Where Do Earthquakes Occur?
Earthquakes can strike any location at any time, but history shows they occur in the same general patterns year after year. The world’s greatest earthquake belt, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81 percent of our planet’s largest earthquakes occur. It has earned the nickname “Ring of Fire”. The Alpide earthquake belt extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. The third prominent belt follows the submerged mid-Atlantic Ridge. The ridge marks where two tectonic plates are spreading apart (a divergent plate boundary).
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
thicknesses 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 fiction and heat, as well as by rending, shearing and tearing. The tearing and shearing 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 killed 25 million people was caused by 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 state. Indeed all the deep earthquakes—those taking place more than 160 miles below the surface—originate around the Pacific trenches. About 90 per cent of the intermediate earthquakes (30 to 160 miles deep) also originate there, as do 40 per cent 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.