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Fractured Fault plane
fracture zone like a piece of cloth being torn. It is called a rupture.
Because the rupture occurs progressively along a line, seismic waves are produced by different parts of the rupture a few seconds apart. The waves produced by the rupture at one point may overlap those produced at another point on the rupture, and this often results in a very complicated pattern of ground movement.
Earthquake waves are normally described in terms of the time it takes for each wave crest to pass. This is called the wave period. Some waves have crests that pass quickly enough for the wave to be audible, so that it is sometimes possible to hear an earthquake as a deep rumble. But the waves that do the damage move more slowly, often with a period of over a second between crests. The slow waves can make the ground pitch and roll; and if this pitching and rolling lasts for long enough, cliffs will crumble, landslides will occur, and buildings will collapse.
Properties of seismic waves
Earthquakes are measured by an instrument called
(Below) A small fault seen in a roadside quarry. Notice the left- hand part of the white band of
rock has been displaced upwards compared to that on the right. Although it is inactive now, this fault was, in the past, a rupture zone and the site of repeated earthquakes.
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