Scientists can't see an earthquake the way meteorologists photograph clouds, so they work out its source from wave arrival times. 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 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. An earthquake can come at any time of year with no warning, and no one can say exactly when one will happen; shaking spreads out from the middle of an earthquake in every direction, like ripples from a rock thrown into a pond. Most earthquakes happen where the edges of Earth's plates meet.
At a transform boundary, two plates slide past each other sideways, and the San Andreas Fault in California is one. At a spreading boundary, hot rock rises and pushes two plates apart, making new crust. 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. Where one plate pushes under another, it sinks down into the hot layer below the crust, and this process is called subduction, the sinking of one plate under another. 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, which is about 8,854 meters tall, and trenches form where plates subduct.
Great earthquakes, those of magnitude 8 or higher, happen about once a year somewhere in the world. As of November 2026, 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 a fault that could produce one would have to reach around most of the planet. Earthquakes can happen in the crust or upper mantle, from the surface to about 800 kilometers (500 miles) deep, that is, from just under the ground to far below it; very large, shallow subduction quakes such as Sumatra in 2004 at magnitude 9.1 and Japan in 2011 at magnitude 9.0 happen on the plate contact above about 60 km depth, which is shallow compared with the deepest earthquakes. Large earthquakes beneath the ocean may not even be felt by people.
A tectonic earthquake happens when strain in rock builds until stress passes the rock's strength and it suddenly fractures, an explanation known as elastic rebound, an idea of Harry Fielding Reid that came 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). Strain builds up in rock like a rubber band pulled too tight, and plates stick at their edges because of friction until the stress gets bigger than the friction. A fault is a crack in the rocks of the Earth 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, whereas S-waves are shear waves that cannot travel through liquids. The slower S-waves and surface waves, at about 2.5 miles per second, 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 between them is what gets recorded; the earliest known earthquake detector, a Chinese seismoscope from A.D. 132, showed that a quake happened but did not record it, so it could tell people that shaking had occurred without keeping any record of the motion.
Seismologists find an earthquake's location by timing when its waves reach different seismograph stations. To locate a quake, they guess a place, depth and time, compare the predicted wave arrival times with the real ones at the stations, and shift the guess to shrink the difference, repeating the process until the two agree. The match between the guess and the real arrival times improves each time. The spot on the ground straight above where an earthquake starts is called the epicenter; magnitude is one number for the size of the earthquake itself, so it does not change from place to place. Intensity, in contrast, measures how strong the shaking is at one place, so it changes with where you stand, and so magnitude and intensity are not the same thing.
Charles Richter of the California Institute of Technology introduced his scale in 1935. Each whole number up in magnitude means 10 times larger wave height on a seismogram, which is a record of ground movement, 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. The Moment Magnitude scale is now the one the USGS reports, and the Richter scale is outdated for large earthquakes; moment magnitude is based on the physical size of the slip, which is the rigidity of the rock times the fault area times the slip distance. Seismologists turn that seismic moment into a number like older scales with the formula Mw = 2/3 (log10(Mo) - 9.1), so that the result reads like a number on the older scales.
Intensity is a rating from people's reports and damage. The USGS used to collect reports by postal questionnaire but now uses an online web form called Did You Feel It? Local geology changes shaking, because bedrock shakes the least and soft mud the most, and liquefaction is when loose, water-soaked sediment loses its strength during strong shaking. Damage depends on design, since it is negligible in buildings of good design and construction and considerable in poorly built or badly designed ones. The strength of shaking also drops with distance from the source, so a quake 500 km deep shakes the surface much less than the same quake at 20 km; 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, which shows how distance from the source weakens the shaking. Engineers make buildings safer by designing new ones to handle shaking and by strengthening old 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. Tsunami waves can move as fast as a jet plane in deep water and slow down as they reach shallow water. Out in deep ocean they are not very tall, but they grow taller as they near land, so the wave that reaches a coast is taller than the wave in the open sea. The name tidal wave is discouraged because tides have little to do with them. If you are near the ocean and feel strong shaking, go to higher ground once the shaking stops, because a tsunami may follow. November 5 is World Tsunami Awareness Day, set by the United Nations to honor a Japanese village chief and farmer whose warning in 1854 saved many lives.
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 is not prediction. It gives its warning only after the quake has begun, and it works because P-waves outrun the damaging S and surface waves, and data travel far faster than seismic waves; in other words, the alert travels as data faster than the shaking travels as seismic waves. People farther from the epicenter have more time to get an alert before the shaking arrives. A phone alert from an early-warning system can come seconds before the shaking, in time to drop, cover, and hold on. Drop, cover, and hold on keeps you from being knocked down and lowers the chance of being hit by falling objects.
The USGS says no one has predicted a major earthquake, and it gives only the probability that a big one will happen in a place within a number of years. 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, while precursors like small-quake swarms or radon often appear without a big quake following. In other words, small swarms and radon do not reliably mean a large quake is coming. Scientists can tell how much strain a fault is building and where past quakes happened to figure the chance of future ones. People cannot prevent natural earthquakes, but they can reduce harm by finding hazards, building safer, and teaching earthquake safety. Families can get ready ahead of time with an emergency kit and a plan for where to meet if they get separated.
Words to know
- seismograph: shaking recorder
- subduction: plate sinking under
- epicenter: spot above start
- intensity: local shaking
- liquefaction: ground turning soupy
- inertia: staying put
