Exactly three years after the devastating Kobe earthquake, which killed 6,300 people, Tokyo appears to be heading towards an even greater disaster, according to a group of Japanese scientists who presented their findings yesterday.
Their conclusions are based upon a study of thousands of smaller tremors, many of them detectable only to the most sensitive seismological instruments, which have been occurring with increasing frequency in the Tokai area, south-west of Tokyo. According to the government-backed National Research Institute for Earth Science and Disaster Prevention (NIED) these may be a precursor of a new Tokai earthquake, which last struck 143 years ago.
"It's pretty alarming," said Dr Yoshimitsu Okada, director of NIED's Earthquake Research Centre. "There are several stages until the Tokai earthquake actually takes place - we think of it like climbing a staircase. Twenty years ago we decided that it was dangerous in the long term. Now we have the impression that we've climbed up a step from long-term alarm to medium-term alarm."
Perched on the convergence point of three tectonic plates on the so-called Ring of Fire, the Japanese islands have active volcanoes, hot springs and geysers, and thousands of earthquakes every year, most of them no more than transitory shudders. But every few decades comes a huge quake which destroys whole cities and kills large numbers of people.
In 1923, 140,000 people died in the Great Kanto Earthquake which had a magnitude of 7.9 on the Richter scale, with an epicentre under the sea off Tokyo and Yokohama. The Kobe disaster, also a submarine quake, measured 7.2.
The Tokai area is some 50 miles south-west of the capital, but the earthquakes which occur there have historically been even more powerful - the last Tokai earthquake occurred in December 1854, with a magnitude of 8.4. It is this fault which the scientists fear may become active again.
In the mid-19th century, Tokyo was still a feudal city of low-rise wooden buildings; today it has absorbed satellite cities to form a megalopolis of 30 million people, with skyscrapers, overhead expressways and millions of tonnes of fuel oil and poisonous chemicals stored in tanks around Tokyo Bay.
Despite thousands of measuring devices all over Japan, it is impossible for seismologists to predict earthquakes as meteorologists predict the weather. "But we can say that the Tokai earthquake will be much bigger, and the space affected much larger, than in Kobe," says the director-general of NIED, Dr Tsuneo Katayama.
Historical records show that previous Tokai quakes have been preceded by unusual seismic events. One of these is an increase in the frequency of moderate earthquakes,with a Richter magnitude of 4 or greater, which cause little damage in themselves. As the graph shows, there were 14 of these in the Tokai area in the 16 years up to 1996. In the last 18 months, however, there have been no fewer than seven such quakes. Other recent observations indicate the rate at which the earth's crust is sinking has slowed during the 1990s, another precursory sign.
Modern buildings in Tokyo are required by law to be "earthquake proof", but the many older buildings have never been tested by a real disaster. Casualty numbers are impossible to predict precisely, but an American projection in 1996, based on a repeat of the 1923 Kanto tremor, painted a worst case picture of 60,000 dead and "staggering" economic losses. "It is impossible to say exactly what will happen," says Dr Okada, "but one thing is certain: sometime in the future the stairway we're climbing will come to an end."Reuse content