UCVM Vp/Vs Ratio
we have diagnosed the issue that was causing the non-physical Vp/Vs ratios that we were seeing earlier. There was a bug in my CCA model UCVM trilinear interpolation code. When querying a point within the model, my code only checked to see if the southwestern grid point lied within the model boundaries. If it did, then for grid point (x, y), it constructed the box from (x + 1, y), (y + 1, x), and (x + 1, y + 1). So for 500m along the northwestern and northeastern boundaries of the model, there were artifacts as the original (x, y) existed in the model but the other grid points did not. Because the model was attempting to do this trilinear interpolation, with non-existent points, it was resulting in bogus material properties and therefore bogus ratios.
I re-did the analysis with the fixed code in UCVM and the results are posted here: https://scec.usc.edu/scecpedia/CCA_Vp/Vs_Check#Vp.2FVs_Ratios_Lower_Than_1.6_From_Fixed_CCA_Model_in_UCVM. Please notice how the blue line at the top no longer exists as that was an artifact of the code.
Most notably with this fix:
- The 1.386 ratio disappears. The minimum throughout the entire model is 1.45. - Most of the other values remain reasonably consistent. The percentage of points under the 1.6 ratio is 0.656% throughout the model.
This was a simple fix and has already been patched in the Central California code. The new released version has been tagged on GitHub: https://github.com/SCECcode/CCA/releases/tag/CS169. It is also available now through UCVM 15.10.0.
Please note, no analysis has been done on points below 1600m/s for Vp, which you suggested should be the absolute minimum Vp throughout the entire model. If you would like to see maps and data of where or if Vp falls below 1600m/s, please let me know.
Scott tells me that CyberShake enforces a 1.45 constraint. For any Vp/Vs ratios that are below 1.45, Vp is set to be Vs * 1.45. Fixing this bug ensures that CyberShake never has to artificially adjust Vp for the CCA region within the CyberShake mesh since the ratios are always 1.45 or higher.
