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Seismic waves
Because the earth’s crust is brittle, when rocks near the surface are put under pressure, they do not bend; they break. An earthquake is the pattern of vibrations in the earth – called seismic waves – that occur when rocks break and move to a new position.
The pattern of seismic waves moves out from
the place where the rocks break, in the same way as waves ripple outwards when a pebble is dropped into a pond of still water.
Waves on the surface (which often shake the ground up and down and from side to side very violently) are merely the most obvious part of what happens. Quite unseen, the shock also sends waves through the earth.
The starting point for the break is called the focus, or hypocentre. The position on the ground surface directly above this starting point is called the epicentre. The
line, or more accurately, the zone, along which the break occurs is called the fracture zone, or fault.
The break begins at a point and then runs along the
(Below) The focus (hypocentre) and epicentre of an earthquake.
Focal depth
Epicentre
Direction of rupture
Focus or hypocentre
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Fault plane