Earthquakes as a release of stored strain
An earthquake is the shaking that results when rock suddenly breaks or slips and releases stored energy. The outer shell of Earth is broken into plates that move slowly, typically a few centimeters per year. That motion is not smooth. In many places, the edges of plates, or the cracks within them, stick together while the surrounding rock keeps moving and deforming. Strain accumulates, a bit like bending a stick, until the rock gives way.
The crack along which the movement occurs is called a fault. When the stuck section finally slips, the rocks on either side snap back toward an unstrained shape, an idea known as elastic rebound. The slip may be a few centimeters or many meters, depending on the size of the earthquake, and it radiates energy outward as seismic waves.
The point underground where the slip begins is the focus (or hypocenter). The point on the surface directly above is the epicenter. News reports usually give the epicenter, but in a large earthquake, the rupture can extend for tens or hundreds of kilometers along the fault.
Where earthquakes occur
Most earthquakes are concentrated along plate boundaries, which is why plate tectonics is central to understanding them. There are three broad types of boundary.
- Convergent boundaries, where plates collide or one sinks beneath another. These produce the largest earthquakes, including those at subduction zones.
- Transform boundaries, where plates slide past each other horizontally. The San Andreas Fault in California is a well-known example.
- Divergent boundaries, where plates move apart. Earthquakes here tend to be smaller and shallower.
Earthquakes can also occur in the middle of plates, along old zones of weakness, although much less often. Human activity can trigger smaller earthquakes in some situations, such as reservoir filling or the injection of fluids underground, and volcanic regions have their own earthquakes from moving magma. Exactly when and where a given earthquake will happen cannot be predicted. Scientists can estimate long-term probabilities for regions, but not the day or hour of an event.
Seismic waves
The energy of an earthquake travels through the Earth as waves.
- P waves (primary waves) are compressional, pushing and pulling the rock in the direction the wave travels. They are the fastest and arrive first, and they can pass through solids and liquids.
- S waves (secondary waves) move the rock from side to side, perpendicular to their travel. They are slower and cannot pass through liquids.
- Surface waves travel along the ground surface. They are slower still but often cause the most damaging shaking.
The fact that S waves do not pass through liquid helped scientists work out that part of Earth’s core is liquid. The way waves bend and reflect inside the planet remains one of the main tools for mapping Earth structure.
Detecting and locating earthquakes
A seismometer records ground motion, and the record it produces is called a seismogram. Because P waves travel faster than S waves, the gap between their arrival times at a station tells how far away the earthquake was. A single station gives only a distance, not a direction. Using the distances from at least three stations, scientists can pinpoint the epicenter where the circles drawn around each station intersect, and today computers do this automatically using many stations.
Some regions use early warning systems that detect the fast-moving P waves and send alerts before the slower, stronger waves arrive. The warning time is typically seconds to tens of seconds, depending on distance, and it is useful only in places where such systems have been built. Near the epicenter, there may be no warning at all.
Measuring size: magnitude
Magnitude is an estimate of the size of the earthquake source, meaning how much energy was released, and it is a single number for each event. The best-known early scale was developed by Charles Richter in the 1930s, based on the amplitude of recorded waves. Today, scientists mostly use the moment magnitude scale, which is based on the physical size of the rupture: the area of the fault that slipped, the amount of slip, and the stiffness of the rock. It works well for large earthquakes, where older scales tend to saturate.
The scale is logarithmic, which has two consequences worth understanding.
- Each whole-number step up corresponds to roughly ten times larger ground-motion amplitude.
- Each whole-number step corresponds to roughly 32 times more energy released. (This comes from 10 raised to the power 1.5, which is about 31.6.)
So a magnitude 7 earthquake releases roughly 32 times as much energy as a magnitude 6, and about 1,000 times as much as a magnitude 5, since 32 times 32 is about 1,024. These are rounded rules of thumb.
Magnitude estimates are often revised in the hours after a large earthquake as more data arrives, which is why early reports sometimes differ.
Measuring effects: intensity
Magnitude does not tell you how strongly the ground shook in a particular town. That is described by intensity, which rates the observed effects at a location, from barely felt to total destruction. The Modified Mercalli scale, which uses Roman numerals, is a common example. A single earthquake has one magnitude but many intensities, which generally decrease with distance from the rupture.
Damage depends on far more than magnitude. Important factors include:
- the depth of the earthquake, with shallow events usually causing stronger shaking at the surface
- the distance from the fault
- local soil, since soft sediments can amplify shaking and may liquefy
- the quality and design of buildings and infrastructure
A moderate earthquake near a poorly prepared city can be more harmful than a larger one in a remote area. Large offshore earthquakes can also cause tsunami waves, which is a separate hazard.
Staying safe
General guidance in earthquake-prone regions is commonly to drop, cover and hold on during shaking, to secure heavy furniture, and to know local plans. Specific advice differs by country and situation, so follow your local emergency agency, and when traveling, check local guidance. No simple method can predict an earthquake, so preparation is the main tool available.