Strong motion earthquake accelerograms, digitized and plotted data, Volume I - uncorrected accelerograms; Part A - Accelerograms IA1 through IA20

Abstract

Preface to the Series The object of the present series is to make available accurate digitized data prepared in a uniform way for all recorded strong-motion earthquakes. This first report is the initial step in a continuing data processing program which will ultimately include all past recorded earthquakes as well as current data as it is collected. In addition to digitized ground acceleration records, certain basic calculated information such as integrated velocity and displacement curves, response spectra, and Fourier spectra, will also be issued in a standard form. The existence of this standardized data will allow all investigators to begin with the same basic numerical data, and should thus reduce the scatter of numerical results, the meaning of which has sometimes been difficult to assess. In addition to an improvement in the consistency of numerical results, several other advantages will be derived from the standardized data. (1) A number of earthquake records will be available which have not hitherto been analyzed in any form; (2) Vertical component data will be available for all earthquakes, digitized on a simultaneous time basis with the two horizontal components; (3)A small overall improvement in data accuracy will result from the inclusion of corrections for baseline distortion on the original record, and for timing mark variations; (4) Basic accelerogram data uncorrected for base line adjustments will be available for research purposes, as well as corrected data using standard adjustments. Accurate measurements of the ground motions of destructive earthquakes form the indispensable starting point for invesitgations in earthquake engineering. The number of such measurements that have been made in the past is unfortunately very small, and there is thus a need to extract the maximum possible information from the relatively few records that do exist. Starting with the first earthquake accelerogram for a destructive earthquake obtained in Long Beach, California, in 1933, the network of accelerographs maintained by the U. S. Coast and Geodetic Survey has gradually accumulated a very important series of accelerograms including some measurements of most of the strong earthquakes occurring in the Pacific Coast States since that time. Although for most of these earthquakes the number of accelerograph records is much smaller than would be desirable, the available records have formed a sound base for the development of current ideas as to the character of destructive earthquake ground motion. The number of such earthquake accelerograms now available for analysis is of the order of one hundred. The importance of the work of the Seismological Field Survey of the U. S. Coast and Geodetic Survey in the development, installation, and servicing of accelerographs and in the highly efficient and reliable collection of this basic data cannot be overestimated. In addition, this group has always made available accurate copies of the original accelerograms to interested investigators throughout the world. The size of the USCGS accelerograph network has gradually grown from about 40 instruments in 1935 to some 300 accelerographs in 19690 This has resulted in a gratifying increase in the number of accelerograms, but has emphasized the need for working out a more comprehensive program for the processing of this data. Both instruments and data processing methods have steadily evolved over the years since the beginning of the network, and the matter of establishing uniform procedures has now become of critical importance. Some of the factors which suggest that a uniform preparation of information is now essential are: (1) The basic accelerograph itself has gone through many stages of development, and the network now contains a wide variety of devices differing considerably in mechanical, electrical, and optical details. Although the basic physical parameters are approximately equivalent, there are significant differences which sometimes can be corrected for if the proper information is at hand. (2) A wide variety of recording media and record sizes are employed in the various accelerographs, and many different techniques have been used over the years to digitize the analog photographic traces for numerical work. (3) Various important earthquake accelerograms have been studied by different investigators using different interpretive techniques. In addition, various corrective methods have been employed, and it is not always clear for a given reproduced accelerogram just how the basic data has been treated. (4) As modern digital computing techniques have been developed, there has been a continuous evolution of the data processing techniques used. For example, response spectrum calculations have been carried out by desk calculators, by analog computer methods, and by digital computers. This may complicate the comparison of old spectrum curves with more recent calculations. Integrated velocity and displacement curves are well known to be particularly sensitive to this kind of computational variation. The foregoing factors, plus the fact that highly efficient digitizing and computing facilities are now available, have provided the motive for the present undertaking. The present series will be issued in the following volumes: Volume IDigitized Accelerograms, Uncorrected, with Print-outs and Plotted Records Volume IIDigitized Accelerograms, Base-line corrected. Integrated Velocity and Displacement Curves in Plotted and Digitized Form Volume IIIResponse Spectrum Curves Additional volumes now being planned will present other forms of the basic data of use in earthquake engineering such as Fourier Spectrum curves. Each volume will in turn be issued in parts A, B, C, etc., each containing 20 earthquakes. In this way it is hoped that useful information can be issued as it is prepared, without the delays associated with the completion of the whole project. The project has involved close cooperation at every stage with the Seismological Field Survey of the U. S. Coast and Geodetic Survey, and the methods and formats used have been arrived at after detailed consultation with Mr. William K. Cloud, Chief of the Seismological Field Survey, and his staff. A work of the present type naturally involves important contributions from a large number of people. Some of these will be identified in specific sections and in the reference material. Anything like a complete list of those who have in this way earned the gratitude of future earthquake investigators is obviously impracticable here. It is a pleasure to acknowledge at this point the support of the Engineering Division of the National Science Foundation for the long-range support which makes a project of the present type possible. Introduction to Volume I Uncorrected Digitized Data - Earthquake Ground Accelerations This volume contains the digitized accelerograms of strong earthquake ground motions as processed from records obtained from the strong-motion accelerograph network maintained by the U. S. Coast and Geodetic Survey. No base-line or instrumental corrections or adjustments have been made - the data may thus be regarded as "uncorrected" in the sense that no modifications have been introduced which involve any hypotheses as to the character of the ground motions or instruments involved. This digitized data is thus believed to be as close a representation of the original, raw information as it is feasible to achieve with a digital process. A significant feature of the compilation is the fact that uniform data processing methods have been applied to all earthquakes. These methods will now be briefly described. Additional details will be found in Appendix I, which reprints a paper presented at the Fourth World Conference on Earthquake Engineering, Santiago, Chile, in January 1969 As a first step, full size contact film negatives were prepared from the original records which were recorded in the field on photographic paper. This reproduction process was carried out by the staff of the Seismological Field Survey of the U. S. Coast and Geodetic Survey in San Francisco, using the original records from the Washington Office of the USCGS. From these film negatives, contact prints were made on a frosted, translucent, Mylar-based film. Measurements have shown that these prints differ in size from the film negatives by less than -7- 0. 1%o, and the distortion involved in going from the original paper record to the contact negative is believed to be no larger than this. The translucent film is mechanically strong, dimensionally stable, and affords excellent optical contrast for setting the cross-hairs on the digitizing machine on a back-lighted glass table. Experiments on photographic development techniques resulted in standard methods for producing an optimum balance between contrast and trace width. Of the approximately one hundred records available, about fourfifths were of good to excellent quality, and permit an accurate digitization with relative ease. About one-fifth are of poorer quality, requiring greater judgment and more effort in the digitizing process, but nevertheless permitting results of satisfactory overall accuracy. The digitizing is performed on a Benson-Lehner 099D Datareducer unit. The cross-hairs are manually set to successive x-y coordinates on the record trace. The coordinates are converted to digital position figures by means of a magnetic readout head, and are stored in a 6-digit accumulator system from which they are automatically read out to an electric typewriter and a card punch. The maximum resolution of the system is about 800 digital counts per inch, corresponding to a least time interval on most records of 1/300 seconds. The 24 inch table length on the digitizer can accommodate 30 seconds of record at the fastest recording speeds. Most of the accelerograms could be digitized in one position on the digitizer table. For those records requiring repositioning, the points in the record at which changes have occurred are noted, using an arrow on the plotted accelerogram, and an asterisk in the print-out. The records have been digitized on an unequal time basis, which has been found to give the best definition of the trace for a given number of sections and in the reference material. Anything like a complete list of those who have in this way earned the gratitude of future earthquake investigators is obviously impracticable here. It is a pleasure to acknowledge at this point the support of the Engineering Division of the National Science Foundation for the long-range support which makes a project of the present type possible.

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