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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.

Quake in Taiwan
The tremor, which measured about 7.6 on the Richter scale, struck inland, close to the central city of Taichung. Like the recent quake which caused devastation across Turkey, it was also very near the Earth's surface.

Already, scientists, many of whom consider Taiwan to be a unique area to study earthquakes, have begun analysing data from seismometers around the world.

Chris Browitt: "Aftershocks will be a worry"
"It looks as if this one was peculiar in some way," said Dr Chris Browitt from the British Geological Survey in Edinburgh. "It looks like it was much shallower than usual and in a different area."

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.

Recent history

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 BBC's Christine McGourty: "Experts say there's no connection between this quake and the one in Turkey"
Recent history shows there have been more than one earthquake a decade of around magnitude 7.5 in the region. The island's last tremor on this scale was on 14 November 1986 but occurred under the ocean and the resulting death toll was restricted to just 15.

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."

Destructive power

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.

Taipei Times journalist Diane Baker: "The main cities were exposed"
Solid rock which normally moves only with the passing of geological ages accelerates briefly to 5,000mph, unleashing huge quantities of energy and creating a shaking movement of up to a metre a second.

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.

Hazadous island

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.

Walter Mooney, a seismologist at the US Geological Survey: "The building codes are better in Taiwan than in Turkey"
"Almost all of Taiwan is so prone to these problems that a great deal of research and engineering work has been done to try and reduce the hazards," said Dr Petley.

"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."

Aftershocks fear

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."

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Internet Links

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