Structure and Mechanics of the Subducted Gorda Plate: Constrained by Afterslip Simulations and Scattered Seismic Waves by Jianhua Gong B.S., Peking University (2012) M.S., Peking University (2015) Submitted to the Department of Earth, Atmospheric, and Planetary Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY and the WOODS HOLE OCEANOGRAPHIC INSTITUTION February 2021 © 2021 Jianhua Gong. All rights reserved. The author hereby grants to MIT and WHOI permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Author............................................................................ Department of Earth, Atmospheric, and Planetary Sciences Massachusetts Institute of Technology & Woods Hole Oceanographic Institution January 4, 2021 Certified by. Dr. Jeffrey Joseph McGuire Research Geophysicist, Earthquake Science Center United States Geological Survey Thesis Supervisor Certified by. Dr. Daniel Lizarralde Associate Scientist with Tenure, Department of Geology & Geophysics Woods Hole Oceanographic Institution Thesis Supervisor Accepted by....................................................................... Dr. Oliver Jagoutz Professor of Geology, Department of Earth and Planetary Sciences Massachusetts Institute of Technology Chair, Joint Committee for Marine Geology & Geophysics 2 Structure and Mechanics of the Subducted Gorda Plate: Constrained by Afterslip Simulations and Scattered Seismic Waves by Jianhua Gong Submitted to the Department of Earth, Atmospheric, and Planetary Sciences Massachusetts Institute of Technology & Woods Hole Oceanographic Institution on January 4, 2021, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract Subduction zones host the greatest earthquakes on earth and pose great threat to human society. The largest slip in megathrust earthquakes often occurs in the 10–50 km depth range, yet seismic imaging of the material properties in this region has proven difficult. This thesis focuses on developing methods to utilize high frequency (2–12 Hz) seismic waves scattered from the megathrust plate interface to constrain its fine-scale velocity structures and to investigate the relationship between velocity structures and megathrust slip behaviors. Chapter 2 investigates the locking condition of the subducted Gorda plate by simulating afterslip that would be expected as a result of the stress changes from offshore strike-slip earthquakes. Chapter 3 develops array analysis methods to identify P-to-S and S-to-P seismic converted phases that convert at the subducted Gorda plate interface from local earthquakes and uses them to constrain the geometry and material properties of the plate boundary fault of the subducted Gorda plate between 5–20 km depth. Chapters 4 and 5 use a dense nodal array and numerical modeling methods to study the seismic guided waves that propagate along the thin low velocity layer at the boundary of the subducted Gorda plate. Taken together, our results indicate that material properties of the subduction plate- boundary fault is highly heterogeneous and the plate-boundary fault is potentially contained in a low velocity layer with significant porosity and fluid content at seismogenic depths. Thesis Supervisor: Dr. Jeffrey Joseph McGuire Title: Research Geophysicist, Earthquake Science Center United States Geological Survey Thesis Supervisor: Dr. Daniel Lizarralde Title: Associate Scientist with Tenure, Department of Geology & Geophysics Woods Hole Oceanographic Institution 3 4 Acknowledgments Coming to the United States for a PhD is a big adventure. There are many people who helped me to get to this point and supported me both academically and emotionally. I am grateful to all of you. First and foremost, I thank my advisor, Jeff McGuire, without whom this thesis would not be possible. Jeff provided the key ideas for each chapter of this thesis and offeredme many opportunities for field works, research cruises, conferences and workshops that helped me to see a bigger picture of a research project and to communicate my work with other people. He taught me to study earthquakes from both the seismological and the geodetic perspectives. Over the past 5.5 years, he has given me endless encouragement to help me overcome the hard times. I will always appreciate him spending his spare time working on my writing. I will never forget the whiteboard in his office on which we laid out the structure of each chapter, and which is also too high that I could never reach the top. I have many things I want to thank Jeff for. But most of all, I thank him for bringing me totheJoint Program. In his career, Jeff has collaborated with many Chinese scholars including several from my alma mater. It is the excellent researchers before me built his trust, and I hope one day I could become one of them! Second, I thank my co-advisor Dan Lizarralde. I could still remember the time when we met in the parking lot one day in the morning and you said to me: “we need to find you a co-advisor at WHOI”, and then “I am happy to do that”. I was thinking to ask you to be my co-advisor at the similar time, but you asked me first! Thank you for your constant checking in through the COVID time. The last few months towards my graduation wouldn’t have been smooth without your encouragement and support. I am grateful to my other committee members—John Collins, Matej Pec and Rob Evans—for giving me support and sharing their expertise. I thank John for giving me a tour to the OBS lab that I had wished for a long time, introducing me to the wavefield decomposition method that is used in the third chapter and the discussions on OBS instrument response and tilting. I thank Matej Pec for the imaging short course at MIT and the discussion of the mélange shear zones. I thank Rob for the EM workshop at Woods Hole and his knowledge of fluids in subduction zones from the EM data. Chapters 3–5 of this thesis would not have been possible without the help from Hao Guo, Wenyuan Fan and Marleen Nyst. I thank Hao Guo for providing the tomography and earth- quake location results near the Mendocino Triple Junction, the study area of this thesis. The method developed in Chapter 3 highly relies on the high-resolution P and S wave velocity models and the accurate earthquake locations from his work. I thank Wenyuan Fan and Marleen Nyst for the design and deployment of the nodal array in Northern California that collects data for Chapters 4 and 5. Thank you for climbing up and down the hills with the heavy nodes and making it a successful experiment. I am grateful to Professor Brian Evans for guiding me through the second general project and allowing me to muck around in his lab. I feel lucky that I could learn the concept of permeability in such a hands-on way. I thank Professor Evans for his kindness, patience and 5 tolerance. I also thank EAPS graduate student Ekaterina Bolotskaya for conducting the experiments together and ERL students and researchers—Chen Gu, Wenlian Xiao, Saied Mighani, Ulrich Mok, Yves Bernabé and Stephen Brown—for various discussions and guid- ance through the project. Going on research cruise is an advantage of studying at WHOI. I thank Jessica Warren and Mark Behn for leading a successful cruise to the Gofar transform fault. I thank the OBS team—Alan Gardner, Dan Knot and Martin Rapa—for letting me operate the transponder. I thank all the graduate students and postdocs on the cruise for their delightful science talks and friendship! I greatly appreciate many WHOI scientists through their teaching and support. I thank Ralph Stephen for his two-student seismology class. I thank Dan Lizarralde, Adam Soule, Olivier Marchal and Liviu Giosan for a really broad Geological Oceanography. I thank Veronique Le Roux and Virginia Edgcomb for a practical presentation training course. I thank Mark Behn and Veronique Le Roux for organizing the special seminar on oceanic crust and leading the SCARF cruise. I thank Pablo Canales for chairing my defense. I am also thankful for the wonderful and challenging classes and seminars I took at MIT and Harvard by Tom Herring, Brian Evans, Brad Hager, Laurent Demanet, Florent Brenguier and Miaki Ishii. I thank Andrew Daly for organizing the geodynamic seminar and the field trip to Josephine Ophiolite and Henry Dick and Professor Tim Grove’s instruction of the geology of western US. Even though I was confused looking at those rocks in the field, it turned out to be very helpful when I read papers on the geology of the Mendocino Triple Junction after we came back. I appreciate the support from the Academic Programs Office. Especially, I would like to thank Meg Tivey and Delia Oppo for the caring meetings with me after Jeff moved to USGS; Julia Westwater and Lea Fraser for many emails on classes, registrations and event scheduling; Valerie Caron and Martha Bridgers for maintaining the tidiness of Oyster Pond complex where I had stayed for 2.5 years; Kris Kipp for booking rooms and all the chats on the 8th floor; and to many who work behind-the-scenes that I don’t really know about. I thank the G&G department admins, in particular Sally Houghton and Christina Cuel- lar, for their prompt help with any office issue. As an international student, our study wouldn’t be smooth without the support from MIT International Student Office. I thank Janka Moss for many emails and several zoom meetings for helping me with the immigration related procedure towards graduation. I would like to thank my Master’s advisor Zengxi Ge for being the first role model as a researcher and other faculty members at the Geophysics Department of Peking University who opened the door for me to seismology.