SCEC Award Number 15035 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Improving High-Frequency Site Response with Ambient Noise
Investigator(s)
Name Organization
Victor Tsai California Institute of Technology
Other Participants Daniel Bowden
SCEC Priorities 6c, 6a, 6d SCEC Groups Seismology, GMP
Report Due Date 11/15/2016 Date Report Submitted 11/13/2016
Project Abstract
 The long-term goal of this project is to achieve improved high-frequency (>0.5 Hz) site amplification response maps by utilizing ambient noise observations. In order to achieve this goal, we have focused on (1) developing a theoretical understanding of ambient noise amplitudes; (2) developing robust methods of wavefield analysis to accurately measure surface-wave amplification; (3) testing the ambient noise results against earthquake data; (4) applying the methods to dense Southern California seismic networks; and (5) understanding how to use the results for improving ground-motion hazards.

Specifically, we have determined that use of an array-normalized Helmholtz method minimizes interferometric biases compared to traditional station-normalized coherency measurements, and that amplitude ratios are also more stable for such array-normalized measurements. We have further demonstrated that dense industrial seismic networks like the Long Beach Array can be used to construct high-frequency noise correlation measurements of local site amplification of surface waves. Initial amplitude-ratio measurements have also been performed over the sparser Southern California Seismic Network, leading to new constraints on shallow crustal structure, and we are beginning to compare high-frequency surface-wave amplification predictions with predictions of local site amplification more commonly used in ground-motion prediction equations.
Intellectual Merit The research has advanced our understanding of how ambient noise correlation amplitudes can be used to measure local site amplification. Through our work, a new method (the Helmholtz-amplitude method) based on array measurements of amplitudes was developed and applied to ambient noise data from dense seismic arrays. Moreover, a better theoretical understanding was gained as to how surface-wave amplification differs from traditional measures of local ground-motion amplification.
Broader Impacts The work has been used as a key example of how scientists are working to better address earthquake risks in a number of outreach talks and tours both at the Seismological Laboratory at Caltech and in local schools. The research also paves the way for improved earthquake zonation based on new knowledge of local ground motion amplification, and will allow society to respond more effectively to earthquake hazards. Finally, aspects of the work were taught by the PI to students at the SCEC/ERI summer schools in both 2015 (Yamanakako) and 2016 (Lake Arrowhead).
Exemplary Figure Figure 3: Maps of relative amplification for southern California, describing 1D amplification factors relative to the hard rock site, PASC, at 0.4 Hz for (A) vertically-incident shear waves, (B) horizontal-component Rayleigh waves, and (C) Love waves. Faults from the U.S. Geological Survey and California Geological Survey are shown by red lines. Figure from Bowden & Tsai (submitted).