An earthquake is the sudden, quick shaking of the ground. Scientists can't see an earthquake the way meteorologists photograph clouds, so they work out its source from wave arrival times; 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; the biggest earthquake ever recorded, as of November 2026, 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. Shaking spreads out from the middle of an earthquake in every direction, like ripples from a rock thrown into a pond; large earthquakes beneath the ocean may not even be felt by people.
Where one plate pushes under another, it sinks down into the hot layer below the crust, and subduction zones have deep-sea trenches, shallow to deep earthquakes and mountain ranges with active volcanoes; depth information from many quakes maps a subduction zone, showing how steeply the plate dips and whether it bends. Earthquakes can happen in the crust or upper mantle, from the surface to about 800 kilometers (500 miles) deep; 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. The northwest U.S. coast, western Canada and southern Alaska sit on a subduction zone. Very deep earthquakes occur in the cold cores of sinking slabs, which stay brittle.
A tectonic earthquake happens when strain in rock builds until stress passes the rock's strength and it suddenly fractures, which is the elastic rebound idea of Harry Fielding Reid, drawn from the 1906 San Francisco earthquake; in that event 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, and in normal faulting the upper block moves down, but 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, while S-waves are shear waves that cannot travel through liquids, and 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 is recorded; the earliest known earthquake detector, a Chinese seismoscope from A.D. 132, showed that a quake happened but did not record it; on a seismogram the horizontal axis is time in seconds and the vertical axis is ground movement, usually in millimeters.
Seismologists find an earthquake's location by timing when its waves reach different stations. To locate a quake, they guess a place, depth and time, compare the predicted wave arrival times with the real ones at stations, and shift the guess to shrink the difference, repeating until the guess fits. The spot on the ground straight above where an earthquake starts is called the epicenter; 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. 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; seismographs and the stations around them show where and how big an earthquake was. Shaking damages buildings most, and how strong it is depends on the quake's magnitude, distance from the fault, local geology and soil.
The Richter scale was introduced in 1935 by Charles Richter of the California Institute of Technology, and it measures the largest wiggle on a record, while other magnitude scales measure other parts of the earthquake; 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, which is the rigidity of the rock times the fault area times the slip distance, and it uses the formula Mw = 2/3 (log10(Mo) - 9.1), which turns seismic moment into a number like the older scales; 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.
Magnitude is one number for the size of the earthquake itself, 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; intensity is a rating from people's reports and damage, and 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, with bedrock shaking the least and soft mud the most, and liquefaction is when loose, water-soaked sediment loses its strength during strong shaking; damage does not usually start until magnitude is above about 4 or 5, and it also depends on distance, soil and building type. Damage is negligible in buildings of good design and construction and considerable in poorly built or badly designed ones; engineers make buildings safer by designing new ones to handle shaking and by strengthening old ones.
Seismology as a science took off in the early 1900s and has answered why and how earthquakes happen; scientists can tell how much strain a fault is building and where past quakes happened to figure the chance of future ones. In the USGS example, if four M7+ quakes struck a region in 200 years at random, the chance of another within 50 years is 50 percent; random timing may not hold, because when strain is released on one part of a fault system it may increase on another. The San Francisco Bay area record has four M6.8+ quakes from 1836 to 1911, then none M6+ for 68 years until 1979, then four M6+ from 1979 to 1989, including Loma Prieta (M7.1).
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, and out in deep ocean they are not very tall, but they grow taller as they near land. Tsunami speed depends on ocean depth, not on how far from the source, and 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, and it works because P-waves outrun the damaging S and surface waves, while data travel far faster than seismic waves. A phone alert from an early-warning system can come seconds before the shaking, in time to drop, cover, and hold on; people farther from the epicenter have more time to get an alert before the shaking arrives. Inside, stay inside and do not run outside while the ground is shaking.
A real earthquake prediction must give three things: the date and time, the location, and the magnitude; 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. Clouds, body aches and slugs have nothing to do with earthquakes, and precursors like small-quake swarms or radon often appear without a big quake following; people cannot prevent natural earthquakes, but they can reduce harm by finding hazards, building safer, and teaching earthquake safety. Drop, cover, and hold on keeps you from being knocked down and lowers the chance of being hit by falling objects; 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
- subduction: plate sinking under
- intensity: local shaking
- liquefaction: ground turning soupy
- precursors: early signs
- epicenter: spot above start
