SCEC Award Number 13010 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
SCEC Priorities 6c, 6a, 6d SCEC Groups Seismology, GMP, GMSV
Report Due Date 03/15/2014 Date Report Submitted N/A
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, ambient noise source heterogeneity and wavefield effects like wave focusing and attenuation must be accounted for. To address these problems, we have focused on four primary research topics: developing a theoretical understanding of ambient noise amplitudes; developing a robust method of wavefield analysis which extends the Helmholtz tomography method to apply to surface wave amplitudes; testing this new Helmholtz-amplitude method on earthquake data; and applying the new method to ambient noise recorded on dense Southern California seismic networks. Our theoretical results suggest that raw cross correlations have smaller interferometric biases compared with coherency measurements, and supports the use of an un-normalized Helmholtz method. We have further shown that the modified Helmholtz method works well on array data from earthquakes. Finally, high-frequency noise correlation results from a couple of dense industrial seismic networks suggest that some of the amplitude variability observed in the Los Angeles Basin can be attributed to site amplification. These results together imply that ambient noise correlation can indeed be used to improve site response maps, and therefore help improve our knowledge of the spatial variability in ground motions across Southern California.
Intellectual Merit The research has advanced our understanding of how ambient noise correlation amplitudes can be used to measure site amplification. Through our work, a new method (the Helmholtz-amplitude method) based on array measurements of amplitudes was developed and applied to earthquake and ambient noise data.
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.
Exemplary Figure Figure 3: Preliminary map of site response at a period of 1 second from the Long Beach Array. Each dot represents a seismic station and the colors represent an estimate of the amount of amplification at each site after wave propagation effects are accounted for. Blue colors have high amplification and red colors have low amplification. The thin black line denotes the trace of the Newport-Inglewood Fault.
Figure from 2013 SCEC Annual Meeting plenary talk by V.C. Tsai, F.-C. Lin and D.C. Bowden.