Site Classification for Seismic Hazard Assessment in Low Seismicity Regions
Total Page:16
File Type:pdf, Size:1020Kb
Site Classification for Seismic Hazard Assessment in Low Seismicity Regions A thesis submitted in fulfilment of the requirements of the Doctor of Philosophy (PhD) degree and the Diploma of Imperial College (DIC) By Sarah Tallett-Williams Department of Civil and Environmental Engineering Imperial College London London SW7 2AZ September 2017 Declaration The work presented in this thesis was carried out in the Department of Civil and Environmental Engineering at Imperial College London. This thesis is the result of my own work and any quotation from, or description of the work of others is acknowledged herein by reference to the sources, whether published or unpublished. This thesis is not the same as any that I have submitted for any degree, diploma or other qualification at any other university. No part of this thesis has been or is being concurrently submitted for any such degree, diploma or other qualification. The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. Sarah Tallett-Williams London, September 2017 1 Abstract Probabilistic Seismic Hazard Assessment of low seismicity regions has significant uncertainty due to the limited catalogue of earthquake records. Part of this uncertainty relates to the modification of ground motions by near-surface geo-materials: site amplification. If the repeatable site amplification from strong ground motion stations (SGMS) is characterised, this uncertainty is reduced. Consequently, the uncertainty demands upon safety critical infrastructure can decrease, enhancing its resilience. However, the characterisation of site amplification effects is complex. Effective parameters can be prohibitively expensive to measure, particularly if full shear-wave velocity (Vs) profiles are required for multiple sites. This thesis aims to investigate an economical method to characterise near-surface site amplification. For 15 of the UK SGMS, geological models are developed and compared to the newly established Global Vs Database. The database is used to examine relationship of Vs with geological parameters. Relationships investigated include age and origin, but were only predominant in depth and grain size. A new Site Specific Proxy probabilistically correlates these characteristics with the geological profiles, making it possible to infer a probability distribution of a time averaged shear wave velocity (Vs30) for each site. Horizontal to Vertical Spectral (HVSR) ratio testing is performed to verify these station models. This in situ method, though indeterminate, is economical and proved accurate when examined against verification testing. The HVSR testing results of the UK SGMS correlate well with the Site Specific proxy estimates, having a mean difference within 18%, which is a significant decrease from other proxy methods. A method of Bayesian updating is developed for refining the Vs30 profiles and shows that one in situ test is generally sufficient to halve the standard deviation of the Site Specific proxy method. Thus, the economical combination of the Site Specific proxy and HVSR testing is recommended for site characterisation in low seismicity regions. 2 Acknowledgements Firstly, I would like to thank my supervisors Clark Fenton and Peter Stafford for their time and generous advice during my research. I am very grateful to Clark for his continuous support despite altered circumstances. His encouragement and expertise have been invaluable to me. I wish him every success in Christchurch. I am also thankful to Peter for taking over the project. His kind patience and immense skill have made a significant contribution to the research. Natalie Campbell made a large contribution to this work through her MSc thesis. She was a challenge to follow as her thesis is of such high quality. The contribution of industry to this project is overwhelming, particularly to the fieldwork campaign. I would like to thank Mike Raines, BGS, for the large amount of time and great care he took teaching me the HVSR method. The HVSR work carried out rests on the foundation of his significant knowledge and I wish him all the best with his much deserved retirement. In addition, I am grateful for contributions of Adam Booth and Roger Clark, Leeds University, which aided the validation of the method. They were brilliant to work with even in the snow and they had much good advice. Special thanks to Darren Ward, In Situ Site Investigation, for donating his rigs for SCPT testing during the research. Without his involvement, no invasive testing would have been possible and I really appreciate his team’s time and excellent work. Anthony Butcher, University of Bristol, aid with MASW testing was also vital to the validation effort. None of this fieldwork would have been possible without the kind funding of the Dixon Scholarship, the Geological Society, the Worshipful Society of Pavior’s Laing Travel Award, Santander Travel Scholarship and the Imperial College Dean’s Fund. In addition, the funding for the EEFIT mission to Nepal came from ESPRC. I learnt so much from the incredible team in the short time we had there. I am also extremely grateful to Brian Baptie, John Laughlin, Stephen Tredwin and Christopher Turbitt for granting access permissions to the UK Strong Ground Motion Stations for testing as well as Brian’s examination of this thesis. In addition, the testing would not have been possible without the permission of the landowners of the UK Strong Ground Motion Stations, including Jersey Water who were so welcoming and allowed me free range with their design data, which was invaluable. Special thanks to Sarah and John Grey and Anthony Martin who endured repeated visits as well as to Rod Bridle for setting up the Jersey visit, Horizon Nuclear Power to allow testing and the Science Museum for access to Wroughton Airfield. Thanks also to Silvia Castellaro and Jeremy Magnon, who enabled the use of the Tromino and their 3 kind donation to the EFFIT mission. Further industry contributions include confidential data and access to sites for which I am indebted, but am unable to mention by name. The initial idea for this project came from Matthew Free, ARUP and I appreciate the time he took with Ziggy Lubrowski, to shaping the output of my work to be useful to industry. The team from ch2m contributed similarly and were very kind and encouraging throughout my research. I am would like to thank Ben Edwards and Andreas Reitbrock, University of Liverpool, for helping me with their model and their enthusiasm in my work. I owe much gratitude to Jamie Pringle and Nigel Cassidy and their introductory geophysics field course organised by NERC. I would also like to thank Alan Yong, Chris Wills, Trevor Allen and Rory Mortimore for their kind advice and helpful discussion on their subjects of expertise. The Imperial Staff has been a great support and kind allies during my time here. I would particularly like to thank Catherine O’Sullivan, Finnoula Ni Dhonnabhain, Marios Christou and Jamie Standing for their generosity towards me as well as Stavroula Kontoe, who has supported me since my undergraduate with her useful comments and kind examination. I need to take this opportunity to thank the wonderful PhDs including the seismic group, Vasia Tsaparli, Bo Han and Evita Skiada for their kind help and technical support with the research, Danny Martinez Calonge for taking the very early trip with me to Jersey, Masahide Ostubo and Giulia Ghiadistri for their warm support and Japanese/Italian translations as well as Roisin Buckley, Klementyna Gawecka, Tingfa Liu, Emil Ushev, Chris Knight, Howard Taylor, Tom Shire, Freya Summersgill and Aine Ni Bhreasail for all the moral support and drinks we had together. I have also been extremely lucky in the support of my sister, Katie and my best friends Ros, Maria and Jenny, who have made this possible. My fiancé Jean Bénard has shown incredible levels of commitment to me throughout this process. I would like to thank him for his patience and the unconditional love he has shown. Finally, and most importantly, I would like to thank my parents, Michael and Pauline Tallett-Williams. This project is a testament to their dedication and generosity. They are the principal funders of this research, but furthermore they have been my fieldwork crew, proof readers, advisors and my support system. This work would not have happened without them and their contribution to geotechnical research should be acknowledged. I am beyond grateful to them both. 4 Table of contents Declaration…………………………………………………………………..………………………...1 Abstract……………………………………………………………………..…………………….…...2 Acknowledgements……………………………………………………..……………………………..3 Table of contents……………………………………………………..…………………....…………..5 List of tables and figures……………………………………….……………………………….……11 List of symbols and abbreviations....……………………….………………………………………..22 Chapter 1: Introduction.………………………………........…………...24 1.1 Importance of site amplification..……………………….……………………………………….24 1.2 Site characterisation in low seismicity regions……………………………….………………….27 1.3 The UK strong ground motion network………………………………..………...……………....28 1.4 Aims and objectives.……………………………….……………………………………….........29