The earth quake because the surface of the Earth is made up of plates that move and grind against one another, and sometimes they stick and then suddenly slip.Earthquakes are caused by sudden movements of the tectonic plates, giant slabs of rock that make up the outer skin of the planet. In some places these plates slide past each other but since they are made of rock they don’t slide smoothly. Instead they get stuck fast at some points, until strain and stress builds up so much that all at once they give, jumping past each other.When this happens a lot of energy is released and movement ripples out through the earth. On the surface these ripples cause earthquakes.Earthquakes can also be caused by hot molten rock rising to the surface in a volcano, and by other catastrophic events such as asteroid impacts.The tectonic plates float on a layer of partially liquid rock called the mantle. This moves very slowly – about one ten-thousandth the speed of the hour hand on a clock. As a result the tectonic plates themselves generally move equally slowly – North America and Europe, for instance, each on different tectonic plates, are moving no faster than a fingernail grows.
This size, or magnitude, is measured with the Richter scale. Low-magnitude quakes are amazingly common. A quake of less than magnitude 3.4 happens over 800,000 times a year, but is too small for anyone to notice. Magnitude 4.3-4.8 quakes will rattle the windows of houses they happen nearly 5,000 times a year. It takes a quake of 6.2 or greater to be really dangerous, and 7.4 to collapse a lot of buildings; fortunately, such a big quake only happens about four times a year.
The amount of energy released by a big quake is terrifying. The energy released in the San Francisco earthquake of 1904 was comparable to one of the largest nuclear bombs ever created exploding under the city, but this was only around magnitude 7.8. The biggest quake ever recorded was the southern Chile quake of 1960, which registered 9.5 on the Richter scale. The Tohoku earthquake that caused the devastating tsunami in Japan in March 2011 was magnitude 9.0.
Great shakes
The Richter scale is a logarithmic scale, which means that each point on it is 10 times greater than the one before. So a magnitude 2 earthquake is 10 times bigger than a magnitude 1 quake, but 100 times smaller than a magnitude 4 quake.
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