The Unified Structural Representation (USR) Focus Area supports SCEC’s science mission by providing digital models of crust and upper mantle structure in southern California for use fault systems analysis, strong ground motion prediction, and earthquake hazards assessment. These efforts include development of a Community Velocity Model (CVM), a Community Fault Model (CFM), and a Community Block Model (CBM), which comprise the USR. Highlights of this past year’s activities include construction and delivery of an improved Community Fault Model (CFM version 2), new alternative velocity models (CVM), and the first Community Block Model (CBM).

Larger imagePerspective view of the SCEC Community Fault Model (CFM version 2). Seismicity is from Hauksson (2000) and color-coded by year of occurrence.

The CFM is an object-oriented, 3-D representation of more than 140 active faults in southern California, defined by surface geology, earthquake hypocenters and focal mechanisms, well bore, and seismic reflection data. CFM version 2.0 includes more than 35 new fault representations (Plesch et al., 2004), guided by contributions from more than 20 SCEC investigators. The model, including its alternative fault representations, were evaluated and approved by the SCEC Community this year in a “virtual workshop,” in which scientists used the LA3D software tool, developed by the SCEC Intern Program, to visualize and analyze the faults. Based on feedback from this evaluation, we defined the inventory of CFM version 2, and are in the process of establishing a set of viable alternative fault models to be used by RELM for earthquake hazards assessment.

The CVM is a 3D description of crustal and upper mantle velocity (vP) structure, with derivative shear wave velocity and density models. The current CVM (Version 3.0) employs a rule-based approach for defining the velocity structure in sedimentary basins (Magistrale et al., 2000), which are embedded in regional tomographic (Hauksson, 2000) and 1D background models. The CVM continues to be widely used by SCEC investigators for numerical simulations of seismic wave propagation (including the TeraShake simulations; Olson et al., 2004), earthquake catalog relocations, and other efforts to characterize earthquake sources. This year, SCEC released a new, alternative velocity parameterization for the CVM, based on petroleum well and seismic reflection data (e.g., Suess & Shaw, 2003). Provision of this new model reflects the commitment of the USR Focus Area to deliver alternative structural representations that reflect epistemic uncertainties. Both models are already being used to generate synthetic waveforms, which compared with observations provide a basis for evaluating the models’ relative performance and their impacts on strong ground motion prediction.

Larger imagePerspective view of the SCEC Community Block Model (CBM), which consists of more than 75 tectonic blocks bounded by major faults, derived from the CFM, and regional topography, base-of-seismicity, and Moho surfaces.

The next generation of SCEC models will provide these and other alternative velocity parameterizations in a framework that is compatible with the fault representations provided by the CFM. To accomplish this, SCEC developed a Community Block Model (CBM), which consists of major fault surfaces from the CFM extrapolated and connected with topographic, base-of-seismicity, and Moho surfaces, to define closed blocks. The CBM is currently being used to generate volumetric meshes that will be used by SCEC’s Crustal Deformation Modeling Group through 3D quasi-static codes to model crustal motions. In addition, the CBM and additional geological surfaces will be used to define fault-bounded blocks in which one or more alternative velocity parameterization may apply, allowing users to develop new property models that are, by definition, compatible with the CFM fault representations. This framework, including fault surfaces and geologic horizons in the CFM and CBM, and compatible property models (CVM), will constitute the Unified Structural Representation.

References

• Hauksson, E., 2000, Crustal structure and seismicity distribution adjacent to the Pacific and North American plate boundary in southern California, Journal of Geophysical Research, v. 105, no. B6, p. 13,875-13,903.
• Magistrale, H., S. Day, R. W. Clayton, and R. Graves, 2001, The SCEC Southern California Reference Three-Dimensional Seismic Velocity Model Version 2: Bulletin of the Seismological Society of America, v. 90, no. 6B, p. S65-S76.
• Olson, K., et al., 2004, SCEC TeraShake — Supporting an Earthquake Storage Intensive Simulation, SCEC Annual Meeting, Palm Springs, CA.
• Plesch, A., Shaw, J. & SCEC USR focus group members, 2004, Community Fault Model (CFM) and Community Block Model (CBM) for Southern California, SCEC Annual Meeting, Palm Springs, CA.
• Süss, M. P., and J. H. Shaw, 2003, P-wave seismic velocity structure derived from sonic logs and industry reflection data in the Los Angeles basin, California, Journal of Geophysical Research, 108/B3.