Hundreds of earthquakes occur on Earth everyday. Most of them are small, barely detectable by most people. But occasionally there is a much more significant quake. On average, a major earthquake—one with a magnitude of 7.0-7.9—strikes somewhere on the planet more than once a month. A great earthquake—with a magnitude of 8.0 or higher—occurs about once a year.

An earthquake can happen anywhere. However, the vast majority of earthquakes occur at the boundaries between tectonic plates. Continental and oceanic plates may move toward each other, scrape past each other, or pull apart as they move slowly across the planet's upper mantle. This movement of the plates, and the pressure that is built up at the boundaries, can result in earthquakes.

The boundaries between plates contains systems of deep cracks, called faults. Most earthquakes occur along these faults. Within a fault, rock masses on either side of the break are pushed by geologic forces in opposite directions. Friction, however, holds the rocks in place, causing stresses to build. Finally, the mounting pressure overcomes the friction and a sudden movement occurs along the fault, releasing a large amount of energy. This is an earthquake.

While the vast majority of earthquakes occur along faults at Earth's plate boundaries, occasionally, a quake occurs in the middle of a plate, far from any boundary. Such quakes make up less than 10 percent of all earthquakes. While these intraplate quakes are not completely understood, scientists theorize that they may result from weaknesses within Earth’s crust from long ago. Although rare and not well understood, these earthquakes are no less devastating than those that occur along plate boundaries. Earthquakes along the New Madrid Fault, along the Mississippi River in the United States, in 1811–1812 were among the strongest quakes ever recorded. More recently, in 2001, an intraplate earthquake in the Gujarat region of northwesternn India killed more than 20,000 people.

Believe it or not, earthquakes aren’t just Earth-bound phenomena. Astronauts who traveled to the moon in the late 1960s and early 1970s installed seismographs, devices used to measure and record vibrations, on the lunar surface. The data radioed back to Earth showed that “moonquakes” occur and can be fairly strong and last much longer than quakes on Earth!