GeoJourna138.3: 349-353. © 1996 (March) Kluwer Academic Publishers. Printed in the Netherlands.

Two surveys for taking measures to cope with the coming earthquake to the Tokyo Metropolis

Matsuda, Iware, Prof. Dr., Kanto Gakuin University, College of Economics, Yokohama, 236 Japan

ABSTRACT: Although Tokyo City has frequently suffered from earthquake disasters, more than 70 years have passed since the last catastrophe of 1923. Because Tokyo has a greater probability now than ever of being hit by another major earthquake, the Tokyo Metropolitan Government has carried out two surveys for taking measures to cope with it; earthquake vulnerability assessment and earthquake damage prediction. The former assesses earthquake vulnerability regionally and is used for disaster-proof city planning. Damage due to the expected earthquake is quantitatively estimated in the latter. The results are used for making a regional disaster prevention plan in an emergency. Both physical and human geographers participate in these interdisciplinary surveys effectively.

1. Short h i s t o r y o f e a r t h q u a k e d i sas t er s

A total of 37 earthquakes which have shaken the city of Tokyo at an intensity of 5 or more on the Japan Meteorological Agency (JMA) Scale since 1595 are detected (Usami 1976). An intensity of 5 on the JMA Scale is nearly equal to that of 8 on the Modified Mercalli 's Scale. Among the 37 earthquakes, 6 earth- quakes recorded an intensity of 6 on the JMA Scale and caused deaths, injuries and destruction of build- ings. They are the earthquakes of 1615, 1649, 1703 (the Genroku Earthquake), 1855 (the Ansei Edo Earthquake), 1894 and 1923 (the Great Kanto Earthquake).

The major two earthquakes of 1703 and 1923 originated in the transform fault along the eastern edge of the Sagami Trough which is the boundary between the Philippine Sea Plate and the Pacific Ocean Plate. The magnitude (M) of the Genroku Earthquake of 1703 was 8.2 on the Richter Scale and that of the Great Kanto Earthquake of 1923 was 7.9. These two earthquakes disastrously damaged the whole southern Kanto area. Tokyo City was a part of the affected district.

The Ansei Edo Earthquake of 1855 and the earth- quake of 1894 were minor. Their magnitude were 6.9 and 7.0, respectively. Nonetheless, they inflicted devastating damage on Tokyo City because their foci were directly under there. Such earthquakes are called a direct-hit type earthquake. The earthquakes of 1615 (M -- 6.4) and 1649 (M = 7.1) are classified into this type.

The most serious damage broke out in 1923. The Great Kanto Earthquake claimed the largest number of deaths and damage ever caused by a natural disaster in Japan. The number of dead and missing reached a total of about 143,000 and 104,000 people were listed as injured. Approximately 128,000 struc- tures were demolished by the shock and as many as 447,000 structures were lost in fires. Although the damage to structures due to strong ground motion were severer in the areas near the epicenter than Tokyo City, fires made damage more far serious in the latter. Soon after the shock, 129 fires broke out simultaneously in Tokyo City. They continued to rage for 40 hours, scorching 44% of the city area. About 60,000 people, namely about 2.4% of the total population of Tokyo City at that time, were lost and about 370,000 structures were burnt out. Fires were to blame for about 89% of the dead and 98% of destroyed houses. This is the main reason why coun- termeasures for fires are regarded as most important against an earthquake.

About 10,000 human lives and 140,000 houses were lost in Tokyo City (Edo in those days) by the Ansei Edo Earthquake. But these damage were due more to the direct impact of ground shaking than fires. In this event, only Tokyo City was devastated and damage in surroundings was not serious.

Because Tokyo has not experienced an earthquake with an intensity of 5 or more since 1929, we can say that Tokyo has a greater probability now than ever of being bit by another major one.

350 lware Matsuda

2. T w o s u r v e y s f o r p l a n n i n g c o u n t e r m e a s u r e s

For the reduction of earthquake damage, it is essen- tial to adopt comprehensive countermeasures being composed of structural and non-structural ones. Tokyo, however, is too overpopulated to adopt such non-constructural countermeasures as landuse man- agement or building regulation. It is a more realistic approach in the case of such an overcrowded city as Tokyo to take countermeasures on the basis of the regional differences in assessed vulnerability or predicted earthquake damage (Matsuda 1990). Two different surveys for planning countermeasures have been carried out by the Tokyo Metropolitan Govern- ment. One is earthquake vulnerability assessment and the other is earthquake damage prediction.

2.1. Earthquake vulnerability assessment

The Tokyo Metropolitan Assembly legislated the Metropolitan Earthquake Disaster Prevention Regula- tion in 1971. Section 17 of the Regulation has made it compulsory for the Tokyo Metropolitan Govern- ment to assess earthquake vulnerability regionally and to announce the results. According to the Tokyo Metropolitan Government (1976), the purposes of the survey are as follows:

a) To make the results of the survey a guideline in working towards a city where damage from earth- quake disasters could be kept to minimum;

b) To further the awareness of the citizens of Tokyo regarding earthquakes and make them more conscious of ways and means to keep damage from the disaster to the minimum;

c) To make the survey a guidepost in selecting areas where the implementation of earthquake and fire countermeasures should be given priority.

In the urbanized areas of the Tokyo Metropolis, 5,017 grid blocks, each measuring 500 m square, were assessed their vulnerability to earthquake disasters. Vulnerability was measured by risk for buildings and people due to ground shaking and fires induced by the expected earthquake with a magnitude of 7.9 and an epicenter in Sagami Bay (same as those of the Great Kanto Earthquake). The results were shown by five grades of 5 to 1 to provide a comparative index of the risk level. The grade 5 means most vulnerable. Although risk due to such a direct-hit type earthquake as the Ansei Edo Earthquake was calculated, it was not used for grading regional vulnerability.

Figure 1 is the flow diagram of earthquake vul- nerability assessment. As a first step the following five basic works were carried out:

a) To make ground shaking hazard maps: after superficial geological conditions were classified into 40 soil types, the maximum ground surface velocity was calculated for each soil type with additional survey of a propensity to cause liquefaction; vulner- ability to slope failure in hilly areas was assessed;

b) To make fire hazard maps: the number of fires breaking out soon after shock, their spreading velocity and capacity of fire fighting were surveyed, because it was assumed that damage due to fires should be a function of these three elements;

c) To make maps showing elements relevant to evacuation: distance to a designated refuge site from the center of a grid block was measured; reduction

l i i assume the expected earthquakes

The Great Kanto Earthquake of 1923 Ms?.9; The maximum acceleration of incident wave is lOOgal The Ansei Edc Earthquake of 1855 M=6.9; The maximum acceleration of incident wave is 60gal

I To make ground shaking hazard maps

I. Soil types 2. Maximum ground surface velocity 3. Vulnerability to liquefaction 4. Vulnerability to slope failure

i i make f i re hazard maps

Number of f i res breaking out Spreading velocity of f i re Capacity of f i re fighting

I )l ( To survey building distribution

1. Number of buildings by structure, use, number of stories, age, etc.

2. Total floor area 3. Building-to-land r a t i o

t I To assess risk for buildings To assess risk for due to ground shaking buildings due to f i res

1. Number of collapsed wooden houses I. Total f loor area of 2. Number of heavily damaged rein- buildings burnt down

forced concrete buildings

I I I

I Total risk assessment for buildings [

To make maps showing elements relevant to evacuation

I. Distance to the designated refuge site 2. Evacuation speed 3. Travel time required to reach the designated refuge si te 4. Number of evacuees

1 Tisurvey population distribution

Nighttime population by age and sex Daytime population by age and sex Population during commuting hours

I I To assess risk for people due I To assess risk for people during to ground shaking |evacuation

t 1. Relative potential for | 1. Total time necessary for that

husan losses induced by | all people reach the designated damage to bum|dings ] refuge s i te

I ]

ITotal risk assessment for peoplel

Figure 1. Flow diagram of earthquake vulnerability assessment.

Earthquakes and the Tokyo Metropolis 351

in evacuation speed due to road congestion and fires was calculated; necessary time to go to a designated refuge site was deduced from dividing the distance by evacuation speed; the number of evacuees were obtained from population distribution;

d) To survey building distribution: the number of buildings by use, structure, number of stories and age was surveyed; the total floor area and the building- to-land ratio which affect the spreading velocity of fire were calculated;

e) To survey population distribution: nighttime and daytime population by age and sex were summed up from the population census data.

Based on the above, the four kinds of risk were assessed for each grid block. They are as follows:

a) Risk for buildings due to ground shaking: using the ground shaking hazard maps and building distri- bution maps, the number of collapsed wooden houses and heavily damaged reinforced concrete buildings were estimated by earthquake engineering method.

b) Risk for buildings due to fires: based on the fire hazard maps and building distribution maps, the total floor area of building burnt down was estimated;

c) Risk for people due to ground shaking: since people are apt to be injured or killed with damage to urban facilities due to ground shaking, relative potential for human losses is assessed by combining ground shaking hazard maps and population dis- tribution maps;

d) Risk for people during evacuation: the total time necessary for that all residents evacuate to the designated refuge site is calculated by multiplying

the travel time required to reach the refuge site and the number of population.

Combining two kinds of risk for buildings due to ground shaking and fires, the total risk assessment for buildings was deduced, and so done risk for people.

The results of survey were announced first in 1975 and revised in 1984 and 1993. Figure 2 shows the result of total risk assessment for buildings in 1993. Grades do not give the extent of damage but the relative degree of risk. It can be seen that high risk areas surround the central part of Tokyo. These areas have been urbanized since the Great Kanto Earthquake of 1923. People who lost their houses moved to suburbs at that time.

2.2. Earthquake damage prediction

The Disaster Countermeasures Basic Law was enacted in 1961 after the heavy damage caused by the Isewan Typhoon of 1959. The Law directed both national and local governments to establish a disaster prevention council to develop a countermeasure plan for natural hazards. The Tokyo Metropolitan Govemment established its council in 1962.

The Law made it compulsory for a disaster pre- vention council to make regional disaster preven- tion plans. Some purposes of the Regional Disaster Prevention Plan for Tokyo Metropolis are as follows: a) To carry out appropriate countermeasures for

disaster prevention; b) To execute necessary actions in an emergency;

Figure 2. Total risk assessment for buildings.

352 Iware Matsuda

c) To execute a quick restoration from the damage; d) To protect the Tokyo Metropolis from natural

hazards; e) To secure a life and property of a citizen of the

Tokyo Metropolis. Since the Regional Disaster Prevention Plan has

to be drawn up in order to take measures to cope with the estimated damage, the Tokyo Metropolitan Government has carried out the survey for quantita- tively estimating earthquake damage since 1965. The first result was announced in 1978 and the latest one in 1991 (Tokyo Metropolitan Government 1991).

The magnitude and epicenter of the expected earthquake were defined to be same as those of the Great Kanto Earthquake. The earthquake was assumed to hit Tokyo City at around 6 o'clock in the evening in winter. Wind was supposed to be blowing from NNW with a velocity of 6 m/s.

Three stages of damage were taken into consider- ation. The first stage is the direct damage for urban facilities due to ground shaking, liquefaction and slope failure. The second stage is loss of human lives and the enlargement of direct damage for urban facilities due to fires, floods, etc., which may be triggered during the first stage. After effects resulting from the losses in the first and second stages persist in the last stage. For example, destruction of infra- structures affects industries as well as residents' everyday life. The third stage is characterized by after effects until everyday life and social activities are restored.

The basic methods for estimating damage in the first and second stages were similar to those adopted in the earthquake vulnerability assessment except for that of calculating the number of casualties. The extent of after effects in the third stage was deduced from the number of affected population and the amount of direct damage to urban facilities in the previous two stages. Adding them, scenarios on everyday life for two modeled households after the event were presented in order to help better under- standing of after effects. Some results of the calcu- lated damage due to the expected earthquake is shown on Table 1.

3. Some countermeasures for earthquake disasters

Soon after the Tokyo Metropolitan Disaster Preven- tion Council was established in 1962, the Council suggested that countermeasures for an earthquake event were urgently required. It's suggestion was based on the "69 year periodicity" presented by late Prof. Kawasumi in 1961. Because fires were a main cause of catastrophic damage in the Great Kanto Earthquake of 1923, countermeasures for fires were considered to be most important. Accord- ingly, construction of disaster prevention bases was planned to secure safe refuge against fires. The

Table 1. Calculated damage of the expected earthquake

Nighttime population Daytime population Estimated population in the evening Total number of buildings

Heavily damaged buildings Wooden houses Reinforced concrete buildings Steel framed buildings

Damage to road bridges Damage to water supply pipes

(main pipes only) Damage to city gas pipes Damage to electric supply facilities Interruption of telephone service Houses burned by fires Dead Heavily injured People unable to go back home People met with difficulty in living

11,785,899 13,963,850 13,676,720 2,396,092

132,787 14,029 8,600

21

8% 87% 33% 28%

632,616 9,363

21,093 2,430,405 3,202,898

Tokyo Metropolitan Government designated 43 refuge bases in 1968. Designation of refuge bases have been revised every five years and 146 bases have been designated in Tokyo City at present.

In 1969, the Redevelopment Plan for the Koto District was announced. At that time, the Koto District located on the deltaic lowland in the eastem part of Tokyo City was regarded as one of the most dangerous districts for an earthquake disaster. Wooden houses had been densely built on soft deposits and were interspersed with factories using combustible materials. Ground height had been lower than the sea level due to land subsidence induced by the withdrawal of ground water. Also, air and water pollution were increasing as problems. Counter- measures for an earthquake together with general improvement of the living conditions were urgently required.

Construction of 6 disaster prevention bases was the main project in the Plan. Each disaster preven- tion base is planned to have an area of 50 to 100 ha and secure open spaces where all residents in the Koto District can take refuge within 30 minutes. Although 6 disaster prevention bases have been planned, construction work has been completed for only one base. A built-up area redevelopment project or a fireproofing promotion project has been under- taken in the other base areas.

The Earthquake Vulnerability Assessment of 1975 identified that countermeasures for fires were neces- sary for not only the Koto District but also the western part of the city. As shown in Figure 2, this condition has not been improved yet. The Tokyo Metropolitan Government drew up the basic Plan for Urban Disaster Prevention Facilities in 1981, to push forward development of a disaster-proof city. The policy adopted in this basic plan was not to secure

Earthquakes and the Tokyo Metropolis 353

safe refuge against fires, but to make evacuation unnecessary. Policy for coping with earthquake disaster has been turned. Three points are given attention in development of a disaster-proof city (Tokyo Metropolitan Government 1986). They are as follows:

a) Disaster-proof city planning should be based on security, but it should be suited to comforts and con- venience which are necessary for daily life;

b) Functions for disaster prevention should be provided not only with works appropriate to disaster prevention, but also with those for long-term urban development and other improvement works for urban facilities;

c) Works for disaster prevention should be carried out together with measures for the community.

These points show that works for an emergency have to fit in with everyday life and at the same time, creation of a community which secures its own safety by itself is indispensable. Because development of a disaster-proof city would take several decades, two practical plans are suggested. One is construction of firebreaks to prevent fires from spreading. The other is improvement of a refuge plan. Also, creation of disaster-proof living zones is recommended.

A network of firebreaks is composed of roads, railways, rivers and parks. Necessary fireproof measures can be carried out along firebreaks. Fire- breaks divide the Tokyo City area into about 700 blocks. Each comprises a disaster-proof living zone which roughly covers an elementary schools or junior high school district. Residents are expected to combat disaster as well as to prevent an outbreak of fire. The refuge plan was revised several times in order to make it possible that all people can find a refuge base within a distance of 3 km. As mentioned earlier, 146 refuge bases have been designated.

4. Concluding remarks

Tokyo City has frequently suffered from earthquake disasters. It has passed more than 70 years since the last catastrophe of 1923. It means that probability being hit by another major earthquake is increasing. Vulnerability pointed out in Tokyo City is as follows:

a) Most of the built-up areas are composed of wooden houses built relatively close together;

b) Roads in the residential areas are narrow, and open spaces are scarce;

c) There are many urban facilities which have not yet experienced a strong earthquake, such as sky- scrapers, underground complexes, highways and so on;

d) The concentration of population, urban facili- ties and economic functions means that it would be difficult to cope with the damage that would result from a big earthquake.

The Tokyo Metropolitan Government carried out two kinds of surveys for taking measures to cope with earthquake vulnerability; earthquake vulnera- bility assessment and earthquake damage prediction. A total of 5,017 grid blocks, each measuring 500 m square, were assessed their vulnerability to earth- quake disasters in the former. The results were shown by five grades to evaluate a relative risk level. Tokyo Metropolitan Government has used the results for disaster-proof city planning. It is intended in this planning that functions for disaster prevention are provided not only with works appropriate to disaster mitigation but also with those for long-term urban development and other improvement works for urban facilities. The main purpose of the latter is to quan- titatively estimate the amount of damage due to the expected earthquake. The results have been used for making a regional disaster prevention plan. In this plan, necessary actions which have to be carried out in an emergency by Tokyo Metropolitan Government are stated.

These surveys can be carried out only by inter- disciplinary cooperation. Both physical and human geographers participate in these surveys effectively.

References

Matsuda, I.: Natural disasters and countermeasures for the Tokyo Lowland. Geographical Review of Japan (Ser. B) 63(1), 108-119 (1990).

Tokyo Metropolitan Government: First step toward a city where damage from disasters can be minimized. Tokyo Municipal News 26(1), 1-3 (1976).

Tokyo Metropolitan Government: Tokyo-to ni Okeru Bosai Toshi Zukuri no Gaiyo (A Summary of Disaster-Proof City Planning in Tokyo), 132 pp. (1986) (J).

Tokyo Metropolitan Government: Tokyo-to ni Okeru Jishin Higai no Sotei ni Kansurn Chosa Kenkyu (Earthquake Damage Prediction for the Tokyo Metropolitan), 223 pp. (1991) (J).

Usami, T.: History of disastrous earthquakes in Edo (Tokyo). Bull. Earthq. Res. Inst. 51,231-250 (1976) (JE).

(J): in Japanese; (JE): in Japanese with English abstract.