N80 – Redwood Creek

N80 – Redwood Creek Velocity Model

Key Data:

  • Site Class – D
  • AVS30 – 218.0 m/s
  • Peak H/V – 0.919 Hz

Survey Details:

  • # of Surveys – 2
  • # of Channels – 18
  • Min Wavelength – 1.0 m
  • Max Wavelength – 231.6 m
  • Data Record Length – 2 hrs

Survey Geometry:

  • Fibonacci L + GPS survey

Survey Notes

On August 24th, 2014 Napa experienced a M6.0 earthquake, with violent shaking erupting across the valley at 03:20:44 in the morning. While performing the two surveys, one of the land owners had some interesting anecdotes to share.

First, his dog started absolutely losing it 90 seconds before the earthquake. Keep in mind it’s 3:20 in the morning, everything is dead quiet. Second, after the earthquake, the water level of Redwood creek rose 1 meter. Having lived here for decades, the was able to know the height by looking because it reached the winter flow mark. The creek is well defined with known history. These are the anecdotes.

In the coherency data, that is the similarity of the data between sensors, there is a large peak rising to a coherency of 1 Hz in most of the data. Except the top is cratered out, the coherencies at 1 Hz take a dive below 0.5. What exactly causes this is to be determined. Take this 80 meter wavelength section for illustration.

Peak H/V on the other hand is at 0.919 Hz. Coincidence I think not. This would appear to indicate that for the ambient 1 Hz sound wavelength that is traveling through the Earth in this area, at least for the top 150m, it travels horizontally.

If an earthquake triggers the release of massive amounts of energy, then in the 1 Hz spectrum, that is 1 pulse per second, for this location the energy will primarily travel horizontally and not vertically. If a structure is given a large shake every second, how long will it hold? The goal with structural engineering is that structures will strongly hold longer than the max expected time of an earthquake.

The H/V plot also indicates that 10 Hz is another zone of interest to structural engineers, for this location. Plan for all the order of magnitudes of the main H/V peak. Vertical and horizontal ground motion during an earthquake needs to be considered. Depending on the building construction materials, say brick, a building will perform much worse horizontally than vertically structurally.

P-waves are fast, S-waves are slower, and surface waves (rayleigh and love) release the most amount of energy. Surface waves make up the 70% of a seismic event. To best understand the geotechnical implications of an area, then passive seismic methods will provide the most useful data, and to depths 5-10x more than conventional active methods.

Phase Velocity Dispersion Curves

N80 – Phase Velocity Dispersion Curve survey 1
N80 – Phase Velocity Dispersion Curve survey 2

Combined Dispersion Curve

Combined Survey 1+2 dispersion curve. RMSE of 13.27 m/s after five inversions.

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