16. Vulnerability to Earthquake Disaster and Countermeasures in Tokyo

Takaaki Kato

16.1 Significant Characteristics of 1995 Great Hanshin-Awaji Earthquake Disaster

First, I would like to look back at the Great Hanshin-Awaji Earthquake of 1995. Kobe, a large city located near epicenter the earthquake, suffered huge damage (Fig. 16-1). The disaster caused huge physical damage and killed more than 6,000 people, although Japan is one of the most devel-oped countries in the world. This disaster made a large impact on society in Japan, as is reflected in the earthquake countermeasures below. I sum-marize the significant characteristics of the Great Hanshin-Awaji Earth-quake disaster from the viewpoint of urban disaster prevention.

The first characteristic is that it was the first urban disaster in which damage from collapsed houses dominated. It had been assumed until then that the typical pattern of urban earthquake disaster in a mega-city would be like the 1923 Kanto Earthquake, in which the dominant damage in was not collapsed houses but rather damage by urban fire-spread. The second

Fig. 16-1. Damage from the Great Hanshin-Awaji Earthquake

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is that building collapse was even more than estimates before the earth-quake. What caused this damage? It is not a problem concerning the pre-sent seismic building code. The buildings which met the present building code had no damage, or little damage. It was main reason that many inade-quate buildings existed. Furthermore, a more fundamental problem was that nonrenewable built-up areas existed. In fact, most damage was con-centrated in densely built-up areas.

The third is that urban fire-spread damage made a large social impact and the reaction of the public was very serious, although fire damage in Hanshin-Awaji Earthquake was not extensive, and in fact was rather small, because it was not windy: wind velocity was only 4m/s maximum. If the winds were stronger, there would have been more severe damage caused by fire. As I will discuss later, if an earthquake at the same level occurred in Tokyo, the volume of fire damage will be one hundred times as the damage in Kobe.

Finally, it is pointed out that the social response to damage in terms of civil engineering construction, lifeline damage, liquefaction, and the other damage was within the scope of estimates.

16.2 Vulnerability to Earthquake Disaster in Tokyo

Tokyo has high earthquake hazard. Fig. 16-2 shows the distribution of earthquake probability of seismic intensity (JMA) 6 upper in the next 30 years. The probability in Tokyo is estimated at more than 26%. Just for reference, the probability just before the Great Hanshin-Awaji Earthquake Disaster was less than 10%. Therefore, we can understand that an earth-quake in Tokyo is an imminent threat.

The Central Disaster Prevention Council, which is the main national or-ganization reported the damage estimation of a Capital inland earthquake in 2005. The history of earthquake occurrence in the Kanto region, which includes Tokyo, shows that an inter-plate earthquake of magnitude 8-class has occurred repeatedly at intervals of a few hundred years (Fig. 16-3). The previous M8-class earthquake was the 1923 Kanto Earthquake; there-fore, Tokyo will not experience an M8-class within this century. However, we should focus attention on the adjacent period before the M8-class earth-quake. We can understand that an intra-plate earthquake of M7 class, which is the same level as the Great Hanshin-Awaji Earthquake, can occur frequently within 100 years before an inter-plate earthquake. Thus, we have to prepare for intra-plate earthquakes of M7 class in the present pe-riod. We will experience a few earthquakes in Tokyo in our lifetime.

Tokyo Metropolitan Government (TMG) has taken countermeasures for

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Fig. 16-2. Distribution of earthquake probability of seismic intensity (JMA) 6 up-per in the next 30 years

Fig. 16-3. History of earthquake occurrence in the Kanto region

earthquakes. Accurate and detailed damage estimation is essential in con-sideration of countermeasures. TMG has reported on earthquake damage estimation three times in the last 20 years. These were for both inter-plate and intra-plate earthquakes. The latest estimation was published in 2007. The damage estimation report shows the damage volume, and describes the situation at each district after an earthquake. The main purpose of the damage estimation is to be used for the premise of the disaster prevention and management plan, which the Basic Act on Disaster Prevention re-quires.

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Fig. 16-4 shows an example of the work flow of the damage estimation. Firstly, a scenario and condition of earthquake occurrence is set: epicenter and magnitude of an earthquake is set; time point and wind conditions, which influence the situation of fire damage, are set. Secondly, various kinds of damage, such as ground shaking, liquefaction, buildings collapse, fire, lifeline damage, and casualties, are calculated based on an engineering model using a huge volume data. Four earthquakes which have different epicenter locations and magnitudes are set as a scenario, and two time points and two wind conditions were set in the TMG estimation in 2005.

According to the TMG report, seismic intensity in Tokyo will not ex-ceed JMA Intensity 6 upper, which is lower than the most damaged area in the Great Hanshin-Awaji Earthquake (Fig. 16-4). Special distribution of seismic intensity shows the difference of the ground structure. The east area in Tokyo is comparatively weak. Liquefaction possibility in the area

Table 16-1. Earthquake scenario in the TMD report

Epicenter Magnitude Time Weather con-dition

North area in Tokyo Bay

M6.9

5:00 in win-ter*1

3m/s*1

West area in Tokyo M7.3*1 18:00 in win-ter

6m/s 15m/s*2

1) The same condition as 1995 Great Hanshin-Awaji Earthquake Disaster 2) The same condition as 1923 Great Kanto Earthquake

Fig. 16-4. Estimation of seismic intensity in each earthquake scenario

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is estimated higher as well. Fig. 16-5 shows the distribution of building collapse. We understand that it concentrates in the east of Tokyo. This is the result from the accumulation of vulnerable buildings on comparative

Fig. 16-5. Estimation of building collapse in each earthquake scenario

Fig. 16-6. Estimation of fire damage in each earthquake scenario

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weak ground. Concerning urban fire-spread, huge damage is estimated beyond com-

parison to the Great Hanshin-Awaji Earthquake. This is due to the huge number of fire break-outs and the existence of large areas vulnerable to fire spread. The number of fire break-outs is estimated to be far more than fire-fighting power, which is considered to have adequate resources for normal situations. In the worst case, it is estimated at more than 1,000 in Tokyo; 500-600 fire break-outs would cause urban spread-fire. This is due to high density of ignitable buildings, and high seismic intensity. On the other hand, vulnerability of urbanized area to fire is influenced by several variables: distribution of buildings structures, building density, road den-sity, and layout of open space. The spatial distribution of fire damage is different from other types of damage, such as building collapse. The highly damaged areas are distributed in a doughnut shape. This area had been built-up without appropriate control under urban planning in the period of economic growth in 1960s. Total fire damage is estimated to extend to as much as 100 times the Great Hanshin-Awaji Earthquake: 100 km2 will be burnt in Tokyo.

Table 16-2 shows the summary of damage estimation. We can see that the situation in Tokyo will be different from that of Kobe in the Great Hanshin-Awaji Earthquake. Building collapse will not be as much as the Hanshin-Awaji Earthquake with consideration of city size; however, dam-age by urban fire spread will be huge beyond comparison to the Great Hanshin-Awaji Earthquake. What would cause this difference? I will explain based on output from Kato (2006). The left image in Fig. 16-7 is a map of fire damaged area in Kobe. The colored area in the map shows the actual burnt area in the Great Hanshin-Awaji Earthquake. The right image is a map of the district where the most fire damage is estimated in Tokyo. Each building is categorized by color. A cluster of the same colored buildings is called a “community sharing its fate” and means that all building belongs to a cluster will be-burnt down in case that more than one fire breaks out from a cluster. We should notice that the scale of the two maps is the same. There is appar-ently a severe shortage of roads in Tokyo and the density of houses ex-tremely is high. The fact that the actual urban fire spread in Kobe would be stopped at the wide road as seen in the map shows that Tokyo has even more vulnerability to urban fire than Kobe. According to our study, the burnt probability of this area in Tokyo is estimated nearly 0.8, and theburnt area is estimated at 2 km2. We should pay attention to this significant dif-ference between Kobe and Tokyo. The present characteristic in Tokyo can be said that fire damage will dominate, like the Kanto Earthquake.

This fact will be applicable to not only physical damage but casualties.

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Table 16-2. Comparison between the damage estimation in Tokyo and actual damage in the 1995 Hanshin-Awaji Earthquake

Building collapse Fire damage Damage estimation in Tokyo*1 Complete collapse 126,523

Half collapse 345,654 310,016

Actual damage in 1995 Great Han-shin-Awaji Earthquake Disaster

Complete collapse 104,906 Half collapse 144,274 7,483

1) Maximum number

Fig. 16-7. Comparison of the situation of urbanized area in Kobe and Tokyo as-pect from urban fire spread based on urban fire

The image of urban earthquake disaster tends to fix on the damage actual-ized in the Great Hanshin-Awaji Earthquake. Disaster will be influenced by all the components which constitute a city; therefore, we must have the correct image regarding urban earthquake disaster corresponding the actual situation of the city, without excess reference to the Great Hanshin-Awaji Earthquake.

I sum up vulnerability in Tokyo. Directly problems are as follows:

- ground form weakness in the east of Tokyo - shortage of roads and open space - existence and expansion of densely built-up area - high density and expansion of flammable buildings - existence of un-renewable built-up area - Most of problems would go back to the period of economic growth. - urbanization without consideration of hazards - imbalance between urbanization speed and infrastructure

development

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- uncontrolled buildings density

Summing up simply, we can say that Tokyo failed to accumulate urban stock at the period of economic growth.

In the upcoming period, we will experience population decline. I think that we should consider this period as a good chance to reduce the risk in the mega-city. Whether the risk in the city will increase or decrease de-pends on urban planning in the next period.

16.3 Earthquake Disaster Mitigation Plan at the City Level

In this section, I will introduce to earthquake mitigation plan at the city level. Tokyo has two kinds of countermeasure aspect from urban planning. One is the evacuation plan to guard citizens’ lives from heat radiation of urban spread fire. Another is the promotion plan to construct a disaster-resistant city.

Fig. 16-8. The evacuation plan in Tokyo

Fig. 16-9. The promotion plan to construct a disaster-resistant city

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Fig. 16-8 shows the evacuation plan in Tokyo. This plan prepares some safe refuge sites from urban fire-spread for all citizens. TMG assigns 170 sites as safe refuge at the present. The area of these sites is said to be ap-proximately more than 10 ha. Safety of all the sites from heat radiation was assured with calculations based on an engineering model. Firstly, flammable zones around a candidate refuge site are assumed, the flame height of urban fire at the zones is calculated, and the intensity of heat ra-diation from flames inside the site is calculated. Moreover, maximum ca-pacity for evacuees is calculated from area which will be less than heat ra-diation intensity for human to endure. The sphere of evacuees is decided corresponding to the capacity. If all citizens were to evacuate to the speci-fied evacuation site, in theory their lives would be guarded from urban fire spread.

Another plan at the city level is the promotion plan to construct a disas-ter-resistant city (Fig. 16-9). This plan has two points. One is to partition the entire urbanized area to urban fire compartments, which is an area of approximately 80 ha. Another is to mitigate vulnerability inside urban fire compartments.

All boundaries of each urban fire compartment are formed by a belt consisting of a wide road and fire-proof buildings by the roadside which will stop urban fire-spread (Fig. 16-10). Inside each compartment, com-munity-based improvements are implemented, such as small open spaces known as pocket parks, collaborative housing renewal, and road improve-ment to widen roads with less than 4 m width to 6 m or 8 m. According to

Fig. 16-10. Image of urban belt to stop fire-spread, through the boundary of com-partments

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Fig. 16-11. Districts with high vulnerability in Tokyo

the Tokyo fire department, approximately 80% of urban belts to stop fire-spread have been built up at the present; however, they are unevenly dis-tributed. Development has made little progress in the west area in Tokyo. As the main reason, it is pointed out that some roads authorized in city planning have not been constructed yet.

Inside compartments, there are also many districts with high vulnerabil-ity, shown as colored areas in Fig. 16-11.

16.4 Earthquake Disaster Mitigation Plan at the District Level for Densely Built-up Wooden Housing: Focusing on New Technologies

Community-based improvement has been implemented inside compart-ments with high vulnerability. This topic is explained in detail in the next chapter. In this section, we are concerned with advanced technologies to support community-based improvement planning, which is becoming popular. I will introduce the community-based planning support system for disaster mitigation based on GIS technology.

The system has three functions: urban spatial database linked to GIS, vulnerability assessment function or disaster simulator, and user-interface to input an alternative plan or countermeasure. The planning process with the planning support system is shown in Fig. 16-12. In the first phase, we recognize the present problems in the community with a vulnerability as-sessment function, such as that shown in Fig. 16-13, and then we set a goal

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Fig. 16-12. The planning process with the planning support system

regarding each problem. In the second phase, we consider an alternative with an input-interface, as shown in Fig. 16-14, and then we check the ef-fectiveness aspect from disaster mitigation with the vulnerability assess-ment function or a simulator. We can reconsider the countermeasures after effectiveness verification, and draw up other alternatives. This cyclic proc-ess refines and optimizes the countermeasures. As a result, the counter-measures or plans will be more suitable to the local characteristics of the community.

We consider that the system would be used by all participants in plan-ning: planners, administrative officers in charge of planning, and the resi-dents, who would not have GIS skills, and that the system has three roles: to make plans and countermeasures, to share common perceptions of haz-ard or risk among users, and to communicate with all participants in this process.

Fig. 16-15 is an example of an alternative plan to widen two streets and to apply new regulations, according to which all new buildings must be fire-resistant or fire-proof. You can draw a plan like this in less than 10 minutes if you are familiar with it. Fig. 16-16 shows the results to check effectiveness. We can easily understand that the fire damage will be re-duced.

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Fig. 16-13. An example of fire-spread simulation for a typical densely built-up area in Tokyo

Fig. 16-14. User-interfaces for consideration

Fig. 16-15. An example of an alternative plan to widen two streets and to apply new regulations, according to which all new buildings must be fire-resistant or fire-proof

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Fig. 16-16. An example of effect verification with fire-spread simulation

The system has been used as a tool for risk communication with resi-dents and residents’ participatory planning workshops, combined with conventional tools such as drawing work and fieldwork. At the present, more than 20 municipalities already use it or will use it soon. This style, which we call vulnerability assessment-based planning, is becoming estab-lished in Japan. The effectiveness of this system has already been demon-strated in some cases, especially in risk-communication with residents; however, concerning plan-making, we need to refine know-how for using it to achieve increased effectiveness, such as how to communicate with residents using the system, how to show the residents the effectiveness of an alternative, and how to program workshops using the system. Planners and administrative officers may need to experience and learn more. From the aspect of software technology, we think that this system has been es-tablished as applied technology based on GIS; however, we will continue to improve the system through actual cases and strive for technical im-provements.

References

Kato, T., Yusuf, Y. and Cheng, H., Yamaguchi, M. and Natori, A. (2006) “A Method Applying to any Different Map-Scale for the Integrated Earthquake Fire Risk Evaluation with the Single Building Data and consideration of Fire Break-out Probability”, Journal of Institute of Social Safety Science, 9 (in Japanese)

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Committee for Promotion and Administration of Community-based Planning Support System for Disaster Mitigation, http://www.bousai-pss.jp (Accessed in 2007)