SCEC Award Number 20035 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Using body wave polarization analysis to constrain the shallow velocity structure in southern California
Name Organization
Yehuda Ben-Zion University of Southern California
Other Participants PhD student Lei Qin
SCEC Priorities 3g, 4a, 2b SCEC Groups Seismology, CXM, GM
Report Due Date 03/15/2021 Date Report Submitted 03/30/2021
Project Abstract
The study aims to image shallow seismic velocities using polarization of body wave from teleseismic events. The analysis is applied to two broadband stations in southern California: BPH06 at Piñon Flats (hard bedrock site) and LGB in the Los Angeles basin (sediment site). The results highlight significant effects of data quality, with only 2-5% and 0.4-0.5% events generating well-polarized P-waves at BPH06 and LGB, respectively. The number of available events decreases and polarization quality of each measurement increases using shorter time windows. The analysis at BPH06 provides stable results for time windows in the range 2-20 sec, implying a simple local structure. Using a 2 sec time window, we obtain frequency-dependent S-wave veloc-ities (4-2.25 km/s at 0.1-2.5 Hz) at BPH06. However, the measurements at LGB show large uncertainties and do not converge to stable results, probably because of the complex local site conditions (e.g. high noise level, converted waves from the subsurface, scattered waves). Simulations based on a two-layer model and a depth profile from the CVMS model suggest that converted waves from subsurface interfaces could introduce considerable bias to the polarization measurements. Therefore, very short time windows (e.g., less than the travel time difference between the direct P and P-to-S converted waves) are required to exclude later arrivals. For high-frequency waves and complex velocity structures with strong scattering effects, even shorter time windows are needed (e.g. ~0.5 sec for waves above 1 Hz for the CVMS model sampled at the Los Angeles basin).
Intellectual Merit The research contributes to development of techniques to the challenging problem of imaging seismic properties of the top crust. The obtained results imply that using polarization analysis re-quires accurate P- and S-wave picks and preferably short time windows (e.g. < 2 sec) to exclude later arrivals and possible local scatterings for better measurements of polarization angles. The performed research indicates that polarization analysis may allow obtaining useful results for relatively simple sites such as Piñon Flats without strong converted waves or local scattering. How-ever, at sites with high noise level and strong local scattering such as the Los Angeles basin, the results are unstable and may be significantly biased.
Broader Impacts The results highlight the difficulties of imaging seismic properties of shallow materials based on earthquake data in basins and urban environments with significant noise. The employed techniques may allow imaging properties of shallow materials at quiet site with relatively simple velocity structure. The project supported a female PhD student (Lei Qin).
Exemplary Figure Figure 2. Results from BPH06 in Piñon Flats using different frequency bands. The blue dots with error bars show the optimized S-wave velocities in each frequency band. The vertical error bar represents the uncertainty of the measured S-wave velocity, and the horizontal error bar indicates the spanning of frequencies in each band. The five small insets show observed polarization angles (dots colored by the polarization significance) and predictions using the optimized S-wave velocity (black lines) in the five frequency bands (labeled on top of each insets). From Qin Lei et al. (work in progress).