Japan is one of those countries most often affected by powerful natural hazards: earthquakes, tsunamis, volcanic eruptions, typhoons, heavy rains, heavy snowfall, tornadoes, etc. The Archipelago is considered a very advanced country in terms of forecasting, prevention and management of natural disasters. A detailed analysis of the reality of recent years is however necessary. In the run-up to the 2020 Olympic Games in Tokyo, among others, a publication on the subject will inform a large number of people wanting to know more about the risks of natural disasters in Japan.
Earth and Fire
Kanto, September 1, 1923. "There's an ocean of fire in Fukagawa", Honjô yells to his friend Jirô and then looks toward the sky disturbed by the wind: "The fire will spread. Tokyo is finished!" A little earlier, Jirô, the young hero of Hayao Miyazaki's animated film The Wind Rises, had found himself on a train when the earthquake occurred. The rails, like the wooden houses, had undulated violently to the rhythm of the propagation of the seismic waves. The train managed to stop without derailing. However, as everyone fled, columns of smoke appeared on the horizon [MIY 13].
"The reason the fires spread so quickly in spite of the profusion of vegetation was that it was noon and that, in every home, the stoves were glowing red with coal", written by Junichirô Tanizaki in A Tuft of Hair, which appeared in 1926 [TAN 97]. The writer knew something about it, since he experienced the event, which even prompted him, several days later, to move from Kanto to Kansai, a region considered safer at that time.
If the great Kanto earthquake, caused by an earthquake with a magnitude of 7.9 followed by numerous and powerful aftershocks, is widely present in the literature of the 1920s and still on the mind of our contemporaries, as Miyazaki's masterpiece shows, it is because it was the origin of the worst natural disaster in Japan's history. There were 105,000 dead, of which 88% would die in the flames. More than 460,000 buildings were completely destroyed. The economic damage represented more than 35% of the GDP [SHI 15, IMA 16].
2.1.1. Japan, principal seismic country
All of Japan's inhabitants have certainly experienced an earthquake. The world leader in terms of number of earthquakes measured, the Japanese archipelago is located in an area where four tectonic plates meet and continuously shift against each other. The brutal shifting between two rock masses, which causes shocks, can appear in various places: between two plates, such as the earthquake in eastern Japan on March 11, 2011; the interior of a plate, such as the Hokkaido Tôhô-Oki earthquake on October 4, 1994; at shallow depths in Earth's crust, such as the Kobe earthquake of January 17, 1995 or the earthquakes in Kumamoto on April 14 and 16, 2016. The latter phenomena appeared due to the movement of active faults, of which 2,000 are currently listed in Japan. Among them, 113 are considered major, which means they may be the cause of an earthquake with a magnitude of 7 or higher.
In Japanese culture, the story is very different: it was said that the country rested on the back of a colossal catfish that caused the earth to shake when it moved. The first known written record of this mythical connection dates from 1592. In a letter, the warrior Hideyoshi Toyotomi (1537-1598), who was one of the unifiers of Japan, wrote the following: "Construction of the Fushimi Castle is not moving forward because of the catfish". This interpretation receded with the development of science, but did not disappear entirely from popular discussions, as the essayist and novelist Aya Kôda (1904-1990) remembered.
"At that time [1919-1920], the adults joked with children by telling them that earthquakes were caused by the movement of a great catfish that lived under the earth. Boys and girls were divided between doubt and belief " [KÔD 96, p. 52].
While magnitude indicates seismic energy released by an earthquake, intensity measures its significance according to the reactions of people, the movement of objects or damage to buildings. Since 1996, the Japanese scale has comprised 10 degrees - as opposed to eight degrees from 1946 to 1995 and seven degrees from 1898 to 1946. Although hundreds of earthquakes occur every day in the archipelago, the vast majority of them are not felt by the population. On the contrary, since the great earthquake of 1923, close to two-thirds of Japan's 47 prefectures have been affected by very violent shocks at least once1.
Figure 2.1. Frequency of earthquakes with an intensity equal or more than the sixth degree by prefecture from 1923 to 20162. For a color version of this figure, see www.iste.co.uk/heimburger/japan.zip
2.1.2. The effects of earthquakes
Japan is very young on the geological scale, and its current shape is only 600,000 years old. The top layer, made up of sediments a few thousands of years old, could be compared to the fragile and soft outer body covering of a cicada that has just accomplished its last molting. The nature of this terrain thus reinforces the effect of terrestrial risks: the ground there trembles more during earthquakes and it slides more easily. Earthquakes can thus cause landslides, fissures in the ground or even liquefaction. This latter phenomenon is the loss of cohesion of certain soils: the shocks disturb grains of sand and water which make up the terrain, which has the result of turning it liquid. It is observed not only in natural areas, such as the shorelines of gulfs, the banks of rivers of the bottom of valleys, but also in anthropic areas, such as embankment. The central part of the city of Yokohama, for example, today rests on embankment, in other words fragile soil, built following the same technique as 100 years ago [ÔKI 15].
Table 2.1. Characteristics of the ten degrees of intensity of the Japanese scale according to the Japan Meteorological Agency Degree of intensity Human perception and reaction Situation indoors Situation outdoors 0
Shaking is imperceptible 1
Shaking rarely perceptible indoors 2
Shaking sometimes perceptible indoors Swinging of hanging objects 3
Shaking perceptible indoors Rattling dishes Light swinging of power lines 4
Shaking perceptible while walking; surprise Strong swinging of hanging objects Strong swinging of power lines 5-
Need to hold onto something stable; fear Risk of falling dishes and books or even furniture toppling over Movement of electrical poles, risk of damage to roads 5+
Walking is difficult Increasing risk of furniture toppling over Collapse of nonreinforced walls 6-
Difficult to remain upright Damage to homes with low earthquake resistance 6+
Impossible to remain upright Increasingly high risk of damage to earthquake-damage to earthquakeresistant homes and collapse non-earthquake-resistant homes 7
Furniture toppling over or even thrown through the air
The direct and natural effects of shocks can then lead to damage to society: damage or destruction of buildings and critical infrastructures; fires, deaths and injuries; decrease in economic activity and anxiety in the population. Since 1990, earthquakes have been responsible for close to 90% of victims of natural disasters in Japan. The major earthquakes of 1995 and 2011 respectively caused an economic impact equivalent to 2.1% and 3.5% of the GDP of the years in question.
Although the trigger mechanism can be a mass movement or a meteorite, the majority of tsunamis are seismic in origin. This involves a wave which travels across a lake, a sea or an ocean. This Japanese word, possibly derived from tsuyonami ("strong waves"), appears for the first time with this meaning in a document published in 1611 [KUR 16]. The speed of a tsunami decreases and its height increases the closer it gets to the coastal areas on which it breaks. The size of this phenomenon depends of several factors, including location and the magnitude of the earthquake, as well as the direction of the fault's movement. In the absence of protection and according to the height of the wave, a tsunami can cause destruction and flooding. Even a tsunami 20 cm high poses a danger, notably for swimmers, due to its pressure. According to the Japan Meteorological Agency, starting at 1 m, a tsunami can cause flooding in buildings or land, and starting at 3 m, major damage to infrastructure is recorded. If someone is caught in a tsunami, the mortality rate increases strongly starting with a phenomenon 50 cm high and reaches 100% for a wave with a height of only 1 m [TSU 16b].
Table 2.2. Categories of tsunamis according to their scale3 Category Height Level of damage - 1
Less than 50 cm No damage 0
Around 1 m Minor damage 1
Around 2 m Damage on the coasts and to...