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observed waveforms (after Olsen and Mayhew, 2010).
SCEC Research Accomplishments | Report
6. Development of a new version of the SCEC Community Fault Model (CFM 4.0) for southern California that includes
updates based on relocated seismicity catalogs and detailed fault traces in the USGS Quaternary Fault and Fold
database.
7. Development of a new version of the statewide Community Fault Model (SCFM 2.0) in coordination with the
California Geological Survey and the USGS in support of the UCERF3 project.
Community Velocity Models (CVM, CVM-SI, CVM-H)
A series of improvements were made to the SCEC Community Velocity Models this past year to improve their performance in
strong ground motion simulations and a range of other applications. These efforts included 3D waveform tomographic
inversions using the F3DT scattering integral method that evaluated the longstanding CVM-S model (Chen et al., 2011, and
Lee et al., 2010). Based on comparisons of observed and synthetic waveforms, perturbations were applied to this model to
generate a revised velocity description (Figure 15). The model is expressed as a 3D grid of 1536 x 992 x 100 cells with a grid
spacing of 500 m and is available as a C/Fortran code and associated files that are downloaded, compiled, and run locally.
This improved model has been imported into UCVM and is referenced with the model label CVM-SI.
In addition, we developed updated versions of the CVM-H (11.2 and 11.9) that are also released as part of UCVM. The latest
version of this model includes a newly compiled Moho surface, addition of the offshore Santa Maria basin, a new detailed
representation of the San Bernardino basin, and much smoother transitions between low and high resolution regions of the
model. The new Moho surface was compiled by Carl Tape from a large number of data sources, including receiver functions
and active-source studies. Improvements to the offshore Santa Maria basin representation in the CVM-H included a revised
definition of the basement surface using reflection seismic data that extends to the western margin of the model. The new
model also includes a representation for the San Bernardino basin that was developed using gravity (Anderson, 2000) and
seismic reflection data (Stephenson et al., 2002). The velocity structure (Vp) in the basin is defined by stacking velocities
(Stephenson et al., 2004) and a 1D velocity profile (Graves, 2008) combined to a basin thickness-depth-velocity function.
Finally, the transitions from the high to the low resolution models were revised by introducing a new smoothing velocity
gradient. This latest model version is being quantitatively assessed by performing seismic wavefield simulations based on
unstructured hexahedral meshing (GEOCUBIT) and the spectral-element method (SPECFEM3D) using a database of synthetic
seismograms for 234 reference earthquakes in southern California (Tape et al., 2011). These synthetic seismograms are filtered
over different period ranges and then compared with observed seismograms using a variety of misfit functions. The misfit
analysis provides an earthquake-based perspective of the quality of CVM-H, and will be used to revise the velocity
parameterizations in future model releases.
Planning for the future of SCEC Community Velocity Models
Comparisons of observed and synthetic waveforms for earthquakes in southern California demonstrate that the SCEC
Community Velocity Models (Magistrale et al., 2000; Süss and Shaw, 2003; Plesch et al., 2009) perform much better than simple
1-D velocity models in accurately simulating seismic wave propagation and forecasting the distribution of hazardous ground
shaking (e.g., Komatitsch et al., 2004; Chen et al., 2007; Olsen and Mayhew, 2010). Furthermore, the new 3D waveform
tomographic inversion methods that are being used to evaluate and improve the current velocity models offer great promise
of offering further improvements in model performance. To help facilitate the process of evaluating and improving these
models, the USR Focus Area has worked closely with SCEC CME group over this past year to coordinate model releases,
enhance the code that delivers the models, and provide tools that can help in evaluating model performance.
Specifically, the UCVM framework now provides a common platform that can be used to access all of community velocity
model versions (CVM-S, CVM-SI, CVM-H), along with other alternative and complementary regional velocity models. This
helps to facilitate direct comparisons between models, as well as use of alternative models in various seismological and hazard
assessment studies that explore the significance of differences between current model versions. In addition, the CME group
has implemented a CVM evaluation system that uses wave propagation simulations of moderate magnitude historical
earthquakes and goodness-of-fit (GOF) measurements that compare simulated and observed waveforms (Figure 16). The
system is designed to automate running of multiple earthquake wave propagation simulations, keeping all-aspects of the
simulations identical except the velocity model used. CVM evaluations are run prior to release of new version of CVM-H, or
2011 SCEC Annual Meeting | 53