UCVM ucvm with sw4 using cvmsi

Followup Questions 11/2025

Region of interest, -118.4232,33.58, -117.3459,34.3519

LBNL TARGET: Performing 5 Hz simulations based on CVM.S4.26.M01 and apply additional GTL with the Ely tapering method. For high frequency simulation requirements, the velocity model needs some additional modifications.

What causes the Ripple-like pattern (bullseyes) in the cvmsi?

 Region of interest: -118.4232,33.58, -117.3459,34.3519

Due to local borehole information that are in the CVMS(rule-based) with its GTL processing

cvms bullseyes vs

cvms5 bullseyes vs

cvmsi bullseyes vs

Using taper with -z 0,700 -L 200,700,1500, to migigate the feature

cvmsi taper vs

Transition depth to high velocity differs between Vp(~400m) and Vs(~250)

LBNL has a variable grid size/spacing based on depth: 50m grid for 0-650m depth, then 100m grid for 650-2500m, 200m grid for 2500-6000m, 400m grid for 6000m and below. The horizontal and vertical spacing is the same within the same depth range.

Region of interest:   33.599659 -117.505759, 34.311847 -117.501624

cvms transition vp

cvms transition vs

cvms transition density

cvms5 transition vp

cvms5 transition vs

cvms5 transition density

cvmsi transition vp

cvmsi transition vs

cvmsi transition density

 https://strike.scec.org/scecpedia/CVM-S4.26#Density_Based_on_Vs

Test adding tapering for cvmsi

Question 9/2025

Question from Rie to Scott,

..conducting SW4 simulations within the LA Basin for both small and large magnitude earthquake events (please see the attached slides for reference where we are interested in). While validating the ground motions, we have come across several features in the velocity model—built from the SCEC model—that we were hoping to better understand with your insight.

Blocky structure north of LA (highlighted in red in the slides):

 We observed a blocky structure characterized by elevated Vs compared to the surrounding region. This structure appears to disappear at 50 m depth and then re-emerges below. Is there a specific reason why this structure is not present at the 50 m depth level?

Vp–Vs relationship of the blocky structure (green color):

 The structure shows higher Vs values, with Vp appearing higher at the surface (Z = 0 m) but becoming lower at 250 m depth. We also noted that the density is relatively low at the surface.

Vp–density relationship west of the blocky structure:

 In this nearby region, the density is higher than that of the surrounding material, but Vp appears to be lower. (I expect Vp correlates with density)

Would you know of any literature or documentation that describe these characteristics?

Background of the posted question

Accessing UCVM velocity model via SW4's UCVM reader branch and then run the SW4 simulation.

 CVM-S4.26.M01 (cvmsi), the velocity values are extracted using UCVM "withSCPBR" branch

internal query parameters were:

 ucvm_query -f conf/ucvm.conf -m cvmsi,elygtl:taper -L 200,700,1500

Is this version of velocity model, recommended for ground motion simulation ? (another question)

Response from Scott

It looks like the blocky structure appears in the original CVM-S4 (https://strike.scec.org/scecpedia/UCVM_ucvm_with_sw4_using_cvmsi#Base_cvm, top row), and therefore also appears in CVM-S4.26 and CVM-S4.26.M01, both of which were derived from it. I believe this is the northern edge of the LA basin as defined in the original CVM-S4. When using this model for CyberShake Study 22.12 with the Ely taper, we didn't see this sharp east-west boundary. You can see our cross sections at https://strike.scec.org/scecpedia/CyberShake_Study_22.12#Cross-sections, in the 3rd column. However, we ran with a minimum Vs of 500 m/s and 100m grid spacing.

For question 1), I think this has to do with the way the merged taper is applied. The taper is constrained by the Thompson et al. (2022) Vs30 values, but every mesh point is evaluated when determining if to apply the taper or not. In other words, at (34.0, -118.0, 0m), the Vs values are calculated with and without the taper, and the method with the lowest Vs is selected. But then this process is repeated at 50m, 100m, etc., so at some depths the taper might be selected and at others not. I pulled Vs profiles of the top 500m for (-118, 34.1), which is in the dark blue region; (-118, 34.2), in the teal region; and (-118, 34.3), in the lime green region (attached as taper 34_1.png, taper 34_2.png, taper 34_3.png). From the smoothness of the profiles, it looks like the taper is always used for 34.1 and 34.3, but not always for 34.2. As a result, sometimes the points in the teal region are closer to the lime green region, meaning the blocky structure disappears, and sometimes they are closer to the dark blue region, meaning the blocky structure reappears.

For 2), it looks to me like at the surface Vp~1100 m/s and Vs~400 m/s, and at 250m depth Vp~4500 m/s and Vs~1800 m/s, since the color bars change dramatically from z=0 to z=250m.

For 3), I extracted velocity profiles with and without the merged taper. In the region you highlighted, even without the taper the Vp value hits the floor at 700 m/s, but density is about 2000 km/m3 (see attached "all props no taper.png"). This is also true in the original CVM-S4 model. So this isn't a result of the taper, but rather a native feature. Since I wasn't involved in the original model construction, I'm not sure what the logic is for these values, but they do seem to be what was intended in CVM-S4 and its derivatives.

Please let me know if you have follow-up questions. We can also discuss modifications to the model if you'd like to avoid some of these features; for example, I know Hu et al. (2022) [1] used an alternative method for deciding where to apply the taper.