SEMIEMPIRICAL ESTIMATION OF PEAK GROUND ACCELERATION FROM LARGE EARTHQUAKES

A simplified method is proposed for synthesizing ground acceleration from a large earthquake. The present method is based on the semi-empirical method of Irikura (1986), which is consistent with the omega-square source model, but is much simplified for engineering use. In this method the fault of the large earthquake is divided into small elements and the acceleration envelope waveforms, instead of the time histories, from the elements are simply determined using the empirical relations. The envelope waveforms from the elements are summed to synthesize the envelope waveform of a large earthquake. The peak acceleration of the large earthquake is obtained from the maximum value of the summed envelope. To test the present method, the peak accelerations are calculated for the 1985 Central Chile earthquake (M(S) 7.8), during which a considerable number of the strong-motion records were obtained close to the source. The calculated accelerations are in good agreement with the observed ones. The attenuation characteristics of the calculated peak accelerations for earthquakes of different magnitudes are examined. The results show that: (1) the shortest distance from the fault plane is an appropriate definition for source distance; (2) the peak acceleration becomes independent of magnitude in the near-field; and (3) the source direction effect on the peak acceleration is not negligible.