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kilometres in the last twenty million years. The average rate at present is about four to five centimetres each year. But in some places the fault has become stuck and moves only occasionally; and when it does move, it does so very violently. One of these seismic gaps is in the area of San Francisco, which is why this city suffered only two earthquakes in the 20th century, but why both were large.
Making use of earthquake
waves
Earthquakes are usually thought of as being destructive events, but they can produce a large amount of information that is of great value to earth science. Not only can they locate the edges of the earth’s plates and tell us about the way in which plates are dipping back into the earth, but they can also tell us about the whole nature of the earth. They are, in effect, a geologist’s stethoscope to the earth’s mantle and core.
To understand how earthquakes can be used in this way, we need to concentrate not on the waves that reach
(Below) All parts of a plate boundary move on average at the same rate. However, there are some parts
of the plate boundaries where earthquakes are infrequent. They are known as seismic gaps. Their locations are shown on this map. When earthquakes do finally occur in seismic gaps, they are likely to
be more powerful than along parts of the plate boundaries where earthquakes happen frequently. In this way a few large earthquakes will allow the seismic gap area to catch up with the more frequent movements elsewhere.
Seismic gap where stresses build Frequent earthquakes relieve stress
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