Two proposals have been made to improve the empirical Green's function method. The first proposal concerns modeling the initial part of the slip velocity time function or a large event. The second proposal concerns modeling soil nonlinearity. A new correction function for the slip velocity is proposed. with a new parameter α representing the abruptness of the initial part of the slip of a large event. The correction function has a preferable spectral characteristic and　is more versatile in representing the initial part of the slip velocity time function of a large event. By means of fitting the slip function obtained with this correction function to Nakamura and Miyatake's (2000) slip function. it is found that the best α value is affected by parameters such as w, the width of the fault. fmax, and N. For all cases considered. the best α value ranges between 0.51-1.51.
　On the other hand. when the empirical Green's function method is applied to seismograms recorded at a site which is located on soft soil layers, it is necessary to take into account the nonlinear behavior or the soft soil layers. In practice, it has often been assumed that the seismic wave is affected by soil nonlinearity only after it's incidence to the local soft soil layers. If we consider a seismic ray connecting the source and the site.however. it might be true that the ray crosses the soft soil layers several times. Therefore. it is reasonable to assume that the seismic wave is affected by soil nonlinearity several times during the transmission from the source to the receiver. This potential phenomena is referred to as multiple nonlinear effects in this article.Taking into account these effects in the empirical Green's function method. the authors propose new parameters v 1 and v 2, which represent the deviation of material properties of the sediments due to soil nonlinearity from linear status and which are used to modify Green's functions. The parameters are applied to the simulation of the 2000 Tottori-ken Seibu and the 1995 Hyogo-ken Nanbu earthquake ground motions. It is found that. although the new parameters are quite simple. they greatly contribute to the improvement of the simulation results.