UCVM Vs30

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Introduction

There may be an issue with how UCVM calculates Vs30 values. Presently UCVM uses the following formula:

  1. sum(Vs sampled from [0,30] at 1 meters, for 31 total values) / 31

Rob has suggested that the correct formula should be:

  1. 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:

  1. 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

UCVMC v18.5 results

-bash-4.2$ ./vs30_query -f ../conf/ucvm.conf -m cvmsi < in.txt
 -118.1712    34.1484    821.328
 -118.1250    34.5570   2573.634
 -118.2860    34.0192    310.574
 -118.9192    34.6402   3393.808
 -120.3602    36.1397   2436.254
 -119.2285    34.2667    283.145


-bash-4.2$ ./vs30_query -f ../conf/ucvm.conf -m cca < in.txt
 -118.1712    34.1484      0.000
 -118.1250    34.5570   2582.267
 -118.2860    34.0192      0.000
 -118.9192    34.6402   3331.251
 -120.3602    36.1397   2279.326
 -119.2285    34.2667    902.013