UCVM Vs30
Introduction
There may be an issue with how UCVM calculates Vs30 values. Presently UCVM uses the following formula:
sum(Vs sampled from [0,30] at 1 meters, for 31 total values) / 31
Rob has suggested that the correct formula should be:
30 / sum(1 / (Vs sampled from [0, 30) at 1 meters, for 30 total values))
This can have a significant impact for Vs30 calculations. For the BBP1D model, UCVM calculates the Vs30 as 932m/s, whereas Rob's formula calculates it as 862m/s.
Another approach was used in CyberShake Study 15.12, in conversations with Rob: Vs30 = 30 / Sum (1/(Vs sampled from [0.5,29.5] in 1 meter increments, for 30 total values)).
Points for comparison
We have selected 6 points to compare Vs30 as generated using the CyberShake approach (using half-meters) to other UCVM approaches.
| lat | lon | Velocity model | CyberShake-generated Vs30 |
|---|---|---|---|
| 34.148426 | -118.17119 | CVM-S4.26.M01 | 838.8 |
| 34.557 | -118.125 | CVM-S4.26.M01 | 2573.6 |
| 34.0192 | -118.286 | CVM-S4.26.M01 | 313.1 |
| 34.64021 | -118.91919 | CCA-06 | 3393.9 |
| 36.13968 | -120.36015 | CCA-06 | 2277.2 |
| 34.26667 | -119.22848 | CCA-06 | 902.3 |
