HK far from nearest seismic belt

Mr Yeung said big tsunamis are caused by great submarine earthquakes that mainly occur along the Circum-Pacific seismic belt, and along the active seismic belt between the Eurasian tectonic plate and the Indian-Australian tectonic plate.

He noted the chance of a big tsunami affecting Hong Kong, which lies within the Eurasian tectonic plate, is slim.

Hong Kong is located far away, about 600 km, from the nearest boundary with the Pacific Plate on the Circum-Pacific Seismic Belt that runs through Japan, Taiwan and the Philippines.

Mr Yeung explained: "The disastrous earthquake that occurred west of northern Sumatra over the Indian Ocean on December 26 last year was located on the active seismic belt between the Eurasian tectonic plate and the Indian-Australian tectonic plate.

Tsunamis in Indian, Pacific Oceans can't reach HK

"While Sumatra and the Malaysian Peninsula will block any tsunami generated over the Indian Ocean from reaching Hong Kong directly, the Philippines and Taiwan will similarly block most of the tsunamis originating from the Pacific Ocean.

"A tsunami that enters the South China Sea through the Luzon Strait will lose energy and weaken when passing over the shallow water over the continental shelf," he added.

Since automatic tide gauges were installed in Hong Kong in the early 1950s, the observatory has detected tsunamis on four occasions, caused by earthquakes in Kamchatka in 1952, Chile in 1960 and 1985, as well as the Luzon Strait in 1988. In each case, the amplitude of the tsunami recorded here was less than half a metre.

HK part of Pacific Tsunami Warning System

Mr Yeung said Hong Kong is part of the Pacific Tsunami Warning System, an international co-operation among countries on the Pacific Rim.

Under the warning system framework, the observatory will transmit the arrival time of seismic waves to the Pacific Tsunami Warning Centre in Hawaii after an earthquake is detected by the seismograph.

The centre, on receipt of this and similar information from other seismograph stations in different countries, will determine the location, depth and magnitude of the earthquake and assess the likelihood of a tsunami.

A message will be sent to member countries and territories with details of any tsunami forecast, including the expected arrival time and amplitude of the tsunami at different locations around the Pacific.

The observatory will, on receiving the message from the centre, issue a tsunami warning to the public if the tsunami expected in here is significant.

The strongest earthquake recorded in Hong Kong so far occurred in 1918, arising from a magnitude 7.3 earthquake in Shantou about 300 kilometres east-northeast of Hong Kong. The tremor caused only minor damage and cracks to buildings locally and its intensity was estimated to be VI or VII on the Modified Mercalli Scale.

The observatory has monitored distant earthquakes since 1921 using long-period seismographs. In 1979, a short-period seismograph network of three stations was installed to detect local tremors, also. The seismological network was upgraded to an advanced digital network with eight stations in 1997.

Two locally felt tremors recorded a year

The observatory said from 1979 to present, a total of 51 locally felt tremors were reported, averaging two a year. Most of these tremors were of Intensity III to IV. The epicentres that caused most locally felt tremors lie outside Hong Kong, with only six with epicenters located in Hong Kong, such as Mai Po and near the east coast of Lantau.

Several scientific studies carried out recently showed that in Hong Kong, the return period for a tremor with Intensity V would be 15 to 20 years, and that for an Intensity VII tremor would be 350 to 400 years.

All buildings in Hong Kong are required to withstand dynamic motion caused by wind gusts of 250 kilometres an hour. Infrastructure including bridges, railways, tunnels and subways have been designed to withstand earthquakes of up to Intensity VII, while reservoirs have been designed to withstand earthquakes of Intensity VIII.

For more details about earthquake and tsunami monitoring in Hong Kong, click here. For safety tips, click here.