The 1995 Hyogo-ken Nanbu earthquake caused severe damage to a lot of buildings, especially in "the heavily damaged zone"; the 1km-wide belt running north-south section through the Kobe urban area. The deep irregular underground structure beneath Kobe City comprises bedrock and sedimentary regions (Osaka group) connected at an almost vertical discontinuity in the fault-normal direction. This induced a local concentration of strong ground motions in "the heavily damaged zone". The configuration of the fault-normal sections of the underground structure slightly varies in the east-west direction. It is important to investigate amplification characteristics of the ground motions in several different areas and to determine their relationship with the actual damage distribution in Kobe City.
First, this study evaluates bedrock motions on an outcrop of bedrock using the 6 ground motions recorded in Kobe City during the 1995 Hyogo-ken Nanbu earthquake. Then, site amplification is investigated for the 6 fault-normal sections of the deep irregular underground structures in Kobe City. The underground structures are modeled faithfully by 2-D FEM based on seismic profiles determined from available geological information. Peak velocities of bedrock motions deconvolved from 6 records show almost the same level, 50-60cm/s, while those of an outcrop of sedimentary regions are 86-137m/s. Ground motions in Kobe City were largely affected by the deep irregular underground structure, even on an outcrop of Osaka group regarded as the engineering bedrock. Then, ground motions in reclaimed areas are successfully simulated by combination of 2-D FEM and 1-D effective stress analyses. This suggests that the deconvolved bedrock motions are reliable and the site amplification can be properly evaluated. For soil responses of Osaka group, the large amplifications can be commonly seen in an area about 1km from the basin edge to the seaside, while the transfer functions from the bedrock vary according to the bedrock shapes. The peak distribution of ground motions account for the actual distributions of damaged structures, indicating that the amplification due to the deep irregular underground structure was one of the main reasons for "the heavily damaged zone". The peak ground acceleration sharply decreases in the reclaimed area due to the nonlinearity of the soft surface soil including liquefaction, which corresponds to the rapid drop of the damage ratios. These results suggest that the ground motions were greatly affected by the underground structure, which must be properly reflected in the evaluation of ground motions for structural design.