UCVM Vs30

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