An earthquake is a tremor in the Earth's crust, caused by movements below its surface. These events can vary widely in intensity, from seismic activity that is barely detectable using sophisticated devices, to devastating temblors that can level cities and trigger tsunamis and sometimes even volcanic activity. The study of these tremors is known as seismology, a word derived from a Greek word meaning “to shake.”
The Earth's outer layer, or crust, is composed of two sections: the lithosphere, a Greek word meaning “rocky sphere,” and the athenosphere, a thick layer of liquid that rests on top of the upper mantle. The liquid rock of the upper mantle keeps the crust in constant motion, with the edges of continental plates being pulled slowly apart or together as they float on the athenosphere. The movement of these plates is what triggers earthquakes. In addition to plate boundaries, temblors also occur along faults, cracks in the lithosphere caused by the stresses created as tectonic plates move.
There are a number of different types of faults, but most can be divided into three categories: strike slip faults, thrust faults, and normal faults. A strike slip fault occurs in an area where two plates are sliding past each other, while a thrust fault happens when plates are being pushed together. A normal fault is the result of plates being pulled apart. The largest normal faults in the world are along the deep sea ocean ridges of the Pacific and Atlantic, where plates are pulling apart, crashing into the continental plates and causing thrust faults. Earthquakes along each fault have different characteristics that help seismologists to identify them.
The roots of a quake lie in stresses placed on the lithosphere as it drifts on the surface of the Earth. Pressure builds up along a fault line, which finally fails, often deep below the crust of the Earth, in an area called the focus. The corresponding spot on the surface of the planet is called the epicenter, and usually the greatest concentration of damage occurs here. When the fault fails, it triggers seismic waves, very low frequency sound waves that come in several shapes, and which can cause the earth to ripple, heave, buckle, or tear apart. The waves can continue for hours after the earthquake was triggered, and aftershocks, further smaller tremors, can continue for months and possibly even years later.
The intensity of an earthquake is called its magnitude. Various scales were proposed to measure this factor until 1935, when the Richter scale was developed. Under this scale, each order of magnitude is 10 times more intensive than the last. A quake that measure a 2 on the Richter scale is 10 times more intense than a 1, while a 3 is 100 times greater. Most quakes around the world are below a 4.5, the magnitude at which it can start to damage buildings, and every year there is at least one greater than an 8, with the largest modern earthquake ever recorded occurring in Chile in 1960; it measured a 9.5.