N84 – North Ave

N84 Velocity Model

Key Data:

  • Site Class – B
  • AVS30 – 809.0 m/s
  • Peak H/V – 20.167 Hz
  • Max Depth – 144 m
  • 111 Spacings, 153 pairs

Survey Details:

  • # of Surveys – 1
  • # of Channels – 19
  • Min Wavelength – 1.0 m
  • Max Wavelength – 98.7 m
  • Data Record Length – 55 mins

Survey Geometry:

  • Fibonacci L with diagonal offsets.

Survey Notes

Survey N84 resulted in some really beautiful data, see the colorful phase velocity plot below. Even though the max wavelength was sub 100 meters, low frequency response was good on-site and data was observed to a depth of 144 meters. This isn’t uncommon in Napa. N84 was surveyed along the top of a long rolling hill off North avenue, and the minimum shear-wave velocity (Vs) recorded was still greater than 600 m/s, which means we’re surveying on volcanics. An open septic tank dig about 3 meters in depth at the neighbors shows the volcanic geology and explosive past of the Napa Valley.

Alluvium (brown) to tuft (tan) interface
Large andesite (gray) inclusion
Andesite close-up

Tuff is a volcanic rock created when ask ejected from a volcano hardens through a process called consolidation. It’s a relatively soft porous rock, though much harden than soil alluvium. Volcanic ash can spread far and wide, and the tuffs of the Napa Valley didn’t necessarily originate from volcanic eruptions locally. The larger the deposit, the closer the proximity to the eruption, and the Alta Heights ridge has been classified as a caldera rim by geologists such as the famed Eldridge Moores. This is a very plausible explanation which explains the geomorpology of north-east Napa and the near-surface geology of the valley.

At a depth of ~20 meters the tuft gives way to a rock type with a higher Vs, and the Vs of the subsurface increases gradually to an inflection point at 120 meters where the Vs increases again. At 120 meters in depth, the increase in the Vs is likely caused from the rock type transitioning from exhibiting small crystal growth and large crystal growth. With hot springs still active in the valley, and The Geysers located nearby, the Napa Valley is still cooling from geologically recent (3 mya) volcanic activity. The Napa geologic basin is made up of highly crystallized rock.

Phase Velocity Dispersion Curve

HV Peak

1st HV peak – 1.251
2nd HV peak – 20.167

The HVSR data at N84 has two characteristic peaks. At 1.25 Hz, the H/V value hits exactly 3, and at 20 Hz the H/V value jumps to five. In-between these two peaks H/V decreases and then rises back up from an even ratio of 1. The NVSP has found that volcanic geology often results in H/V graphs similar to N84 here, with multiple H/V peaks. Volcanics are especially good at achieving H/V peaks in the higher frequency ranges.

Dispersion Curve

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