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Shallow quake amplified Taiwan damage level

HOPES WERE fading, as Ground Engineering went to press, of finding alive any of the 2,500 people believed still buried beneath collapsed buildings following the massive earthquake that hit Taiwan last month.

The magnitude 7.6 earthquake hit just before 2am on 21 September near the town of Puli in the centre of the country, 150km south west of the capital Taipei. This was the worst hit area, with an estimated 98% structural damage.

Thousands of people were forced to leave their homes after reports that the skin of the Moon Lake reservoir in the foothills of the mountains surrounding the town was reported to be cracked. Water was being released as a precaution.

The capital Taipei emerged relatively unscathed with only a block of flats and a hotel suffering extensive damage.

As the death toll reached 2,000, with 4,000 injured, the island was hit by a series of major aftershocks up to 6.8 on the Richter scale, triggering landslides and further building collapses, hampering rescue attempts.

First estimates were that the earthquake was shallow, between 5km and 10km below the ground surface, producing high vertical accelerations and giving rise to the extensive damage, although the strength of the aftershocks almost certainly contributed to the collapse of some buildings. United States Geological Survey geophysicist Waverly Pearson said that buildings that resisted the first shock may have been weakened and brought down by aftershocks.

Taiwanese construction is considered to be better engineered for seismic risk than Turkey's, where shoddy construction was partly blamed for the amount of damage sustained in August's earthquake (see News), but many of the collapsed buildings in Taiwan were modern. Reports indicated that foundations of high-rise apartment blocks had failed at ground level, causing buildings to topple.

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