The earth is made up of four distinct layers, and the top two, the mantle and the crust are constructed from a collection of pieces called plates. These tectonic plates, shift, move, press against one another, and become wedged together. When they become unstuck, they produce an earthquake.. Earthquakes occur when blocks, or plates, that make up these top layers of the earth shift and move against each other along fault lines. The area beneath the surface of the earth where this occurs is called the hypocenter, and on the surface, it is known as the epicenter. In many cases, there is a forewarning of the major quake to come, in the form of foreshocks,which are less severe than the main shock.
Shaking, on the earth’s surface, of course, causes damage to structures, increases the chances of a tsunami, and normally opens large cracks and crevasses in the crust. The shaking itself is caused by the release of energy when the blocks become unstuck, and one block shifts over, past, or downward in relation to another plate, or block. This energy travels to the surface in the form of seismic waves, causing the earth to shake. Once the main earthquake has taken place, the plates may still move, producing aftershocks that can go on for weeks, or in some cases, decades.
The size of the earthquake is determined by the size of the fault, and the amount of movement that has occurred. However, it is impossible to actually view the tectonic plates or the fault lines, so a seismograph records the amount of shaking that resulted from the quake, and determines the quake’s magnitude.
During an earthquake, both primary (P) waves, and secondary (S) waves occur. Primary waves move the earth back and forth violently, while secondary waves move in an up and down, wavy motion. These are used to determine exactly where the quake initiated. P waves move faster, and are the first to be noticed, followed by the S waves. The area where both occur nearly simultaneously, is the epicenter of the quake. This is determined by the seismograph, and the measuring of the distances and time that the P and S waves were felt around the quake area.
While scientists are well aware of where many fault lines and tectonic plates are located throughout the world, there is no way to predict when these tectonic plates will shift, collide, or produce another quake.