The short answer
An earthquake is the shaking of the ground caused by a sudden release of energy in the Earth’s crust or upper mantle. In most cases, that energy has been stored for years, decades or centuries as rock slowly deforms, and it is released when the rock suddenly slips along a break called a fault.
Faults and stored stress
The outer shell of the Earth is divided into moving plates, and their slow motion is explained in the discussion of plate tectonics. Most earthquakes occur along plate boundaries, where plates grind past, pull away from or push into each other. A fault is a fracture in the rock where one side can move relative to the other.
Faults are usually not smooth. Friction holds the rock on either side in place even while the plates keep moving, so the surrounding rock bends and stores elastic energy, much as a stick bends before it snaps. When the stress exceeds the friction, the fault slips suddenly, and the rock springs back toward an unstressed shape. This explanation, called elastic rebound, was developed after studies of the 1906 San Francisco earthquake.
Earthquakes also occur inside plates, on older faults far from boundaries, and in some cases they are triggered by human activity, such as filling large reservoirs or injecting fluids deep underground, although such quakes are usually small to moderate.
Focus, epicenter and seismic waves
The point underground where the slip begins is called the focus or hypocenter. The point on the surface directly above it is the epicenter. Shallow earthquakes are generally more damaging at the surface than deep ones of the same size, since the energy has less distance to travel.
The released energy spreads outward as seismic waves, of several types:
- P waves (primary waves) are compressional, squeezing and stretching rock in the direction they travel, a bit like sound waves moving through air. They are the fastest and arrive first. They can travel through solids and liquids.
- S waves (secondary waves) shake the ground from side to side or up and down, at right angles to their direction of travel. They are slower than P waves and cannot travel through liquids.
- Surface waves travel along the ground surface. They are slower still but often produce the strongest shaking and most of the damage.
Because P and S waves travel at different speeds, a seismometer records them arriving at different times, and the gap indicates how far away the quake was. Readings from at least three stations allow scientists to find the location by triangulation. Early warning systems in some regions use the fact that damaging waves travel more slowly than electronic signals to give seconds of notice, but only to places some distance from the epicenter.
Studying how seismic waves bend and bounce inside the planet has also taught scientists most of what is known about Earth’s interior, including the fact that the outer core is liquid. Similar instruments have even recorded marsquakes, which have added to knowledge gained through Mars exploration.
Magnitude: how big was it
Magnitude is a number describing the size of an earthquake, based on the energy released or the movement on the fault. Early versions were developed in the 1930s by Charles Richter, and the familiar “Richter scale” is often mentioned in news reports. Today seismologists mostly use the moment magnitude scale, which better represents the largest earthquakes. It is based on the area of the fault that slipped, the amount of slip and the rigidity of the rock.
These scales are logarithmic, so each whole number is a large step up. Each increase of one in magnitude corresponds to about 10 times larger ground motion recorded on a seismogram and roughly 32 times more energy released. So a magnitude 7 earthquake releases about 32 times as much energy as a magnitude 6, and about 1,000 times as much as a magnitude 5, because 32 times 32 is about 1,000. Earthquakes below magnitude 3 are usually not felt by many people, while those above about magnitude 7 can cause serious damage near the epicenter.
Intensity is different from magnitude
Each earthquake has one magnitude, but the shaking varies from place to place. Intensity describes the effects at a given location, such as how strongly people felt it and how much damage occurred. It is often rated with the Modified Mercalli scale, using Roman numerals. Intensity depends on the distance from the epicenter, the depth, the local soil and rock, and how buildings are constructed. Soft soil can amplify shaking, and well-designed buildings can survive shaking that would destroy weaker ones.
Related hazards
Large earthquakes beneath or near the sea can displace the seafloor and generate tsunamis, series of waves that can cross whole oceans. These are not the same as ocean tides, which are caused by gravity from the Moon and Sun. Earthquakes can also trigger landslides and, in some areas, make water-saturated sandy soil behave like a liquid, which is called liquefaction.
Can earthquakes be predicted?
Scientists can estimate where earthquakes are likely and give long-term probabilities, such as the chance of a major quake on a particular fault over several decades. They cannot currently predict the exact time, place and size of a specific earthquake in advance. Aftershocks, smaller quakes that follow a larger one, are expected, and they typically decline in number over days to months.
This is general educational information. In an earthquake-prone area, local emergency agencies provide guidance on preparing and on what to do when shaking starts.