 An earthquake is caused by a sudden slip on a
fault. A fault is a fracture in the crust of the earth
along which rocks on one side have moved relative to those on
the other side. Stresses in the earth's outer layer push the
sides of the fault together, builds up and the rocks slips
suddenly, releasing energy in waves that travel through the
rock to cause the shaking that we feel during an earthquake.
Earthquakes tend to be concentrated in narrow zones. There
are 7 major crustal plates on earth, about 80 km (50
miles) thick, all in constant motion relative to one another.
They move at between 10 and 130 mm (from less than one half to
5 inches) per year.
It is estimated that there are several million earthquakes
in the world each year. Many of these earthquakes go
undetected because they occur in remote areas or have very
small magnitudes. The USGS Earthquake Info Center locates 12
000 to 14, 000 earthquakes each year (about 35 per day). On
average, about 60 earthquakes per year are classified as
significant, with 19 classified as major. A significant
earthquake is one of magnitude 6.5 or higher or one of lesser
magnitude that causes casualties or considerable damage. Major
earthquakes have a magnitude larger than 7.0. |
地殼運動造成的斷層
外層板塊
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