What Is an Earthquake?

An earthquake is a sudden shaking of the ground caused by the movement of huge slabs of rock deep inside the Earth. The slabs are called tectonic plates, and they are constantly drifting around the planet at a few centimetres per year. Most of the time the plates move smoothly. But at their edges, they often lock together and pressure builds up over years or centuries. When the rock finally cannot take any more, it slips suddenly, releasing huge amounts of energy as seismic waves that shake the ground. Around 500,000 earthquakes are detected each year worldwide, but most are far too small for anyone to feel.

• Earthquakes per yearApprox. 500,000 detectedMost are too small to feel
• Felt by peopleApprox. 100,000 per yearMost cause no damage
• Strongest ever recorded9.5 magnitudeValdivia, Chile, 22 May 1960
• Famous deadliestShaanxi, China, 1556Approx. 830,000 deaths
• Seismic wave speed3 to 13 km/sThrough different rocks
• Measured withSeismometerDetects ground vibrations

How earthquakes happen

Earth's outer shell (the crust) is broken into about 15 large tectonic plates plus several smaller ones. The plates float on hot, partly molten rock in the layer below, and they are constantly being pushed around by slow currents in that hot rock. The plates move only a few centimetres per year (about as fast as your fingernails grow), but the edges are rough and often lock together.

Pressure builds up at locked plate boundaries for years, decades or even centuries. Eventually the rock cannot take the strain and it slips suddenly. The slip releases the stored energy as seismic waves that travel outwards through the Earth in all directions, shaking everything in their path.

Parts of an earthquake

• Hypocentre (or focus): the exact spot underground where the rock slipped. Can be just below the surface or hundreds of kilometres deep.
• Epicentre: the point on the surface directly above the hypocentre. Usually the worst-shaken area.
• Seismic waves: vibrations that travel outwards from the hypocentre.
• Aftershocks: smaller earthquakes that often follow a big one as the rock settles into a new position.

Types of seismic waves

Earthquakes produce several different kinds of seismic wave.

• P-waves (primary): the fastest. Push and pull through rock like sound through air. Arrive first; do little damage.
• S-waves (secondary): slower. Shake the ground sideways. Cannot travel through liquids.
• Surface waves: slowest but most damaging. Travel along the surface, making the ground roll and shake.

The fact that P-waves arrive a few seconds before the destructive S-waves and surface waves is what makes early-warning systems possible: seismometers near the epicentre can detect the P-wave and send warning signals to cities further away before the destructive waves arrive.

Fact The strongest earthquake ever measured was the Valdivia earthquake in Chile on 22 May 1960, measuring magnitude 9.5. It lasted around 10 minutes, killed approximately 5,700 people and triggered a tsunami that crossed the entire Pacific Ocean. Waves over 10 metres tall hit Chile, while smaller waves struck Hawaii (10,500 km away), Japan (17,000 km away) and the Philippines (16,000 km away).

Where earthquakes happen

Almost all earthquakes happen at the boundaries between tectonic plates. The famous Ring of Fire around the Pacific Ocean (the edge of the Pacific Plate) sees about 75% of all earthquakes. Other major earthquake zones include the boundary between the African and Eurasian Plates (running through the Mediterranean), and the long collision zone between the Indian Plate and Eurasia (which is still raising the Himalayas).

Britain sits in the middle of a tectonic plate, so it rarely has serious earthquakes. The biggest UK earthquake in recent history was the 1931 Dogger Bank earthquake (magnitude 6.1) in the North Sea, which damaged buildings as far away as Hull.

Did you know? Some animals seem to detect earthquakes before humans do. There are many recorded cases of dogs, cats, snakes, fish, birds and farm animals behaving oddly hours or days before a big earthquake. Scientists are still investigating what they might be sensing: possible candidates include slight ground vibrations, changes in electrical fields, gases escaping from cracks, or changes in the water table.

Deeper dive: how we predict earthquakes (and why we mostly cannot)

One of the great frustrations in geology is that we still cannot reliably predict earthquakes. We can identify earthquake-prone areas, estimate the likelihood of a big earthquake over decades, and warn people seconds before the damaging waves arrive. But we cannot say "there will be a magnitude 7 earthquake near San Francisco on this date in this year".

The problem is that earthquakes happen deep in the rock, where we cannot easily watch what is going on. Even after decades of research, no reliable warning signs have been found. Possible precursors (ground tilting, changes in groundwater chemistry, electromagnetic signals, foreshocks) sometimes appear before earthquakes and sometimes do not, and they often appear when no earthquake follows.

Modern earthquake science has focused instead on preparation rather than prediction. By understanding which areas are at risk, building codes can require quake-resistant construction. Early-warning systems use the fact that fast P-waves arrive before destructive S-waves: a network of sensors near the epicentre can detect the P-waves and send warnings to cities further away, giving 10 to 60 seconds for people to take cover and for trains and gas pipelines to shut down automatically. Japan, Mexico and parts of California now have such systems running.

For more, see tectonic plates, the Richter scale and tsunamis.