Strong-Motion Instrumental data on the Borrego Mountain Earthquake of 9 April 1968

The Borrego Mountain earthquake triggered 114 strong-motion accelerographs in the southern California region. Hitherto the maximum number of instruments in the Pacific Coast network which had simultaneously operated was 35 accelerographs during the main shock of the Kern County earthquakes of 1952. The unprecedented nature of the instrumental coverage makes it desirable to ensure a prompt distribution of the complete basic data. As the first large scale test of the field installation and servicing conditions, as well as of the instruments themselves, the data are of unusual interest. Even though the large distance from the epicenter (135 miles) to the densest portion of the accelerograph network in the Los Angeles area resulted in many records of very low magnitude, significant studies of the distribution of ground motion can be made. In particular, the distribution pattern of the earthquake-triggered accelerographs gives valuable information on the characteristics and reliability of the starting devices incorporated into the instruments. It will be noted that in addition to a number of important ground motion records at El Centro, San Diego, San Onofre, etc., numerous accelerograms were obtained in upper floor positions in tall buildings in the Los Angeles region. Although these records show low acceleration levels, they permit accurate determination of building response period. It will be of great interest to compare these earthquake -excited period measurements with those obtained from other structural tests and from calculations. Because of the general interest in instrumental characteristics as well as in the distribution of ground motion, it has been decided to reproduce photographs of all record traces, no matter how small the indicated motions. It is realized that the small scale of many of the figures precludes their direct use for accurate measurements. It is intended that they will serve as an index to the available records, and will indicate to a prospective investigator the particular records which he might wish to obtain in the full-scale form from the U. S. Coast and Geodetic Survey. Since only a limited number of full-scale records can be so distributed, the present report should thus contribute to an optimum use of existing data-handling capabilities. . . . Since means have not yet been found to enable the Seismological Field Survey to expand its activities in the preparation and distribution of accelerograph records to the extent called for by the present size of the instrumental network, the present report is a cooperative effort with the California Institute of Technology under a grant from the Engineering Division of the National Science Foundation for basic studies in earthquake engineering.