EARTHQUAKE IN JAPAN: THE CAUSES: `Early warning' system eludes scientists

  • @SteveAConnor
The earthquake's destructiveness was due to its relatively shallow depth rather than magnitude, seismologists said yesterday. Its epicentre also struck almost directly beneath a city with traditional timber-framed houses and heavy tiled roofs whic h collapsed easily and burned readily in a conflagration caused by fractured gas and fuel pipes.

The British Geological Survey in Edinburgh said the earthquake measured 7.2 on the Richter scale. "We get about 20 of them a year worldwide, so it's not that big," Alice Walker, a survey seismologist, said.

But it struck at a relatively shallow depth of 30km almost directly beneath the city, which sent seismic waves to the surface that shook and splintered buildings, roads and mains.

"Usually the earthquakes in Japan are deeper and so produce less impact at the surface," Ms Walker said. The last great earthquake to hit Japan, in 1923, measured 8.2 and killed an estimated 143,000 people after fires consumed most of Tokyo. Like the 192

3 quake, most of the deaths in Kobe can be attributed to fires.

Scott Steedman, director of engineering at Gibb consulting engineers in Reading, said the older timber houses in Japan made its cities especially vulnerable.

Dr Steedman, who has just returned from Japan, said its level of preparedness is probably the best in the world. A programme is also underway to strengthen older timber-framed housing. "The problem is it takes time. It takes generations to strengthen theentire building stock.''

Japan lies in one of the most active regions for earthquakes. It sits at the junction of three tectonic plates - the Pacific, Eurasian and Philippine Sea plates - that are slowly sliding into one another. This creates stresses in rock which are periodically released as earthquakes.

The principal movements are due to the "subduction" of the Pacific plate from the east under the Eurasian plate in the west. A great tongue of land descends up to 300km under Japan at an angle of about 40 degrees, melting the rock as it descends.

Japan owes its existence to this geological process. As molten rock rises to the surface it causes volcanic outcrops that have risen above sea level.

Wayne Thatcher, a seismologist at the US Geological Survey in Menlo Park, California, said the Kobe earthquake happened on a minor fault above the major subduction zone. "It is an area with a strike-slip fault where there is lateral movement rather than vertical movement," he said.

About 1,500 earthquakes are recorded each year in Japan but only a quarter are strong enough to be felt at the surface. Violent earthquakes are less frequent, heavy ones occurring once every 10 or 30 years.

Attempts to predict earthquakes have largely failed. Although seismologists can estimate the probability of an earthquake in a particular area, they cannot give a time or date, rending their predictions useless in practice. Research aimed at improving the accuracy of predictions rests on being able to identify any seismic signs of an impending earthquake, yet many earthquakes have occurred without any precursory seismic movements.