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Ethan F. Williams

Ph.D. Candidate in Geophysics
Seismological Laboratory
California Institute of Technology
Office:
    358 South Mudd
Mail:
    MS 252-21
    1200 E. California Blvd.
    Pasadena, CA 91125
Email:
    efwillia (at) gps.caltech.edu

I am a Ph.D. candidate in geophysics at the Caltech Seismo Lab, working with Prof. Zhongwen Zhan. In my research, I develop diverse applications of ambient noise in engineering and environmental seismology.

Much of my recent work involves distributed acoustic sensing (DAS), an emerging technology that converts ordinary fiber-optic cables into dense arrays of broadband strainmeters. In urban Pasadena, I am working with a multi-year continuous DAS recording on a 37-km fiber seismic network to map shallow shear-wave velocity structure and monitor velocity changes as a proxy for subsurface hydrologic conditions. With ocean-bottom DAS recordings from offshore Belgium and Spain, I am extending the techniques of seismic interferometry to ocean surface gravity waves in order to measure spatio-temporal variations in current speed and sea state. Other projects include wind turbine localization from passive ocean-bottom recordings, long-term structural health monitoring of Caltech's Millikan Library with ambient vibrations, and investigating the physics of microseism geneneration with ocean-bottom DAS data.

(LEFT) PDF of noise spectra on one channel of Caltech's Pasadena DAS array over three weeks. (RIGHT) Earthquake spectra observed on the same channel, compared with the median noise spectrum and nearby broadband PASC.
Ocean surface gravity waves observed on an ocean-bottom DAS array in the North Sea represented in the f-k domain. Waves propagating landward across the array appear faster than waves travelling the opposite direction, suggesting that they are traveling in a mean flow (ocean current).