Ethan Williams's Home Page

Ethan F. Williams

About me:

In summer 2025, I will be joining UC Santa Cruz as an assistant professor in the Department of Earth & Planetary Sciences. If you are a prospective postdoc or graduate student interested in working with me, please send me an email.

Until then, I am a postdoc at the University of Washington working with Brad Lipovsky and Marine Denolle. I previously completed my Ph.D. in geophysics at the Caltech Seismo Lab, advised by Zhongwen Zhan.

I apply fiber-optic sensing to problems in seismology and oceanography. My research interests include wave physics, Earth structure, ocean dynamics, ocean-solid Earth interaction, and geohazards.

• 2025- Assistant Professor of Earth & Planetary Sciences, UC Santa Cruz
• 2023-2025 GHI Postdoctoral Scholar, University of Washington

• 2023 Ph.D. in Geophysics, California Institute of Technology
• 2019 M.S. in Geophysics, California Institute of Technology
• 2017 B.S. in Geophysics, Stanford University (minor in Geological Sciences)
• 2017 B.A. in Music, Stanford University

Research:

• Most of my work utilizes distributed acoustic sensing (DAS), which records strain or temperature perturbations along ordinary optical fibers (such as used in telecom applications), up to a distance of 100+ km with spatial resolution as fine as 1 m. When deployed on terrestrial or seafloor fiber-optic cables, DAS provides an unprecedented multi-scale view of solid-earth and ocean dynamics which can easily span 4-5 orders of magnitude resolution in both space and time. DAS is therefore naturally suited to studying wave propagation problems, for example surface wave inversion from sub-array dispersion measurements or localization of point scatterers with wavefield imaging.
• While I don't develop fiber-optic sensing instrumentation directly (very good DAS systems are now widely commercially available), I do continuously attempt to improve my understanding of the technology. One recent project has focused on quantifying the effect of cable construction and installation on DAS measurements from a theoretical standpoint, modeling the cable as a layered elastic rod embedded in an elastic or acoustic whole-space under different incident wavefields. You can read some of the initial findings here.

• For subduction zones like Cascadia and Alaska with a large distance from the coast to the trench (or deformation front, in the case of Cascadia), onshore seismic and geodetic instrumentation struggles to constrain the degree of locking on the plate interface in the interseismic period as well as the distribution of shallow slip in great earthquakes. With numerous seafloor telecom cables crossing active subduction zones around the world, fiber-optic sensing is opening the door to continuous offshore seismo-geodetic monitoring at a tiny fraction of the cost of conventional cabled arrays, like Japan's S-net.
• In 2021 and 2024, I was involved with two community DAS experiments on the NSF Ocean Observatories Initiative's Regional Cabled Array offshore central Oregon. In addition to acoustic signals from marine mammal vocalizations (see Wilcock et al., 2023) and regional T waves (see Shen and Wu, 2024), we recorded a diverse array of ambient seismic and ocean signals. In a study led by Jiaqi Fang at Caltech (preprint forthcoming), we developed a shear-wave velocity model for forearc sediments from ambient noise interferometry, illuminating microbasin structures associated with order-of-magnitude variability in site amplification. Integrating this 2D model with shear-wave velocity measurements from other cables in Cascadia, I am beginning to look at structural controls on the distribution of offshore strong ground motion, in order to understand how shelf morphology influences earthquake triggering of submarine landslides and turbidity currents.
• In another study led by Qibin Shi at UW (preprint forthcoming), we demonstrated a successful application of multiplixed DAS on lit fiber, meaning that DAS data can be acquired on the RCA without interrupting the operation of the seafloor cabled observatories. In the coming years, I am excited to acquire more DAS data on the OOI RCA, which can be utilized for offshore geodetic monitoring at low frequencies and real-time earthquake detection. With long time series, it will also be possible to perform time-lapse inversion and coda-wave dv/v measurements to search for velocity changes associated with pore pressure transients or stress evolution across the seismic cycle.

• In shallow water (~200 m or less), seafloor DAS commonly records cable strain induced by wind waves and ocean swells propagating above. For several buried (and thereby uniformly coupled) cables, we have demonstrated that DAS strain scales in proportion to seafloor pressure (see here and here). Because the transfer function from sea surface displacement to seafloor pressure is well known, it is therefore possible to recover the statistics of ocean surface waves (e.g. significant wave height) from DAS data if the relationship between cable strain and seafloor pressure can be constrained. In a recent study with colleagues from OSU, UW, and WHOI, we compared wave spectra measured with DAS against wave spectra measured with collocated conventional instrumentation (e.g. wave buoys and pressure sensors), demonstrating the validity and generalizability of this approach. However, this study also revealed a staggering 10 order of magnitude variability in the empirical transfer functions among different experiments. I am currently working on modeling (e.g., here) with the goal of deriving a fully analytical transfer function that can explain the empirical variability in DAS sensitivity to ocean waves.
• Because DAS constitutes an array with multi-scale resolution, it is possible to capture phase-resolved propagation of ocean waves in space and in time, something not typically possible with buoys or remote sensing methods. Adapting an approach from acoustics and seismology, we demonstrated (here) that the ocean surface wave Green's function can be recovered from the ambient records of waves propagating over a DAS array. This method has several uses: (1) water depth along a cable can be recovered from dispersion, (2) the along-array component of the mean flow (ocean currents) can be recovered from non-reciprocity in dispersion, and (3) scattering and nonlinear transformation over bathymetry are easily distinguished based on travel-time and spatial coherence.
• Wave buoys typically measure both the sea surface displacement and the directional distribution of wave energy, using multi-component vector sensors. DAS is only a single-component sensor, but array processing like beamforming or frequency-wavenumber analysis can recover some information about the directional distribution of wave energy. I am currently working on a 1.5 year dataset from Lower Cook Inlet, southern Alaska, where intermediate-to-shallow water waves are observed over more than 300 km of cable and are strongly modulated by tidal currents. After benchmarking the directional sensitivity of DAS against a nearby wave buoy, linear and nonlinear models of wave-current interaction will be tested against the observations. In particular, the long-time evolution of waves over currents has never been observed at this scale and should prove interesting.
• Infragravity waves offshore Oregon... under construction!

• DAS temperature sensing of internal solitary waves in the Strait of Gibraltar...
• DAS temperature sensing of internal tides on the near-critical slopes of Gran Canaria...
• Remote monitoring of abyssal turbulence with cable vibrations...

• High-frequency secondary microseism generated by local wave-wave interactions, novel sources, and sea ice...
• Observational constraints on the primary microseism source from DAS arrays across continental shelves...
• Imaging the solid Earth with seismic ambient noise...

• Geotechnical characterization and remote structural monitoring for offshore wind...
• Time-dependent and nonlinear elasticity in civil structures...
• Seasonal to decadal velocity changes at a liquefiable site in south Seattle...

Publications:

Submitted

• Glover, H.E., M.M. Smith, M.E. Wengrove, E.F. Williams, J. Thomson, M. Ifju, and B.P. Lipovsky (in review) Comparisons of seafloor distributed fiber-optic sensing datasets and empirical calibrations for inferring ocean surface gravity waves. Submitted to Journal of Atmospheric and Oceanic Technology.
• Shi, Q., M. Denolle, Y. Ni, E.F. Williams (in review) Denoising offshore distributed acoustic sensing using masked auto-encoders to enhance earthquake detection. Submitted to Journal of Geophysical Research: Solid Earth.

Journal Articles

1. Biondi, E., W. Zhu, J. Li, E.F. Williams, and Z. Zhan (2023) An upper-crust lid over the Long Valley magma chamber. Science Advances 9 (42), eadi9878.
2. Williams, E.F., A. Ugalde, H.F. Martins, C. Becerril, J. Callies, M. Claret, M.R. Fernandez-Ruiz, M. Gonzalez-Herraez, S. Martin-Lopez, J.L. Pelegri, K.B. Winters, and Z. Zhan (2023) Fiber-optic observations of internal waves and tides. Journal of Geophysical Research: Oceans, 128 (9), e2023JC019980.
3. Fang, J., Y. Yang, Z. Shen, E. Biondi, X. Wang, E.F. Williams, M.W. Becker, D. Eslamian, and Z. Zhan (2023) Directional sensitivity of DAS and its effect on Rayleigh-wave tomography: A case study in Oxnard, California. Seismological Research Letters, 94 (2A), pp. 887-897.
4. Biondi, E., X. Wang, E.F. Williams, and Z. Zhan (2023) Geolocalization of large-scale DAS channels using a GPS-tracked moving vehicle. Seismological Research Letters, 94 (1), pp. 318-330.
5. Williams, E.F., T.H. Heaton, Z. Zhan, and V.R. Lambert (2022) Variability in the natural frequencies of a nine-story concrete building from seconds to decades. The Seismic Record 2 (4), pp. 237-247.
6. Williams, E.F., Z. Zhan, H.F. Martins, M.R. Fernandez-Ruiz, S. Martin-Lopez, M. Gonzalez-Herraez, and J. Callies (2022) Surface gravity wave interferometry and ocean current monitoring with ocean-bottom DAS. Journal of Geophysical Research: Oceans, 127 (5), e2021JC018375.
7. Yang, Y., J.W. Atterholt, Z. Shen, J.B. Muir, E.F. Williams, and Z. Zhan (2022) Sub-kilometer correlation between near-surface structure and ground motion measured with distributed acoustic sensing. Geophysical Research Letters, 49, e2021GL096503.
8. Wang, X., Z. Zhan, E.F. Williams, M. Gonzalez-Herraez, H.F. Martins, and M. Karrenbach (2021) Ground vibrations recorded by fiber-optic cables reveal traffic response to COVID-19 lockdown measures in Pasadena, California. Communications Earth & Environment 2 (160).
9. Williams, E.F., M.R. Fernandez-Ruiz, R. Magalhaes, R. Vanthillo, Z. Zhan, M. Gonzalez-Herraez, and H.F. Martins (2021) Scholte wave inversion and passive source imaging with ocean-bottom DAS. The Leading Edge 40 (8), pp. 576-583.
10. Li, Z., Z. Shen, Y. Yang, E.F. Williams, X. Wang, and Z. Zhan (2021) Rapid response to the 2019 Ridgecrest earthquake with distributed acoustic sensing. AGU Advances 2, e2021AV000395.
11. Wang, X., E.F. Williams, M. Karrenbach, M. Gonzalez-Herraez, H.F. Martins, and Z. Zhan (2020) Rose Parade seismology: Signatures of floats and bands on optical fiber. Seismological Research Letters 91, pp. 2395-2398.
12. Williams, E.F., M.R. Fernandez-Ruiz, R. Magalhaes, R. Vanthillo, Z. Zhan, M. Gonzalez-Herraez, and H.F. Martins (2019) Distributed sensing of microseisms and teleseisms with submarine dark fibers. Nature Communications 10 (5778).
13. Gurnis, M., H. Van Avendonk, S.P.S. Gulick, J.M. Stock, R. Sutherland, E. Hightower, B. Shuck, J. Patel, E.F. Williams, D. Kardell, E. Herzig, B. Idini, K. Graham, J. Estep, and L. Carrington (2019) Incipient subduction at the contact with stretched continental crust: the Puysegur Trench. Earth and Planetary Science Letters 520, pp. 212-219.
14. Williams, E.F., C.M. Castillo, S.L. Klemperer, N. Raineault, and L. Gee (2018) Sycamore Knoll: A wave-planed pop-up structure in a sinistral-oblique thrust system, Southern California Continental Borderland. Deep Sea Research, Part II: Topical Studies in Oceanography 150, pp. 132-145.

Review Papers

1. Fernandez-Ruiz, M.R., H.F. Martins, E.F. Williams, C. Becerril, R. Magalhaes, L.D. Costa, S. Martin-Lopez, S. Jia, Z. Zhan, and M. Gonzalez-Herraez (2022) Seismic monitoring with distributed acoustic sensing from the near-surface to the deep oceans. Journal of Lightwave Technology 40 (5), pp. 1453-1463.
2. Fernandez-Ruiz, M.R., M.A. Soto, E.F. Williams, S. Martin-Lopez, Z. Zhan, M. Gonzalez-Herraez, and H.F. Martins (2020) Distributed acoustic sensing for seismic activity monitoring. APL Photonics 5 (3), 030901.

Expanded Abstracts

1. Williams, E.F. and B.P. Lipovsky (2024) Toward cable response for DAS. IEEE Photonics Society Summer Topicals Meeting Series (SUM), 1-2.
2. Callens, C., B. Stuyts, T. Lanckriet, and E.F. Williams (2023) Characterisation of small-strain shear modulus through DAS Offshore Site Investigation Geotechnics 9th International Conference Proceeding, pp. 1416-1421.
3. Karrenbach, M., Z. Shen, Z. Li, S. Cole, E.F. Williams, A. Klesh, X. Wang, Z. Zhan, L. LaFlame, V. Yartsev, and V. Bogdanov (2020) Rapid deployment of distributed acoustic sensing systems to track earthquake activity. SEG Technical Program Expanded Abstracts 2020, pp. 490-494.
4. Fernandez-Ruiz, M.-R., E.F. Williams, R. Magalhaes, R. Vanthillo, L. Costa, Z. Zhan, S. Martin-Lopez, M. Gonzalez-Herraez, and H.F. Martins (2019) Teleseism monitoring using chirped-pulse φOTDR. Seventh European Workshop on Optical Fibre Sensors, 1119921.
5. Martins, H.F., M.-R. Fernandez-Ruiz, L. Costa, E.F. Williams, Z. Zhan, S. Martin-Lopez, and M. Gonzalez-Herraez (2019) Monitoring of remote seismic events in metropolitan area fibers using distributed acoustic sensing (DAS) and spatiotemporal signal processing. Optical Fiber Communication Conference, M2J.1.

Other Publications

1. Williams, E.F. (2023) Probing solid-earth, ocean, and structural dynamics with distributed fiber-optic sensing Ph.D. Thesis, California Institute of Technology.
2. Williams, E.F. (2022) Listening to the seafloor through optical fibers. Physics Today 75 (10), pp. 70-71.