CyberShake BBP Verification

This page documents our CyberShake Broadband Platform validation work, running the BBP validation events through CyberShake.

Events

We plan to run the following events from the BBP:

• Northridge
• Landers
• North Palm Springs
• Whitter
• Chino Hills

Verification and Validation

Planned process will involve verification and validation stages.

• Verification: Compare CyberShake broadband seismograms to Broadband Platform seismograms
• Validation: Compare CyberShake broadband seismograms to observed seismograms

Validation Process

For each of the 5 BBP validation events listed above, go through the following process:

• Run the BBP validation event with the Graves & Pitarka modules to produce BBP results for comparison.
• Review the BBP validation data for the event, including station list and seismogram files and validation results from GP method for this event.
• Identify about 10 stations from the BBP station list that have observed seismograms to run in CyberShake.
• Modify station list low-pass filter to 50 Hz.
• Use the Vs30 values from the BBP station list for both the low-frequency and high-frequency site response in CyberShake, for consistency.
• We will use CVM-S4.26.M01 as the velocity model for CyberShake.
• Run one realization using the SRF file which is generated by the BBP when run with one realization.
• Create an ERF in the CyberShake database containing all the validation events, with a unique source and rupture ID for each one.
• Save the SGTs in case we want to run additional realizations later.
• To interface with the BBP tools for generating data products, convert the CyberShake seismograms into BBP format seismograms

Data Products

The CyberShake method calculates two component seismograms. Broadband platform supports three component seismograms. We will convert CyberShake 2 components to 3 components by duplicating the N/S component in the vertical component data.

We will produce the following data products to compare CyberShake results with BBP results:

• ts_process plot, comparing traces and response spectra.
• RotD50 comparison plots
• Goodness-of-fit bias plots
• Map of GoF, by station

Northridge

A KML of the Northridge stations is available here. Selected stations are in orange. The BBP station list file for these stations is available here.

We made the following modifications to the default BBP platform to support better comparison to the BB CS results:

• Changed the BBP LF and HF seismogram length to 500 sec / 50,000 timesteps
• Modified the filtering so the merge frequency is 1.0 Hz and phase=0

3D comparisons

Below are comparisons using the CVM-S4.26.M01 model in CyberShake.

Stage	Seismogram comparison
Filtered high-frequency acceleration with site response
Low-frequency velocity
Broadband velocity

BBP LA Basin 1D comparisons

Below are comparisons using the BBP 1D LA Basin model in CyberShake with no interpolation. We also:

• Modified the CyberShake filtering scheme to match the BBP filtering scheme, which uses a high-pass filter for the HF at around 0.89 Hz and a low-pass filter for the LF at about 1.11 Hz.
• Modified the CyberShake low-frequency site response to use the high-frequency PGA values, consistent with the BBP approach.

We ran SCE with both the original 1D model, and a modified version (File:Modified nr02-vs500.txt)

Stage	BBP vs CS, original 1D	BBP vs CS, modified 1D
Low-frequency (velocity)
Low-frequency with site response (velocity)
Low-frequency with site response and filtering (velocity)
High-frequency with site response and filtering (velocity)
Broadband results (velocity)

Comparable dt

We reran our comparisons with the same dt (0.1) for SCE in CyberShake and the BBP:

Stage	BBP vs CS, original 1D
Low-frequency with site response and filtering (velocity)
High-frequency with site response and filtering (velocity)
Broadband results (velocity)

Other sites

	RO3 (5/170 interchange)	PEL (Santa Monica/Highland)
Low-frequency (velocity)
Low-frequency with site response (velocity)
Low-frequency with site response and filtering (velocity)
High-frequency with site response and filtering (velocity)
Broadband results (velocity)

Halfspace comparisons

Seismograms

Point source. Red = AWP, Black = RWG.

Unfiltered
Filtered

Northridge BBP validation event. Red = AWP, Black = RWG.

Unfiltered
Filtered

SGTs

	X-component	Y-component
Adjusted mu in AWP
Unadjusted mu in AWP		File:SCE hspace sgts rwg v awp uncorrmu y.png

Uncorrected mu

Modified 1D velocity model

RWG and AWP SGTs, using SGT headers with uncorrected mu values

Filtered seismograms (red = AWP, black = RWG)

Point source
Northridge

SGTs (red = AWP, black = RWG)

X	Y

1Hz source plots

To reduce ringing effects, we started using an impulse low-pass filtered at 1 Hz.

All the plots below are of SGTs.

Slow halfspace

The plots below were calculated using a halfspace model with Vp=1500, Vs=750, rho=2200.

Qs=37.5, Qp=75.0

X	Y

Qs=10000, Qp=20000

X	Y

4-layered model

Next we ran with a 4-layered model:

Thickness(km)   Vp      Vs      rho     
0.2500          2.4000  1.0000  2.2500  
0.7000          3.7000  1.8000  2.4500  
1.5000          4.7500  2.4000  2.6000 
2.5000          5.4000  3.0000  2.6500 
999.00          6.2000  3.5000  2.7200 

Qs=10000, Qp=20000

X	Y

Qs=0.05Vs, Qp=2Qs

X	Y

Qs=0.05Vs, Qp=2Qs

Unadjusted mu and lambda used for stress->strain conversion. This was accomplished by removing adjustment factors in the SGT kernel.

Depth	X	Y
Shallow (5 km)
Deep (15 km)

12-layered model

   Vp      Vs      rho         Depth to layer bottom (km)
  2.4000   1.0000   2.2500     0.250001
  3.1000   1.4000   2.3000     0.450001
  3.3495   1.5560   2.3750     0.550001
  3.7000   1.8000   2.4500     0.950001
  3.9190   1.9285   2.5125     1.050001
  4.2000   2.1000   2.5750     1.450001
  4.4483   2.2343   2.5875     1.550001
  4.7500   2.4000   2.6000     2.450001
  4.9150   2.5565   2.6100     2.550001
  5.1000   2.7500   2.6200     3.450001
  5.2426   2.8661   2.6350     3.550001
  5.4000   3.0000   2.6500     4.950001
  6.2000   3.5000   2.7200     9999.999999

Qs=0.05Vs, Qp=2Qs

Unadjusted mu and lambda used for stress->strain conversion.

Depth	X	Y
Shallow (5 km)
Deep (15 km)

14-layered model

   Vp      Vs      rho      Depth to layer bottom (km)
  1.6447   0.5378   2.0600     0.050001
  1.8893   0.7115   2.1500     0.150001
  2.3653   0.9766   2.2500     0.250001
  3.1000   1.4000   2.3000     0.450001
  3.3495   1.5560   2.3750     0.550001
  3.7000   1.8000   2.4500     0.950001
  3.9190   1.9285   2.5125     1.050001
  4.2000   2.1000   2.5750     1.450001
  4.4483   2.2343   2.5875     1.550001
  4.7500   2.4000   2.6000     2.450001
  4.9150   2.5565   2.6100     2.550001
  5.1000   2.7500   2.6200     3.450001
  5.2426   2.8661   2.6350     3.550001
  5.4000   3.0000   2.6500     4.950001
  6.2000   3.5000   2.7200  9999.999999

Qs=0.05Vs, Qp=2Qs

Unadjusted mu and lambda used for stress->strain conversion.

Depth	X	Y
Shallow (5 km)
Deep (15 km)

Forward/Reverse Tests

To narrow down potential issues further, we performed a series of forward/reverse tests. In these, we place a force at one location and observe the velocities at another, then place a force at the 2nd location and observe the velocities at the first. As long as the correct components are being compared, the resulting seismograms should be very similar.

Results from Rob's 4th-order code (14-layer model), using the points at 5 km and the surface, and 15 km depth and the surface.

Depth	Results
5 km
15 km

Rob repeated the experiment with 2nd-order operators and got agreement to floating-point error.

Using the AWP-CS code, we got the following results. We averaged the Z=1 and Z=2 layers to correctly get the free surface velocities. For some reason, one of the seismograms needs to be inverted in order to match correctly.

Model	Results for 5 km/source pair
14 layer
12 layer	|
4 layer
halfspace

Velocity source insertion

We determined that the offset is due to the way the AWP staggered grid is set up. The normal stresses are in the center of the grid cell, but the X velocity is in the center of the side, in the +x direction. So inserting a source at location A and recording velocities at location B is actually recording velocities at B+.5, and doing the reverse tests means velocities are measured at A+.5. So in the first test, the distance traveled is (B+.5)-A = (B-A)+.5, and in the second it's B-(A+.5) = (B-A)-.5, a difference of 1 grid point. If the force is inserted by modifying velocities,

To illustrate this, the plots below show the same test with a stress insertion and a velocity insertion. Both points have the same Y and Z indices (depth=1km), only the X is varied.

Stress insertion
Velocity insertion

Here's a plot of the relative differences:

We also tried increasing Q to 1e10:

Since the above results looked good, we tried moving one of the points to the surface:

The differences in the late arrivals are ~3%.

We repeated these velocity insertion tests for the 4, 12, and 14-layer models, with one point at the surface and one point at depth.

Model	X results	Y results
4-layer
12-layer
14-layer

I ran a model at 33m resolution. Here are the difference plots, and comparison to the 100m model, for a halfspace.

Kim made some updates to the symmetries in the kernel. I reran the 14-layer tests with the new kernel.

We recomputed results using 100m and 33m spacing, and compared them to the FK results for the 14-layer model. We chose the AWP points (432, 365, 1) and (317, 453, 52) in the 100m run and (1294, 1093, 1) and (948, 1362, 154) in the 33m run.

100m self-reciprocity, X component
33m self-reciprocity, X component
100m vs 33m
33m vs FK

SGT Tests

Using the 14-layer model, we ran tests to generate SGTs at 100m and 33.333m grid spacing, using both velocity and stress insertion

AWP 100m vs RWG 100m, X component
AWP 33m vs AWP 100m, X component
AWP 33m vs RWG 100m, X component
AWP 100m velocity insertion vs AWP 100m stress insertion, X component

BBP Comparisons

For these comparisons, we used the 14-layer model in the BBP, and a 24-layer pre-processed version of this model better for finite difference codes.

BBP vs AWP-CS
BBP vs AWP-OOR (modified source insertion)
BBP vs RWG reciprocity

Related Entries

• CyberShake
• Broadband Platform