Keydwords: Building Collapse Risk, the 1995 Kobe Earthquake, Tokyo, Reliability
Analysis, Fragility Curve
Abstract: “The Earthquake Area Vulnerability Assessment of Tokyo”, which was published
by Tokyo Metropolitan Government (TMG) recently, shows relative seismic risks in Tokyo.
“The area danger levels” were determined with respect to building collapse, fire outbreak and
spread, human casualties and access for evacuation, considering soil conditions, building
inventory and social conditions. The method on the report was applied to Nada Ward, Kobe City
using the inventory before the 1995 Hyogoken-Nanbu (Kobe) Earthquake. The obtained
“building collapse risk” was compared with the actual damage of the ward due to the earthquake.
The result of the comparison indicated that the weights used in the TMG method should be
adjusted since the obtained collapse risk was strongly influenced by the building density of each
city block. Then a revised method to evaluate “building collapse risk” was proposed. The new
building collapse risk corresponds to the severe damage ratio of buildings and it reflects the
characteristics of buildings and sites. The proposed method may be useful for seismic risk
assessments by local governments in Japan.
1. Introduction
Since the 1995 Hyogoken-Nanbu Earthquake, many local and national governments of
Japan have been more respected the studies of earthquake damage assessment. An earthquake
causes fires, liquefaction, land slides and lifeline interruptions as well as damage to buildings
and infrastructures. It is very important for local governments, not only of prefectures but also
of cities or wards, to grasp seismic vulnerability of each city block. Recent GIS technologies
enable us to assess seismic risk and to predict earthquake damage visually considering several
kinds of inventories and other natural and social data.
This study deal with building collapse risk in “The Earthquake Area Vulnerability
Assessment of Tokyo” [TMG, 1998]. This assessment is different from damage assessments
for scenario earthquakes [TMG, 1997] in these points: (1) The former assumes no specific
earthquake source model. (2) Rather simple model is used in the former.
In this study, in order to examine the assessment method of building collapse risk due
to earthquakes, the method used in the TMG’s report was applied to the Nada Ward, Kobe
City using building inventory before the 1995 Kobe Earthquake. The obtained “building
collapse risk” was compared with the actual damage of the ward due to the earthquake and a
revised method to estimate “building collapse risk” which fits the actual damage was
proposed.
2. Application of the TMG Method to Nada Ward
2. 1 Building Collapse Risk by the Method of TMG
The building collapse risk by the method classified all the district blocks in Tokyo into
five levels due to the amount of risk calculated with weights for building characteristics and
site characteristics. These elements are determined by experiences of six professionals
considering seismic vulnerability of buildings. The amount of risk (simply call "risk"
hereafter) for building collapse was calculated by the following equations (1) and (2):
Qkn = Dk (1-Wkn·Ukn) (1)
Uk n= u1 n·u2 n·u3 n·u4 n·u 5 n (2)
in which k is the building category (1-19), Q is the amount of risk, D is the number of
buildings per 1 km
2 , W is the weight for earthquake resistant capacity of buildings of the
category k, U is the weight for site characteristics, u
i's are the weights for subsurface condition
(u
1), for possibility of liquefaction
(u
2), for large-scale cut-and-fill
(u
3), for reclaimed land
(u
4),
and for steep slope
(u
5), n is each value by the 6 specialists. By adding
Q
k’s for all the
structural types in a block, the amount of risk for the block is evaluated.
2. 2 Comparison between Building Collapse Risk and Actual
The building collapse risk of the Nada Ward in Kobe City was calculated using the
inventory and soil data by the method of TMG. Soil conditions of each district block in Nada
Ward were classified within TMG’s classification in choosing similar conditions in order to
use the weights. As a result, the building collapse risk was determined as shown in Figure 1(a).
Figure 1(b) shows the ratio of severely damaged buildings due to the 1995 Kobe Earthquake
investigated by Kobe City. It is noticed that the distribution of building collapse risk evaluated
by the method of TMG (Figure 1(a)) and the density of buildings (Figure 1(c)) look quite
similar. It is observed that these two factors, the building collapse risk and the actual severe
damage ratio, are not highly related. Although the former is strongly influenced by the
building density of each district block. “The collapse of a building” itself is usually not
influenced by the density of buildings in an area. Hence a revised amount of risk, which
divides the evaluated amount of risk (by TMG method) by the total number of buildings in the
block, is proposed.
(a) Building collapse risk by the method of TMG
(b) Severe damage ratio of buildings due to the Kobe Earthquake
(c) Number of buildings
Figure 1: Building collapse risk and actual damage in Nada Ward
