The earthquake occurred directly under Tangshan at a depth of 8km (5 miles).

A magnitude 7.1 aftershock 15 hours later caused further destruction and killed many people trapped in collapsed buildings.

news.bbc.co.uk...

The night preceding the earthquake, July 27-28, many people reported seeing strange lights as well as loud sounds. The lights were seen in a multitude of hues. Some people saw flashes of light; others witnessed fireballs flying across the sky. Loud, roaring noises followed the lights and fireballs. Workers at the Tangshan airport described the noises as louder than that of an airplane.2

history1900s.about.com...

28 July 1976

"Just before the first tremor at 3:42 am, the sky lit up like daylight. The multi-hued lights, mainly white and red, were seen up to 200 miles away. Leaves on many trees were burned to a crisp and growing vegetables were scorched on one side, as if by a fireball."

Some New Details on China's Quake

There is another possible - if wildly improbable - cause of the mysterious event at Tunguska in 1908 (7 September, p 14). One of Nikola Tesla's great projects was the wireless transformation of energy over large distances. He believed that this could be harnessed in war to destroy incoming attacks from over 300 kilometres away.

Tesla built his "death ray" at Wardencliffe on Long Island, and it is a possible that he tested it one night in 1908. The story goes something like this. At the time, Robert Peary was trekking to the North Pole and Tesla asked him to look out for unusual activity. On the evening of 30 June 1908, Tesla aimed his death ray towards the Arctic and turned it on. Tesla then watched the newspapers and sent telegrams to Peary, but heard about nothing unusual in the Arctic.

However, he did hear about the unexplainable event in Tunguska, and was thankful no one was killed, as it was clear to him that his death ray had overshot. He then dismantled his machine, as he felt it was too dangerous to keep it. See www.parascope.com/en/1096/tesdeth.htm for the full story.

www.newscientist.com...

The curious effect of the Tunguska explosion on the trees near ground zero was replicated during atmospheric nuclear tests in the 1950s and 1960s. These effects are caused by the shock wave produced by large explosions. The trees directly below the explosion are stripped as the blast wave moves vertically downward, while trees further away are felled because the blast wave is travelling closer to the horizontal when it reaches them.

Soviet experiments performed in the mid-1960s, with model forests (made of matches) and small explosive charges slid downward on wires, produced butterfly-shaped blast patterns strikingly similar to the pattern found at the Tunguska site. The experiments suggested that the object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north and had exploded in mid-air.

en.wikipedia.org...

Name Symbol Quantity Expression in terms of other units Expression in terms of SI base units