Photograph by National Geographic Television & Film

If you’re lucky, the quake is a mild one. The ground rumbles as if a large truck is driving by, and you sleep undisturbed.

If you’re one of the unlucky ones, the ground starts to shake, jolting your house off its foundation. Bookcases fall, trees are uprooted, cars are crushed, roads ripple, bridges collapse. After it’s all over—in mere seconds—people lay dead or dying, industries are devastated, cities leveled.

You’ve just experienced one of nature’s most violent phenomena, an earthquake.

Earthquakes happen when plates, or rocks, within the Earth suddenly break or shift under stress, sending shock waves rippling. Most quakes are unnoticeable by people on the surface—in fact, thousands of quakes occur every day around the globe, most of them too weak to be felt.

Every year, scientific instruments detect about 500,000 quakes worldwide. People feel only a small fraction of those.

On average, a magnitude 8 quake happens every year somewhere on Earth. But severe earthquakes that cause widespread destruction happen on average once every five years. These great quakes can destroy cities, burst dams, trigger volcanic eruptions, and set off landslides and giant sea waves.

The deadliest quake ever occurred in China in 1557, when an estimated 830,000 people were killed. Every year about 10,000 people, on average, die as a result of earthquakes.

Most earthquakes occur along fractures in the Earth's crust called faults. Most faults occur along the edges of major plates, huge slabs of rock that make up the Earth's lithosphere, or shell.

Some earthquakes happen far from plate boundaries. These intraplate quakes happen when stress builds up and the Earth's crust is stretched or squeezed together until it rips.

In 1811 and 1812, three large intraplate earthquakes shook New Madrid, Missouri. Shock waves were felt as far as Canada. Chimneys were knocked down in Cincinnati, Ohio, about 370 miles (600 kilometers) away.

The largest quake belt—responsible for an estimated 80 percent of temblors, or earthquakes—is where plates carrying the Pacific Ocean come into contact with plates carrying the continents surrounding the Pacific.

This belt includes the most active zone in the United States—California and other states along the Pacific coastline, including Alaska and Hawaii.

Another earthquake belt passes through the Mediterranean Sea region between Europe and North Africa and continues eastward through Asia.

	Photograph by S.D. Ellen/USGS

The Earth's plates are constantly moving and interacting in a process called plate tectonics—they push against each other, pull apart, or grind past one another.

Usually the plates move very gradually. Occasionally, however, stress builds up and is released suddenly in a great burst of seismic waves. These vibrations move through the rock, causing an earthquake.

The sudden movement along the fault causes the ground to move forward and backward, heave up and down, or shift from side to side.

California's San Andreas Fault, visible from the Earth's surface, forms the boundary between two moving plates.

It is the "master" fault of an intricate fault system that stretches more than 800 miles (1,290 kilometers) long and at least 10 miles (16 kilometers) down into the Earth.

Thousands of earthquakes, most too small to be felt, occur along the San Andreas Fault every year.