Dense mapping of shallow velocity structure in the Raymond Basin using the Pasadena Distributed Acoustic Sensing Array

Ethan F. Williams, Zhongwen Zhan, Martin Karrenbach, Steve Cole, & Lisa LaFlame

Submitted August 14, 2018, SCEC Contribution #8543, 2018 SCEC Annual Meeting Poster #023

With the increasing ability of ground-motion simulations to accurately model earthquake wavefields at high frequencies and the dependence of the earthquake engineering community on site characterization metrics like Vs-30, the resolution of existing maps of shallow velocity structure in Southern California, such as the SCEC CVM geotechnical layer (GTL), is inadequate. Here, we combine information from active-source surface waves, teleseismic and regional earthquakes, and noise correlation functions to map near-surface shear wave velocity and site response in the Raymond Basin (northern San Gabriel Valley) using the Pasadena Array. The Pasadena Array employs distributed acoustic sensing (DAS) technology to convert a >25-km fiber-optic telecommunications cable into an array of horizontal, single-component linear strainmeters, which can record dynamic strains with spatial resolution as fine as 1 m and instrument noise levels comparable to a geophone. Exciting the resonant modes of Caltech's Millikan Library using a torsional shaker installed on the roof of the nine-story building, we measure surface wave dispersion between 1 and 5 Hz along the Pasadena Array and observe variations in Rayleigh wave speed as significant as 200 m/s across spatial distances of less than 100 m. Above 5 Hz we compute ambient noise correlation functions which further constrain the spatial scale of shallow velocity variations. We supplement our velocity study with a preliminary map of site amplification using the standard spectral ratio method for a small catalog of teleseismic and regional earthquakes. As a case study, our results demonstrate the tremendous value of dense DAS strainmeter arrays leveraging existing telecommunications infrastructure to address the current and future needs of Southern California earthquake science.

Key Words
site amplification, surface waves, distributed acoustic sensing, dense array

Citation
Williams, E. F., Zhan, Z., Karrenbach, M., Cole, S., & LaFlame, L. (2018, 08). Dense mapping of shallow velocity structure in the Raymond Basin using the Pasadena Distributed Acoustic Sensing Array. Poster Presentation at 2018 SCEC Annual Meeting.


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