An earthquake is caused by a sudden slip on a fault, or fracture, in the earth's surface. The tectonic plates on the surface are always slowly moving, but they get stuck at their edges because of friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and create the shaking we feel. An earthquake's magnitude is a measured value of its size and is the same no matter where you are, or how strong or weak the shaking was in different locations. An earthquake's intensity is a measure of the shaking it creates, and varies with location. A magnitude of 8 or higher defines a "great" earthquake; 7 to 7.9 is considered "major"; 6 to 6.9 is "strong"; 5 to 5.9 is "moderate"; 4 to 4.9 is "light"; 3 to 3.9 is "minor"; and less than 3 is "micro." Experts have said that a million people died in earthquakes in the 20th century and that this century might see 10 times as many deaths, with as many as a million killed in a single quake. That is, unless major efforts are made to fortify the world’s growing cities, which are expected to be homes to billions of added residents. Even though the rate of earthquakes over time seems to be more or less unchanging, the world's population explosion means that more people are moving into quake zones, which are often near coasts. The result, the experts say, is the prospect of continuing trauma. "It is inevitable," Klaus H. Jacob, an earthquake expert at Lamont-Doherty, the earth sciences research center of Columbia University, said at the end of the last century. "More and more people, and more and more buildings, are at stake. As the world gets more populous and richer, allowing a more built-up environment, higher buildings and all the infrastructure that supports our civilization, communications and the like, the risk goes up."