N32 – Napa River Velocity Model
- Site Class – C
- AVS30 – 395.3 m/s
- Peak H/V – 8.162 Hz
- # of Surveys – 1
- # of Channels – 13
- Min Wavelength – 12.9 m
- Max Wavelength – 203.3 m
- Data Record Length – 2400 s
- GPS survey on each side of the river
To survey across the Napa River, Atom Seismographs were placed on both sides of the river at water level. In fact, the tide rose significantly during the survey, and 3 of the Atoms had to be rescued from rising waters. Two of the Atoms, SN’s 24 & 35 experienced a 2 second time shift, and this was identified and corrected with post processing.
The goal of this survey was not to establish an accurate AVS30, but rather to acquire data significantly deeper. There is only one spacing in-between 10-20 meters, with better spacing coverage beginning after 23 meters. Therefore it is likely the AVS30 value of this data is higher than it actually is, and nearby N surveys (N1, N2, N5) would peg it closer to ~300 m/s. Data was collected well beyond 30 meters though, with a maximum depth of 155 meters being achieved. The velocity model for 30-155 meters is most likely accurate for all the Napa downtown area.
Beyond 70 meters in depth, the shear wave velocity increases gently from 500 m/s to 600 m/s. Considering the tufts and rhyolites that make up the Montecito Ridge 1.5 km to the north-east, it seems likely that weathered volcanics underlay the Napa River. The clay-dominant geologic layers of the Mayacamas to the west and volcanics from the east form an angular nonconformity, and it is the Napa River will flows and meanders on top of this nonconformity as it passes through the City of Napa.