Electronic Theses and Dissertations

Nima Nazemi

Date

2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Civil Engineering

Committee Chair

Shahram Pezeshk

Committee Member

Roger Meier

Committee Member

Arash Zandieh

Committee Member

Chris Cramer

Committee Member

Sabyasachee Mishra

Abstract

This study uses unique characteristics of coda waves to estimate seismic wave attenuation in New Madrid Seismic Zone and its extended area. Coda waves are backscattered waves from reflections and refractions of seismic waves by material discontinuities such as material changes, cracks, etc. These waves are eventually recorded at the end of the seismographs. Coda waves sample the same path, pass through the same site, and finally are recorded by the same instrument. This allows us to easily isolate the path, site, and instrument responses and estimate the attenuation. In chapter 1, this method, called Coda normalization, is used to estimate Lg waves. This type of wave, due to its propagation characteristics, is representative of attenuation at regional distance, and sample deeper layers of the crust. In Chapter 2, the Coda normalization method is again used to estimate the quality factor of P and S waves. We studied these waves at near-source distances. Evidently, near-source attenuation is stronger than Lg waves at regional distance. Within the New Madrid Seismic Zone, due to higher seismic activity compared to its extended area, it would be valid to assume that scattering has a dominant contribution to the quality factor. In order to test this assumption, in chapter 3, we used the temporal variation of scattered energy density to better understand higher order scattering. At three different lapse time windows, energy density levels from observations are matched with theoretical energy density function driven by energy transport theory (Zeng et al., 1991). The best matching pair of seismic Albedo and extinction length at each frequency of interest is then obtained through a grid search. The final product of this analysis is separate frequency-dependent quality factors for intrinsic and scattering in the area of interest.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest.

Notes

Open Access

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