Ambient Noise Tomography and Microseism Directionalities across the Juan de Fuca Plate by Ye Tian B.S., Geophysics, University of Science and Technology of China, China, 2006 A thesis Submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Physics 2016 i This thesis entitled: Ambient Noise Tomography and Microseism Directionalities across the Juan de Fuca Plate written by Ye Tian has been approved for the Department of Physics Michael H Ritzwoller Shijie Zhong Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. ii Abstract Ye Tian (Ph.D, Geophysiscs [Department of Physics]) Ambient Noise Tomography and Microseism Directionalities across the Juan de Fuca Plate Thesis directed by Professor Michael H. Ritzwoller Ambient noise tomography has been well developed over the past decade and proven to be effective in studying the crust and upper mantle structure beneath the Earth’s continents. With new seismic array deployments beginning in the oceans, the application of the tomographic methods based on ambient noise observed at ocean bottom seismometers (OBSs) has become an important topic for research. In this thesis, I investigate the application of ambient noise tomography to oceanic bottom seismic data recorded by the Cascadia Initiative experiment across the Juan de Fuca plate. With higher local noise levels recorded by OBSs, I find that traditional data processing procedures used in ambient noise tomography produce measurable Rayleigh wave Green’s functions between deep ocean stations, whereas the shallow water stations are severely contaminated by both tilt noise and compliance noise and require new methods of processing. Because the local noise level varies across the study region, four semi- independent studies are conducted to both utilize the quieter deep-water stations and to address the problem posed by noisy shallow water stations. First, I construct an age-dependent shear wave speed model of the crust and uppermost mantle with 18 deep-water stations near the Juan de Fuca Ridge. The model possess a shallow low shear velocity zone near the ridge and has its sedimentary thickness, lithospheric thickness, and mantle shear wave speeds increase systematically with age Second, I investigate the locations and mechanisms of microseism generation using ambient noise cross-correlations constructed between 61 OBSs and 42 iii continental stations near the western US coast and find that the primary and secondary microseisms are generated at different locations and possibly have different physical mechanisms. Third, I show that tilt and compliance noise on the vertical components of the OBSs can be reduced substantially using the horizontal components and the differential pressure gauge records. Removal of these types of noise improves the signal-to-noise ratio of ambient noise cross-correlations significantly at beyond 10 sec period. Lastly, I present a new single- station method to estimate the microseism Rayleigh wave strength and directionality based on the horizontal-to-vertical transfer function. The high spatial and temporal resolution of this method may open up the microseism Rayleigh waves for a wider range of studies. iv Acknowledgments I’m taking this opportunity to express my gratitude to everyone who supported me throughout the six years I spent in the department of Physics at CU Boulder. Among those, I particularly would like to thank my advisor Michael Ritzwoller for his continuous support of my Ph.D study and research and for his patience, motivation, and immense knowledge. His guidance helped me to become a better researcher and also, as a foreign student, to write better scientific articles. This thesis would not be possible without his help and support. I would like to thank Professor Anatoli Levshin, Professor Anne Sheehan, Professor Michael Calkins, and Professor Shijie Zhong for composing my dissertation committees. Anatoli has great knowledge in the field of geophysics and is a very kind person too. I had many discussions with him during the years I spent at CU and he constantly provides insight related to my research. Shijie is an expert in geodynamics and he provided much help in explaining the thermal process related to my first paper. I’ve also learned a lot about seismology and inversion from Anne during the courses she taught and the discussions I had with her. I would also like to thank Vera Schulte-Pelkum for being in my committee for my comprehensive exam. Professor John Wahr was not able to be in my committee but he was a very kind person and had always been supportive of my research. Many people in our group have helped me throughout the years. Misha Barmin, as our computer administrator, provided numerous help in solving technique problems. The codes he provided related to my research saved me a great amount of time. During my early years at CU, Fan-chi Lin and Weisen Shen provided many help as I learned and developed the research and programing skills needed to complete my Ph.D works. Teri Keeler is a very good administrator and helped me in various aspects on my study and research. I would also like to thank Jiayi Xie, Xi v Liu, Chuan Qin, Lili Feng, Hongda Wang, and many others who have been my colleagues and friends at CU. My study and life at Boulder are made much easier with the discussions and the endless helps from them. All the data used in this thesis are provided by the Cascadia Initiative experiment and are downloaded from the IRIS DMC. The work has been supported by numerous scholars in the CI team. My everlasting love goes to my wife Jing Tian and my parents Mingquan Tian and Yongxue Wang. They are my greatest support anytime. It would be impossible for me to complete my Ph.D work without their love and support. vi Contents Chapter 1 Introduction .................................................................................................................... 1 1.1 Scientific motivations and objectives ............................................................................... 1 1.2 Roadmap ........................................................................................................................... 3 Chapter 2 Lithospheric age-dependent crustal and uppermost mantle shear velocity structure adjacent to the Juan de Fuca Ridge ................................................................................................. 6 2.1 Summary ........................................................................................................................... 6 2.2 Introduction ....................................................................................................................... 7 2.3 Methods............................................................................................................................. 8 2.3.1 Data Processing ...................................................................................................... 8 2.3.2 Bayesian Monte Carlo Inversion ......................................................................... 16 2.4 Discussion and Conclusions ........................................................................................... 26 Chapter 3 Directionality of Ambient Noise on the Juan de Fuca Plate: Implications for source locations of the primary and secondary microseisms ................................................................... 31 3.1 Summary ......................................................................................................................... 31 3.2 Introduction ..................................................................................................................... 32 3.3 Data and Measurements .................................................................................................. 36 3.4 The Strength and Directionality of Ambient Noise ........................................................ 42 3.4.1 Signal Level ......................................................................................................... 45 3.4.2 Seasonal Variability ............................................................................................. 47 3.4.3 Azimuthal Content ............................................................................................... 47 3.5 Discussion ....................................................................................................................... 48 3.5.1 Are the primary and secondary microseisms generated at the same locations? .. 48 3.5.2 Are ambient noise signals generated in deep water of the North Pacific? ........... 49 vii 3.5.3 Are ambient noise signals generated locally in shallow waters on the Juan de Fuca Plate and its surroundings?................................................................................... 53 3.5.4 Does the ambient noise wavefield extend from the oceanic to the continental parts of the study region? .............................................................................................. 62 3.6 Conclusions ..................................................................................................................... 65 Chapter 4 Towards extraction of high quality ambient noise cross-correlations in the noisy ocean bottom environment across the Juan de Fuca plate .....................................................................