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i
Multi-Scale Imaging of the Fault Zone Velocity Structure: Double-difference
Tomography, Inversion of Fault Zone Headwaves, and Fault Zone Sensitivity
Kernels
by
Amir A. Allam
A Dissertation Presented to the
Faculty of the USC Graduate School
University of Southern California
In Partial Fulfillment of the
Requirements for the Degree
Doctor of Philosophy
(Geological Sciences)
August, 2013
Object Description
| Title | Multi-scale imaging of the fault zone velocity structure: double-difference tomography, inversion of fault zone headwaves, and fault zone sensitivity kernels |
| Author | Allam, Amir A. |
| Author email | aallam@usc.edu;aallam@usc.edu |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Geological Sciences |
| School | College of Letters, Arts And Sciences |
| Date defended/completed | 2013-05-15 |
| Date submitted | 2013-07-08 |
| Date approved | 2013-07-08 |
| Restricted until | 2013-07-08 |
| Date published | 2013-07-08 |
| Advisor (committee chair) | Ben-Zion, Yehuda |
| Advisor (committee member) |
Miller, Meghan S. Haydn, Nicolai |
| 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. |
| Keyword | tomography; plate boundary; seismic imaging; fault zone; sensitivity kernels; tectonics; seismic inversion |
| Language | English |
| Format (imt) | application/pdf |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m |
| Contributing entity | University of Southern California |
| Rights | Allam, Amir A. |
| 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 author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright. The original signature page accompanying the original submission of the work to the USC Libraries is retained by the USC Libraries and a copy of it may be obtained by authorized requesters contacting the repository e-mail address given. |
| Repository name | University of Southern California Digital Library |
| Repository address | USC Digital Library, University of Southern California, University Park Campus MC 7002, 106 University Village, Los Angeles, California 90089-7002, USA |
| Repository email | cisadmin@usc.edu |
| Filename | etd-AllamAmirA-1745.pdf |
| Archival file | uscthesesreloadpub_Volume7/etd-AllamAmirA-1745.pdf |
Description
| Title | Page 1 |
| Contributing entity | University of Southern California |
| Full text | i Multi-Scale Imaging of the Fault Zone Velocity Structure: Double-difference Tomography, Inversion of Fault Zone Headwaves, and Fault Zone Sensitivity Kernels by Amir A. Allam A Dissertation Presented to the Faculty of the USC Graduate School University of Southern California In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy (Geological Sciences) August, 2013 |
