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Earthquakes occur when stress accumulated along geological fault lines is suddenly released. Tectonic plates — the massive sections of Earth's crust — are constantly moving, typically a few centimetres per year. Where plates meet, they grind against each other. Friction prevents smooth sliding, so stress builds up over decades or centuries until it releases catastrophically.
The point underground where the earthquake originates is the hypocentre (or focus). The point on the surface directly above is the epicentre, which is what news reports refer to. Seismic waves radiate outward in all directions from the hypocentre.
The Richter scale and more modern moment magnitude scale measure energy released. The scale is logarithmic — a magnitude 7 is about 32 times more energy than a magnitude 6. A magnitude 9 releases about 1,000 times more energy than a magnitude 7.
Earthquakes are concentrated along plate boundaries: the Pacific Ring of Fire (Japan, Indonesia, Chile), the Alpine-Himalayan belt, and mid-ocean ridges. But intraplate earthquakes can occur far from boundaries and are often harder to predict.
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