Why do earthquakes happen

The sudden shaking of the ground that occurs when masses of rock change position below the Earth’s surface is called an earthquake, sometimes called tremblors. Globally, earthquakes occur almost continuously. The vast majority are not destructive and are felt at the Earth’s surface as small tremors. Most earthquakes can be detected by sensitive instruments, known as seismographs.

The majority of larger earthquakes are caused directly by changes in the shape of the Earth’s outermost shell, particularly the crust. The continental plates that comprise the Earth’s crust are in continual and very gradual motion. When these enormous plates ease up against each other, the results can be catastrophic. The strongest and most violent earthquakes are associated with ruptures in the Earth’s crust, known as faults. Internal pressures caused by tectonic motion strain the great rock masses beneath the Earth’s surface. The strain continually builds until it is released through the sudden movement of the rock masses along fault lines. The rock masses rub up against each other in opposite directions, shaking the ground above. The motion of the rocks comprising the jagged fault line is far from smooth. Sometimes the masses stick a little and internal energy begins to build up until it is able to overcome resistance, sometimes breaking off rock mass along the fault line. These events are among the most powerful seismic activity. At the surface of the Earth, the ground may be displaced by a few inches to hundreds of feet.

A small proportion of earthquakes can be attributed to human activity. Atomic or large dynamite explosions can sometimes cause mild tremors. Also, the injection of liquid waste deep into the Earth’s crust, collapsing mines and the pressures resulting from holding vast amouts of water in reservoirs behind large dams can also lead to small amounts of seismic activity. As the rock of the Earth’s crust shifts it causes shock waves, called seismic waves. In larger earthquakes, the shocks can be felt hundreds, even thousands, of miles away.

There are two broad classes of seismis waves: interior and surface, and there are three basic types of seismic waves: primary waves, secondary waves and surface waves.

Primary waves spread in the crust from the point of rupture, often called the focus of the earthquake. The point on the Earth’s surface immediately above the focus is called the earthquakes’ epicenter. Primary waves expand and contract the rock through which they pass and vibrate in the same direction in which the wave travels.
Secondary waves vibrate perpendicularly to the direction of motion of the wave. These secondary waves are responsible for shaking areas up and down. Primary and secondary waves are classed as internal seismic waves.

Surface waves follow the primary and secondary waves. They have greater amplitude than primary and secondary waves and are therefore responsible for much of the violent shaking that occurs far from the earthquake’s epicenter. Surface waves are the most powerful shock waves by far.