The ground beneath our feet seems solid and still, but it is not. Earth's outer shell is broken into giant pieces that move slowly, a few centimeters each year, about as fast as your fingernails grow. Over time, this slow movement builds up enormous stress in the rocks. When the rocks suddenly break and slip, the energy is released as seismic waves that shake the ground. That is an earthquake. Earthquakes can be gentle tremors felt only by instruments or devastating events that reshape coastlines and cities. This explainer describes plate tectonics, how earthquakes happen, how they are measured and how people can reduce their risks.
Earth's moving shell
Earth is made of layers: a thin, rocky crust, a thick, hot mantle, and a metallic core. The crust and the uppermost part of the mantle together form the rigid lithosphere, which is broken into about a dozen large tectonic plates and many smaller ones. These plates float on a hotter, softer layer of the mantle called the asthenosphere, which flows very slowly over long periods.
The plates move because of heat from Earth's interior. Several processes contribute, including the pull of dense, cold plates sinking into the mantle at subduction zones, the push of new crust forming at mid-ocean ridges, and slow convection currents in the mantle.
The theory of plate tectonics, developed in the 1960s, built on Alfred Wegener's earlier idea of continental drift. It explains why continents fit together like puzzle pieces, why mountains and ocean trenches form where they do, and why earthquakes and volcanoes are concentrated in certain belts around the world.
Types of plate boundaries
| Boundary type | Plate movement | Typical features | Examples |
|---|---|---|---|
| Divergent | plates move apart | mid-ocean ridges, rift valleys, mostly shallow earthquakes | Mid-Atlantic Ridge, East African Rift |
| Convergent | plates collide | subduction zones, deep trenches, mountains, volcanoes, the largest earthquakes | Japan, Andes, Himalayas |
| Transform | plates slide past each other | strike-slip faults, shallow earthquakes | San Andreas Fault, California |
Most earthquakes happen along plate boundaries, though some occur within plates, along older zones of weakness.
How an earthquake happens
The edges of tectonic plates do not slide smoothly. Friction locks them together, while the plates keep moving. Stress builds up in the rocks, which bend and deform elastically, like a stretched rubber band. Eventually, the stress becomes greater than the strength of the rocks or the friction holding them, and they suddenly slip along a fracture called a fault. The rocks snap back toward their unstrained shape, releasing stored energy. Scientists call this the elastic rebound theory.
The point underground where the rupture begins is the hypocenter or focus. The point on the surface directly above it is the epicenter. A large earthquake rupture can extend along a fault for hundreds of kilometers.
Earthquakes are often followed by aftershocks, smaller quakes that occur as the crust adjusts to the new stresses. Sometimes, smaller foreshocks precede a large earthquake, but they cannot be identified as foreshocks until the main shock happens.
Seismic waves
The energy released in an earthquake travels through Earth as seismic waves. There are two main kinds:
Body waves
- P waves (primary waves) are compressional waves that push and pull rock in the direction they travel, like a slinky being pushed. They are the fastest seismic waves and can travel through solids, liquids and gases.
- S waves (secondary waves) move rock up and down or side to side, perpendicular to their direction of travel. They are slower than P waves and cannot travel through liquids.
The fact that S waves do not pass through Earth's outer core was key evidence that the outer core is liquid.
Surface waves
When body waves reach the surface, they create surface waves, which travel along the ground. They are slower but often cause the most damage, because they produce strong rolling and side-to-side shaking.
Measuring earthquakes
Seismographs
Instruments called seismometers detect ground motion, and the recordings, called seismograms, show the arrival of different waves. By comparing the arrival times of P and S waves at different stations, seismologists can calculate the location and depth of an earthquake.
Magnitude
The size of an earthquake is described by its magnitude, which measures the energy released. Today, scientists use the moment magnitude scale, which replaced the older Richter scale for most purposes, especially for large earthquakes.
The magnitude scale is logarithmic. Each increase of one whole number corresponds to about ten times more ground motion and about 32 times more energy. A magnitude 7 earthquake releases about 32 times more energy than a magnitude 6, and about 1,000 times more than a magnitude 5.
| Magnitude | Typical effects | Approximate number per year worldwide |
|---|---|---|
| below 3 | usually not felt | millions |
| 3 to 3.9 | often felt, rarely causes damage | many thousands |
| 4 to 4.9 | noticeable shaking, minor damage possible | about 10,000 |
| 5 to 5.9 | damage to poorly built structures | about 1,000 to 1,500 |
| 6 to 6.9 | destructive in populated areas | about 100 to 150 |
| 7 to 7.9 | major, serious damage over large areas | about 10 to 20 |
| 8 or more | great earthquake, can devastate areas hundreds of km wide | about 1 |
The largest earthquake ever recorded struck Chile in 1960, with a magnitude of 9.5.
Intensity
While magnitude measures the size of an earthquake, intensity describes how strong the shaking was at a particular place and what effects it had. The Modified Mercalli Intensity Scale uses Roman numerals from I, not felt, to XII, total destruction. One earthquake has one magnitude but different intensities in different places, depending on distance, depth, local soil and building quality.
Tsunamis
When a large earthquake occurs beneath the ocean, especially at a subduction zone, the seafloor can suddenly rise or fall, displacing a huge volume of water. This generates a tsunami, a series of long, powerful waves. In the deep ocean, a tsunami may be less than a meter high but travel as fast as a jet airplane, about 500 miles per hour (800 kilometers per hour). As it reaches shallow coastal waters, it slows and grows dramatically in height.
The 2004 Indian Ocean earthquake and tsunami killed about 230,000 people in more than a dozen countries. The 2011 Tōhoku earthquake in Japan, magnitude 9.0 to 9.1, generated a tsunami that caused enormous destruction and the Fukushima nuclear accident.
Natural warning signs of a tsunami include strong shaking near the coast and the sea suddenly drawing back from the shore. If you notice them, move to high ground immediately.
Can earthquakes be predicted?
Despite decades of research, scientists cannot yet predict the exact time, place and size of an earthquake. What they can do is estimate probabilities: the likelihood of a significant earthquake in a region over the coming decades, based on fault locations, past earthquakes and the rate of plate movement. These forecasts guide building codes and emergency planning.
Earthquake early warning systems are a different and valuable tool. They detect the fast P waves of an earthquake that has already begun and send alerts before the slower, more damaging waves arrive. Depending on distance, this can give seconds to tens of seconds of warning, enough to slow trains, stop elevators and allow people to take cover. Systems operate in Japan, Mexico and parts of the United States, among other places.
Reducing earthquake risk
Earthquakes themselves rarely kill people directly. Most deaths and injuries come from collapsing buildings, falling objects, landslides, fires and tsunamis. That means good preparation saves lives.
- Building codes: Earthquake-resistant design, such as flexible frames, reinforced walls and base isolation, helps buildings survive shaking.
- Securing furniture: Anchor bookcases, cabinets and water heaters to walls, and store heavy objects low.
- Emergency kits: Keep water, food, a flashlight, a radio, first aid supplies and medications.
- Family plan: Agree on meeting points and how to contact each other.
During an earthquake
Emergency agencies recommend Drop, Cover and Hold On:
- Drop to your hands and knees.
- Cover your head and neck, ideally under a sturdy table or desk.
- Hold on until the shaking stops.
Stay away from windows and heavy objects that might fall. If you are outdoors, move away from buildings, trees and power lines. If you are near the coast and feel strong shaking, move inland or to higher ground as soon as the shaking stops.
Questions readers ask
What causes earthquakes? Most earthquakes happen when stress built up by moving tectonic plates causes rocks to suddenly slip along a fault, releasing energy as seismic waves.
What is the difference between magnitude and intensity? Magnitude measures the energy released at the source. Intensity measures how strongly the shaking is felt at a particular place.
Where do most earthquakes happen? Along plate boundaries, especially around the Pacific Ring of Fire.
Can animals predict earthquakes? There are many stories, but scientific studies have not shown that animal behavior can reliably predict earthquakes.
Is the Richter scale still used? Scientists now mostly use the moment magnitude scale, which measures large earthquakes more accurately, but the term "Richter scale" is still common in the media.
In short
Earth's outer shell is made of tectonic plates that move slowly, building stress along faults. When rocks suddenly slip, they release energy as seismic waves, causing an earthquake. P waves arrive first, followed by S waves and damaging surface waves. Magnitude describes the energy released on a logarithmic scale, while intensity describes local shaking. Undersea earthquakes can generate tsunamis. Earthquakes cannot yet be predicted precisely, but probability forecasts, early warning systems, strong buildings and simple preparations such as Drop, Cover and Hold On save countless lives.