An earthquake is the sudden, quick shaking of the ground, and it happens when rock under the ground breaks and moves. The outer layer of the Earth is broken into big pieces called plates. That outer layer averages about 70 kilometers thick and is made of about a dozen large plates. These plates slide over, under and past each other on a partly molten layer. The plates of the Earth's crust push and slide against each other very slowly and they can get stuck.

Plates stick at their edges because of friction. Strain builds up in the rock like a rubber band pulled too tight, until the stress gets bigger than the friction and the rock suddenly breaks and slips. The energy of that slip goes out as waves that shake the ground. A fault is a crack in the rocks of the Earth where one side can move past the other, and a tectonic earthquake happens when the strain on a fault passes the strength of the rock. This explanation is called elastic rebound, an idea of Harry Fielding Reid that came from the 1906 San Francisco earthquake, when 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).

Faults move in several different ways. In strike-slip faulting the rocks slide past each other sideways, while in dip-slip faulting they move up or down the slope of the fault. In normal faulting the upper block moves down, and in reverse or thrust faulting it moves up. The edges of plates behave in their own ways as well. At a transform boundary, two plates slide past each other sideways. The San Andreas Fault in California is one of these. At a spreading boundary, hot rock rises and pushes two plates apart, making new crust, and in subduction one plate pushes under another and sinks down into the hot layer below the crust.

Earthquakes make four main kinds of waves. Two body waves travel through the Earth, and two surface waves, called Love and Rayleigh waves, travel along the surface. P-waves are compressional waves, like sound, while 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, but the slower S-waves and surface waves, which travel about 2.5 miles per second, are generally more damaging. The spot on the ground straight above the place where an earthquake starts is called the epicenter. The shaking spreads from the middle of the earthquake in every direction, like ripples from a rock thrown into a pond.

A machine called a seismograph records the shaking of the ground, and it works by inertia: the ground and the frame shake, but the mass on a spring stays put. The difference between them is what gets recorded. On a seismogram, the horizontal axis shows time in seconds and the vertical axis shows ground movement, usually in millimeters. The earliest known earthquake detector was a Chinese seismoscope from A.D. 132, which showed that a quake happened but did not record it. Seismologists find the location of an earthquake by timing when its waves reach different stations.

Magnitude measures the size of the earthquake at its source, so it does not change from place to place, while intensity measures how strong the shaking is at one place, so it changes with where you stand. 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. Moment magnitude is now the scale the USGS reports, and the Richter scale is outdated for large earthquakes. Moment magnitude is based on the physical size of the slip: the rigidity of the rock times the fault area times the slip distance. Intensity, in contrast, is a rating built from people's reports and damage, which the USGS now collects through an online web form called Did You Feel It?

Damage does not usually start until magnitude is above about 4 or 5, but it also depends on distance, soil and building type. The strength of shaking drops with distance from the source, so a quake 500 km deep shakes the surface much less than the same quake at 20 km. In 1994 a magnitude 8.3 quake 636 km under Bolivia was felt as far as Minnesota and Toronto, but it caused no major damage because it was so deep. Local geology changes the shaking as well, since bedrock shakes the least and soft mud the most, and liquefaction is when loose, water-soaked sediment loses its strength during strong shaking. Damage is negligible in buildings of good design and construction, but considerable in poorly built or badly designed ones.

Most earthquakes happen where the edges of Earth's plates meet. Less than 10 percent happen inside plates, away from the edges.

Subduction zones have deep-sea trenches, earthquakes from shallow to deep, and mountain ranges with active volcanoes. The northwest U.S. coast, western Canada and southern Alaska sit on one.

Earthquakes in the central and eastern U.S. are less common because the crust there is thicker, colder, older and farther from plate boundaries, yet big ones have happened, at New Madrid in 1811 and 1812 and at Charleston in 1886.

People can also cause earthquakes. Filling reservoirs, mining, pumping fluids out and injecting fluids underground can all induce them.

Whether anyone can know when the next earthquake will come depends on what is meant by prediction. A real prediction must give three things, the date and time, the location, and the magnitude. The USGS says no one has predicted a major earthquake, offering only the probability that a big one will happen in a place within a number of years. Clouds, body aches and slugs have nothing to do with earthquakes, and vague predictions always seem to match some quake afterward. Precursors such as swarms of small quakes or radon often appear without a big quake following. Early warning is a different thing, because ShakeAlert detects an earthquake that has already started and estimates where it is, how big it is and how strong the shaking will be. It works because P-waves outrun the damaging S and surface waves while data travel far faster than seismic waves.

A tsunami is a series of ocean waves, and a big earthquake under the sea can cause one, but only a shallow undersea earthquake that moves the seafloor will do it. Thrust-type quakes are far more likely to cause one. Tsunami waves can move as fast as a jet plane in deep water and then slow down as they reach shallow water, and the name tidal wave is discouraged because tides have little to do with them. Anyone near the ocean who feels strong shaking should go to higher ground once the shaking stops, because a tsunami may follow.

Words to know

• seismograph: shaking recorder
• epicenter: spot above start
• magnitude: quake size
• subduction: plate sinking under
• liquefaction: ground turning soupy