N26 – Silverado School

Key Data:

  • Site Class – C
  • AVS30 – 493.1 m/s
  • Peak H/V – 1.146 Hz
  • Max Depth – 270 m

Survey Details:

  • # of Surveys – 2
  • # of Channels – 10, 18
  • Min Wavelength – 7.2 m
  • Max Wavelength – 371.4 m
  • Data Record Length – 30, 60 mins

Survey Geometry:

  • Survey 1 – 50m triangle
  • Survey 2 – GPS J-survey

Survey Notes

Silverado Middle School has a series of large fields that are relatively far removed from car traffic, and they maintain a flat elevation. Site conditions for passive-seismic at Silverado School are ideal. Two surveys so far have been done to ascertain the geology of the site. On 5/20/20, a 10-channel Koichi Triangle (50m) survey was setup to record 2 Hz vertical data. The survey collected data from 18:30 – 19:00 pm. On 7/26/20, a second survey was setup in order to observe deeper geologic information. 18 Atom Seismographs (seventeen 1C, one 3C) were deployed via GPS in a J-shaped array totaling 510 meters in length. Size the second survey was much larger and eight of the sensors were closer to the road, it was done at night from 21:45-22:45 pm.

Survey 2 geometry
Survey 1 HV Peak (day)
Survey 2 HV Peak (night)

Above is the HVSR data from surveys 1 and 2 at Silverado School. As can be seen, the general shape is similar between the two, but the HV data from survey 1 bulges after 0.3 Hz and has a peak value of 2.372 Hz, though it’s more of a shelf from 1.2 – 3.5 Hz. HV data for survey 2 is relatively flat, reaching a low at 10 Hz, where the vertical ground motion exceeds the amplitude of the horizontal. This HV dip below 1 at 10 Hz is shared by both HV plots, and we can expect that with 10 Hz waveforms, vertical ground motion will exceed that of horizontal. When the two HV plots are combined and averaged, we observe the following.

From 0.3 to 5 Hz the data effectively flattens at a H/V value of 2. At ~35 meters the Vs increases from 574 to 974 m/s. The eastern side of Napa is gently rolling hills with thick clay alluvium on top, but underneath it is the volcanic rocks that provide the land it’s aesthetically pleasing geomorphology. At Silverado School, this boundary from alluvium to volcanics is at 35 meters. 5 Hz is the 35 meter inflection point, so we can draw the conclusion that with the volcanics of the east side of Napa, the horizontal amplitude is double that of the vertical from 0.3 – 5 Hz. A large scale 3C love-wave survey would be the next passive-seismic survey recommended to be done at Silverado School.

Phase Velocity Dispersion Curves (1 & 2)

Survey 1 PVDC (Koichi 50m Triangle)
Survey 2 PVDC (GPS J-Survey)

Combined Dispersion Curve

Dispersion curve data from surveys 1 and 2 were combined pre-inversion, average, and smoothed in order to generate the final velocity model and dispersion curve.

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