Broadband CyberShake Validation

This page follows on CyberShake BBP Verification, moving from 1D comparisons to 3D CyberShake comparisons with both the BBP and observations.

1D BBP comparisons

We calculated 64 realizations for Northridge for these 38 stations.

Some of the differences between the 1D BBP and 3D CyberShake results can be attributed to differences in site response, which is calculated based on the reference velocity ('vref') and the vs30 of the site. A spreadsheet with vref and Vs30 for both the BBP and CyberShake is available File:BBP v CyberShake Northridge vs30.xls

V1 (2/14/22)

Initially, we used vref=500 for both the high-frequency and low-frequency site response. However, this is incorrect; vref for the low-frequency should vary depending on the properties of the 3D velocity mesh.

V2 (2/25/22)

We continued to use vref=500 for the high-frequency site response. For the low-frequency site response, we are now using the same vref we used in Study 15.12:

vref_LF_eff = Vs30 * [ VsD5H / Vs5H ]

Vs30 = 30 / Sum (1/(Vs sampled from [0.5,29.5] in 1 meter increments))

H = grid spacing

Vs5H = travel time averaged Vs, computed from the CVM in 1 meter increments down to a depth of 5*H

VsD5H = discrete travel time averaged Vs computed from 3D velocity mesh used in the SGT calculation over the upper 5 grid points

So, for H=100m Vs5H would be:

Vs500 = 500 / ( Sum ( 1 / Vs sampled from [0.5,499.5] in 1 meter increments ))

And then VsD5H is given by:

VsD500 = 5/{ 0.5/Vs(Z=0m) + 1/Vs(Z=100m) + 1/Vs(Z=200m) + 1/Vs(Z=300m) + 1/Vs(Z=400m) + 0.5/Vs(Z=500m) }

Below are the ts_process plots for a subset of 10 stations, comparing the 3D CyberShake with 1D BBP results.

Site	TS Process plot
SCE
SYL
LDM
PAC
PKC
SPV
WON
KAT
RO3
ANA

V3 (3/2/22)

Next, we recalculated the CyberShake results for 10 sites, using the BBP vs30 values for both the low-frequency and high-frequency elements. Note that the vref low-frequency value for CyberShake is still being derived from the velocity model.