What Is an Earthquake?

An earthquake is the shaking produced when rock inside the Earth suddenly breaks or slips along a fracture, releasing energy that has been building up for years, decades or centuries. That energy travels outwards as seismic waves, and it is those waves passing through the ground beneath you that you feel as shaking.

How strain builds and releases

The Earth's outer shell is broken into tectonic plates that move relative to one another at roughly the rate your fingernails grow — a few centimetres per year. Where two plates meet, the boundary is rarely smooth. Friction locks the two sides together while the plates behind them keep moving, so the rock near the boundary deforms elastically, storing energy like a bent spring.

Eventually the accumulated stress exceeds the friction holding the fault closed. The rock slips, sometimes by a few centimetres, sometimes by tens of metres in the largest events, and springs back towards its unstrained shape. This model is called elastic rebound, and it explains why earthquakes recur on the same faults: the plates never stop moving, so strain simply begins accumulating again the moment an earthquake ends.

The point underground where the rupture begins is the focus, also called the hypocentre. The point on the surface directly above it is the epicentre — the coordinates you see reported for every event. The distance between them is the focal depth, and it matters enormously for how strongly the surface shakes.

What seismic waves actually do

Rupture releases several distinct types of wave, and they arrive in a predictable order.

P-waves (primary waves) travel fastest, at roughly 6 km per second in the crust. They are compressional: the ground moves back and forth along the direction the wave is travelling, the same way sound moves through air. P-waves are usually felt as a sharp jolt or a bang, and they can travel through both solid rock and liquid.

S-waves (secondary waves) follow at roughly 3.5 km per second. They shear the rock sideways, perpendicular to their direction of travel, and they carry considerably more energy than P-waves. The strong side-to-side motion that damages buildings is mostly S-wave energy. S-waves cannot travel through liquid, which is how seismologists first established that the Earth's outer core is molten.

Surface waves arrive last and travel along the ground rather than through it. Love waves shift the ground horizontally; Rayleigh waves roll it in an elliptical motion rather like an ocean swell. Surface waves decay more slowly with distance than body waves, so at long range they dominate — and because tall buildings resonate with their long periods, they cause much of the damage in large distant earthquakes.

The gap between the P-wave and S-wave arrival is what makes earthquake early warning possible: detect the harmless P-wave, calculate the event, and issue an alert before the destructive S-waves arrive.

Why the same magnitude produces different shaking

Two earthquakes of identical magnitude can produce completely different outcomes. Several factors decide this.

Depth. A magnitude 6.0 at 10 km can be devastating; the same magnitude at 300 km may barely be noticed. The energy has much further to spread before it reaches anyone.

Distance. Shaking intensity falls off rapidly with distance from the epicentre. Being 50 km away rather than 5 km away typically makes an enormous difference.

Local ground conditions. Soft sediment amplifies shaking dramatically compared with solid bedrock. Basins of loose sediment can also trap and reverberate seismic waves, extending the duration of strong shaking. In extreme cases saturated sandy soils undergo liquefaction and temporarily behave like a fluid.

Building construction. By far the largest determinant of casualties. Unreinforced masonry performs terribly; modern seismically designed structures perform very well. This is why earthquakes of similar size can kill tens of thousands in one country and almost nobody in another.

This is also why seismologists distinguish magnitude from intensity. Magnitude is a single number describing the energy released at the source. Intensity — usually expressed on the Modified Mercalli scale from I to XII — describes how strongly the ground shook at a particular place, and a single earthquake has many different intensities at different locations.

How often earthquakes happen

Earthquakes are far more common than most people assume. Global networks locate several million events a year, the overwhelming majority far too small to be felt. The frequency follows a strikingly regular pattern known as the Gutenberg–Richter relationship: for each whole step down the magnitude scale, roughly ten times as many earthquakes occur.

In round numbers, the planet averages about one magnitude 8 or greater per year, around 15 in the magnitude 7 range, roughly 130 in the magnitude 6 range, and well over a thousand in the magnitude 5 range. Below that the counts climb into the tens and hundreds of thousands.

Earthquakes that aren't tectonic

Not every recorded event is caused by plate motion. Volcanic earthquakes accompany magma moving underground and often occur in swarms. Collapse earthquakes follow mine or cavern failures. Induced seismicity is triggered by human activity — most commonly deep wastewater injection, and also reservoir impoundment behind large dams. Seismic catalogues also record quarry blasts and other explosions, which is why event records carry a type field distinguishing a genuine earthquake from a human-made source.