Vertical and horizontal displacements of Los Angeles from InSAR and GPS time series analysis: Resolving tectonic and anthropogenic motions

Abstract

We resolve the complex ground deformation associated with tectonic and anthropogenic activities in Los Angeles, California by combining Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) measurements acquired from 2003 to 2007. The three-dimensional (3-D) cumulative displacement velocity field is first derived based on Weighted Least Squares (WLS) InSAR analysis of both ascending and descending ENVISAT ASAR data aided by GPS measurements at 54 Southern California Integrated GPS Network (SCIGN) stations. Clear subsidence can be observed from the results in the Pomona and San Gabriel areas, respectively, while uplift in the Santa Fe Springs. The displacements are mainly due to the extraction and injection of groundwater or oil. The northwestern horizontal displacement detected also suggests that the area had experienced interseismic strain accumulation related to the San Andreas Fault. The varying northeastern displacement magnitudes indicate increasing displacement accumulation away from the San Andreas Fault. The results are validated with GPS measurements at 15 independent stations, with the root mean squares (RMS) residuals of the discrepancies being 2.4, 1.5 and 0.9mm/yr in vertical, northern and eastern directions, respectively. The seasonal displacement in Santa Ana Basin due to anthropogenic activities is examined with eight pairs of ascending and descending InSAR LOS displacement estimations selected from the WLS InSAR solution. It is clear that almost the entire Santa Ana Basin experiences significant seasonal vertical displacement with the maximal displacement reaching 43mm. The Newport-Inglewood Fault forms a sharp boundary of the seasonal vertical displacement field. In addition, seasonal oscillations of 10–20mm in east direction are detected near the margin of the Santa Ana Basin, across the Newport-Inglewood Fault. Strong correlations are found between the groundwater levels and the seasonal displacements at three wells, yielding an average elastic skeletal storage coefficients of 5.9×10−4. This indicates that the Santa Ana basin is dominated by the elastic deformation.