2001 Volume 110 Issue 6 Pages 885-899
The real cause of heavy damage on buildings during the Hyogo-ken Nanbu (Kobe) earthquake of 1995 is presented and the future direction for disaster mitigation is discussed.
First a seismological approach for strong motion prediction is shown and the simulated strong motions for the Hyogo-ken Nanbu (Kobe) earthquake are presented. It is found that the most important feature of the near-fault motion is the ocurrence of high-amplitude velocity pulses with a peak ground velocity more than 100 cm/sec and a peak ground acceleration more than 800 cm/sec2. To predict the amplitude and period of such a velocity pulse we need to predict the size of an asperity (a patch with a large slip) and the slip velocity within it.
Once we obtain ground motions simulated in the whole Kobe area, we use them as input to our theoretical models that are capable of simulating the damage to buildings. When we use ultimate strengths for design, the estimated damage is much larger than the observed. We estimate the actual ultimate strengths of buildings so that we can obtain the same damage ratios in Kobe for different building heights and for different construction ages.
Then we apply the established building models to the near-fault ground motions observed during the Tottori-ken Seibu earthquake of 2000 in order to verify their applicability. We found that the models do not show any heavy damage except for one site where soft surface soil strongly amplified the ground motion. This corresponds to the fact that we did not observe any heavy damage in the area.
Finally we discuss the problems of the current anti-seismic design practices and countermeasures for earthquake disaster based on the facts described above. Basic consensus in the structural engineering community on the seismic safety of our buildings is that they are sufficiently safe as long as we design buildings based on the current seismic code that was enforced after 1982. It is true that the buildings constructed after 1982 performed quite well, but we have shown that it was due to their additional strengths not considered in the original design. It is apparent that we have a big gap between theoretical models for design and the actual buildings we have constructed. Unless we do quantify the gap, we cannot take any effective countermeasures for future destructive earthquakes.
