• Abstract and Figures
• Public Full-text

INVESTIGATION OF THE I960 CHILEAN TSUNAMI ON THE PACIFIC COAST OF CANADA by RONALD HAROLD LOUCKS B. Sc., University of British Columbia, 1961 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE RETIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of PHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1962 Txi presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely- available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of PrWVSKS The University of British Columbia, Vancouver 8, Canada. Date flC60Sr 17, IKJZ i ABSTRACT A set of closely-spaced tide-gauge records of the I960 Chilean tsunami w^<3i obtained on the Pacific Coast of Canada. The object of this study was to glean as much as possible of the information contained in these records. The investigation was carried on by power spectrum analysis, cross- spectrum analysis and visual inspection of the tide-gauge records. An interpretation is offered which invokes the mechanisms of wave build• up due to shoaling, clapotis effect, resonance, viscous dissipation, and shift of energy between wave and current by Reynolds stresses to explain the form of the power spectra. In the appendices are given a formula for the response characteristics of stilling wells, an application of an electrical engineering result for the response characteristics of pressure gauges, an elucidation of the conver• sion of power estimates to the positive frequency range, an interpretation of the phase difference from cross-spectra, and a formula for prewhitening the covariances before performing the Fourier transform in the spectral an• alysis. 20 ACKNOWLEDGEMENTS The author wishes to express his appreciation to Drs. R. W. Burling and G. L. Pickard of the Institute of Oceanography of the University of British Columbia for their valuable advice, to the Canadian Hydrographic Service, Vic• toria, British Columbia, for their most helpful cooperation, to the Computing Center of the University of British Columbia for assistance in carrying out the digital computations, and to Mr* V. K. Jain for the stimuli from our many conversations. TABLE OF CONTENTS ABSTRACT LIST OF ILLUSTRATIONS JaNTRODUCTION PROCEDURE RESULTS DISCUSSION A. Arrival Times B. Wave Amplitudes C. Power Spectra D. Those Records Not Studied in Detail E. Cross-Spectra CONCLUSIONS APPENDICES A. Response Characteristics of a Stil• ling Well B. Response Characteristics of a Fox- boro Diaphragm-Box Level Recording System. C. Measurement of Spectra 1. Power Spectra 2. Cross-Spectra D. Derivation of a Formula for Prewhit- ening Covariances ACKNOWLEDGEMENTS BIBLIOGRAPHY CITED iii LIST OF ILLUSTRATIONS Figure 1: Map of the Pacific Coast of Canada with tide-gauge positions circ• led. Figure 2: Some tide-gauge records of the tsunami. Figure 3: More tide-gauge records of the tsunami. Figure 4: The power spectra. Figure 5: Coherence and phase difference between the Tofino and Barkley Sound records. Figures 1, 2 and 3 are copied, with permission, from Wigen. In figure 1, the locations of Nakwakto Rapids, Slingsby Channel, Outer Narrows, Cape Scott, Mayne Bay, Cape Flattery and the Gulf Islands were labelled by me. 1 INTRODUCTION On May 22, i960, there was an earthquake with epicenter in Chile at *fl* south latitude and 73.5* west longitude. This earthquake generated an ocean wave or tsunami which propagated" away from the source region with celerity, c, given by Here, g is the acceleration due to gravity, k is the wave number, and D is the depth of the ocean. It happened that, at this time, the Canadian Hydrographic Service had in operation, along the Pacific Coast of Canada, twenty-one closely-spaced tide-gauges. Seventeen hours after the earthquake, the tsunami arrived on this coast and was subsequently recorded by seventeen of the gauges. These tide-gauge records appear in a manuscript report by Wigen and are reproduc• ed, with permission, as figures Z 3. The one minute differences between arrival times of the first wave of the tsunami at those points on the outer coast indicate that it approach• es from the southwest with the first crest about parallel to the coast. The arrival times of the first wave at points on the inner coast agree, with the exception of the set of records up inlet from Nakwakto Rapids, with the times required for travel from the outer coast predicted using distances and depths from the charts and the celerity formula for long waves. (Long waves are those for which the depth is less than about one tenth of a wavelength. For long waves, c=gD.) The initial influence of the tsunami on this coast was an elevation of the sea-surface. The wave of largest amplitude usually occurred some five hours after the initial disturbance. A feature most apparent in the records; from the outer coast is the series of from one to three long period waves which occurred during the two hours immediately following the arrival of." the tsunami. After two hours, the records show characters which are different for different records and which persist for a few days. The Cape St. James rgtsord shows innumerable waves of about five min• utes period. The Alert Bay record has higher waves of longer period and is smooth• ed compared to Cape St. James. 2: The Tofino and Barkley Sound records appear to be similar in that there are larger waves of very long (15 to 150 minutes) period and smaller waves of long (2 to 15 minutes) period than there are at Cape St. James. The Victoria record shows waves decreased in amplitude and smoothed compared to the Barkley Sound record. Fulford Harbour shows less tsunami action than Victoria. Two gauges showed almost no tsunami influence. Prince Rupert was ef• fectively sheltered by the Queen Charlotte Islands; Caulfield, by the Gulf Islands. The McKenney Island Record resembles the Cape St. James record except that the very long waves are more apparent and the long waves, less apparent. The Klemtu record is smoothed compared to the McKenney record. The Griffin Passage record shows evidence of a 25 minute seiche with resonant amplifi• cation. Johnson Point, Nugent Sound and Mereworth Sound show a surprising amount of tsunami action. Seymour and Belize Inlets were influenced only slightly by the Tsunami. (The location of all place names are shown on the map in figure 1.) This study is an attempt to explain these features. 3> PROCEDURE It was decided early in the course of the investigation to attempt power spectrum analysis of some of the tide-gauge records. This yields an estimate, in each of many frequency bands, of the mean square value of the water surface elevations which is referred to here as the power of the tsu• nami. With twelve hours of tsunami record sampled at one minute intervals, it was possible to calculate the power spectra to within close confidence limits. Thus, the statistical nature of the waves could be used to obtain a more concrete representation of the character of the tsunami. Before power spectrum analysis could be performed, two questions had to be resolved. The first question was the following. What are the distort• ing influences of the tide-gauges, if any, and how do we measure and correct for them? The answer is that the distorting influences can be measured and compensated for, for the two types of tide-gauges used, using the techniques described in Appendices A and B. The second question was this. Are the tsunami records accurately re• produced? The original tide-gauge records are on linear or circular plots, depending on the type of gauge, and have been marked for other purposes. The records presented in Wigen are copies of the originals drawn clearly on uniform format. These records were compared with the originals and seem to be very carefully done. They were chosen to form the basis of this study. Seven stations were chosen for power spectrum analysis. These were, from north to south, Cape St. James, Johnson Point, Nugent Sound, Alert Bay, Tofino, Barkley Soun<2, and Victoria. It can be seen from figure 1 that these stations might provide information on the development of the tsunami along the coast and shoreward. The power spectra were calculated by the method given in Appendix CI. The first spectrum completed, for Victoria, consisted of some power estimates, for components of period less than three minutes, which were negative (-31.2 cms. per J-^cycles per minute). This was disturbing since negative power is difficult to interpret. At length, the following reasoning was developed. Since a power spectrum analysis is known to be unstable when the spectrum has high power and steep slopes, and, since the Victoria spectrum is seen to have these, perhaps the negative power is the result of instability in the analysis. A need for prewhitening is indicated. Prewhitening reduces the power 4; in the high-power bands and amplifies the power in the low-power bands so that the Fourier transform process, referred to in Appendix C 1., is perform• ed on a more nearly smooth spectrum with correspondingly more stability. The spectrum is later restored to its actual shape. The derivation given in Appendix D yields a method for prehwitening the autocovariances rather than the individual data. This was convenient because I had already calcu• lated the autocovariances with unprewhitened data.

Load more

  28

Copy Link