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.