An earthquake is one of the most terrifying phenomena that nature can dish up. We generally think of the ground we stand on as "rock-solid" and completely stable. An earthquake can shatter that perception instantly, and often with extreme violence.

Up until relatively recently, scientists only had unsubstantiated guesses as to what actually caused earthquakes. Even today there is still a certain amount of mystery surrounding them, but scientists have a much clearer understanding.


Photo courtesy USGS
A section of Interstate 880 in Oakland, California, damaged by the magnitude 7.1 earthquake that shook the
San Francisco area in 1989.

There has been enormous progress in the past century: Scientists have identified the forces that cause earthquakes, and developed technology that can tell us an earthquake's magnitude and origin. The next hurdle is to find a way of predicting earthquakes, so they don't catch people by surprise.

In this edition of How Stuff Works, we'll find out what causes earthquakes, and we'll also find out why they can have such a devastating effect on us.