An earthquake is the quick shaking of the ground. It happens when rock under the ground breaks and moves. The plates of Earth's crust push and slide against each other very slowly, and they can get stuck. Strain builds up in rock like a rubber band pulled too tight, until the rock suddenly breaks and slips. That sudden slip is an earthquake, and an earthquake can come at any time of year with no warning. A fault is a crack in the rocks of the Earth where one side can move past the other.

The Earth's outer layer averages about 70 kilometers thick. It 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. At a transform boundary, two plates slide past each other sideways, as at the San Andreas Fault. In subduction, one plate pushes under another and sinks down into the hot layer below the crust.

Plate movement also builds mountains. India crashed into Asia about 50 million years ago, and the crust crumpled up to form the Himalayas. Western U.S. mountains and coasts are young because they sit near active plate boundaries. The East's features formed at plate edges long ago.

Earthquakes can happen in the crust or upper mantle, from the surface to about 800 kilometers (500 miles) deep. 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.

How do people record shaking? The earliest known earthquake detector was a Chinese seismoscope from A.D. 132. It showed that a quake happened but did not record it. A seismograph does record it. It works by inertia: the ground and frame shake, but a mass on a spring stays put, and the difference is recorded. On a seismogram, the horizontal axis is time in seconds and the vertical axis is ground movement, usually in millimeters.

Shaking spreads out from the middle of an earthquake in every direction, like ripples from a rock thrown into a pond. Earthquakes send out waves that travel at different speeds. 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.

Two ideas are easy to mix up. Magnitude measures the size of the earthquake at its source. 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. Damage does not usually start until magnitude is above about 4 or 5, but it also depends on distance, soil and building type.

How big can they get? The biggest earthquake ever recorded was magnitude 9.5 in Chile on May 22, 1960, on a fault almost 1,000 miles long, as of November 2026. Earthquakes of magnitude 10 or larger cannot happen, because no fault is long enough. It would have to reach around most of the planet. Great earthquakes 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.

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 is not prediction. Scientists can tell how much strain a fault is building and where past quakes happened, and from that they can figure the chance of future ones. Warning and chance are both different from knowing the day.

People cannot prevent natural earthquakes, but they can reduce harm by finding hazards, building safer, and teaching earthquake safety. Engineers make buildings safer by designing new ones to handle shaking and by strengthening old ones. When the ground shakes, the safe moves are drop, cover, and hold on. This keeps you from being knocked down and lowers the chance of being hit by falling objects. Families and classes practice drop, cover, and hold on in earthquake drills.

Words to know

• seismograph: shaking recorder
• magnitude: quake size
• intensity: shaking strength
• subduction: plate sinking under
• inertia: staying put