Abstract
　In this paper, a semi-empihcal synthesis method is developed fopr statistically estimating longperiod ( 2 to 20 sec ) surface waves during the mainshock from records for small events such as aftershocks. Normal mode theory and fault dislocation theory aer used. In this method, effect related to rupture propagation over the fault, the correction for radiation amplitudes of surface waves due to the difference in mechanisms btween a mainshock and an aftershock, and the removal of ghostly oscillatory motions in the high frequency range, etc., can be considered. Ground motions for the mainshock aere synthesized using aftershock records obtained by low-magnification seismographs of Central Meteoronogical Observatory for the Nankaido earthquake ( Ms=8.l ) of December 20, 1946. Long-period ground motions and acceleration response spectra with 2% critical spectra are in good agreement with those calculated for the mainshock by Inoue et al. Under certain circumstances, horizontal components of the acceleration response spectra at Sumoto during a great earthquake along the Nakai trough may exceed the design spectra for the Honshu-Shikoku bridges. This synthesis method implies the possibility of prediction of long-period ground motions for future large earthquakes.

*京都大学工学部
Faculty of Engineering, Kyoto Univ.