what is a earthquake?
The shifting masses send out shock waves that may be powerful enough to
- alter the surface of the Earth, thrusting up cliffs and opening great cracks in the ground and
- cause great damage ... collapse of buildings and other man-made structures, broken power and gas lines (and the consequent fire), landslides, snow avalanches, tsunamis (giant sea waves) and volcanic eruptions.
Frank Thorp, a US citizen in Haiti, says he drove about 100 miles (160km) to Port-au-Prince immediately after Tuesday's earthquake to rescue his 23-year-old wife Jillian, an aid worker.
Jillian Thorp was trapped underneath about a foot of concrete
"I'd spoken to her on Skype for about 10 seconds, she said that she was trapped. And that's all that I knew. It was absolutely terrifying," Mr Thorp told CBS's Early Show.
Mr Thorp says that their entire house - a three-storey concrete building - had collapsed and Jillian and one other person were trapped under 30cm (12in) of concrete.
We had to plough bricks and bricks and bricks... and doors and metal away for at least an hour before we were able to get her and her co-worker out
"I jumped into the hole and I was able to see her wave her hand; I couldn't see her whole body... I could hear her voice..."
He said his wife was saying: "Just get me out of here!"
"We had to plough bricks and bricks and bricks... and doors and metal away for at least an hour before we were able to get her and her co-worker out,"