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of plate tectonics see the book Plate tectonics in this set.)
Each type of plate boundary proves to have its own distinctive types of earthquakes. At spreading boundaries, where two plates pull apart, earthquakes occur at shallow depths (within 30 kilometres of the surface). In part this is because the crust at a spreading boundary is very thin. The fault lines tend to be nearly vertical.
Where plates grind past one another, the earthquakes also tend
to occur at shallow depths and
form fairly straight long lines. Here
the earthquakes tend to be shallow. The San Andreas Fault is the world’s best-known example of a line on which earthquakes are common.
At destructive plate boundaries, where plates collide, and where one plate pushes the other downwards into the mantle, earthquakes occur at a wide range of depths, from shallow to very deep.
Ninety per cent of all earthquakes
occur at plate boundaries. The rest
occur within plates, but they are not
insignificant. Often they form along
very ancient plate boundaries that have
subsequently become part of new plates. The great New Madrid earthquake of 1812 was one of them. Others occur where plumes of mantle material rise up underneath
a plate. They are called hot spots. The area near the northern end of the East African Rift Valley is one of them.
Earthquakes occur at greatly varying depths. The depth is often called the focal depth, meaning the distance vertically from the surface to the earthquake focus (see diagram on page 6). Earthquakes with focal
(Above} This computer-generated image shows the many faults that lie across the San Francisco Bay region. It is a classic seismic gap, where earthquakes are rare because the ground is locked more firmly together than other places on the San Andreas Fault. When they do occur, the earthquakes are of larger magnitude than elsewhere because they have a long period of catching up to do.
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