(CNN) - A tsunami which struck southern China around a thousand years ago nearly wiped out civilization in what is now one of the most densely populated regions of the planet, according to a new study by Chinese scientists.
The devastation caused by the giant wave raises new questions over the potential vulnerability of the region today, particularly in the wake of last month's Indonesian tsunami and the ongoing expansion of nuclear power plants in southern China.
Writing in the journal Chinese Science Bulletin this month, researchers at the University of Science and Technology of China (USTC) and East China Normal University said archaeological and sedimentary evidence, backed up by brief mentions in the historical record, pointed to a massive wave hitting what is now Guangdong province around 1076 AD, during the Song Dynasty.
The effects of the tsunami appear to have been devastating, resulting in a "drastic cultural decline" which did not rebound until some five hundred years later.
The research team, led by Sun Liguang and Xie Zhouqing from USTC, first became interested in a potential paleotsunami during a 2013 survey of Lincoln Island, part of the Paracel chain in the hotly-disputed South China Sea.
"We found sedimentary and biological (evidence) indicating that a tsunami occurred in the South China Sea and struck the island about a thousand years ago," they said.
This included large rocks and coral which had been deposited on the island far away from the coast, and could not have got there but for a massive wave.
Computer modeling suggested the tsunami was sparked by an earthquake in the Manila Trench, a seismically active 5,400 meter (17,700 foot) deep depression off the coast of Luzon and Mindoro in the Philippines. The resultant wave is believed to have struck islands across the South China Sea, as well as the coasts of what is now Guangdong and Hainan provinces in China.
For their latest study, Sun and Xie's team turned to Nan'ao Island in southeastern Guangdong "to study the influence of the South China Sea tsunami."
Again, the researchers found shell, coral and rock deposits that had no right to be there. "Giant boulders, typically requiring strong waves to transport, were (also) distributed near the sampling sites," they wrote. "All this evidence strongly supports that the southeastern coast of the island was struck by the paleotsunami."
While ancient coins and pottery were found at the site, the historical record appeared to be largely cut off following the tsunami, with major activity not resuming for centuries.
"We analyzed the temporal distribution of the cultural relics of Nan'ao Island and found that the amount of the relics significantly decreased after the tsunami and remained low until the late Ming dynasty," the researchers wrote.
Beyond their archaeological and historical importance, the findings provide a real warning of the potentially devastating effect of future tsunamis on Guangdong and other parts of southern China.
Today, more than 68 million people live in the Greater Bay Area alone, a 56,500 square kilometer (21,800 square mile) stretch of southern China encompassing Hong Kong, Macau, Zhuhai, Shenzhen, Guangzhou and six other major cities in Guangdong province.
As well as being heavily built up, the southern Chinese coast also plays host to seven nuclear power plants, with a further four currently under construction, according to the World Nuclear Association.
While Chinese nuclear companies have defended their plants as "tsunami-proof," the South China Morning Post reports, the 2011 Fukushima disaster in Japan is a pertinent reminder of the devastation a tsunami can cause when combined with a nuclear reactor.
At Fukushima, massive waves crashed over the plant's 10 meter (32 foot) sea wall, flooding its emergency generators and knocking vital cooling systems offline, eventually leading to a major meltdown, the worst nuclear disaster since the 1986 Chernobyl incident.
Chinese authorities suspended the approval of new plants following the Fukushima disaster, and ordered a comprehensive safety review. During that audit, multiple reactors were found not to meet safety standards for flood and earthquake resistance, while others did not have sufficient tsunami protections in place.
Following the adoption of new standards, approvals for new reactors have since resumed, including for floating nuclear plants for offshore Chinese islands.
Sun and Xie's team said their study showed the risk of tsunamis in the South China Sea, which "should be considered in future planning and construction of nuclear power plants."
In a 2007 paper examining seismic risks in the South China Sea, US and Chinese researchers pointed to the Manila Trench as the most likely cause, the same earthquake-prone region believed to have sparked the paleotsunami which struck Guangdong in 1076.
"Tsunami earthquakes take place most likely in trench regions with a large tectonic movement and young folded crustal belts," the researchers wrote. "The South China Sea with the adjacent Philippine Sea plate bordered by the Manila Trench is an excellent candidate for such devastating waves to occur."
Moreover, they added, "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 meters."
As much of the region is built on reclaimed land, particularly in Hong Kong, Shenzhen and Zhuhai, the potential risks are even greater, and the researchers warned those cities could suffer a "large-scale disaster, even if the height of the tsunami wave were moderate."
A tsunami which struck Indonesia last month was particularly devastating as it arrived without much warning, due in part to the extremely poor condition of the country's tsunami warning system.
Much of the system -- introduced in the wake of the devastating 2004 Indian Ocean tsunami -- "has not been operational since 2012," officials said in the wake of the disaster. Upgrades are now underway.
China has also been ramping up its preparedness for future tsunamis, with new buoy systems deployed around the Manila Trench to warn of seismic activity in May last year.
Most water-based tsunami detection systems feature a pressure recorder anchored to the ocean floor and a buoy on the surface. When a tsunami passes over the recorder, the instruments detect and record changes in water pressure. That data is then transmitted to the surface buoys, which relay the message to the wider tsunami detection system.
The new South China Sea systems, experts told state media, "can issue an alert two hours before seismic sea waves arrive." Such a warning could save millions of lives, as an earthquake of 8.5-magnitude or higher could cause waves of up to 10 meters, similar to that which flooded the Fukushima plant.
"Previous tsunami warning systems using seismic and water level data, always made forecasts relying on people's experience, which resulted in many errors. The new warning system with numerical simulations and tsunami buoys, improves efficiency and precision," Shang Hongmei, a researcher at China's National Ocean Technology Center, told the news agency.
However, while the modern citizens of southern China would hopefully have significantly more warning that a giant wave was headed their way than their ancestors did, buildings, nuclear power plants, and other infrastructure would still be in the path of the tsunami, meaning it could be equally as devastating.
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