Earthquakes occur mainly in the regions of the earth where mountains are being formed, and where the earth’s crust is under strain.
Some mountains are formed of great thickness of folded sedimentary rock laid down beneath the sea. Heat currents deep within the earth are thought to suck down sections of the undersea crust and so produce great trenches thousands of feet deep. When the heat currents die away the material forming the bottom of the trench begins to rise because it is lighter in weight. Eventually it is thrust up as a mountain range.
This is never a smooth process but is accompanied by great friction and heat, as well as by rending and shearing and tearing of deep underground rocks connected with mountain formation cause earthquakes. Even small underground movements may produce violent surface shocks. The great Tokyo earthquake of 1923 which is believed to have killed 25 million people was caused by the twisting of a section of the earth’s crust in Sagami Bay.
As might be expected, ocean trenches are the seat of a great many earthquakes, for there the earth’s crust is in an unstable sate. Indeed all the deep earthquakes those taking place more than 160 miles below the surface-originate around the Pacific trenches. About 90% of the intermediate earthquake (30 to 160 miles deep) also originates there, as do 40% of the shallow earthquakes (less than 30 miles deep).
Some shallow and intermediate earthquakes are caused by volcanoes or by a slight shifting of layers of rock at a weak place or “fault” on the earth’s surface. One of the most famous and widely publicized of these is the San Andreas Fault on which San Francisco is built.
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