Sarah Beritich, age 10, of Tacoma, Wash., for her question:
WHAT EXACTLY IS AN EARTHQUAKE?
Most earthquakes aren't even noticed when they hit. Lots of very minor quakes can be mistaken for a passing truck. Big ones do happen, however, with the most destruction in terms of loss of life being one in the
Shensi Province of China in 1556. More than 830,000 people were killed in that earthquake. Second most disastrous quake hit Northern Persia, now Iran, in 1755, taking more than 300,000 lives.
More than a million earthquakes occur each year, most of them occurring beneath the surface of the sea and going almost unnoticed.
An earthquake is a sudden rolling, shaking or shock of the earth's surface.
The plate tectonics theory of earthquakes says the earth is made of about 20.rigid plates that move slowly past each other. The motion of the moving plates squeezes and stretches rocks at the edges of the plates. When the force becomes too great, the rocks will break and shift, causing an earthquake. The breaks are called faults, with most of them lying well below the surface of the ground.
Energy produced by earthquakes travels away from the faults in waves called seismic waves. Near the focus, or the place where the break began, vibrations of the seismic waves can be very destructive. Seismographic stations are located around the world and are manned by scientists called seismologists. Here earthquakes are measured on what is called the Richter scale.
Energy released by a large earthquake can be greater than that in 200 million tons of TNT. This would be 10,000 times greater than the first atomic bomb.
Seismologists can locate the focus or centerpoint of an earthquake by studying the time intervals at which the seismic waves reached a number of different seismographic stations. Some quakes happen on the surface of the earth, while others occur 400 miles below the surface.
The world's most geologically active regions are located along the boundaries where plates separate, collide or slide past each other. New mountain ranges, volcanoes and deep ocean trenches occur along the edges of the plates as well as earthquakes. In contrast, the flat parts of the continents and ocean floors are stable regions that have few quakes.
Most of the world's earthquakes take place within two limited regions. One is called the circum Pacific belt which lies along plate boundaries around the Pacific Ocean. The second is called the Alpid belt. This region follows plate boundaries across the southern part of Europe and Asia.
Earthquake prediction is not possible yet, but scientists believe they will find a method in the not too distant future.