As in the ancient times, even today earthquakes continue to exercise the human imagination powerfully.It is difficult to imagine another phenomenon that is at once subject to such wide scientific scrutiny, and yet capable of arousing such profound dread as well as curiosity in the popular mind. In all of history, humans have constructed all manners of hypotheses in way of understanding it. The earliest belief was that the   earth was supported by an animal whose occasional movement resulted in earthquakes. In Japan the animal was thought to be a spider; in India, a mole; in certain parts of South America, a whale; and for North American Indians it was a tortoise. But it was the great empiricist Aristotle who framed, what is perhaps the first "rational" explanation: that earthquakes occurred due to sudden release of entrapped air from within earth's body, which caused the ground to shake and open.
 
  Of course, it is now known that an earthquake is caused by a slip on a fault, i.e. a fracture between two large blocks of rock. Such faults can extend from a few centimeters to thousands of kilometers. They allow the two blocks of rock to move relative to each other,either slowly,over geologic times; or suddenly, which results in an earthquake. Some of the most "famous" faults across the world include the Californian San Andreas Fault that runs about 650 miles long and 10 miles deep. Closer to home, amongst others, a significant section of the Himalayan belt, is known to be situated on a fault line of relatively high susceptibility.
   The reason why fault lines give away suddenly can be understood in terms of earth's inner structure. Our planet was formed nearly 4 billion years ago by a massive conglomeration of materials from space, and is cooling off even today.Over the aeons, heavier materials like iron sank towards centre of the earth; while lighter silicates, other oxygen compounds and water rose to the surface. As we see it today, the earth is comprised of four concentric layers: the inner core, outer core, mantle and crust. The core contains mostly iron and about 10% sulfur. Being under extreme pressure,the inner core is solid, while the outer core exists in a highly hot and molten state. Beyond the core, the mantle is a semi-solid mass (composed of iron, magnesium and silicate compounds), which is capable of plastic deformation. The crust,which is the outermost layer, is the thinnest and least dense of the layers. It is relatively cold, rocky (i.e. deposits of calcium, sodium and aluminium-silicate minerals) and brittle; and, thus, can fracture easily during earthquakes.

For nearly half a century it was thought that the faults in the earth's crust were the main reason behind earthquakes. It now appears that a very significant proportion of earthquakes have their origin in the earth's mantle.And that, the opening of faults,in the crust is a secondary phenomenon: it is damage produced by deformation of the more mobile mantle below.It is this continual slow movement of the outer layer of earth - technically known as "plate tectonics"-that eventually lead to earthquakes. It is responsible for causing volcanoes as well. The very same motions have also created the greatestmountain ranges across the world!