Abstract
After the 1987 Whittier Narrows1 and 1994 Northridge2 earthquakes revealed that blind thrust faults represent a significant threat to metropolitan Los Angeles3, a network of 250 continuously recording global positioning system (GPS) stations4,5 was deployed to monitor displacements associated with deep slip on both blind and surface faults. Here we augment this GPS data with interferometric synthetic aperture radar imagery to take into account the deformation associated with groundwater pumping and strike-slip faulting. After removing these non-tectonic signals, we are left with 4.4âmmâyr-1 of uniaxial contraction across the Los Angeles basin, oriented Nâ36°âE (perpendicular to the major strike-slip faults in the area). This indicates that the contraction is primarily accommodated on thrust faults6 rather than on the northeast-trending strikeâslip faults. We have found that widespread groundwater and oil pumping obscures and in some cases mimics the tectonic signals expected from the blind thrust faults. In the 40-km-long Santa Ana basin, groundwater withdrawal and re-injection produces 12âmmâyr-1 of long-term subsidence, accompanied by an unprecedented seasonal oscillation of 55âmm in the vertical direction and 7âmm horizontally.
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Acknowledgements
This work was supported by NASA Solid Earth and Natural Hazards Program. Water well data was provided by the Orange County Water District. We acknowledge the Southern California Integrated GPS Network and its sponsors, the W. M. Keck Foundation, NASA, NSF, USGS and SCEC, for providing GPS data used in this study. GPS velocity data were provided by SOPAC with assistance from Y. Bock. We thank C. Wicks for his InSAR expertise, R. O. Castle, P. Hseih, J. Langbein, W. Prescott, F. Riley, J. C. Savage, R. Simpson and J. Shaw for reviews, and J. Shaw and A. Plesch for Puente Hills thrust fault data.
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Bawden, G., Thatcher, W., Stein, R. et al. Tectonic contraction across Los Angeles after removal of groundwater pumping effects. Nature 412, 812â815 (2001). https://doi.org/10.1038/35090558
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DOI: https://doi.org/10.1038/35090558
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