Earthquakes occur when a build-up of pressure or strain between sections of rocks within the earth’s crust is suddenly released, causing minor or severe vibrations on the surface of the land. The point at which layers of rock shift and reposition in relation to one another is called the focus; this is the effective centre of the earthquake. Directly above the focus, a second point called the epicentre marks the corresponding point of highest-intensity shock on the surface. Shock waves propagate like ripples from the focus and epicentre, decreasing in intensity as they travel outwards. The main types of seismic waves are primary waves (P waves) and shear waves (S waves). P waves cause particles to vibrate in the same direction as the shock wave (left). P waves are the first to be recorded in an earthquake because they move faster than S waves (right), which cause vibrations perpendicular to the direction of travel

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