THE explosion was picked up by the Royal Observatory's seismographic monitoring equipment, but the blast was not strong enough to trigger the earthquake monitor. Worried people called the forecasters asking if there had been an earthquake or if further tremors were likely, but they were told there had been no significant movement. While residents near the border said they felt the ground and their buildings shake, the duty officer at the Royal Observatory last night said the vibrations had been minor ones. ''If tremors are strong enough they will set off an alarm here,'' he said. ''This was too weak to register as an earthquake, though it did appear on our records.'' The continuous seismograms taken at the Kowloon headquarters of the Royal Observatory revealed a small vibration at 1.26 pm. There are no earthquake-recording facilities at weather stations nearer the border. The duty officer added that the prevailing winds were from the southwest, so any poisonous gases, dust or debris would be blown away from Hong Kong. An expert warned that a toxic cloud could contain a vicious cocktail of gases. The liquified petroleum gas would simply have blown up, leaving little trace, but the ammonium nitrate blast could have formed a number of harmful combinations, said the environmental specialist. Hundreds of people living and working in the area immediately surrounding the blast could have been affected by gaseous by-products of the explosion, such as nitrogen dioxide and nitrogen trioxide gases. Such substances would not be as harmful as chlorine gas, which can asphyxiate people up to half a kilometre away, but they are unpleasant and could cause side effects such as streaming eyes. The toxic cloud was too far away to harm anyone in Hong Kong, the Deputy Director of the Environmental Protection Department, Rob Law, said last night. ''I would be extremely surprised if any effect from an incident like that could be detected in the urban areas of Hong Kong,'' he said. ''Even fairly toxic gases would be unlikely to have any effect after they travelled for a few kilometres as they would be diluted all the way with air.''