The danger stems from where the bed of the South China Sea ducks under the Manila Trench, a boundary in the notoriously seismic Philippine Sea plate of the Earth's crust.

An earthquake on the Manila Trench could cause giant waves that would radiate across the shallow sea, placing major cities in harm's way, according to computer simulations by scientists in China, the United States and Japan.

According to their calculations, a 7.5-magnitude quake occurring on parts of the Trench southwest of the Philippines would unleash a wave of over two metres (6.5 feet), which would endanger a 400-kilometre (250-mile) swathe of the coast of Guangdong province, from Macau and Hong Kong in the south to Shantou in the north.

The risk of this happening over the next century is 10.12 percent for Hong Kong and Macau and 13.34 percent for Shantou, scientists believe.

For waves ranging from one to two metres (3.25 to 6.5 feet), the risk over the next century is put at 17.19 percent for Hong Kong and Macau and 30.65 percent for Shantou.

The study, which appears in the August issue of a specialist journal Physics of the Earth and Planetary Interiors, says the South China Sea "is an excellent candidate" for a tsunami and calls for an alert system to be set up for the highly populated, economically vital region.

The threat has been largely overlooked, even though the historical record is clear, it says.

Since AD 171, 11 tsunamis have hit the Chinese mainland coast and Taiwan, the biggest of which was a wave 7.5 metres (24.4 feet) high that struck Keelong, Taiwan, on December 18, 1867.

"The coastal heights along the South China Sea are generally low, thus making it extremely vulnerable to incoming tsunami waves with a height of only a couple of metres," the study's authors point out.

"In particular, many economically important coastal cities, such as Hong Kong and Macau, are only a couple of metres above the sea level and would suffer a large-scale disaster, even if the height of the tsunami wave were moderate."

On the other hand, Taiwanese cities are less at risk than their mainland counterparts, for the historical seismic records show that quakes with a magnitude higher than 7.0 are rare in the ocean bed near Taiwan.

The calculations see a risk of 3.44 percent that a two-metre (6.5-feet) tsunami would hit the Taiwanese cities of Tainan, Kaoxiong and Nawan over the next hundred years.

The paper is lead-written by Yingchun Liu of the Graduate University of the Chinese Academy of Sciences in Beijing. The group included geophysicist David Yuen of the University of Minnesota and Angela Santos, of the Tsunami Engineering Laboratory at Tohoku University, Sendai, in Japan.

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