SCEC Award Number 14176 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Toward Implementation of a Stochastic Description of Fine Scale Basin Velocity Structure in the SCEC Community Velocity Model (CVM-H)
Investigator(s)
Name Organization
John Shaw Harvard University Thomas Jordan University of Southern California
Other Participants Andreas Plesch, Xin Song
SCEC Priorities 6a, 6c SCEC Groups GMP, Seismology, CS
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 We present a statistical description of fine-scale velocity structure in the sedimentary basins of southern California that is intended to support high frequency ground motion simulations for earthquake hazards assessment. 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 (Magistrale et al., 1996; Süss and Shaw, 2003; Brocher, 2005, Plesch et al., 2014; Song et al., 2014). 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 robust, deterministic regional model. Thus, we have developed a statistical description of fine-scale velocity structure, informed by more than one million measurements in borehole sonic logs and geological correlations. Specifically, we have defined the variability in both Vp and Vs, and established vertical and horizontal correlation lengths for fine-scale velocity structures using wells across the basin as well as in tightly clustered oil fields. We anticipate that these results will be used to develop a statistical representation of fine scale wavespeed structure as an enhancement to the SCEC community velocity models (CVM, CVM-H). We have developed an effective medium theory that accounts for the effects of heterogeneities much smaller than a seismic wavelength.
Intellectual Merit This research directly supports the development of improved 3D structural models of southern California that serve as the basis for seismic wave propagation modeling and other studies. Specifically, our work provides the basis for statistically defining small scale velocity heterogeneities in these models that can be used to support more accurate simulations at higher frequencies. Our analysis provides a general approach that can be applied to current and future model iterations. In addition, we have developed an effective medium theory that accounts for the effects of heterogeneities much smaller than a seismic wavelength, thereby helping to provide a basis to include these statistical representations of structures in future simulations. These efforts directly address SCEC priorities included in 6 “Seismic wave generation and scattering: prediction of strong ground motions.”
Broader Impacts This work directly addresses SCEC’s primary mission to improve seismic hazards assessment in southern California by developing 3D velocity models that can be used to simulate ground motions for future earthquakes to higher frequencies. More accurate simulations at higher frequencies are essential to better understand how the built environment will respond to shaking from future large earthquakes, thereby improving seismic hazard and risk assessments.
Exemplary Figure Figure 3: (left) Histogram of deviations of shear wave slowness relative to the background model. The distribution has a standard deviation of 0.073, equivalent to a ratio data/background of e 0.073 = 1.076 or a variability of +/- 7.6 % at the 1 m length scale. (right) The combined vertical variogram of shear wave variability shows a minimum correlation distance of ca. 15m and a maximum of 40m. The oscillatory nature of the variability beyond these distances likely represents cyclicity in the rocks related to stratigraphic sequences.