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

The pattern of energy release during the Imperial Valley, California, earth-quake 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 calcu-lated 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 re-sulting from the present sequence of shocks would be significant only for struc-tures with relatively long natural period of vibration. However, progressive weak-ening effects may also lead to greater damage for multiple event earthquakes. A A (t) a(t) A t