ENG-1976 — Page 280

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THE ENVIRONMENT

nautical miles of Hong Kong, it can be located and monitored by the observatory's radars mounted on the top of Tate's Cairn (580 metres above mean sea level). An iso-echo device is available to map rainfall intensities and a video-time-lapse system enables the recent past movement of the echoes to be studied. Apart from tropical cyclone and heavy rain or thunderstorm warnings, the Royal Observatory also issues strong monsoon, fire hazard, frost, and low temperature warnings whenever necessary.

The observatory is responsible for Hong Kong's Time Service. Six pip signals from a special crystal clock, accurate to 0.05 of a second, are broadcast every 15 minutes on a frequency of 95 MHz and are relayed by radio and television stations.

Instruments and Measurements

The seismological section of the observatory operates 12 seismographs in a specially constructed cellar. These instruments record vibrations of both long and short periods transmitted through the ground. On average, tremors from about 800 earthquakes occurring all over the world are detected and analysed each year. Other tremors from underground nuclear explosions, storm microseisms, local blasting or pile-driving are also recorded. Hong Kong lies just outside the circum-Pacific seismic belt and has not suffered serious earthquake damage since 1918. However, an average of two to three minor tremors may be felt each year by residents in certain locations such as on balconies of high buildings. In 1976 there was one such tremor, on February 22, and its intensity was one to two in the Modified Mercalli Scale of 12.

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The observatory prepares bulletins on all earthquake tremors recorded and participates in the Tsunami Warning System for the Pacific area. Tsunamis are seismic sea waves that are caused by earthquakes. Whenever an intense earthquake is recorded, with its epicentre anywhere in the Pacific Ocean or the South China Sea, a special warning message is issued.

The disastrous earthquakes in China and the Philippines in July and August led to the Royal Observatory maintaining a special 24-hour watch in order to provide immediate information to alleviate anxiety, particularly for people having relatives in the affected areas. In addition to the normal continuous recording of earthquakes, the observatory made arrangements to issue reports on any significant earthquakes which occurred at any time of the day or night. Frequent news releases were issued and, because of the considerable public interest, talks were given explaining the nature of earthquakes and the seismicity of the region. The earthquake at Tangshan, in the Peking-Tientsin region of China, which occurred on July 28, was of magnitude 8.2 on the Richter scale. This was followed by an aftershock of magnitude 7.9 on the same day and numerous other smaller magnitude aftershocks. The earthquake and tidal wave in the southern Philippines occurred in mid-August, and there were also serious earthquakes in Guatemala and Italy during 1976.

Geomagnetic measurements are made at the geomagnetic station near Tate's Cairn, where magnetic variation is also recorded. This is a joint project by the University of Hong Kong and the Royal Observatory, and was made possible by a donation from the Nuffield Foundation.