Seismic structure along transitions from flat to normal subduction: central Mexico, southern Peru, and southwest Japan Thesis by Sara L. Dougherty In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 2014 (Defended May 23, 2014)
ii c 2014 Sara L. Dougherty All Rights Reserved
iii Acknowledgments First and foremost, I would like to sincerely thank my advisors, Rob Clayton and Don Helmberger, without whom this thesis would not have been possible. Don taught me to appreciate every wiggle of a seismogram and introduced me to complex waveforms and the fine-scale structures they can resolve. Rob taught me to appreciate the bigger picture and to explore the possible tectonic implications of observed fine-scale structures. Their support and guidance throughout my time at Caltech has been immensely valuable. I greatly appreciate Rob s patience, encouragement, and accessibility, especially as this thesis was coming together. I would like to thank my academic advisor, Jennifer Jackson, for her assistance with selecting courses and applying for candidacy and for the thought-provoking questions that she poses in my thesis committee meetings. I am also grateful to Joann Stock for serving on my committee and always providing insightful comments. The former grad students and postdocs who helped me in the initial stages of my research and along the way deserve special thanks: Alex Song, Risheng Chu, Min Chen, YoungHee Kim, and Ting Chen. Shengji Wei has also been a great help. I would like to thank Xyoli Pérez-Campos and Steve Skinner for insightful comments and valuable discussions. I am grateful for the support staff in the Seismo Lab and Tectonics Observatory, especially Rosemary Miller, Donna Mireles, Viola Carter, Evelina Cui, and Lisa Christiansen. Thanks also to Mike Black for all the IT support. Special thanks to Jason Saleeby and Jean-Philippe Avouac for leading the Pahoehoe field trip to Hawaii and the division enrichment trip to Greece, respectively, both of which were great experiences. Finally, my deep thanks to my husband, Walt Stelz, for his continuous support and understand-
iv ing, especially during the busy times. His ability to always make me laugh has been invaluable, and the occasional coding advice didn t hurt either.
v Abstract The fine-scale seismic structure of the central Mexico, southern Peru, and southwest Japan subduction zones is studied using intraslab earthquakes recorded by temporary and permanent regional seismic arrays. The morphology of the transition from flat to normal subduction is explored in central Mexico and southern Peru, while in southwest Japan the spatial coincidence of a thin ultra-slow velocity layer (USL) atop the flat slab with locations of slow slip events (SSEs) is explored. This USL is also observed in central Mexico and southern Peru, where its lateral extent is used as one constraint on the nature of the flat-to-normal transitions. In western central Mexico, I find an edge to this USL which is coincident with the western boundary of the projected Orozco Fracture Zone (OFZ) region. Forward modeling of the 2D structure of the subducted Cocos plate using a finite-difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure in this region and confirms the location of the USL edge. I propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South Cocos plate along the projection of the OFZ, by a process analogous to that which occurred when the Rivera plate separated from the proto-cocos plate 10 Ma. In eastern central Mexico, observations of a sharp transition in slab dip near the abrupt end of the Trans Mexican Volcanic Belt (TMVB) suggest a possible slab tear located within the subducted South Cocos plate. The eastern lateral extent of the USL is found to be coincident with these features and with the western boundary of a zone of decreased seismicity, indicating a change in structure which I interpret as evidence of a possible tear. Analysis of intraslab seismicity patterns and focal mechanism orientations and faulting types provides further support for a possible tear in the South Cocos slab. This potential tear, together with the tear along the projection of the
vi OFZ to the northwest, indicates a slab rollback mechanism in which separate slab segments move independently, allowing for mantle flow between the segments. In southern Peru, observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest a smooth contortion of the slab. Concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry. The lateral extent of the USL atop the horizontal Nazca slab is found to be coincident with the margin of the projected linear continuation of the subducting Nazca Ridge, implying a causal relationship, but not a slab tear. Waveform modeling of the 2D structure in southern Peru provides constraints on the velocity and geometry of the slab s seismic structure and confirms the absence of any tears in the slab. In southwest Japan, I estimate the location of a possible USL along the Philippine Sea slab surface and find this region of low velocity to be coincident with locations of SSEs that have occurred in this region. I interpret the source of the possible USL in this region as fluids dehydrated from the subducting plate, forming a high pore-fluid pressure layer, which would be expected to decrease the coupling on the plate interface and promote SSEs.
vii Contents Acknowledgments iii Abstract v List of Figures x List of Tables xiii 1 Introduction 1 2 Seismic structure in central Mexico: Implications for fragmentation of the subducted Cocos plate 7 2.1 Abstract........................................... 7 2.2 Introduction......................................... 8 2.3 Data Analysis........................................ 10 2.3.1 Data......................................... 10 2.3.2 1D Velocity Modeling............................... 10 2.3.3 Ultra-slow Velocity Layer............................. 14 2.3.4 2D Velocity Modeling............................... 20 2.3.5 Seismicity and Slab Dip Across USL Edge.................... 27 2.4 Discussion.......................................... 27 2.5 Conclusions......................................... 36 2.6 Supplemental Figures................................... 36 References............................................. 41
viii 3 Seismicity and structure in central Mexico: Evidence for a possible slab tear in the South Cocos plate 48 3.1 Abstract........................................... 48 3.2 Introduction......................................... 49 3.3 Tectonic Setting...................................... 51 3.4 Data Analysis........................................ 55 3.4.1 Data......................................... 55 3.4.2 Slab Dip....................................... 58 3.4.3 Ultra-slow Velocity Layer............................. 60 3.4.4 Seismicity...................................... 64 3.4.5 1D Velocity Modeling............................... 70 3.4.6 2D Velocity Modeling............................... 71 3.5 Discussion.......................................... 79 3.6 Conclusions......................................... 85 3.7 Supplemental Figures................................... 86 References............................................. 94 Appendix............................................. 105 4 Seismic structure in southern Peru: Evidence for a smooth contortion between flat and normal subduction of the Nazca plate 107 4.1 Abstract........................................... 107 4.2 Introduction......................................... 108 4.3 Data Analysis........................................ 111 4.3.1 Data......................................... 111 4.3.2 Seismicity...................................... 113 4.3.3 Slab Dip....................................... 117 4.3.4 Slab Transition................................... 119 4.3.5 Ultra-slow Velocity Layer............................. 122
ix 4.3.6 2D Velocity Modeling............................... 127 4.4 Discussion.......................................... 138 4.5 Conclusions......................................... 146 4.6 Supplemental Figures................................... 148 References............................................. 150 5 Further evidence for P-wave complexity in a region with slow slip events 158 5.1 Abstract........................................... 158 5.2 Introduction......................................... 159 5.3 Data and Method...................................... 161 5.4 Results............................................ 162 5.5 Discussion and Conclusions................................ 166 5.6 Future Work........................................ 168 References............................................. 169 6 Conclusions 173
x List of Figures 1.1 Overview map of study locations.............................. 2 2.1 Map of western central Mexico with event and station locations............ 11 2.2 1D P- and S-wave velocity models tested for western central Mexico.......... 14 2.3 Comparison of 1D modeling results for the five models tested in western central Mexico 15 2.4 1D modeling results for the ncm velocity model..................... 16 2.5 Profile across the MARS array............................... 17 2.6 Raypath schematic and examples of complex P waveforms in western central Mexico. 19 2.7 Maps of local S-to-P conversion points in western central Mexico............ 21 2.8 2D velocity model across the MARS array......................... 23 2.9 2D modeling results..................................... 24 2.10 Comparison of 2D modeling results for five different models............... 25 2.11 Seismicity and slab dip across the western USL edge................... 28 2.12 3D schematic of two-tear plate model........................... 30 2.13 Supplemental: 1D modeling results for the SoCal velocity model............ 37 2.14 Supplemental: 1D modeling results for the SoCalx velocity model........... 38 2.15 Supplemental: 1D modeling results for the Kim et al. (2010) velocity model...... 39 2.16 Supplemental: 1D modeling results for the Song et al. (2009) velocity model..... 40 3.1 Map of eastern central Mexico with event and station locations............. 52 3.2 Topographic-bathymetric map of eastern central Mexico................. 54 3.3 Lateral variation in slab dip across eastern central Mexico................ 59
xi 3.4 Raypath schematic and examples of complex P waveforms in eastern central Mexico. 62 3.5 Eastern lateral extent of ultra-slow velocity layer in central Mexico........... 63 3.6 Intraslab seismicity in eastern central Mexico....................... 65 3.7 Focal mechanisms for intraslab earthquakes in eastern central Mexico......... 66 3.8 Cross-sections of CAP focal mechanisms in eastern central Mexico........... 69 3.9 1D velocity models tested in eastern central Mexico................... 71 3.10 Comparison of 1D modeling results in eastern central Mexico.............. 72 3.11 1D modeling results for ncmc velocity model....................... 73 3.12 2D velocity model cross-section locations and results for eastern central Mexico.... 75 3.13 Example 2D velocity model for eastern central Mexico.................. 76 3.14 Comparison of 2D modeling results for example event.................. 77 3.15 3D schematic of slab rollback process and plate tearing through time.......... 84 3.16 Supplemental: Map of MASE, VEOX, SSN, and OXNET station locations and names 87 3.17 Supplemental: Enlarged map of the eastern lateral extent of the USL in central Mexico with event labels....................................... 88 3.18 Supplemental: Additional 1D velocity models tested in eastern central Mexico.... 89 3.19 Supplemental: 1D modeling results for ncmc velocity model without station overlap. 90 3.20 Supplemental: Additional example 2D velocity model tested for eastern central Mexico 91 3.21 Supplemental: Comparison of 2D modeling results for additional example event.... 92 3.22 Appendix: Summary map of USL extent in central Mexico............... 106 4.1 Map of southern Peru with event and station locations.................. 109 4.2 Topographic-bathymetric map of southern Peru..................... 110 4.3 Seismicity pattern in southern Peru............................ 114 4.4 Focal mechanisms for intermediate depth earthquakes in southern Peru........ 116 4.5 Lateral variation in slab dip across southern Peru.................... 118 4.6 Map of along-strike seismicity in southern Peru...................... 120 4.7 Along-strike variation in seismicity in southern Peru................... 121
xii 4.8 3D view of the flat-to-normal transition in the Nazca slab................ 123 4.9 Raypath schematic and examples of complex P waveforms in southern Peru...... 126 4.10 Lateral extent of ultra-slow velocity layer in southern Peru............... 128 4.11 S-to-P conversion point locations from the top of the Nazca slab............ 129 4.12 1D velocity models tested in southern Peru........................ 130 4.13 2D velocity model cross-section locations......................... 131 4.14 2D velocity models for event 26............................... 136 4.15 Comparison of 2D modeling results for event 26..................... 137 4.16 2D velocity models for event 20............................... 139 4.17 Comparison of 2D modeling results for event 20..................... 140 4.18 Comparison of slab strain in central Mexico and southern Peru............. 147 4.19 Supplemental: Cross-section of seismicity illustrating shallow cluster.......... 148 4.20 Supplemental: Complete set of trench-parallel seismicity cross-sections......... 149 5.1 Map of southwest Japan with event and station locations................ 160 5.2 Raypath schematic and examples of complex P waveforms in southwest Japan.... 163 5.3 Raypath schematic and examples of teleseismic underside reflections.......... 164 5.4 Map of teleseismic short-period array and broadband station locations......... 165 5.5 Map of local S-to-P conversion points and underside reflection bounce points..... 166
xiii List of Tables 2.1 Events used in western central Mexico and their source parameters........... 12 3.1 Events used in eastern central Mexico and their source parameters........... 56 4.1 Events used in southern Peru and their source parameters................ 112 4.2 Model parameters and results for each 2D profile in southern Peru............ 133