INCIDENT BOUNDARY AND AVERAGED AMPLIFICATION FACTORS OF SH WAVE FOR GROUND WITH IRREGULAR SUBSURFACE TOPOGRAPHY

The amplification factor due to multiple reflection has been widely used to estimate the characteristics of ground motions. However, as had been observed during the 1995 Hyogo-ken Nanbu earthquake disaster, the amplification of the ground motion due to irregular subsurface topography has to be considered. From this point of view, an incident boundary amplification factor was firstly introduced wherein it was revealed that contributions from the irregular boundary appear not only in the immediate vicinity of the irregular boundary but the whole of the sedimentary region considered in this study. Next, an averaged amplification factor to distributed incident plane waves was formulated, and the amplification characteristics of the ground were estimated from it. It has been concluded that when the incident wave field is not predictable, the averaged amplification factor is more reliable than the amplification factor due to either one-dimesional multiple reflection or an incident plane wave in a ground with irregular subsurface topography.