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Published February 1970 | Published
Journal Article Open

Complexity of energy release during the Imperial Valley, California, earthquake of 1940


The pattern of energy release during the Imperial Valley, California, earthquake of 1940 is studied by analyzing the El Centro strong motion seismograph record and records from the Tinemaha seismograph station, 546 km from the epicenter. The earthquake was a multiple event sequence with at least 4 events recorded at El Centro in the first 25 seconds, followed by 9 events recorded in the next 5 minutes. Clear P, S, and surface waves were observed on the strong motion record. Although the main part of the earthquake energy was released during the first 15 seconds, some of the later events were as large as M = 5.8 and thus are important for earthquake engineering studies. The moment calculated using Fourier analysis of surface waves agrees with the moment estimated from field measurements of fault offset after the earthquake. The earthquake engineering significance of the complex pattern of energy release is discussed. It is concluded that a cumulative increase in amplitudes of building vibration resulting from the present sequence of shocks would be significant only for structures with relatively long natural period of vibration. However, progressive weakening effects may also lead to greater damage for multiple event earthquakes.

Additional Information

Copyright © 1970, by the Seismological Society of America. Manuscript received June 30, 1969. We wish to express our appreciation to Professors G. W. Housner, D. E. Hudson, and C. F. Richter for valuable discussion. They have also read the manuscript critically and offered many valuable suggestions. Mr. W. Cloud of USCGS kindly supplied us with a copy of the accelerogram record from El Centro strong motion station, of the Imperial Valley, California, 1940 earthquake. Professor C. Allen kindly provided the data for the measured field offset along the Imperial Valley fault given in Figure 9. This study was supported by the National Science Foundation grant GP 1087 (Earthquake Mechanism) and National Science Foundation grant GK 1197X (Engineering Mechanics Program).

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