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The Mechanisms and Triggering of Earthquakes in the Ridge-Transform Environment

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The Mechanisms and Triggering of Earthquakes in the Ridge-Transform Environment The Mechanisms and Triggering of Earthquakes in the Ridge-Transform Environment Danielle Frances Sumy Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2011 © 2011 Danielle Frances Sumy All rights reserved ABSTRACT The Mechanisms and Triggering of Earthquakes in the Ridge-Transform Environment Danielle Frances Sumy The theory of plate tectonics introduced a paradigm shift in the way we view and study our planet. Many of the world’s plate boundaries, however, are beneath our oceans making data collection, the key to furthering our knowledge about these zones, difficult. Recent advances in research vessels, seismic data acquisition techniques, and equipment built to withstand the temperatures and pressures within the oceans and on the seafloor, have made a huge impact in helping us understand the complicated structure and dynamics of our Earth. In the field of seismology, precise earthquake locations can illuminate regions of active seismic deformation, and help us better understand the orientation, mechanics, and kinematics of plate boundary zones. Although ocean bottom seismometers have been in use since the late 1930s, the instruments did not have the recording capacity and endurance to withstand being placed on the seafloor for a large span of time. Today, ocean bottom seismometers are deployed in densely spaced arrays that record seismic signals for approximately a year. The high- precision seismic data now available can help us redefine plate boundaries and further our understanding of the internal processes and deformation within these zones. In this dissertation, I aim to use ocean bottom seismometer data to explore the Pacific-North America plate boundary within the Gulf of California, and the internal workings of the 9º50’N East Pacific Rise high- temperature hydrothermal system. The first chapter of my dissertation uses data collected from an ocean bottom seismometer array deployed along the plate boundary within the Gulf of California from October 2005 to October 2006. In this study, I detect and locate ~700 earthquakes mainly located on the NW-SE striking oceanic transform faults that delineate the plate boundary. In addition, we calculate regional moment tensors for ~30 of these events, and find that the majority are right- lateral strike-slip events consistent with observed transtensional plate motion. Chapter 2 investigates the relationship between tides and ~3500 microearthquakes recorded on six ocean bottom seismometers deployed in the vicinity of the 9º50’N East Pacific Rise high-temperature hydrothermal vent system from October 2003 to April 2004. I find unequivocal evidence for tidal triggering of microearthquakes with maximum extensional stresses induced by the solid Earth tide at this site. Although tides are not the underlying cause of earthquake nucleation within the region, the modulation of microearthquakes by these small amplitude tidal stresses indicates that the hydrothermal system is a high-stress environment that is maintained at a critical state of failure due to on-going tectonic and magmatic processes. In Chapter 3, I further investigate the role of tides in triggering microearthquake activity at the 9º50’N East Pacific Rise high-temperature hydrothermal vent site, and observe systematic along-axis variations between peak microearthquake activity and maximum predicted tidal extension. I interpret this systematic triggering to result from pore-pressure perturbations propagating laterally through the hydrothermal system, and from this result and a one- dimensional poroelastic model, I provide an estimate of bulk permeability at this site. This observation may allow for more sophisticated investigations into the heat and chemical exchange between the newly formed oceanic crust and hydrothermal fluids, and may provide insight into the plumbing supporting the subsurface biosphere. Table of Contents List of Figures................................ ………………………………………………………………ii List of Tables…………………………………………………………………………………......iii Acknowledgements……………………………………………………………………………….iv Dedication…………………………………………………………………………………………v Introduction………………………………………………………………………………………..1 Chapter 1: The Mechanics of Earthquakes and Faulting in the southern Gulf of California…………………………………………………………………....5 Chapter 2: Pulse of the Seafloor: Tidal triggering of microearthquakes at 9º50’N East Pacific Rise……………………………………………………….......64 Chapter 3: Systematic along-axis tidal triggering of microearthquakes observed at 9º50’N East Pacific Rise.......................................................................................................74 References………………………………………………………………………………………..86 i List of Figures Chapter 1 Figure 1: Map of Gulf of California and Baja California.............................................44 Figure 2: Global catalogued events within the Gulf of California................................45 Figure 3: Map showing Hypoinverse relocated events.................................................46 Figure 4: Velocity-depth profile......................................................................... ..........47 Figure 5: Double-difference relocated hypocenters and RMTs....................................48 Figure 6: Regional Moment Tensor Solution Examples................................... ...........49 Figure 7: Moment Magnitude vs. Local Magnitude.....................................................50 Figure 8: Northern Guaymas Transform Events...........................................................51 Figure 9: Southern Guaymas Transform and Guaymas Basin Events..........................52 Figure 10: Cross-sections of southern Guaymas Basin and Transform events.... ........53 Figure 11: Carmen Transform events................................................................. ..........54 Figure 12: Magnitude Plots and Spatial Sequences on the Carmen Transform.... .......55 Figure 13: Farallon Transform Events..........................................................................56 Figure 14: Alarcon Ridge-Transform events................................................................57 Figure 15: August 2006 Baja California earthquake sequence.....................................58 Figure 16: Map comparison between our study and Castro et al., 2010......................59 Chapter 2 Figure 1: Bathymetric Map of 9º50’N East Pacific Rise..............................................66 Figure 2: Relationship between Earthquakes andTides................................................67 Figure 3: Definition of Tidal Phase..................................................................... .........68 Figure 4: Percentage of Excess Events during times of Encouraging Stress................72 Chapter 3 Figure 1: Bathymetric Map with Double-Difference Relocated Epicenters.................76 Figure 2: Along-Axis Observations of Tidal Triggering..............................................77 Figure 3: Mean Tidal Phase Angle Against Along-Axis Latitude.................................78 Figure 4: Permeability as a Function of Porosity and Temperature................... ..........78 Figure 5: Volumetric Tidal Stress over Time...............................................................85 ii List of Tables Chapter 1 Table 1: The 695 Hypoinverse events divided into qualitative bins............................ .....60 Table 2: Focal mechanism solutions for the 31 earthquakes.............................................61 Table 3: Parameters for total effective seismic stress drop calculation.............................63 Chapter 2 Table 1: Statistical Parameters Associated with the Tidal Components...........................71 Table 2: Relationship between Peak-to-Peak Tidal Stresses and Microearthquakes........73 iii Acknowledgements Thank you to my friends and family who were gracious enough to see me through this process, especially Jennifer Arbuszewski, Adrienne Block, Anna Foster, Lauren Friedman-Way, Heather and Paul Garcia, Kate and Jeff Hoops, Robert and Elizabeth Kearns, Jennifer Levy-Varon, Kylene and Jeff Lopo, Jacqueline and Steve Phillips, Sandy Rivera, Ashley Shuler, Erika and Jonathan Thornton, and Andrew Way. I would especially like to thank my parents, Chuck and Jean Stroup, who nurtured my love of earth science. Thank you for letting me choose Charles Richter as my role model! I would also like to thank my advisor committee, Jim Gaherty, Bill Menke, and Maya Tolstoy. Thank you for letting me tirelessly ask questions and for never treating me badly because of it. Thank you for always considering me as a whole complicated person, and not just a graduate student. Your thoughts, encouragement, and constant advice have made me not only a better seismologist, but also a better person. Thank you to Del Bohnenstiehl, who encouraged me to apply to Lamont, and even though he left me in the dust after my first year, still let me call and nag him frequently. Thank you for continuing to serve on my committee, and for taking the time and energy to help me understand my science more thoroughly. Thank you to Jeff McGuire for serving on my defense committee, and for your encouragement. I cannot honestly read an entire oceanic transform paper without seeing your work cited, and your work has transformed my own personal knowledge of oceanic faults. Finally, two of the chapters in this thesis were previously published in Geophysical Research Letters, and are printed here with permission granted
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