South-west Taiwan faces tsunami risk: Experts

Prof Adam Switzer indicating where the 18th-century tsunami took place on a touch screen. He says the area that included the cities of Tainan, Kaohsiung, Donggang and Kenting was hit by the giant wave. The wavy line denotes the Manila Trench, and the
Prof Adam Switzer indicating where the 18th-century tsunami took place on a touch screen. He says the area that included the cities of Tainan, Kaohsiung, Donggang and Kenting was hit by the giant wave. The wavy line denotes the Manila Trench, and the area circled is where the tsunami took place. PHOTO: LIANHE ZAOBAO

Area includes the cities of Kaohsiung and Tainan and the site of a nuclear power plant

Scientists have warned that the south-western coast of Taiwan is a potential tsunami zone, after unravelling the mystery surrounding a tsunami which occurred there in the 18th century.

That giant wave was the most devastating ever reported in the South China Sea, killing more than 40,000 people when it struck between 1781 and 1782.

The scientists from Nanyang Technological University's Earth Observatory of Singapore (EOS) have now found that it was likely caused by an underwater landslide on the upper portion of the continental slope offshore from southwestern Taiwan. This was likely triggered by an earthquake.

"A similar event today like that in the 18th century would endanger millions of lives in the coastal cities like Kaohsiung and Tainan and damage infrastructure located at the south of Taiwan," warned Assistant Professor Adam Switzer of EOS, who led the research into the 18th- century Taiwan tsunami.

The cities of Kaohsiung and Tainan have a total population of over 4.5 million people. The Maanshan Nuclear Power Plant is also located on that same coast.

The latest prediction is an added worry for anti-nuclear activists who have been campaigning for the abolition of nuclear energy on the earthquake-prone island.

The team's findings were published earlier this month in Geophysical Research Letters, a peer-reviewed scientific journal published by the American Geophysical Union. Such devastating underwater landslides happen when sediment accumulates on a steep slope on the ocean floor. They can get dislodged by an earthquake, displacing a large volume of water when they fall, in turn causing a tsunami.

Prof Switzer said an earthquake of around 7-magnitude is likely able to trigger a tsunami by means of an underwater landslide. Southwest Taiwan has experienced three such powerful 7-magnitude temblors in the last century.

The latest, in 2006, was a 7.1-magnitude earthquake off the southern coast of Taiwan, which caused an underwater landslide that damaged several submarine cables, disrupting Internet and telecommunication services throughout East Asia. That earthquake also triggered a tsunami, but only of a height of around 40cm.

The scientists say it is difficult to predict when a devastating tsunami will strike. Based on history, earthquakes and tsunamis which have happened before are likely to occur again in the same spot.

"Can it happen again? Absolutely. Will it happen tomorrow or in the next 100 years? I can't tell you that," said Prof Switzer, who is also from the Asian School of the Environment.

"But there is a very clear danger that that region would experience a magnitude-7 earthquake which can generate a sub-marine landslide."

Already, Taiwan has taken steps to prevent a Fukushima-type disaster by building emergency cooling towers and increasing the height of the sea walls surrounding its Maanshan Nuclear Power Plant. And last month, Taiwan tested its tsunami alarm to get the public familiar with its sound.

But Prof Switzer said more can be done to educate people about the tsunami threat. In the case of southwest Taiwan, residents should not wait for a tsunami warning as it could take just half an hour for a tsunami to hit the coast.

Even a slight tremor should be a signal for people to seek higher ground of at least three storeys, he said. "So the earthquake is the warning," said Prof Switzer. "If there is an earthquake, seek higher ground."

A version of this article appeared in the print edition of The Straits Times on October 23, 2015, with the headline 'South-west Taiwan faces tsunami risk: Experts'. Print Edition | Subscribe