SCEC Award Number 16076 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Characterizing seismic site conditions in southern California based on topographically induced stress and bedrock fractures
Name Organization
Seulgi Moon University of California, Los Angeles Lingsen Meng University of California, Los Angeles
Other Participants 2 graduate students
SCEC Priorities 6a, 6c, 6e SCEC Groups CME, GMP
Report Due Date 03/15/2017 Date Report Submitted 03/22/2017
Project Abstract
The seismic shear-wave velocity in the upper 30 m of the Earth (hereafter, VS30) is widely used to characterize local seismic site conditions and predict ground-motion amplification. Previous studies have mapped the spatial distribution of VS30 based on its correlation with lithology and topographic slope. While this approach provides a reasonable first-order prediction of VS30, appreciable discrepancies still exist between predicted and measured VS30 values. Our project evaluated the hypothesis that topographically induced bedrock fracture and weathering patterns influence the spatial variability of VS30 in southern California. We took a two-step approach to examine the controls of local variations of VS30 in southern California. First, we measured and compiled VS30 from P-wave seismic refraction surveys. Second, we calculated elastic topographic stress distribution beneath the topographic profile using a boundary element model. Our initial results showed that the VS30 of the San Bernardino Mountains is slower than the VS30 of the San Gabriel Mountains. In our SGM site, the spatial distribution of stress model proxy shows similar distribution with the P-wave tomography from our seismic refraction survey. The initial results of this work suggest a potential control of topographic stress on bedrock weathering and VS30, which we will continue to investigate this control. Our work will provide a systematic understanding of the spatial variation of bedrock weathering in mountainous sites in southern California, which will improve the characterization of subsurface shear-wave velocity and seismic site conditions.
Intellectual Merit Our research develops a new method for predicting VS30 that is based on integrated geologic, geomorphic, and tectonic stress (from the Community Stress Model) data. Our initial results showed a potential, and we are currently working to examine whether this control can be applicable to larger areas. The refined VS30 predictions will substantially improve the assessment of local seismic hazards, which will implement SCEC information and techniques in seismic hazard, earthquake engineering, risk analysis, and ultimately loss mitigation.
Broader Impacts Our SCEC project supported two graduate students in UCLA (Jessica Lin and Tian Feng). Lin is working on bedrock weathering in both SGM and SBM in southern California, and Feng is working to improve the characterization of the seismic site amplification effect based on microseismic data. In addition, our projects provide research and educational opportunities for undergraduate students at UCLA. UCLA undergraduate students from groups underrepresented in the Earth sciences (female and/or minority) participated field trips to conduct P-wave seismic refraction surveys in SGM and SBM, and learned about near-surface seismic techniques to image subsurface velocity structures (Figure 3.).

Exemplary Figure
Figure 2. A comparison of the topographic stress proxy (failure potential for shear fractures) and seismic P-wave velocity for the SGM site. (a) The distribution of the failure potential from a horizontal stress value of 2 MPa and the seismic velocity. (b) The measured P-wave velocity (c) The average S-wave velocity for the upper 30 m using the relationship Vp=√3Vs.