Monitoring and Imaging Seismic Velocity Changes Across Temporal and Spatial Scales
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mao-maos-PhD-EAPS-2021-thesis.pdf
Description
Thesis PDF
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30.05 MB
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Author(s)
Mao, Shujuan
Advisor(s)
van der Hilst, Robert D.
Campillo, Michel
Date Issued
September 2021
Publisher
Massachusetts Institute of Technology
Abstract
Relative changes in seismic velocity (dv/v) are associated with changes in mechanical properties of crustal materials, and can reflect the perturbations in stress fields, rock damage, and fluid content. Interferometry of seismic ambient noise enables the continuous monitoring of dv/v.
In this thesis, I first explore the sensitivity and resolution limits of noise-based monitoring of dv/v. By employing dense seismic arrays at La Réunion Island, I demonstrate the feasibility of using noise-based interferometry to detect tidally-induced deformation (volumetric strain ∼ 10-8, dv/v ∼ 10-4) with ∼ hourly time resolution. I further extend the applications of noise-based monitoring by not only detecting the temporal changes but also imaging the spatial variations of dv/v. Based on the spacetime dv/v observations, I investigate groundwater fluctuations in the Coastal Los Angeles Basins during 2000–2020. The spatial imaging of dv/v reveals pronounced seasonal variability in confined aquifers. The spatial patterns of dv/v are consistent with surface deformation inferred from InSAR but also constrain aquifers and their hydrology at different depths. Moreover, I propose a method for measuring seismic travel-time shifts based on wavelet cross-spectrum analysis. This new method provides stable time-shift measurements with optimal time-frequency joint resolution that can enhance the high-resolution spatial imaging of dv/v.
This dissertation advances the techniques and applications of temporal monitoring as well as spatial imaging of dv/v via seismic interferometry. Monitoring dv/v in time and space is shown to be a promising tool, which can be used in concert with other geophysical observations, to identify and decipher dynamic processes in the crust.
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
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