An earthquake is the sudden, quick shaking of the ground, and it happens when rock under the ground breaks and moves. A fault is a crack in the rocks of the Earth where one side can move past the other. A tectonic earthquake happens when strain in rock builds until stress passes the rock's strength and it suddenly fractures. This is the elastic rebound idea of Harry Fielding Reid, from the 1906 San Francisco earthquake. In 1906 the San Andreas Fault slipped along a plane 430 km (270 miles) long, and the ground moved sideways by as much as 6 meters (20 feet).

First, the outside layer of the Earth is broken into big pieces called plates. In strike-slip faulting the rocks slide past each other sideways, and in dip-slip faulting they move up or down the slope of the fault. At a transform boundary, two plates slide past each other sideways, and the San Andreas Fault in California is one. The San Andreas Fault is more than 650 miles long and goes down at least 10 miles. The Pacific Plate and the North American Plate slide past each other there at about two inches a year.

The Earth's outer layer averages about 70 kilometers thick and is made of about a dozen large plates. Most earthquakes happen where plates meet, and less than 10 percent of earthquakes happen inside plates, away from the edges. Earthquakes and volcanoes mostly happen along plate boundaries, and plate movement also builds mountains. Subduction is where one plate pushes under another and sinks down into the hot layer below the crust. Trenches form where plates subduct, and the Challenger Deep in the Mariana Trench is nearly 11,000 meters deep, deeper than Mount Everest (about 8,854 m) is tall.

The plates of Earth's crust push and slide against each other very slowly, and they can get stuck at their edges because of friction. When the stress gets bigger than the friction, the rock slips and sends out energy as waves that shake the ground. Earthquakes make four main kinds of waves. Two body waves travel through the Earth, and two surface waves, Love and Rayleigh waves, travel along the surface. P-waves are compressional waves, like sound. S-waves are shear waves that cannot travel through liquids. P-waves travel about 3.7 miles per second and usually do not shake hard. The slower S-waves and surface waves travel about 2.5 miles per second and are generally more damaging.

A machine called a seismograph records the shaking of the ground. A seismograph works by inertia: the ground and frame shake, but the mass on a spring stays put, and the difference is recorded. Seismographs and the stations around them show where and how big an earthquake was. The spot on the ground straight above where an earthquake starts is called the epicenter. Shaking spreads out from the middle of an earthquake in every direction, like ripples from a rock thrown into a pond. Seismologists find an earthquake's location by timing when its waves reach different stations.

Magnitude measures the size of the earthquake at its source, while intensity measures how strong the shaking is at one place, so it changes with where you stand. Magnitude is one number for the size of the earthquake itself, and it does not change from place to place. The Richter scale was introduced in 1935 by Charles Richter of the California Institute of Technology. Each whole number up in magnitude means 10 times larger wave height on a seismogram and about 31 to 32 times more energy. The Modified Mercalli scale has 12 levels, written in Roman numerals, and was developed in 1931 by Harry Wood and Frank Neumann.

Shaking damages buildings most. How strong it is depends on the quake's magnitude, distance from the fault, local geology and soil. Damage does not usually start until magnitude is above about 4 or 5. Liquefaction is when loose, water-soaked sediment loses its strength during strong shaking.

Can anyone predict a quake? The USGS says no one has predicted a major earthquake, and gives only the probability a big one will happen in a place within a number of years. Scientists can tell how much strain a fault is building and where past quakes happened to figure the chance of future ones. A real earthquake prediction must give three things: the date and time, the location, and the magnitude. Clouds, body aches and slugs have nothing to do with earthquakes, and vague "predictions" always seem to match some quake. An earthquake can come at any time of year with no warning.

Words to know

• seismologists: quake scientists
• liquefaction: ground turning soupy
• subduction: plate sinking under
• magnitude: quake size
• inertia: staying put