FOCUSING OF SEISMIC-WAVES ALONG THE UEMACHI FAULT - DAMAGE IN THE OSAKA AREA CAUSED BY THE 1995 HYOGO-KEN NANBU EARTHQUAKE AND A SUBSURFACEGEOLOGIC STRUCTURE

The 1995 Hyogo-ken Nanbu earthquake of JMA magnitude 7.2 caused seriou s damage widely throughout the Hanshin and Awaji areas in the Kinki di strict of southwest Japan. Damage to artificial structures was observe d widely, even in the Osaka area 30-50 km east of the epicenter. Most of the damage was accompanied by ground failures such as liquefaction, lateral spreading and landslides of reclaimed lands along the coastal zone and river channels and ponds in the inland areas. Damage to arti ficial structures not accompanied by ground failure was also observed in several areas. Most of this type of damage was concentrated in narr ow zones along the Butsunenji-yama fault and the Uemachi fault trendin g in the N-S direction. As the concentration of damage suggests that s eismic waves were strongly amplified in limited zones around faults, w e examined the focusing effect of seismic waves as one of possible int erpretation. A velocity structure model composed of four homogeneous l ayers with curved interfaces referring to the result of a seismic refl ection survey was utilized for this purpose. Two-dimensional calculati ons of wave paths and surface amplitudes support the possibility that vertical displacement of a subsurface geologic structure along the fau lt caused seismic waves to focus at the margin of the subsided block, resulting in the narrow zone of damage.