Seismic Imaging of the Hayward Fault Near the California
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SEISMIC IMAGING OF THE HAYWARD FAULT NEAR THE CALIFORNIA STATE UNIVERSITY AT EAST BAY HAYWARD CAMPUS ________________________ A University Thesis Presented to the Faculty of California State University, East Bay ________________________ In Partial Fulfillment of the Requirements for the Degree Master of Science in Geology ________________________ By Collin Quesenberry May 2019 Abstract The Hayward Fault (HF) is an active, right-lateral, strike-slip fault that strikes ~325° through the eastern San Francisco Bay Area (East Bay) from Milpitas to Point Pinole, California. The more broadly defined Hayward Fault Zone (HFZ) includes the HF and lesser subparallel faults, which structurally separate major lithologies in the East Bay. The dominant lithologies west of the HFZ are metamorphosed sedimentary and igneous rocks, whereas clastic sedimentary and igneous rocks dominate areas to the east. The core of the HFZ is a 1- to 3-km-wide terrane, dominated by ophiolite rocks and oriented along the eastern side of the HF, which I refer to as the San Leandro Block (SLB). The western margin of the SLB, specifically the active trace of the HF, is the focus of my research. I collaborate with the California State University at East Bay (CSUEB) and the United States Geological Survey (USGS) Earthquake Science Center at Menlo Park, California. Our work advances the goal of using seismic velocities to interpret a three-dimensional structural model of the HFZ. This study continues efforts by the CSUEB and USGS to evaluate the near surface structure of the SLB on and around the CSUEB Hayward campus. We use seismic refraction methods, which are especially adept at identifying the high-angle fault zones and complexly deformed geologic materials that characterize the SLB. I analyze P-wave and S-wave velocity data (VP and VS) from two nearly coincident seismic refraction surveys that we conducted across the active trace of the HF, in 2013 and 2017 respectively. The 2013 dataset consists of low-resolution VP and VS data from ten 4.5-Hz three-component seismographs (vertical, horizontal-transverse, and ii horizontal-longitudinal), spaced at ~30-m-intervals perpendicular to the HF. The 2017 dataset consists of high-resolution VP data from sixty-three 4.5-Hz vertical-component geophones along a seismic profile that was similar to the 2013 seismic profile. I use seismic refraction analyses to produce cross-sectional models of VP, VS, VP/VS ratios, and Poisson’s ratios for the 2013 data and the 2017 data. I correlate these results with the known seismic characteristics of geologic materials, and previous geologic mapping, to interpret the geometry of geologic materials and fault segments in the near surface. Refraction tomography models show that VP/VS ratios > 2.0 and Poisson’s ratios > 0.3 generally coincide with VP > 2.0 km/s and VS > 0.7 km/s, which we expect as the seismic expression of bedrock. These relationships, when compared to geologic mapping, indicate that the HF separates highly weathered gabbro and/or basalt to the east from alluvium to the west. Based on the velocity models and geologic mapping, the active HF likely dips ~75° toward the northeast at the study area. At least two splay faults, one ~50-m-west of the HF and another ~150-m-east of the HF, further separate the lithologies at the study area. The eastern splay is likely a west- dipping fault that places basalt structurally above gabbro, whereas the western splay is likely an east-dipping fault that places basalt structurally above alluvial sediments. There is no mapped evidence for the western splay; however, a study by Catchings et al. (2006) has shown that there are many buried faults within the sediments west of the HF. A buried fault ~50-m-west of the active trace of the HF would increase the seismic hazard for the businesses and residences that are adjacent to the western end of the study area. iii SEISMIC IMAGING OF THE HAYWARD FAULT NEAR THE CALIFORNIA STATE UNIVERSITY AT EAST BAY HAYWARD CAMPUS By Collin Quesenberry iv Acknowledgements Thank-you, Dr. Luther Strayer and Dr. Jean Moran, for the guidance you gave toward focusing my research and academic interests during my time in the CSUEB Earth and Environmental Sciences Graduate Program. Thank-you, Dr. Rufus Catchings, Ms. Joanne Chan, and Mr. Mark Goldman, for the opportunities you gave to me as mentors at the USGS Earthquake Science Center. Thank-you, my fellow graduate students Adrian McEvilly and Ian Richardson, for the close assistance you gave as I conducted my research and completed this thesis. I want to extend my thanks to all the volunteers who assisted me in bringing my research to fruition. v Table of Contents Abstract ……………………………………………………………………… ii Signatures ……………………………………………………………………… iv Acknowledgements ……………………………………………………………… v Lists of Figures and Appendices ……………………………………………… vii Introduction ……………………………………………………………………… 1 Tectonic Setting ……………………………………………………… 1 Tectonic Framework ……………………………………………………… 6 San Leandro Block ……………………………………………………… 9 Background and Purpose ……………………………………………… 12 Study Area ……………………………………………………………… 13 Seismic Methods ……………………………………………………………… 18 Seismic Waves ……………………………………………………… 18 Seismic Surveying Techniques ……………………………………… 20 Seismic Data Processing ……………………………………………… 24 Data Acquisition ……………………………………………………………… 26 Seismic Surveys ……………………………………………………… 26 Data Overview ……………………………………………………… 36 Data Analyses ……………………………………………………………… 37 Seismic Refraction Analysis …………………………………………… 37 Multi-channel Analysis of Surface Waves ……………………………… 41 VP/VS ratios and Poisson’s ratios ……………………………………… 44 Results ……………………………………………………………………… 48 VP models ……………………………………………………………… 48 VS and MASW models ……………………………………………… 50 VP/VS ratio and Poisson’s ratio models ……………………………… 52 Interpretations ……………………………………………………… 56 Discussion ……………………………………………………………………… 65 Conclusion ……………………………………………………………………… 71 References ……………………………………………………………………… 74 Appendices ……………………………………………………………………… 85 vi List of Figures Figure 1: Important faults of the central San Francisco Bay Area ……………… 2 Figure 2: The Hayward Fault Zone ……………………………………………… 3 Figure 3: Geologic overview of the central East Bay Area ……………………… 8 Figure 4: The San Leandro Block ……………………………………………… 11 Figure 5: Location of the Carlos Bee study area ……………………………… 14 Figure 6: Three versions of fault positions across the study area ……………… 16 Figure 7: Geology of the study area based on Graymer (2000) ……………… 17 Figure 8: Geology of the study area based on Jennings et al (2010) …………… 18 Figure 9: Snell’s Law for seismic waves ……………………………………… 20 Figure 10: Seismic wave propagation for a refraction survey ……………… 21 Figure 11: Cross-sectional diagram of a geophone ……………………………… 22 Figure 12: Example shot gather ……………………………………………… 23 Figure 13: Diagram of shot gather trends ……………………………… 23 Figure 14: Seismic profiles for 2013 and 2017 ……………………………… 27 Figure 15: Operation of a hand auger ……………………………………… 29 Figure 16: Preparation of a real-time kinematic GPS device ……………… 30 Figure 17: Example geophone station in the seismic profile ……………… 31 Figure 18: Geode seismograph station ……………………………………… 32 Figure 19: View of the field headquarters ……………………………………… 33 Figure 20: Operational view of the survey monitoring software ……………… 34 Figure 21: Betsy Seisgun™ emplacements ……………………………………… 35 vii Figure 22: Preparation of an explosive charge emplacement ……………… 35 Figure 23: Example shot gather for an explosive charge source ……………… 36 Figure 24: Example shot gather data for a Betsy Seisgun™ shot ……………… 37 Figure 25: Example first arrival trend for a Betsy Seisgun™ shot ……………… 38 Figure 26: Travel path coverage model for 2017 ……………………………… 39 Figure 27: P-wave model for 2017 (P17) ……………………………………… 40 Figure 28: P-wave model for 2013 (P13) ……………………………………… 41 Figure 29: Example MASW processing steps ……………………………… 42 Figure 30: MASW model for 2017 (M17) ……………………………………… 43 Figure 31: S-wave models for 2013 ……………………………………… 44 Figure 32: VP/VS ratio and Poisson’s ratio models for 2017 ……………… 45 Figure 33: VP/VS ratio models for 2013 ……………………………………… 46 Figure 34: Poisson’s ratio models for 2013 ……………………………………… 47 Figure 35: VP structures of P17 ……………………………………………… 49 Figure 36: VP structures of P13 ……………………………………………… 49 Figure 37: VS structures of 2013 VS models ……………………………… 51 Figure 38: VS structures of M17 ……………………………………………… 52 Figure 39: VP/VS ratio and Poisson’s ratio anomalies for 2017 ……………… 53 Figure 40: VP/VS ratio anomalies for 2013 ……………………………………… 54 Figure 41: Poisson’s ratio anomalies for 2013 ……………………………… 55 Figure 42: Interpretations of P17 and P13 ……………………………………… 58 Figure 43: Interpretations of M17 ……………………………………………… 60 viii Figure 44: Interpretations of 2013 VS models ……………………………… 61 Figure 45: Interpretations of 2017 VP/VS ratio and Poisson’s ratio models …… 63 Figure 46: Interpretations of 2013 VP/VS ratio models ……………………… 64 Figure 47: Interpretations of 2013 Poisson’s ratio models ……………………… 65 Figure 48: Simplified geologic cross-section ……………………………… 71 ix List of Appendices Appendix 1: Glossary of acronyms ……………………………………… 85 Appendix 2: Equipment used for the 2017 survey ……………………………… 86 Appendix 3: Geophone stations for the 2017 survey ……………………… 87 x 1 Introduction Tectonic Setting The Hayward Fault (HF) is an active, right-lateral, strike-slip fault that strikes ~325° through the eastern San Francisco Bay Area (East Bay) (Figure 1). The HF extends along the western margin of the East Bay Hills, from near Milpitas in the south to San Pablo Bay near Point