I refer to the report ('Is HK safe from tsunamis? Experts say no', April 20).
With the sheltering effect of the land masses of Taiwan and Luzon against tsunamis originating from the Pacific, the most likely earthquake source zone that may generate a tsunami to affect Hong Kong significantly is the Manila Trench over the South China Sea to the west of Luzon.
Since the commencement of instrumental detection in the late 19th century, the strongest earthquake on or near the Manila Trench was a magnitude-7.6 earthquake on February 14, 1934. This earthquake generated a tsunami that was observed at San Esteban and south of Vigan in the Philippines but not in Hong Kong. Studies also revealed that there was no indication of a damaging tsunami affecting Hong Kong in recorded history (for example, see 'Tsunamigenic Earthquakes in China 1931 BC to 1980 AD' by Zhou and Adams, 1986, Science of Tsunami Hazards). The assumption of a magnitude-9 earthquake occurring in the South China Sea and generating tsunami waves up to eight metres to hit Hong Kong in the next 50 to 100 years requires more solid substantiation. Regarding the claim about a 10-metre tsunami affecting southern Taiwan in 1782 killing many people, we would like to point out that there was no record of a damaging earthquake or tsunami affecting the island that year according to earthquake catalogues compiled by the Chinese authorities.
On the other hand, a flooding event caused by a typhoon-related storm surge in Taiwan in 1782 might be the cause of a tsunami legend which later emerged. This event was documented in a report to the Qing emperor by the naval commander who governed Taiwan at that time.
In spite of there being no record of a damaging tsunami in history, we have instituted a tsunami warning system in Hong Kong. Over the past few years, the Observatory has strived to increase its earthquake detection capabilities and improve its tsunami warning system. In February 2010, we commissioned a new seismograph station in a deep tunnel off Po Shan Road, Mid-Levels, which has become a member of the Global Seismographic Network, to enhance the detection of earthquakes in the South China Sea round-the-clock. We have also implemented a numerical prediction system to estimate possible tsunami heights in Hong Kong based on parameters of earthquakes detected in the South China Sea.
In the event that a significant tsunami with a height exceeding 0.5 metres is expected to affect Hong Kong, the Observatory will issue tsunami warnings to the public via the established information channels. The Observatory has also published educational pamphlets and webpages, and conducted seminars to promote tsunami knowledge among members of the public.