Annual Meeting - SCEC.org

Poster Abstracts | Group 2 – USR
Group 2. Poster Abstracts
Plaza Ballroom, Hilton Palm Springs Resort
Unified Structural Representation (USR)
2-001
SCEC COMMUNITY VELOCITY MODEL (CVM-H 5.5) Plesch A, Shaw JH, Hauksson E,
and Tanimoto T
We present a new version of the SCEC Community Velocity Model (CVM-H version 5.5) that
incorporates several enhancements to better facilitate its use in strong ground motion prediction
and seismic hazards assessment. These improvements include updates to the background models,
basin structures, geotechnical layer, and the code that delivers the CVM-H. The extent of the model
was expanded to an area between longitudes 120d52'20"W and 113d19'16"W and latitudes
30d56'49"N and 36d37'17"N to accommodate larger numerical experiments.
Improvements to the basin structures include new Vp, Vs, and density parameterizations within
the Santa Maria and Ventura basins that are based on direct velocity measurements from
petroleum well logs and seismic reflection data. These new basin structures were used as input for
the development new P and S wave tomographic velocity models, and a new upper mantle
teleseismic and surface wave model. The CVM-H thus consists of the revised basin representations
embedded in self-consistent regional crust and upper mantle models. In addition, we also
enhanced the geotechnical layer (GTL) representation. The GTL in the major sedimentary basins
remains unchanged, following the approach of the SCEC CVM 4.0 (Magistrale et al., 2000). In
bedrock sites, however, we implemented a new GTL based on the depth-velocity relations of Boore
& Joyner (1997). In this implementation, we used the empirical velocity gradient from Boore &
Joyner (1997) to scale upwards from the base of the GTL (top of basement). This bedrock GTL
implementation results in gradual velocity gradients and variable velocities at the surface.
In addition, we provide a series of enhancements to the C-code that delivers the CVM-H. This code
specifies Vp, Vs, and density values at arbitrary points (x,y,z) defined by the user by locating the
nearest neighbor grid point in the appropriate CVM-H voxet. The new model version consists of
high (250m) and medium (1000m) resolutions voxets, or regular grids, defining both Vp and Vs
structure (density is derived using a scaling relationship from Vp). To support the needs of SCEC
scientists who employ the code to help parameterize their computational grids, we enhanced the
code to deliver several additional functions. First, the code now provides the location, in addition
to the value, of the nearest neighbor grid point. This allows users to identify the locations where
the values were initially parameterized in the CVM-H, ensuring data integrity and supporting the
use of a variety of interpolation schemes that can be tailored to the users application. A sample
implementation of an interpolation routine which handles material boundaries correctly is
provided. Second, the code now provides the depths (distances) from the arbitrary points to the
surfaces used to construct the CVM-H, namely the surface topography/bathymetry, the top of
crystalline basement, and the Moho. This information is of particular value when using the CVM-H
to guide the construction of computational meshes.
142 | Southern California Earthquake Center