An earthquake is the sudden, quick shaking of the ground, and it happens when rock under the ground breaks and moves. An earthquake can come at any time of year with no warning, and no one can say exactly when one will happen.

Great earthquakes, those of magnitude 8 or higher, happen about once a year somewhere in the world. Large earthquakes beneath the ocean may not even be felt by people. The biggest earthquake ever recorded was magnitude 9.5 in Chile on May 22, 1960, on a fault almost 1,000 miles long. Earthquakes of magnitude 10 or larger cannot happen, because no fault is long enough, and one would have to reach around most of the planet.

The Earth's outer layer averages about 70 kilometers thick and is made of about a dozen large plates that slide over, under and past each other on a partly molten layer. At a spreading boundary, hot rock rises and pushes two plates apart, making new crust. The Mid-Atlantic Ridge is one, where plates move apart about 2.5 centimeters a year. Over time the Earth does not grow or shrink, so crust must be destroyed, recycled at convergent boundaries, at about the rate new crust is made. GPS and other satellite measurements track plate motion directly, because plate motion is global in scale. Plate spreading rates range from less than 2.5 cm a year at the Arctic Ridge to more than 15 cm a year at the East Pacific Rise.

India crashed into Asia about 50 million years ago, and the crust crumpled up to form the Himalayas. Where one plate pushes under another, it sinks down into the hot layer below the crust. Subduction zones have deep-sea trenches, shallow to deep earthquakes and mountain ranges with active volcanoes. The Challenger Deep in the Mariana Trench is nearly 11,000 meters deep, deeper than Mount Everest, at about 8,854 m, is tall. The northwest U.S. coast, western Canada and southern Alaska sit on a subduction zone.

Strain builds up in rock like a rubber band pulled too tight, until the 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 which the San Andreas Fault slipped along a plane 430 km long and moved the ground sideways by as much as 6 meters. A fault is a crack where one side can move past the other. 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.

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, and they travel about 3.7 miles per second and usually do not shake hard. S-waves are shear waves that cannot travel through liquids, and they and the surface waves travel about 2.5 miles per second and are generally more damaging. A seismograph works by inertia: the ground and frame shake, but the mass on a spring stays put, and the difference is recorded. Seismologists find an earthquake's location by timing when its waves reach different stations. The spot on the ground straight above where it starts is the epicenter.

Magnitude measures the size of the earthquake at its source, while intensity measures how strong the shaking is at one place, so intensity changes with where you stand. Charles Richter of the California Institute of Technology introduced his scale in 1935, and each whole number up in magnitude means 10 times larger wave height on a seismogram and about 31 to 32 times more energy. Richter's scale measures the largest wiggle on a record, but other magnitude scales measure other parts of the earthquake. Moment magnitude, now the scale the USGS reports, is based on the physical size of the slip: the rigidity of the rock times the fault area times the slip distance. Intensity comes from people's reports and damage, which the USGS now collects through an online web form, 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. Earthquakes can happen from the surface to about 800 kilometers deep, and the strength of shaking drops with distance from the source. A magnitude 8.3 quake 636 km under Bolivia in 1994 was felt as far as Minnesota and Toronto but caused no major damage, because it was so deep. 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 and considerable in poorly built ones.

A tsunami is a series of ocean waves, and it needs a shallow undersea earthquake that moves the seafloor, with thrust-type quakes far more likely to cause one. In the deep ocean tsunami waves are not very tall, but they can move as fast as a jet plane, and they grow taller and slow down as they near land. Tsunami speed depends on ocean depth, not on how far from the source. 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.

Earthquake early warning, forecasts, probabilities and prediction are four different things, and early warning is issued after a quake starts. 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, and data travel far faster than seismic waves. People farther from the epicenter have more time to get an alert before the shaking arrives.

A real earthquake prediction must give the date and time, the location and the magnitude, and the USGS says no one has predicted a major earthquake. It gives 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.

Earthquakes in the central and eastern U.S. are less common because the crust 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.

Human activity can also induce earthquakes, through filling reservoirs, mining, pumping fluids out and injecting fluids underground.

People cannot prevent natural earthquakes, but they can reduce harm by finding hazards, building safer and teaching earthquake safety.

Words to know

• intensity: local shaking
• trench: deep ocean valley
• tsunami: giant ocean wave
• liquefaction: ground turning soupy
• epicenter: spot above start