There are two main types of earthquakes: natural and man-made. Naturally occurring(tectonic) earthquakes occur along tectonic plate lines(fault lines) while man-made earthquakes are always related to explosions detonated by man.
Tectonic earthquakes will occur anywhere there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. Plate boundaries move past each other smoothly and aseismically if there are no irregularities or asperities along the boundary that increase the frictional resistance; however, most boundaries do have such asperities that lead to stick-slip behavior. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and stored strain energy around the fault surface. The energy increases until the stress breaks through the asperity, suddenly allowing sliding over the plate and releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating, and cracking of the rock, which all adds up to an earthquake. This process is called the elastic rebound theory. It is estimated that only 10 percent or less of an earthquake’s total energy is radiated as seismic energy. Most of the earthquake’s energy is used to power the fracture growth or is converted into heat generated by friction.
Occasionally, naturally occurring earthquakes happen away from fault lines. When plate boundaries occur in continental lithosphere, deformation is spread out over a much larger area than the plate boundary, so earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace. Also, all tectonic plates have internal stress fields caused by their interactions with neighboring plates and sedimentary loading or unloading. These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.
The other type of earthquake is the artificial or man-made quake. This type of quake has been felt all over the world after the detonation of a nuclear weapon. There is very little actual data that is readily available on this type of quake, but, of the two types of of earthquakes it is the only type that can be easily predicted and controlled.
The slower wave through the body of rock is called the secondary or S wave. As an S wave propagates, it shears the rock sideways at right angles to the direction of travel. If a liquid is sheared sideways or twisted, it will not spring back, hence S waves cannot propagate in the liquid parts of the earth, such as oceans and lakes.
The actual speed of P and S seismic waves depends on the density and elastic properties of the rocks and soil through which they pass. In most earthquakes, the P waves are felt first. The effect is similar to a sonic boom that bumps and rattles windows. Some seconds later, the S waves arrive with their up-and-down and side-to-side motion, shaking the ground surface vertically and horizontally. This is the wave motion that is so damaging to structures.
The third general type of earthquake wave is called a surface wave, reason being is that its motion is restricted to near the ground surface. Such waves correspond to ripples of water that travel across a lake.
Surface waves in earthquakes can be divided into two types. The first is called a Love wave. Its motion is essentially that of S waves that have no vertical displacement; it moves the ground from side to side in a horizontal plane but at right angles to the direction of propagation. The horizontal shaking of Love waves is particuly damaging to the foundations of structures.
The faster of these body waves is called the primary or P wave. Its motion is the same as that of a sound wave in that, as it spreads out, it alternately pushes (compresses) and pulls (dilates) the rock. These P waves are able to travel through both solid rock, such as granite mountains, and liquid material, such as volcanic magma or the water of the oceans.
The second type of surface wave is known as a Rayleigh wave. Like rolling ocean waves, Rayleigh waves wave move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.