Isotropic and Anisotropic Seismic Tomography Using Active Source and Earthquake Records Springer Theses

Springer Theses Recognizing Outstanding Ph.D. Research Yu-Pin Lin Isotropic and Anisotropic Seismic Tomography Using Active Source and Earthquake Records Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent field of research. For greater accessibility to non-specialists, the published versions include an extended introduction, as well as a foreword by the student’s supervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria • They must be written in good English. • The topic should fall within the confines of Chemistry, Physics, Earth Sciences, Engineering and related interdisciplinary fields such as Materials, Nanoscience, Chemical Engineering, Complex Systems and Biophysics. • The work reported in the thesis must represent a significant scientific advance. • If the thesis includes previously published material, permission to reproduce this must be gained from the respective copyright holder. • They must have been examined and passed during the 12 months prior to nomination. • Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. • The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. More information about this series at http://www.springer.com/series/8790 Yu-Pin Lin Isotropic and Anisotropic Seismic Tomography Using Active Source and Earthquake Records Doctoral Thesis accepted by National Taiwan University, Taipei, Taiwan 123 Author Supervisors Dr. Yu-Pin Lin Prof. Li Zhao Department of Earth Sciences, Southern Institute of Earth Sciences California Earthquake Center Academia Sinica University of Southern California Taipei Los Angeles, CA Taiwan USA Prof. Shu-Huei Hung Department of Geosciences National Taiwan University Taipei Taiwan ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-981-10-5067-1 ISBN 978-981-10-5068-8 (eBook) DOI 10.1007/978-981-10-5068-8 Library of Congress Control Number: 2017943094 © Springer Nature Singapore Pte Ltd. 2018 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore Supervisor’s Foreword One of the major goals of seismology is to gain knowledge on the internal structures of the Earth from seismic signals recorded by instruments deployed on the surface of the globe. Ever since the introduction of digital seismometers in the late 1970s, a huge amount of data has been collected which, with the availability of increasingly powerful and affordable computers, has led to the creation of numerous tomography models, allowing us to peek into the Earth’s global, regional and local structures. In keeping with the improvement in seismic tomography models, our perception of the Earth’s internal constitution and dynamics has been constantly shaped and reshaped. However, at the same time we have also seen attempts that are little more than straightforward applications of an established method to a more or less the same old dataset (sometimes with additions of a number of earthquakes and/or stations). The outcome of this practice is a simple accumulation of models for the same region with little discernible advance in science. Take Taiwan as an example: an incom- plete search of literature in the past two decades yields at least eight different tomography models covering roughly the same area of Taiwan and its immediate surroundings. They share some common large-scale features, but have drastic differences in details. This situation is by no means unique to Taiwan. As “new” models are created, there has clearly been a lack of (1) rigorous, objective and quantitative assessments on the validity of the models and (2) exploration of new techniques to qualitatively improve the robustness of tomography models. The research presented in this thesis represents two efforts in those two aspects. The first part of this thesis (Chaps. 2 and 3) describes the research in which a dataset of the first (P-wave) arrival times from active sources were used to make quantitative assessments of existing tomography models for Taiwan. These P-wave arrival times are regarded as “ground truth” because they are from active sources (explosions) whose locations and origin times are exactly known. Through tedious manual picking from waveforms recorded at nearly 1400 sites all over Taiwan, the author acquired more than 6000 P-wave arrival times, which were then compared with predictions by existing tomography models to evaluate the performance of the models. Indeed, different levels of performance can be seen not only in different v vi Supervisor’s Foreword tomography models but also in different parts of the same model. This “ground truth” dataset was also used to conduct a tomography study for the shallow crustal structure in Taiwan. The resulting model (presented in Chap. 3) has much lower residual and good correlation with surface geology. It was nevertheless turned down for publication at an international journal due to the objection by an anonymous but “well-known expert in active-source seismology”. Shunryu Suzuki, founder of the San Francisco Zen Center, once wrote, “In the beginner’s mind there are many possibilities, in the expert’s mind there are few”. The second part of this thesis (Chaps. 4 and 5) deals with the development and application of a new, full-wave approach to imaging the three-dimensional anisotropic structures using the widely available shear-wave splitting measurements. Traditional modelling and interpretation of shear-wave splitting in terms of anisotropy have been largely based on ray theory which presumes a uniform distri- bution of anisotropy. The full-wave approach based on normal-mode theory developed here leads to spatially varying sensitivities of shear-wave splitting on anisotropic structures. The application of this approach to data collected at a seismic network in Southern California, coupled with a multi-scale inversion practice, demonstrates that this new approach is very effective in imaging the three- dimensional variations of anisotropy, which in turn provides strong seismic con- straints on the state of stress and strain in the lithosphere and the dynamics of the mantle flow in the asthenosphere. At the present time, full-wave seismic tomography is one of the most active research frontiers in seismology. Another new generation of Earth models are being created offering new insights into the inner workings of our planet. I am confident that the author of this thesis has much to contribute in her future endeavours. Taipei, Taiwan Li Zhao February 2017 Abstract This thesis involves two regional seismic tomography studies: One for the isotropic P-wave velocity in the upper crust beneath Taiwan using first-arrival times from active sources; the other for the shear-wave anisotropy structure under Southern California using SKS splitting intensity measurements. In the isotropic tomography for Taiwan, we use waveform records from the 10 explosions in 2008 conducted by the TAiwan Integrated GEodynamics Research (TAIGER) project. A large dataset of high-quality ground-truth first-arrival times are hand-picked from the active-source records at *1400 sites throughout Taiwan, which greatly enhance our capability to determine the crustal velocity beneath Taiwan with unprecedented accuracy and resolution, especially along the two north-south and two east-west island-wide linear transects with densely deployed receivers.

Load more