ENG-1978 — Page 293

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THE ENVIRONMENT

tropical cyclone comes within 400 kilometres of Hong Kong, it can normally be seen on the observatory's radars mounted on top of Tate's Cairn, 580 metres above mean sea-level. Time-lapse movies of the radar display are taken during tropical cyclones, and a video time-lapse system is used in the Central Forecasting Office to record and play back the radar display in real-time.

Instruments and Measurements

The Seismology Section of the observatory operates seismographs at three sites in the New Territories and also in a specially constructed cellar at Royal Observatory Head- quarters. The instruments record vibrations of both long and short periods transmitted through the ground. On average, tremors from about 800 earthquakes all over the world are detected and analysed each year. Other tremors from underground nuclear explosions, storm microseisms, local blasting or pile-driving also are recorded. Hong Kong lies just outside the circum-Pacific seismic belt and has not suffered significant earthquake damage since 1918. However, an average of two to three minor tremors are felt each year by residents in certain locations and especially in high-rise buildings. Two such tremors occurred in 1978. Both were of intensity four to five in the Modified Mercalli Scale of 12.

The observatory prepares bulletins on all earthquake tremors recorded and partic- ipates in the Tsunami Warning System for the Pacific region. Tsunamis are seismic sea waves that are caused by earthquakes. Whenever an intense earthquake is recorded, with an epicentre in the Pacific Ocean or the South China Sea, a special warning message is issued. A tsunami on August 19, 1977, caused severe damage to southern Indonesia.

To study the response of different geological structures to seismic waves, three strong motion accelerographs are installed on bedrock, decomposed granite and reclaimed land. Sites have been chosen at Tate's Cairn, the Royal Observatory Headquarters and the Royal Hong Kong Auxiliary Police Force Headquarters at Wan Chai. In co-operation with the University of Hong Kong, the Royal Observatory also takes geomagnetic measurements at a station near Tate's Cairn.

Air pollution measurements made at King's Park, Yau Ma Tei, include daily measurement of particulates and a continuous record of the concentration of sulphur dioxide in the atmosphere. This information is needed for a scientific study on the effects of local weather on the concentration of pollutants. The observatory also monitors radioactivity. Measurements of beta and gamma activity in the atmosphere, in rainfall and in tap water, have been made at King's Park since 1961. The general level of atmospheric radioactivity was low during 1978.

The observatory maintains meteorological instruments at various locations through- out Hong Kong. Anemometers are installed at 12 sites for wind information which is especially important during tropical cyclones and is also useful in connection with a variety of engineering projects. The observatory co-operates with the University of Hong Kong in operating wind towers at Cape D'Aguilar to record the vertical struc- ture of the winds, especially in relation to wind stress on buildings. As the majority of these instruments - both electronic and non-electronic - are unique in Hong Kong, all repairs, calibration and maintenance are done by observatory staff.