. This uplift at 4 mm/yr between 1992-2000 and >1 mm/yr in 2000-2011, illustrates the end of the poroelastic rebound following recovery of hydraulic heads after the 1960s low stand, leading to uplift one order of magnitude smaller than its preceding subsidence. The second and third PCs correspond to seasonal deformation (Figure 1b bottom). PC2 deformation encompasses most of the confined aquifer west of the SCF with an average amplitude of ~2-2.5 cm (Figure 1c score map PC2). Peaks in PC2 occur immediately after rainfall, suggesting elastic deformation of a highly permeable aquifer system. PC3 is limited to a ~3 km wide region west of the Silver Creek Fault (SCF) with an average peak-to-peak amplitude of ~0.5-1 cm and peaks lagging by ~105 days, suggesting a later phase of deformation, possibly due to delayed aquitards deformation.
Using hydraulic head data from the Santa Clara Valley Water District at 50 wells we normalize the seasonal ground deformation (PC2+PC3) by the head changes to constrain the storativity of the aquifer. Higher storativity are observed near the shoreline, over the Holocene Bay mud and around the SCF, with values representative of clays. These observation confirm the SCF is a barrier to across fluid flow likely due to smearing resulting from dragged clay layers or ductile flow along the fault plane. Finally, we evaluate how well seasonal water level changes can be predicted from the observed deformation and calculated storativity. The 2006-2011 hydraulic head changes predicted from InSAR data agree within 70% with the actual hydraulic head changes measured at wells through the Valley, which demonstrates InSAR's benefits for groundwater management.
|Tools||InSAR, groundwater well data, PCA|
|Geographic Location||Santa Clara Valley, California|
|Project Duration||Summer 2013-Summer 2014|
|Group Members Involved||Estelle Chaussard, Roland Bürgmann|
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