Precautions to be taken during earthquakes

Precautions to be taken during earthquakes
(27 December 2006)

Dr W T Wong, Senior Scientific Officer of the Hong Kong Observatory held a media briefing today (27 December 2006) in response to enquires from some members of the public concerning the precautions to take during earthquakes.

At 8:26 p.m. last night (26 December 2006), a severe earthquake of magnitude 7.2 occurred over the Luzon Strait. Some local residents reported to the Observatory to have felt the earth tremor. After analyzing the reports, it was determined that the local intensity of the earth tremor was III to IV on the Modified Mercalli Scale. Details on the Modified Mercalli Scale are shown in Annex I.

The Hong Kong Observatory's seismograph recorded the earthquake at 8:28 p.m. last night. Thereafter, the Observatory issued the following series of information related to the earthquake.

According to Dr Wong, under the current state of technology, there is no reliable way to predict earthquakes. So press releases on an earthquake can only be issued after the earthquake has occurred. Depending on the time required for receiving and analyzing earthquake information, the Observatory generally takes about half an hour to issue an earthquake press release after the occurrence of a locally felt tremor. As the damage of an earthquake is inflicted within a few minutes of its occurrence, if members of the public feel an intense earth tremor and decide to take safety precautions, they should do so immediately without waiting for a press release on the earthquake.

In the vast majority of cases, the intensity of earth tremors in Hong Kong is under V on the Modified Mercalli Scale. No damage is expected and there is no need to take any precautions. If a more intense earth tremor is encountered, members of the public can take those precautions given in Annex II, just in case. The same information is given on the Observatory's website at
http://www.weather.gov.hk/en/gts/equake/eq_safety.htm.

Dr Wong pointed out that Hong Kong was not situated in a seismically active area. The chance for an intense earth tremor occurring in Hong Kong is very low. A tremor with an intensity of VII on the Modified Mercalli Scale occurs about once every 350 to 400 years. Locally felt tremor occurs on average twice a year. A total of 53 locally felt earth tremors have occurred since 1979. The one with the highest intensity was V to VI on the Modified Mercalli Scale, from an earthquake that occurred in the southern part of the Taiwan Strait on 16 September 1994. Another one with intensity of IV to V on the Modified Mercalli Scale was associated with an earthquake that occurred over the sea areas off the east coast of Lantau Island on 11 May 1995. The intensity of the remaining 51 locally felt earth tremors was IV or below.

Members of the public often confuse between the magnitude of an earthquake and the intensity at a location. The magnitude of an earthquake reflects the total energy released by the earthquake. An increase of one step in the magnitude corresponds to a 32-fold increase in the energy released. Theoretically, there is only one magnitude for an earthquake. However in practice different seismological centres may determine slightly different magnitudes for an earthquake. The intensity of an earthquake at a particular location reflects the effects of earth motion there. The Modified Mercalli Scale adopted by the Observatory has twelve intensity levels (see Annex I). The intensity at a particular location is determined from reports by residents. Generally speaking, the further away a location from the epicentre of the earthquake is, the lower the intensity.