Delineating the Near-Surface West Napa Fault in St Helena, California Using Vp/Vs and Guided Waves

We used Vp/Vs ratios and guided waves to locate the northern extension of the West Napa Fault in St. Helena, California. We acquired two high-resolution, P- and S-wave seismic profiles (Profiles 1 and 2) using multiple hammer hits to generate both P- and S-wave seismic energy along Profile 1 (215 m long) at 109 shot points, each spaced 2 m apart and co-located with the 109 geophones. We also generated additional P-wave energy along Profile 1 at every fifth geophone using a 227-kg accelerated weight drop (AWD). Profile 2 was a shorter (75 m), higher resolution (1 m shot and geophone spacing) profile located about 70 m northward and parallel to Profile 1. For both profiles, we recorded P-wave data using 40-Hz, vertical-component geophones and S-wave data using 4.5-Hz, single-component geophones. The resulting data sets allowed us to develop P- and S-wave refraction tomography and reflection models along the seismic profiles. From the P- and S-wave tomography models, we developed Vp/Vs and Poisson's ratio models that delineate the fault most clearly at about 15 m depth as areas of very high ratios. In addition, we generated guided waves within the fault zone 220 m north of Profile 1 and 150 m north of Profile 2 using the AWD source, and we recorded them with the S-wave geophones. We observed high-amplitude guided waves on both profiles in the vicinity of the fault trace, as shown by the tomography and Vp/Vs ratio models. Our data indicate that the West Napa Fault extends northward into the St. Helena area, but the fault may not extend to shallow (< 10 m) depths, as suggested by the trenching results of (Philibosian et al., 2019).