What is an earthquake? A simple explanation
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An earthquake is a sudden release of energy in Earth's crust. Stress that has built up over years or centuries is released in seconds — and the ground shakes.
How the shaking starts
Earth's outer layer is in constant, slow motion. When two blocks of crust lock against each other, the rock bends and strains like a spring. One day the stress exceeds the friction: the rock snaps back along a fracture — a fault — and the energy spreads as waves in every direction. Those waves are what you feel as shaking. The vast majority of quakes happen along the boundaries between Earth's tectonic plates.
Epicentre and hypocentre
The rupture itself happens underground — that point is the hypocentre (or focus). The point directly above it on the surface is the epicentre — that is what maps show. Depth matters a lot: a quake 5 km down feels far more violent than the same quake 300 km down.
The waves of a quake
A quake sends out several kinds of waves: P waves (fastest, felt as a jolt), S waves (slower, stronger) and surface waves (slowest, but the ones that sway and damage). Because P waves arrive first, early-warning systems can give seconds of notice before the strong shaking hits.
How often does it happen?
More often than most people think: hundreds of thousands of quakes are registered worldwide every year — on EarthquakeIndex's live map you typically see 300–400 per day. The vast majority are so small that only instruments notice them. How strong a quake is, is measured in magnitude — read what the number means here.