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Seismology Near and Far: Local Earthquake Monitoring and Teleseismic Reflection Imaging

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Abstract

In a subduction zone, intermediate- and deep-focus earthquakes generate upgoing waves that reflect from discontinuities such as the crust-mantle boundary. When these reflections are detected, they can be used to map crustal and upper mantle structure thousands of kilometers away from where they are recorded. One of the main goals of this dissertation is expanding the applicability of this method to new regions; the other main focus is an investigation into local seismicity using a network deployed in Ithaca, New York. Chapter 1 introduces necessary topics in seismology for the reader whose background is in physics and situates the imaging method used in this thesis in the context of other seismic imaging methods. Chapters 2 and 3 describe the use of depth phase precursors to map lithospheric structure without deployment of seismometers to the target region. In chapter 2, the underside reflection from the crust-mantle boundary pmP is used to measure crustal thickness in the Tonga subduction zone using a dense network of short-period, high-sensitivity seismometers in Japan, Hi-net. Previous researchers were unable to resolve pmP for thin oceanic crust using long-period recordings. However, in this chapter the ability to do so with short-period recordings is demonstrated in Tonga, with results in agreement with previous refraction studies. In chapter 3, this same method is used to measure crustal thickness in the Aleutian subduction zone. Here, underside reflections from intracrustal discontinuities are also observed, which can be associated with nearby active volcanoes in the region and are likely generated by magma bodies. These represent the first seismic reflection measurements of magma bodies in the Aleutians and add to our understanding of the crustal plumbing systems under Aleutian volcanoes. Shifting focus from far to near, Chapter 4 describes the results of the analysis of CorNET recordings of earthquakes and anthropogenic events in Ithaca and the surrounding region, for the purpose of assessing the safety of a planned geothermal energy project on Cornell's campus. A small number of natural earthquakes are found in the Ithaca area, with magnitudes too small to be previously detected by permanent networks in the region, but these do not show any clustering on known faults in the area. A number of likely anthropogenic events are also identified; the possibility that some of these represent induced seismicity on a fault on the base of the Fir Tree anticline is discussed.

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269 pages

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Date Issued

2021-08

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Keywords

Earth sciences; Physical sciences; Reflection imaging; Seismic arrays; Seismic imaging; Subduction zones

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Union Local

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Committee Chair

Brown, Larry Douglas

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Committee Member

Elser, Veit
Wittich, Peter

Degree Discipline

Physics

Degree Name

Ph. D., Physics

Degree Level

Doctor of Philosophy

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Government Document

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dissertation or thesis

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