An earthquake is a tremor in the Earth's crust, caused by movements below its surface. Earthquakes can vary widely in intensity from seismic activity which is barely detectable using sophisticated devices, to devastating earthquakes which can level cities and trigger tsunamis and sometimes even volcanic activity. The study of earthquakes 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 which 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, earthquakes 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 which help seismologists to identify them.

The roots of an earthquake 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 of the earthquake. The corresponding spot on the surface of the earth 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 which come in several shapes 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 earthquakes, can continue for months.

The intensity of an earthquake is called its magnitude. Various scales were proposed to measure earthquakes until 1935, when the Richter Scale was developed. Under the Richter scale, each order of magnitude is 10 times more intensive than the last. A two is 10 times more intense than a one, while a three is one hundred times greater. Most earthquakes around the world are below a 4.5, the magnitude at which a quake can start to damage buildings, and every year there is at least one earthquake greater than an eight, with the largest modern earthquake ever recorded occurring in Chile in 1960; it measured a 9.5.