University of Southern California Dissertations and Theses

Multi-scale imaging of the fault zone velocity structure: double-difference tomography, inversion of fault zone headwaves, and fault zone sensitivity kernels

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Multi-scale imaging of the fault zone velocity structure: double-difference tomography, inversion of fault zone headwaves, and fault zone sensitivity kernels

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Abstract (if available)

Abstract In spite of the close relationship between fault zone structure and earthquake mechanics, fault zone structure at seismogenic depths remains poorly understood. How does localization of the primary slip zone vary with depth? Is there a signature of broad persistent damage zones at seismogenic depths? How does fault zone structure merge with regional structure? To answer these questions, we utilize multiple imaging techniques. We apply high-resolution double-difference tomography to the San Jacinto fault zone, invert for velocity structure along the Hayward fault using fault zone head waves, and use analytical results for idealized geometries to validate sensitivity kernels of fault zone phases for use in adjoint tomographic inversions. ❧ Double-difference tomography uses the arrival times of P and S waves to invert simultaneously for compressional velocity, shear wave velocity, and source location in three dimensions. We present results in the southern California plate-boundary area, with a focus on the San Jacinto fault zone, which incorporate arrival times of 247,472 P- and 105,448 S-wave picks for 5493 earthquakes recorded at 139 stations. Starting with a layered 1D model, and continuing in later iterations with various updated initial models, we invert the data for Vp and Vs in a 270 km long, 105 km wide and 35 km deep volume using a spatially variable grid with higher density around the San Jacinto. Our final velocity results show zones of low-velocity and high Vp/Vs ratios associated with various fault strands and sedimentary basins, along with clear velocity contrasts across the San Jacinto. While both features are limited to the upper 10km, the low velocity zones generally have higher amplitude and broader distribution in geometrically complex areas, while the velocity contrasts are more pronounced for Vp than Vs. ❧ Along the Hayward fault in the San Francisco Bay region, we identify fault zone head waves at eight stations on the northeastern side of the fault using particle motion polarization analysis and waveform comparison. For three-component broadband and strong-motion instruments, head waves are identified with polarization analysis that detects early arrivals from the fault direction before the regular body waves with polarizations in the source-receiver direction. A systematic moveout between the head waves and direct P waves along a roughly 80 km long fault section suggests a single continuous fault interface over that distance. We then directly invert the observed travel times for the velocity contrast, obtaining a spatially variable contrast 5-12% along the central portion of the Hayward fault, with the southwest side having faster P wave velocity. This is in agreement with previous tomographic results, but provides much higher resolution evidence that the velocity contrast is highly localized and persistent along-strike. ❧ In order to understand exactly what portion of fault zone structure each phase interacts with, we construct sensitivity kernels for head waves and trapped waves based on a variety of idealized models and tomographic models. Beginning with models for which analytical solutions exist, we validate numerical modeling with waveform comparisons and demonstrate numerical accuracy up to 10Hz. In contrast to P waves, which have little or no sensitivity to fault zone structure, head wave sensitivity kernels have the highest values near the fault in the faster medium. While surface wave kernels show the broadest spatial distribution of sensitivity, trapped wave kernels are extremely narrow with sensitivity focused entirely inside the low-velocity zone. Because of this, adjoint tomography based on high-frequency trapped wave and head wave kernels will much more tightly constrain fault zone structure than has previously been possible. Here, we demonstrate the feasibility of such an inversion and provide the basic framework for carrying it out.

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University of Southern California Dissertations and Theses

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Asset Metadata

Creator Allam, Amir A. (author)

Core Title Multi-scale imaging of the fault zone velocity structure: double-difference tomography, inversion of fault zone headwaves, and fault zone sensitivity kernels

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Geological Sciences

Publication Date 07/08/2013

Defense Date 05/15/2013

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag fault zone,OAI-PMH Harvest,plate boundary,seismic imaging,seismic inversion,sensitivity kernels,tectonics,tomography

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Ben-Zion, Yehuda (committee chair), Haydn, Nicolai T. A. (committee member), Miller, Meghan S. (committee member)

Creator Email aallam@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-285044

Unique identifier UC11292653

Identifier etd-AllamAmirA-1745.pdf (filename),usctheses-c3-285044 (legacy record id)

Legacy Identifier etd-AllamAmirA-1745.pdf

Dmrecord 285044

Document Type Dissertation

Format application/pdf (imt)

Rights Allam, Amir A.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA