Estimating creep rate from velocity profiles

In the vicinity of Santa Rosa, we observe a step in velocity across the Rodgers Creek fault. We investigate this feature, which we interpret as representing surface fault creep, by plotting cross-fault profiles through our reduced PS-InSAR dataset at 5 km intervals (Figure 2.39b,c). To estimate fault offset rates, we fit parallel straight lines to windows of datapoints on either side of the fault, and calculate the separation between them. The gradients of the lines reflect the regional component of deformation, along with any residual error in satellite orbital position, and are used to detrend the profiles. Assuming pure right-lateral strike-slip motion, we then convert the LOS velocities to creep rates, propagating the uncertainties of the measurements through the calculation.

We obtain rates which peak at $6.0\pm0.6$ mm/yr immediately north of Santa Rosa (profile G-G'), and die off to the north and south. It is possible that the southernmost profile (H-H') appears less steplike than when plotted in plan view (Figure 2.39b) due to the fault bend at Santa Rosa. These rates, which represent the averages over an 10 year interval, are comparable with the estimate of 4.3 mm/yr recently obtained from alignment array measurements at site RC1 (Figure 2.39b) which do not overlap in time with our measurements.

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