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.