SCEC Award Number 13018 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Stochastic Descriptions of Basin Velocity Structure from Analyses of Sonic Logs and the SCEC Community Velocity Model (CVM-H)
Name Organization
John H. Shaw Harvard University Thomas Jordan University of Southern California
Other Participants Andreas Plesch
SCEC Priorities 6 SCEC Groups GMP, USR, Seismology
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
This work aims to develop and implement a stochastic description of fine-scale velocity structure in the SCEC Community Velocity Models (CVMS and CVMH). The trend of numerical wave propagation studies to higher frequencies (> 2 Hz) has been facilitated by sustained increases in computational power, and has created a demand for higher resolution velocity models. Building such models is challenging, in part because geologic and seismologic data indicate that fine-scale elastic inhomogeneities can be strong in sedimentary basins and have spatially anisotropic statistical distributions. While we have local measures of fine-scale velocity structure (down to meter scales) along boreholes with sonic logs, there is not a sufficient density of such samples to facilitate the development of a deterministic regional model. Thus, we are developing a statistical description of fine-scale velocity structure, informed by these local observations and geological correlations, to enhance the community models so that they can support higher-frequency simulations. Our current results define the standard variation (6.5%) in velocity that is represented in the wells but not the current CVM-H, and show that this variation is markedly non-Gaussian. In addition, we define vertical and horizontal correlation lengths for velocity structures within the sedimentary basin of 80 and 2000 meters, respectively. Our goal is to use these characteristics to implement a fine-scale velocity structure in the CVM-H to support wave propagation and strong ground motion simulations to higher frequencies.
Intellectual Merit This research is focused on developing a statistical description of small-scale velocity structure in southern California that is informed by well measurements and can be added to the CVM's. Representation of these small scale structures is needed in the CVM's to effectively simulate wave propagation and strong ground motions at higher frequencies.
Broader Impacts These efforts will ultimately help to improve the ability of wave propagation simulations to accurately forecast strong ground motions at frequencies that are important for building response. Thus, the ultimate goal of this research is to improve seismic hazard assessments.
Exemplary Figure Figure 1: Perspective view of well log database in an oilfield in the southwestern LA basin that was used in our analysis. There are 70 well paths in an area of 7km x 2.5km with more than 400,000 samples of interval travel times by logging tools (converted to Vp); logs in yellow and stratigraphic tops as spheres; 3:1 vertical exaggeration.