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
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 friction and heat, as well
as by rending and shearing and tearing. The tearing and shearing of deep
under-ground 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 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
(30to160 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.
Comments
Post a Comment