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The Earth’s outer layer is called the crust. It covers the Earth like a cracked eggshell. The pieces are called geological plates that fit together like a jigsaw puzzle, with the joints known as fault lines. The plates naturally and continually move against each other. Usually they glide smoothly, but in places they catch and pressure gradually builds up. Eventually the pressure becomes too strong and the huge masses of rock that form the plates suddenly shift, releasing waves of energy that race out in concentric circles, rather like the waves in a pond if you throw a stone into the water. This is an earthquake. You can get the idea of how the plates shift if you snap your fingers. Pressure between the fingers builds until . . . snap!
The point at which the plates shift is known as the hypocenter. It can be many miles below the surface. The epicenter is the point on the earth's surface directly above the hypocenter. There are different types of shock waves caused by an earthquake. Some travel across the surface, causing the trembling and shaking of the ground. Another type travels vertically, while still others move underground. The waves lose energy as they travel so, although there may be major damage at the epicenter, many miles away there may be only a slight shake felt.
An earthquake can have many aftershocks, which can happen over a period of years, depending on the size of the main shock. There may be several the first hour after a big earthquake, but ten days later there will be only a tenth as many. The reason they happen is that the main shock changes the pressure along the fault line and can trigger sudden shifting of the rock masses at different points on the fault.
The size of an earthquake is measured on a Richter scale of magnitude. Seismographs are basically pens suspended over a paper-covered rotating drum. When the earth trembles the pen makes a larger squiggle on the drum, allowing the size of the shaking to be measured. Each whole number on the Richter scale represents an earthquake 30 times larger than the number below it. Earthquakes that measure less than 3.0 are not usually felt, while one of 5.0 produces the same amount of energy as the Hiroshima atomic bomb. The largest recorded quake happened in Chile in 1960 and measured 9.5. The shock waves traveled around the world for days!
Most earthquakes happen within the ‘Ring of Fire’ around the Pacific Ocean. Alaska experiences a 7.0 earthquake at least once a year. California gets about 10,000 quakes a year, but most are unnoticed except by the seismograph machines.
It is thought that there are as many as 500,000 earthquakes every year, although only 100,000 can be felt at all and only about 100 of those actually cause any damage.
All around the world people are using their better understanding of earthquakes to design safer buildings and transport systems so, although we can’t do anything to stop the Earth shaking, we can minimize the damage caused.
The San Francisco earthquake of 1906 was probably the most destructive earthquake in American history. This earthquake occurred at 5:12 a.m. on April 18, 1906, and measured 7.9 magnitude. San Francisco was devastated by the damage caused by the earthquake. However, the resulting fire from broken gas lines caused a great deal more damage to the city.