Wednesday, September 22, 1999 Published at 10:35 GMT 11:35 UK
Earthquake 'unusual', say scientists
Seismologists say that Taiwan's devastating earthquake which has killed thousands and rocked the capital Taipei was unusual, breaking a trend of quakes occurring hundreds of kilometres out at sea.
Already, scientists, many of whom consider Taiwan to be a unique area to study earthquakes, have begun analysing data from seismometers around the world.
Tremors can occur as deep as 700 km and as recently as 1993 Taiwan experienced a quake that was more than 150 km down. This one was probably just a few kilometres down.
Initial assessments timed the quake at 0147 (1747GMT Monday), with its epicentre - the point on the Earth's surface directly above the quake - located at 23.7 degrees north latitude and 121.1 degrees east longitude, about 12.5 km (7.75 miles) west of the scenic Sun Moon Lake in central Nantou county.
The last time that Taiwan experienced an earthquake of this magnitude on land was in 1935, resulting in more than 3,000 deaths.
"A magnitude 7.5 earthquake will occur maybe four times a year somewhere in the world," said Dr Browitt.
"It's just that this one happened to hit at a place where a lot of people and buildings are exposed."
The destructive power of an earthquake comes from the momentum gathered when two opposing "faults" or plates of rock, which may have been locked together for decades, suddenly move apart.
Taiwan sits in a very active seismological region where the relatively small Philippine plate is slowly grinding into the very large Eurasian plate.
To the north of the island, the Philippine plate is being forced under the Eurasian plate in what seismologists call a subduction zone. To the south, the opposite is happening with the Eurasian plate being forced under the Philippine plate.
Dr David Petley, a geologist who has worked on earthquake hazard-reduction projects in Taiwan since 1991, said that the plate movement is as much as 7cm a year - literally forcing the island's mountains to rise annually by 5mm.
While in layman's terms this appears to be a minuscule figure, Dr Petley said that the twin phenomenon of significant seismic activity and an aggressive tropical climate continually leads to major landslides, rock falls and hazards throughout the island.
"The trouble is that the magnitude of the problems is so great. In mountainous areas, the topography is so serious and the frequency of medium sized earthquakes is so high it is extremely difficult to mitigate against the hazards.
"Almost everywhere is a hazardous zone. The materials upon which the cities are built upon are relatively weak, material eroded from the mountains and deposited below, and have a strange effect on the earthquake waves.
"And that's why Taipei has been so badly affected because the buildings are constructed on poorly consolidated material. They are really fighting a losing battle."
Chris Browitt added that the major fear would be the potential for powerful aftershocks which could bring down already weakened buildings.
Five such tremors all measuring about six on the Richter scale occurred within 30 minutes of the initial earthquake.
"From the previous history it looks as though there is the potential for other earthquakes on this size or even larger to occur over the next few months," said Chris Browitt.
"Back in 1935 there was a magnitude 7.1 which killed over 3,000 people in the west of Taiwan and it had an aftershock about three months later which killed just under 3,000 people."