Inversion for Creep Rate

To obtain an idea of the distribution of creep rate, we perform inversions using both the InSAR stack and GPS velocities. The CSAF is well-covered by continuous (Plate Boundary Observatory) and campaign GPS sites. Initially, separate InSAR and GPS inversions are carried out, as preliminaries to a joint inversion. The InSAR stack is downsampled by a factor of 20 in the north and east directions. Slip is constrained to be positive (i.e. right-lateral), and an upper bound of 40 mm/year on the slip rate is imposed. In the joint inversion, a maximum shallow creep velocity of 33 mm/year occurs in the centre of the segment, and to first order tapers off on either side, more rapidly to the south than to the north. This mirrors the pattern illustrated in Figure 3 of Titus et al. (2005), which is a compilation of surface geodetic slip rate estimates from different workers since the 1970s. Shallow creep rate falls to very low values ($<$ 10 mm/year) around Parkfield. Intermediate depth creep rates reach a maximum of 38 mm/year just north of centre, tapering off to the north, and decreasing before rising again towards Parkfield. The deep slip rate is about 35 mm/year.