California North Coast Offshore Wind Studies Overview of Geological Hazards This report was prepared by Mark A. Hemphill-Haley, Eileen Hemphill-Haley, and Wyeth Wunderlich of the Humboldt State University Department of Geology. It is part of the California North Coast Offshore Wind Studies collection, edited by Mark Severy, Zachary Alva, Gregory Chapman, Maia Cheli, Tanya Garcia, Christina Ortega, Nicole Salas, Amin Younes, James Zoellick, & Arne Jacobson, and published by the Schatz Energy Research Center in September 2020. The series is available online at schatzcenter.org/wind/ Schatz Energy Research Center Humboldt State University Arcata, CA 95521 | (707) 826-4345 California North Coast Offshore Wind Studies Disclaimer This study was prepared under contract with Humboldt State University Sponsored Programs Foundation with financial support from the Department of Defense, Office of Economic Adjustment. The content reflects the views of the Humboldt State University Sponsored Programs Foundation and does not necessarily reflect the views of the Department of Defense, Office of Economic Adjustment. This report was created under Grant Agreement Number: OPR19100 About the Schatz Energy Research Center The Schatz Energy Research Center at Humboldt State University advances clean and renewable energy. Our projects aim to reduce climate change and pollution while increasing energy access and resilience. Our work is collaborative and multidisciplinary, and we are grateful to the many partners who together make our efforts possible. Learn more about our work at schatzcenter.org Rights and Permissions The material in this work is subject to copyright. Please cite as follows: Hemphill-Haley, M.A., Hemphill-Haley, E. and Wunderlich, W. (2020). Overview of Geological Hazards. In M. Severy, Z. Alva, G. Chapman, M. Cheli, T. Garcia, C. Ortega, N. Salas, A. Younes, J. Zoellick, & A. Jacobson (Eds.) California North Coast Offshore Wind Studies. Humboldt, CA: Schatz Energy Research Center. schatzcenter.org/pubs/2020-OSW-R16.pdf. All images remain the sole property of their source and may not be used for any purpose without written permission from that source. Overview of Geological Hazards ii California North Coast Offshore Wind Studies CONTENTS 1 EXECUTIVE SUMMARY................................................................................................................... 1 2 INTRODUCTION ................................................................................................................................ 7 3 STRONG MOTION .............................................................................................................................. 7 3.1 Potential Sources of Seismicity and Strong Shaking ................................................................. 10 3.1.1 Cascadia subduction zone (CSZ) .......................................................................................... 10 3.1.2 San Andreas fault (SAF) ....................................................................................................... 15 3.1.3 Mendocino Fault (MF) .......................................................................................................... 20 3.1.4 Gorda plate ............................................................................................................................ 21 3.1.5 Faults in the Fold and Thrust Belt of the Accretionary Wedge ............................................. 23 3.1.5.1 Table Bluff Fault and Anticline .................................................................................... 27 3.1.5.2 Little Salmon Fault ....................................................................................................... 29 3.2 North Coast Earthquakes >M6 Since 1960 ............................................................................... 32 3.2.1 1980 M7.2 Earthquake .......................................................................................................... 36 3.2.2 1992 M7.2 Earthquake .......................................................................................................... 38 3.2.3 2010 M6.5 Earthquake .......................................................................................................... 41 4 SURFACE RUPTURE ....................................................................................................................... 43 5 GAS HYDRATES AND SHALLOW-SEAFLOOR FREE GAS....................................................... 52 5.1 Gas Hydrate Occurrence ............................................................................................................ 53 5.2 Gas Hydrates Offshore of Northern California ......................................................................... 56 6 LIQUEFACTION ............................................................................................................................... 59 6.1 Overview of Liquefaction Processes and Related Ground Failure ............................................ 60 6.2 Liquefaction in the Marine Environment .................................................................................. 62 6.3 Documented Liquefaction from North Coast Earthquakes........................................................ 63 7 SUBMARINE LANDSLIDES ........................................................................................................... 69 7.1 Overview of Submarine Landslides........................................................................................... 69 7.2 Slumps and Slides ...................................................................................................................... 69 7.3 Unstable Sediment Masses ........................................................................................................ 71 7.4 Turbidity currents ...................................................................................................................... 74 8 TSUNAMIS ........................................................................................................................................ 76 8.1 Overview of Tsunami Hazards .................................................................................................. 76 8.2 Tsunami Record for the North Coast at Humboldt Bay ............................................................ 80 9 COSEISMIC LAND-LEVEL CHANGES ......................................................................................... 81 9.1 Coseismic Subsidence ............................................................................................................... 81 9.2 Coseismic Uplift ........................................................................................................................ 85 9.3 Interseismic Subsidence at Humboldt Bay ................................................................................ 86 10 RECOMMENDATIONS FOR FUTURE WORK.......................................................................... 87 11 REFERENCES ............................................................................................................................... 90 Overview of Geological Hazards iii California North Coast Offshore Wind Studies FIGURES Figure 1. Map showing the Northern California Offshore Wind (NCOW) study area, including the location of proposed offshore wind farms (Humboldt Call Area) and proposed locations of associated infrastructure (“NCOW facilities”). .............................................................................................................. 2 Figure 2. Map of the intersection of the Gorda, Pacific, and North America plates at the Mendocino triple junction. ........................................................................................................................................................ 8 Figure 3. Map showing the extent of the Cascadia subduction zone off northwestern North America. ..... 11 Figure 4. USGS base map of the San Andreas fault in California. ............................................................. 16 Figure 5. Map showing the 1906 rupture length of the San Andreas fault and area of impact from the earthquake. .................................................................................................................................................. 17 Figure 6. Modified Mercalli Intensity shake map of northern California for the 1906 San Andreas fault earthquake. .................................................................................................................................................. 18 Figure 7. Probabilistic seismic hazard model showing a 10% probability of peak ground accelerations exceeded 0.4-0.8 g in coastal Northern California over the next 50 years. ................................................ 20 Figure 8. Map of the Gorda plate ("Gorda deformation zone") by Rollins and Stein, 2010.. ..................... 22 Figure 9. Cascadia subduction zone accretionary prism relative to the location of the Humboldt Call Area.. ........................................................................................................................................................... 25 Figure 10. Uninterpreted and interpreted seismic reflection profile constructed in NE to SW azimuth roughly parallel to the coastline. ................................................................................................................. 26 Figure 11. Multi-channel seismic profile constructed in a NE-SW azimuth that transects the