Key Data:
- Site Class – B
- AVS30 – 785.9 m/s
- Peak H/V – 1.367 Hz
- Max Depth – 88 m
Survey Details:
- # of Surveys – 1
- # of Channels – 18
- Min Wavelength – 1.0 m
- Max Wavelength – 83.8 m
- Data Record Length – 45 mins
Survey Geometry:
- Fibonacci line & half circle
Survey Notes
Seventeen Atom-1C Seismographs and one Atom-3C Seismograph were deployed across Esther Deaver Park in Alta Heights to record the ambient ground motion present. Geologically, Alta Heights is a volcanic ridge comprised of brecciated rhyolite, tuff breccia, and massive breccia deposits. These materials have a high shear-wave velocity, and this is well observed in the phase velocity dispersion curve below. The minimum shear wave velocity present is greater than 650 m/s, whereas downtown Napa has minimum shear wave velocities below 200 m/s.
In the velocity model, three clearly distinct geologic layers are visible, from 11-20 meters, 19 to 28 meters, and 28-88 meters. Shear wave velocity increases by 100 m/s or more with each layer, and correlate nicely with the known geology.
Because of the increased stiffness of the Alta Heights region compared to lowlands Napa, the risk of earthquake damage is lesser here than lower velocity sections of Napa. The intensification of shaking from surface waves is minimal here, if a stress plane is faulted, there will be significant offset that occurs. During the 2014 M6.0 earthquake, some areas of Napa experienced displacement up to 10 cm as measured by NASA ARIA GPS.
Survey Geometry
It’s worth discussing the survey geometry for N60 as this passive-seismic survey is unique in design and resulted in excellent data quality given the geologic conditions. Typically higher velocity materials results in lower data, as measured by the clarity of the phase velocity dispersion curve and dispersion curve RMS error. This is governed by a variety of factors
A 55 meter fibonacci line was positioned extending out from a nine Atom half circle configuration. The half circle provided excellent 3D site coverage, and the fibonacci line allowed higher frequency data to be collected cleanly, as well as extend the lower frequency resolution. Remember frequency, wavelength, and depth surveyed are highly correlated together.
Phase Velocity Dispersion Curve
Dispersion Curve
HV Peak
