New Method for Building Collapse Risk Assessment Based on Damage Data due to The 1995 Kobe Earthquake

3. 4 New Building Collapse Risk of Tokyo
Reapplying the method proposed above to Tokyo, the building collapse risk in Tokyo was evaluated. In calculating the new building collapse risk, the median of PGV in mountain areas was assumed to be 30cm/s, which was used in the damage assessment study of TMG (1997) for near-field earthquakes. The median values of PGV in other topographical conditions were obtained by multiplying the amplification factors proposed by Yamazaki et al. (1999). The logarithmic standard deviation of PGV was assumed as the record-to-record (intra-event) variation in the attenuation relation of PGV proposed by Molas and Yamazaki (1995).

Under these assumptions, the new building collapse risk of Tokyo was calculated and it was compared with that by TMG as shown in Figure 4. It is seen in the figure that the risk level in the eastern part of Tokyo (lowland with soft soil) is large by the proposed method while the high-risk blocks are distributed outside of Yamanote Line circle (highly built-up wooden building areas) by the TMG method. Soil condition and building type are dominant factors to determine the collapse risk in the proposed method. However, since the Urban Planning Bureau of TMG uses this building collapse risk in the selection of areas for urban redevelopment, the risk by the TMG method may have different meaning from the risk evaluated by the proposed method.

(a) Building collapse risk by TMG (1998)

(b) Building collapse risk by the proposed method

Figure 4: Comparison of building collapse risk in Tokyo by two methods
4. Conclusions
In this study, the building collapse risk of the Nada Ward, Kobe City was calculated based on the method of Tokyo Metropolitan Government (TMG) using building inventory and soil condition data. The evaluated building collapse risk was compared with the actual building damage due to the 1995 Kobe Earthquake. Since the number of buildings in unit area is a dominant factor to determine the building collapse risk by the TMG method, the evaluated risk was high for densely built-up areas and it was not very good agreement with the actual damage distribution. Considering the result of comparison, a revised method, which divides the evaluated amount of risk by the total number of buildings in an area (removing the effect of building density in an area), was proposed. Besides we adopted the basic reliability analysis theory to calculate the weights.

The physical meaning of building collapse risk was interpreted properly by the proposed approach and it is generally applicable to entire Japan. Assessment of building collapse risk in urban areas is one of the important factors to be considered in urban planning. Hence the proposed method may be conveniently used by local governments in Japan for the seismic risk assessment of district level.

References

Molas, G. L., Yamazaki, F. (1995), “Attenuation of Earthquake Ground Motion in Japan Including Deep Focus Events,” Bulletin of the Seismological Society of America, 85, 5, pp. 1343-1358.
Murao, O., and Yamazaki, F. (1999), “Re-estimation of Strong Ground Motion in Nada Ward based on Inventory and Building Damage Data”, Journal of Structural and Construction Engineering, AIJ, 523, pp. 141.148 (in Japanese).
Murao, O., and Yamazaki, F. (2000), “Development of Fragility Curves for Buildings Based on Damage Survey Data of a Local Government after the 1995 Hyogoken-Nanbu Earthquake”, Journal of Structural and Construction Engineering, AIJ, 527 (in Japanese), (in printing).
Tokyo Metropolitan Government (1997), The Report on the Damage Assessments for Scenario Earthquakes in Tokyo due to Near-field Earthquakes (in Japanese). Tokyo Metropolitan Government (1998), The Fourth Report of Survey of the Earthquake Area Vulnerability Assessment (in Japanese).
Yamazaki, F., Wakamatsu, K., Onishi, J., and Yamauchi, H. (1999), “Relationship between Geomorphological Land Classification and Soil Amplification Ratio Based on JMA Strong Motion Records”, Bulletin of Earthquake Resistant Structure Research Center, No. 32, Institute of Industrial Science, The University of Tokyo, pp. 17-33.

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