University of Pennsylvania ScholarlyCommons Publicly Accessible Penn Dissertations 2014 Diatom-Based Reconstructions of Megathrust Earthquake Deformation and Tsunami Inundation in Central and South-Central Chile Cristina Dura University of Pennsylvania, [email protected] Follow this and additional works at: https://repository.upenn.edu/edissertations Part of the Ecology and Evolutionary Biology Commons, Geology Commons, and the Paleontology Commons Recommended Citation Dura, Cristina, "Diatom-Based Reconstructions of Megathrust Earthquake Deformation and Tsunami Inundation in Central and South-Central Chile" (2014). Publicly Accessible Penn Dissertations. 1265. https://repository.upenn.edu/edissertations/1265 This paper is posted at ScholarlyCommons. https://repository.upenn.edu/edissertations/1265 For more information, please contact [email protected]. Diatom-Based Reconstructions of Megathrust Earthquake Deformation and Tsunami Inundation in Central and South-Central Chile Abstract The timespan of instrumental and historical records of earthquakes and tsunamis limit our understanding of long-term subduction zone behavior. Coastal stratigraphy, incorporating diatom sea-level indicators, has provided some of the most detailed geological reconstructions of earthquake induced land-level change and tsunami inundation, often over multiple seismic cycles. In central Chile, I extended the seismic history through a study of a lowland stratigraphic sequence at Quintero (32.5°S). I documented six laterally continuous sand beds dated to 6200, 5600, 5000, 4400, 3800, and 3700 cal yr BP, probably deposited by high tsunamis. Sediment properties and diatom assemblages of the sand beds--anomalous marine planktonic diatoms and upward fining of silt-sized diatom valves and sediments--point to a marine sediment source and high-energy deposition. I inferred coseismic uplift concurrent with the deposition of the sand beds based on an increase in freshwater siliceous microfossils in units overlying the beds. Our record indicates the recurrence interval of large tsunamigenic earthquakes in central Chile is ~500 years, implying that the frequency of historical earthquakes (~80 year recurrence) in central Chile is not representative of the greatest earthquakes the subduction zone can produce. My study sites in south-central Chile are located in the overlap of the 1960 (Mw 9.5) Valdivia segment and the 2010 (Mw 8.8) Maule segment ruptures. My paleoseismic investigations from the Tirúa (38.3° S) and Quidico (38.1°S) rivers were consistent with eyewitness accounts of tsunami inundation (AD 2010 and 1960) and historical accounts of coseismic land-level change (AD 2010, 1960, 1835, 1751, and 1575). The vertical deformation inferred from diatom analysis suggests that Maule segment earthquakes result in uplift at our sites (e.g., 2010, 1835, and 1751), and Valdivia segment earthquakes result in no deformation (e.g., 1960) or subsidence (e.g., 1575). I identified four prehistoric tsunami deposits dated to AD 1457-1575, 1443-1547, 256-461, and 176-336. Diatoms indicate uplift coincident with sand deposition in AD 1457-1575 and AD 256-461, which we attribute to the Maule segment. Subsidence coincident with sand deposition in AD 1443-1547, and no change in elevation in AD 176-336, is attributed to the Valdivia segment. Degree Type Dissertation Degree Name Doctor of Philosophy (PhD) Graduate Group Earth & Environmental Science First Advisor Benjamin P. Horton Keywords Diatoms, Earthquake, Paleoecology, Paleoseismology, Sea level, Tsunami Subject Categories Ecology and Evolutionary Biology | Geology | Paleontology This dissertation is available at ScholarlyCommons: https://repository.upenn.edu/edissertations/1265 DIATOM-BASED RECONSTRUCTIONS OF MEGATHRUST EARTHQUAKE DEFORMATION AND TSUNAMI INUNDATION IN CENTRAL AND SOUTH- CENTRAL CHILE Tina Dura A DISSERTATION in Earth and Environmental Science Presented to the Faculties of the University of Pennsylvania in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy 2014 Supervisor of Dissertation ____________________________________________________ Benjamin P. Horton Professor, Rutgers University Graduate Group Chairperson ____________________________________________________ Douglas Jerolmack Associate Professor, Earth and Environmental Science Dissertation committee Douglas Jerolmack, Associate Professor, Earth and Environmental Science Jane K. Willenbring, Assistant Professor, Earth and Environmental Science Alan R. Nelson (external), Project Chief, USGS DIATOM-BASED RECONSTRUCTIONS OF MEGATHRUST EARTHQUAKE DEFORMATION AND TSUNAMI INUNDATION IN CENTRAL AND SOUTH- CENTRAL CHILE © COPYRIGHT 2014 Tina Dura This work is licensed under the Creative Commons Attribution- NonCommercial-ShareAlike 3.0 License To view a copy of this license, visit http://creativecommons.org/licenses/by-ny-sa/2.0/ DEDICATION To my Momma. Thank you for making it possible for me to pursue everything I ever dreamed of. All of this hard work is for you. You are my person for always. Antes morir que perder la vida. iii ACKNOWLEDGMENTS First I want to sincerely thank my adviser, Ben Horton, the most organized man I know. His ability to field countless paper and figure drafts and still provide valuable feedback is remarkable. Thank you for always pushing me to do my best, and for being patient through the struggles. I would also like to thank my external supervisor, Alan Nelson. From our first fieldwork trip to Chirikof, he has gone above and beyond the call of duty to guide me through my scientific endeavors, and I am very grateful. I look forward to many years of fruitful research with both of you. I also express my gratitude to my committee members, Doug Jerolmack and Jane Willenbring. Both of them are incredible role models in their passion for science. I greatly appreciate all the academic and logistical support from the faculty and staff at Penn. A special thanks goes to Joan and Arlene for their support. I would like to thank Don Charles at the Academy of Natural Sciences of Drexel University for providing access to the diatom preparation lab and microscopes there, Elena Colon for her assistance in the preparation lab, and Mihaela Enache and Sonja Hausmann for their assistance with diatom analyses. To the Chirikof crew (Alan, Rich Briggs, Simon Engelhart, Guy Gelfenbaum, and Spoony the fox), thank you for an epic trip and the support and guidance since. To the Chile crew (Marco Cisternas, Lisa Ely, Rob Wesson, Isabel Hong, Jessica Pilarczyk, Ed Garrett, Diego Muñoz, Lorena Rebolledo and family, and the students at Universidad Católica de Valparaíso), thank you for many enjoyable field seasons. I have learned so much from the experience and appreciate your generosity in advice. Marco, I iv am forever grateful for your hospitality and support. Even though you call me “the weed,” I have thoroughly enjoyed working with you. Viva Chile...! I am indebted to my fellow sea-level research lab-mates, past and present: Simon, Nicole Khan, Andrea Hawkes, Andy Kemp, and Jessica Pilarczyk. You’ve been invaluable with your eagerness to answer questions and give advice, your support in the field, and importantly, your friendship. To my fellow grad students (Brandon Hedrick, Kim Miller, Vanessa Boschi, Maddie Stone, Raleigh Martin, Collin Phillips, and Rachel Valleta), it’s been a pleasure crossing paths with all of you. Thank you for the friendship and support. To Ivan, I know being in the same house with me while I wrote my dissertation was not the most enjoyable thing you have ever experienced. But, we made it, so now we can survive pretty much anything. I love you, Meat. To Bodhi, you are my angel starship. Finally, I want to thank my family and friends in the US, Spain, and Croatia for your support and understanding. I love you guys. This dissertation would not have been possible without funding from the National Science Foundation, University of Pennsylvania, National Ocean Science Accelerator Mass Spectrometer Facility and the Penn Stipend for Summer Paleontology. Lastly, I would like to acknowledge research support from the Greg and Susan Walker Endowment at the University of Pennsylvania. No thanks to the wasp. v ABSTRACT DIATOM-BASED RECONSTRUCTIONS OF MEGATHRUST EARTHQUAKE DEFORMATION AND TSUNAMI INUNDATION IN CENTRAL AND SOUTH- CENTRAL CHILE Tina Dura Benjamin P. Horton The timespan of instrumental and historical records of earthquakes and tsunamis limit our understanding of long-term subduction zone behavior. Coastal stratigraphy, incorporating diatom sea-level indicators, has provided some of the most detailed geological reconstructions of earthquake induced land-level change and tsunami inundation, often over multiple seismic cycles. In central Chile, I extended the seismic history through a study of a lowland stratigraphic sequence at Quintero (32.5S). I documented six laterally continuous sand beds dated to 6200, 5600, 5000, 4400, 3800, and 3700 cal yr BP, probably deposited by high tsunamis. Sediment properties and diatom assemblages of the sand beds—anomalous marine planktonic diatoms and upward fining of silt-sized diatom valves and sediments—point to a marine sediment source and high-energy deposition. I inferred coseismic uplift concurrent with the deposition of the sand beds based on an increase in freshwater siliceous microfossils in units overlying the beds. Our record indicates the recurrence interval of large tsunamigenic earthquakes in central Chile is ~500 years, implying that the frequency