Technically, an earthquake is a vibration that travels through the Earth's crust. Quakes can be caused by a variety of things, including meteor impacts and volcanic eruptions, and even sometimes man-made events like mine collapses and underground nuclear tests [source: Hamilton]. But most naturally occurring earthquakes are caused by movement of pieces of the Earth's surface, which are called tectonic plates. (We'll learn more about those plates on the next page.)
The U.S. Geological Survey estimates that, each year, there are as many as 1.3 million quakes with a magnitude greater than 2.0, the threshold at which humans can feel the vibrations [source: USGS]. The vast majority of them are very small, and many occur in remote areas far from people, so we don't usually even notice them. The earthquakes that capture our attention are the rare big ones that strike near heavily populated areas. Such earthquakes have caused a great deal of property damage over the years, and they've claimed many lives. Over the last decade alone, earthquakes and the tsunamis, avalanches and landslides caused by them -- have killed 688,000 people around the world [source: Stoddard].
Perhaps the most lethal quake in history had a magnitude of 8.0 and struck China's Shanxi Province in 1556. According to historical accounts, city walls, temples, government buildings and houses all crumbled, and more than 830,000 people were killed. A scholar named Qin Keda, who survived the quake, later provided what may have been the first earthquake preparedness advice in history: "At the very beginning of the earthquake, people indoors should not go out immediately," he recommended. "Just crouch down and wait for chances. Even if the nest is collapsed, some eggs in it may still be kept intact" [source: Science Museums of China].
On the next page, we'll examine the powerful forces that cause this intense trembling and we'll discuss why earthquakes occur much more often in certain regions.