Journal of Natural Disaster Science
Journal of Natural Disaster Science, Volume 18, Number 2, 1996, pp.99f.
LESSON FROM THE 1995 HYOGO-KEN NANBU EARTHQUAKE: WHY WERE SUCH DESTRUCTIVE MOTIONS GENERATED TO BUILDINGS?
(Received 17 March, 1997 and in revised form 30 May, 1997)
Abstract
The 1995 Hyogo-ken Nanbu earthquake struck Kobe City and its surrounding, heavily populated areas, the Hanshin district, Japan, killing more than 6,400 people and destroyed more than 150,000 buildings. Surface breaks associated with the earthquake were found for about 15 km along the Nojima fault, Awaji Island; but, none were found in the Kobe area where earthquake damage was much more heavy than on Awaji. Aftershocks were distributed along already mapped active faults known as Rokko faults in the Kobe area as well as on Awaji. The most heavily damaged zones in the Kobe area did not, however, always coincide with the aftershock zones, being offset about 1 km to the south, sediment areas. Two questions had to be answered: Where are the causative faults that generated the destructive ground motions, in particular, did buried faults beneath the Kobe area rupture? In answer, the rupture process from the waveform inversion of the strong motion data shows concentrated slips in the shallower portion of the Nojima fault of Awaji Island but only in the buried portions of the Rokko faults of Kobe. The second question is why are the heavily damaged zones in Kobe not consistent with the surface fault traces associated with the buried faults. So far, earthquake damage zones are commonly believed to be directly related to causative faults, but it was not the case. We found that ground motions in the near-fault area in Kobe were characterized by two large-pulses with a duration of 1 to 2 seconds. These durations are related to the size of the asperities from the inverted slip distribution. Ground motions near the heavily damaged zones have especially distinctive pulses showing larger amplifications than those in less damaged zone as an observational fact. We consider that these large pulses may have caused the collapse of buildings and bridges in Kobe during the mainshock. We used the 3-D finite difference method to simulate near-source ground motions based on the inverted slip distributions and a 3-D velocity structure of the Kobe area. We found that impulsive ground motions of extremely large amplitudes in the period range of 1 to 2 seconds are generated and coincide with heavily damaged zones offset about 1 km from the basin edges. We concluded that these destructive motions were caused by both forward rupture directivity and basin edge effects. Special consideration of ground motions amplified by the coupling of the source and geological structure effects should be made when mapping seismic hazards in urban areas.
Key words
the Hyogo-ken Nanbu earthquake, strong ground motion, "Damage Belt", basin edge effect, empirical Green's function