Geodetic Measuremenof Iorizontal Crustal Deformation
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GEODETIC MEASUREMENOF IORIZONTAL CRUSTAL DEFORMATION ASSOCIATED WITH THE OCTOBER 15, 1979, IMPERIAL VALLEY (CALIFORNIA) EARTHQUAKE a thesis submitted by Christopher Neil CROOK for the degree of DOCTOR OF PHILOSOPHY of The University of London Geophysics section, Department of Geology, Imperial College of Science and Technology, London. July 1984 ABSTRACT Since 1971 the Geophysics section of Imperial College has surveyed a trilateration network in the Imperial Valley, southern California, ten times in order to measure horizontal ground deformation associated with the Imperial fault. These surveys provide a uniquely detailed record of the deformation occurring before, during, and after the magnitude 6.6 October 15, 1979, Imperial Valley earthquake. The trilateration network now comprises over 300 stations on an 800 in spacing square grid of agricultural roads, and extends to more than 20 km from the Imperial fault. The main instrument used in the surveys is the mekometer, a high-precision short-range electro-optical distance measurement equipment. A detailed analysis of the survey data indicates that the mekometer introduces systematic errors of three types; a long term drift, a temperature dependent error, and daily drift. The long term drift is explained by water vapour entering the microwave cavity which generates the reference frequency fundamental to the distance measurement, and the temperature dependent error may arise from thermal expansion of the cavity. No satisfactory cause is found for the daily variation, though it apparently arises in the mekometer rather than the environment. After appropriate corrections have been applied the standard error of measurements in the most recent (and most accurate) survey is less than 1 ppm. There is no clear precursor to the 1979 earthquake in the survey data, although shear strain accumulation was measured centred on the Imperial fault, which was slipping about 6 mm/yr in episodic creep events. The rate of strain accumulation may have been increasing. The earthquake caused 0.6 in right lateral displacement on the Imperial fault within the network. Right lateral displacement of points further from the fault were greater, with a maximum of 1.3 in between points 3 km either side of the fault. A zone of intense deformations was observed reaching to 6 km from the fault with strain changes up to 200 ppm. Unexpectedly large displacements perpendicular to the fault were measured in this zone. These movements are not consistent with simple models of displacements and may result from an inelastic response of weak surface materials to the shaking during the earthquake. Following the earthquake the fault continued to slip at a rate inversely proportional to the elapsed time after the earthquake. This slip is modelled as the delayed propagation of co-earthquake slip from depths of about 4 km. ACKNMLEDG4ENTS I joined the Geophysics section of Imperial College in 1977, and commenced my research course in 1978. At this time the Imperial Valley project had been established for about 8 years. It was my very good fortune to become involved in the work just before the earthquake in 1979, at a time when the labours of the previous workers who had built and surveyed the Imperial Valley network were just beginning to bear fruit. My thanks go to Professor Ron Mason, whose inspiration the project was and who has supervised my research, and to the numerous people who have contributed to the work in the past. The surveying work in the Imperial Valley requires a minimum of two people, as a consequence of which I have had the pleasure of working with Torn Crompton, Graham Pearce, George Gebski, Tim Sanderson, Peter and Judy Wood, Caroline Wood, Jonathon Madden, and Andy Pullen. My particular thanks go to Torn Crompton who taught me the necessary principles and practices of surveying, and to Peter Wood, with whom I shared the excitement of the survey immediately after the earthquake. Hans Jensen and Bernard Bell helped Andy Pullen in surveys after spring 1982, the results of which I have used. The people in the Imperial Valley have always been very helpful to us, and I would thank especially the Wiyninger family, who undoubtedly make the best iced tea in El Centro. I am grateful to Shawn Biehler and the Earth Sciences department of the Riverside Campus of the University of California for storing equipment between surveys, to John Cheney and the staff of the Building Research Station, Garston, for the use of their baseline, and to Messrs J. Hurnphreys and K. Hawkins of ComRad Electronic Equipment, Slough, for making the inekometer available on often very short notice, and for repairing it promptly on numerous occasions. I was initially supported in this work by a studentship from the Natural Environment Research Council, and for six months was employed as a research assistant under a contract from the United States Geological Survey. The survey work has been funded by the USGS, generally from the Earthquake Hazards Reduction Fund. I am particularly grateful to my wife, Val, without whose support and encouragement the preparation of this thesis would not have been possible. CONTENTS Page ABSTRACT ........................................................... 2 AKNOWLEDGEMENTS .................................................... 3 CONTENTS ........................................................... 4 LISTOF FIGURES .................................................... 7 LISTOF TABLES ..................................................... 11 CHAPTER ONE: Introduction. 1.1. The role of surveying in earthquake prediction research .. 12 1.2. The Imperial Valley - a natural earthquake laboratory .... 15 CHAPTER TWO: The Imperial Valley mekometer network. 2.1. The history of the network 22 2.2.Thestations 24 2.3. Station names 26 2.4. Locating stations 29 2.5. Stability of the stations 30 CHAPTER THREE: The mekometer. 3.1. Introduction 35 3.2. Operating principle 35 3.3. Theoretical corrections to mekometer distance measurement 40 3.4. Frequency calibration of the mekometer 45 3.5. Measurement of cavity air temperature and pressure 50 3.6. The range comparison test 51 3.7. The system constant calibration 52 Page CHAPTER FOUR: Survey procedures. 4.1. The typical pattern of a survey 55 4.2. The distance measurement procedure 56 4.3. Centring equipment over stations 57 4.4. Meteorological measurements 60 4.5. Special distance measurement procedures 63 4.6. Angle measurements 68 4.7. Field correction and checking of the data 71 CHAPTER FIVE: An analysis of systematic errors in rnekometer data. 5.1. Random survey errors 72 5.2. Method of analysis of systematic errors 75 5.3. The model of systematic errors 78 5.4. Statistical tests of the model 82 5.5. Discussion of the results 84 5.6. Frequency drift and water vapour in the mekometer reference cavity 88 5.7. Temperature dependent systematic errors 90 5.8. Pressure dependent errors 93 5.9. Time dependent errors 93 5.10.Absolute scale errors in the surveys 100 5.11.Conclusions and recommendations 101 CHAPTER SIX: Analysis and comparison of survey data. 6.1. Introduction ............................................. 105 6.2. Station elevations ....................................... 106 6.3. Calculation of station coordinates ....................... 107 6.4. Displacement vectors 112 6.5. Calculation of strain parameters of ground deformation 114 6.6. Estimation of fault displacement 118 'I Page CHAPTER SEVEN: Ground deformation associated with the Imperial fault. 7.1 Reintroduction to the Imperial fault .....................119 7.2 Fault displacements before the 1979 earthquake ...........125 7.3 Pre-earthquake strain accumulation ........................134 7.4 Predominantly co-earthquake deformation ..................146 7.5 Co- and post- earthquake fault displacements .............166 7.6 Post-earthquake strain changes ...........................177 7.7 Resumé ...................................................185 CHAPTER 8: Modelling and discussion. 8.1 Dislocation modelling ....................................188 8.2 Conclusions and reccommmendations ........................197 APPENDIX A: Least squares approximation and the solution of large systems of normal equations. A.1. Least squares theory .....................................202 A.2. The solution of large sparse systems of Normal equations .. 204 APPENDIX B: Surmnary of the 1978 to 1983 surveys. B.1. The 1978 survey ..........................................208 B.2. The 1979A survey .........................................210 B.3. The 1979B survey .........................................211 B. 4. The 1980 survey ..........................................212 B.5. The 1982A survey .........................................213 B.6. The 1982B survey .........................................214 B.7. The 1983 survey ..........................................215 APPENDIXC: Corrections to the data ................................227 APPENDIX D: The data from the 1971 to 1983 surveys (microfiche) REFERENCES.........................................................235 LIST OF FIGURES Page Fig. 1.1. Location of the Imperial Valley ..........................16 Fig. 1.2. The boundary of the N.Ainerican and Pacific plates ........17 Fig. 1.3. The faults in the Imperial Valley ........................19 Fig. 1.4. Model of the plate boundary as a series of 'leaky transform faults' ...................................21 Fig. 2.1. The development of the Imperial Valley mekometer network .. 23 Fig. 2.2. The design of the stations