A SEISMIC HAZARD ASSESSMENT AND MICROZONATION OF BUNDABERG By Michael Lloyd Turnbull BAppSc(Distinctions) GAIP JP(Com.Dec.) A thesis submitted in fulfilment of the requirements for the Degree of Master of Applied Science (Seismology) Central Queensland University Faculty of Engineering and Physical Systems Copyright © M.L.Turnbull, September 2000, April 2001, all rights reserved. Abstract ABSTRACT A Seismic Hazard Assessment and Microzonation of Bundaberg by Michael Lloyd Turnbull BAppSc(Distinctions) GAIP JP(Com.Dec.) This thesis investigates the statistical seismic hazard that exists within the Bundaberg area and derives microzonation information for Bundaberg City, suitable for conjoint use with AS1170.4 – 1993, and its future replacements, in determining Earthquake Loading for design and construction engineering. A brief history of significant seismic events that have occurred in the Bundaberg area is provided, including presentation of an isoseismal map for the 1997 Bundaberg earthquake, and the regional geography is outlined. The effect of ground motion amplification and how it increases the seismic risk at sites within the Bundaberg City area is examined. The use of spectral ratios of ambient seismic noise, calculated from seismograms of microtremors, in characterising local site response to ground motion is discussed in detail. Specifically, horizontal to vertical spectral ratios (HVSR), otherwise referred to as Nakamura Spectra, are used to determine the local site responses of engineering interest (microzonation), for a 1 km grid of the Bundaberg City area. A methodology and associated computer software is developed to calculate Nakamura Spectra and to carry out the microzonation analysis. The results are presented in map form, suitable for viewing on a Geographical Information System (GIS). The Nakamura Spectra are also used to estimate the known depths of sedimentary deposits in the Bundaberg area. The estimated depths show a positive correlation with known depths thus verifying the applicability of the Nakamura Spectra for the purposes of microzonation. ii Dedication DEDICATION I was the first member of my family to gain a University education. My parents could not afford to educate me past year 10 but they both instilled in me an ethic for work, a thirst for knowledge and a drive for achievement that money cannot buy and rote learning cannot accomplish. This work is dedicated to my parents: Robert James Turnbull, and Daphne Jane Elizabeth Turnbull (Ne Griffith). ii i Table of Contents TABLE OF CONTENTS Abstract .................................................................................................................................ii Dedication............................................................................................................................iii Table of Contents...............................................................................................................iv List of figures ....................................................................................................................viii List of tables ........................................................................................................................xi Acknowledgments.............................................................................................................xii Declaration .........................................................................................................................xv Chapter 1 Introduction..................................................................................................1 Bundaberg’s Seismicity in the World perspective. .................................................................................. 1 The Australian Earthquake Building Codes............................................................................................. 3 Overall research goal................................................................................................................................ 4 The importance of relative risk assessment.............................................................................................. 5 Regional perspective ................................................................................................................................ 6 Theoretical perspective ............................................................................................................................ 7 Summary of research achievements presented in this Thesis................................................................... 7 Chapter 2 Geology of the Bundaberg Area...............................................................9 Introduction.............................................................................................................................................. 9 Onshore Structural Elements and Solid Geology..................................................................................... 9 Geological History ................................................................................................................................. 14 Recent Volcanism .................................................................................................................................. 17 Summary ................................................................................................................................................ 19 Chapter 3 Seismic History of the Bundaberg Region............................................20 Introduction............................................................................................................................................ 20 Seismic Activity in Southeast Queensland............................................................................................. 20 iv Table of Contents Significant Seismic Events..................................................................................................................... 22 Summary ................................................................................................................................................ 27 Chapter 4 An Isoseismal map of the Bundaberg 1997 Earthquake ...................28 Introduction............................................................................................................................................ 28 Putting the Bundaberg 1997 earthquake into perspective. ..................................................................... 28 The felt effect reports ............................................................................................................................. 29 Analysis of felt reports ........................................................................................................................... 30 Isoseismal map ....................................................................................................................................... 31 Implications for Bundaberg.................................................................................................................... 31 Chapter 5 Earthquake Hazard in the Bundaberg Area, Estimated Using Gumbel Type I Statistics..........................................................34 Introduction............................................................................................................................................ 34 The Gumbel Type I Extreme Event Probability Distribution................................................................. 35 Estimation of and ............................................................................................................................. 36 Extrapolation of Results......................................................................................................................... 38 Estimating Seismic Hazard .................................................................................................................... 40 The average recurrence period ............................................................................................................... 40 Confidence of recurrence period ............................................................................................................ 41 Probabilities of Multiple Events............................................................................................................. 44 Earthquake hazard.................................................................................................................................. 44 Summary and Conclusions..................................................................................................................... 45 Chapter 6 Characterisation of ground motion amplification...............................47 Introduction............................................................................................................................................ 47 What is Ground Motion Amplification................................................................................................... 47 Horizontal and vertical ground motion .................................................................................................. 48 Rayleigh wave particle orbits................................................................................................................. 49 Amplification spectra ............................................................................................................................. 51 Other causes