Identifying vertical and horizontal components of surface displacement along the Maacama and Rodgers Creek faults using InSAR

Fault creep, i.e. slow aseismic slip, has been observed along the Maacama (MF) and Rodgers Creek faults (RCF) in northern California, initially through offset cultural features (e.g. offset sidewalks) and more recently, through the use of InSAR. Accurately mapping the distribution of fault creep is important because creeping regions have the potential to reduce the overall strain accumulation on a fault as well as reduce the area of fault that could rupture in an earthquake.

Fault creep was previously mapped along the RCF using data from the ERS and Envisat missions. InSAR data coverage is incomplete in those studies due to decorrelation from inconsistent repeat times of the satellites and the dense vegetation over the majority of the region. The Sentinel-1A/B satellites, launched in 2014 and 2016, have helped mitigate some decorrelation issues by having regular 12-day repeat observations. In earlier work, we showed line-of-sight (LOS) velocities along the MF and RCF using Sentinel-1 descending data. With the addition of a second viewing geometry, we can now distinguish between vertical velocities, most likely due to anthropogenic deformation processes, and fault-parallel horizontal velocities, which are likely due to fault creep.

Here we present new results combining ascending and descending viewing geometries of Sentinel-1, from 2015 to 2018. We use the JPL ISCE software to produce over 400 descending and over 300 ascending interferograms (IFG), with baselines < 100 m and time spans < 1 yr. We input the IFGs from each track, separately, into the SBAS code in the StaMPS/MTI software to produce time series of ground deformation at high spatial density in a zone ~10 km wide along the faults. We then estimate relative LOS velocity on fault-perpendicular profiles at regular short intervals along strike. The descending data shows varying LOS offset rates along the extent of both faults, implying creep rates on the MF of 3 - 6 mm/yr near Willits, Ukiah, and Geyserville and up to 11 mm/yr near Cloverdale and creep rates on the RCF of 2 - 6 mm/yr near Santa Rosa, if we assume all LOS deformation is fault-parallel. We will test this assumption by solving for the vertical and fault parallel offset rates using both the ascending and descending datasets and estimating the fault creep rate for each profile.