Velocity Decomposition
In order to compute the horizontal components of strain rate we must first construct 3 components of surface velocity (east, north and up) from only two components of LOS velocity. To accomplish this, we adopt the assumption that the direction of surface displacement matches the direction from the GNSS velocity model [Wright et al. , 2004;Tymofyeyeva & Fialko , 2018; Shen & Liu , 2020]. This utilizes the local azimuth of a horizontal GNSS velocity predicted from the interseismic slip model of Zeng & Shen [2017] as a constraint to reduce the required degrees of freedom in the measurements from three to two. The decomposed velocity field (Fig. 6) reveals detailed spatial variations in the east-west component together with dramatic vertical motions. Some of the variations in horizontal component are associated with hydrological signals (e.g., inside Central Valley). Subsidence inside the Central Valley is greater than 30 cm/yr with other areas like Geysers, Heber and Cerro Prieto Geothermal Fields standing out in the map. Uplifts that occur at ground water recharging sites (Long Beach, Santa Clara Valley, etc.) are usually a direct reflection of human activities, some of which cannot be well represented by a single velocity (e.g., Fig 2c., GNSS station SACY). Since the velocity is derived as a linear fit, a positive rate does not necessarily result from an overall uplift. Due to the near-polar orbits, InSAR satellites are generally not very sensitive to north-south motion. Thus, the decomposed north-south component absorbs most of its information from the GNSS model, resulting in an oversmoothed field.