The internal structure of the Earth is often likened to the cross section of an egg or apple. The surface of the planet is covered in a crust made up of what are known as tectonic plates. Continental plates are 30-100 km thick, and oceanic plates 5-7 km thick. However, the Earth is some 13,000 km in diameter, so relatively speaking these plates are indeed as thin as the peel of an apple or the shell of an egg. And it is on this thin shell that we all live.
Internal structure of the Earth: a lot like the cross section of a soft-boiled egg?
Earthquakes occur when a force of some kind causes the Earth's crust to break or buckle. The Earth's surface is covered by more than ten tectonic plates, all of which are constantly moving in different directions, bumping into each other, and rising and falling. Powerful forces are at play in places where these plates meet, and once these forces reach a certain level, the Earth's crust is unable to contain them and ruptures. In other words, most earthquakes occur along the boundaries of tectonic plates, and there is little seismic activity elsewhere. This is the reason why the distribution of earthquakes around the world is so uneven, with some places experiencing many and others hardly any at all. Large seismic disturbances are particularly common in places like the Himalayas where two or more plates collide, and in places like the Japan Trench where plates are sinking. On the globe at left, try displaying the distribution of "Earthquakes in 2005" and "Tectonic plates" at the same time. You should find that the places where earthquakes occurred are concentrated along plate boundaries. (For the names of the various plates and the directions in which they are moving, see the plate distribution map below.)
How are plates born and how do they die? Extremely hot matter from the Earth's interior seeps to the surface through fissures known as submarine ridges forming plates some 10-100 km thick. Plates extend out on both sides of these submarine ridges, moving at a speed of several centimeters a year. In the Pacific Ocean, for example, there is a submarine ridge to the east of Hawaii from which emerges a plate (the Pacific Plate) that is moving slowly at a speed of around 9 cm a year in the direction of Japan, and which sinks back under the ocean floor along the Japan Trench where it hits the Eurasian Plate. Once it sinks below the Earth's surface, a plate heats up as it travels through the Earth's interior until it eventually melts and disappears. In other words, submarine ridges are where plates are born, while trenches are plate graveyards. Hawaii sits on top of the Pacific Plate and is moving slowly towards Japan along with the plate underneath. At its present speed it is said it will collide with Japan in 60 million years.
We know that earthquakes occur when the Earth's crust fractures in places where plates are moving, colliding with each other, and sinking. But why do plates move? Just as under the shell of an egg there is albumen (white), directly beneath the Earth's crust (tectonic plates) there is a layer of hot, molten rock known as the mantel. Because this soft mantel is constantly moving around beneath the Earth's surface, the plates that lie on top of it are also slowly moving. In other words, we can think of the earthquakes that we experience as occurring because the Earth is still a relatively young celestial body whose interior has yet to cool down completely. The interior of the moon, for example, has cooled completely and solidified, and so earthquakes like those experienced on the Earth do not occur on the moon (although it is said that the moon experiences tremors of a different kind). So in actual fact the Earth as it is now is a relatively earthquake-prone planet.