Introduction
When the earth moves in what we term an earthquake, it usually does so as a result of tectonic forces deep beneath the surface of the planet. In a strict definition of the term, earth movements triggered by volcanoes, nuclear blasts, mine explosions or meteor strike can also be termed earthquakes, but the earth manifests a means of movement that remained frighteningly mysterious to people for centuries. These are earthquakes caused by tectonic forces. Tectonic earthquakes are the result of movements of the earth’s tectonic plates on which rest all the continents and oceans. These plate boundaries do not follow the boundaries of the continents or oceans and can frequently be identified by lines of mountains or what are known as subduction zones. To understand earthquakes, one must understand the way that the earth moves, the way that scientists have learned to measure earthquakes and how these movements affect the planet.
Body
There are basically three types of movement along the plate boundaries: transform, divergent and convergent. When the plates move horizontally to each other, a person standing on the fault line may experience the land sliding past on the other side of the line as the plates pass by each other. This is a relatively smooth movement as long as there are no irregularities in the line. An example of this type of fault is the San Andreas Fault in California. Divergent faults are often characterized by the presence of volcanic or outgassing activity, breaking through or directly below the fault line. In this type of fault, the plates are spreading apart and new material is being created. Proof that magma is not the only material that might emerge at these zones of construction, locations such as the Mid-Atlantic Ridge are characterized more by the release of nitrates in what are termed ‘black smokers’ than they are by the release of magma. Finally, convergent boundaries are those boundaries that are coming together in some way, destroying land.
When plates come together in convergent zones, they have a few options of their own. One plate may begin to sink under the layer of the other plate or both plates may meet head on and sink in subduction zones causing deep rifts such as the Marianas Rift in the Pacific Ocean. They might also collide and move upward, causing huge mountain ranges. In most cases, one plate sinks under the other, usually causing trenches on the ocean side, whose denser material tends to sink under the more buoyant material of the continental plate and volcanic mountain ranges on the land side as materials in the subducted oceanic plate begins to heat up and escape upward through the continental material. An example of this type of plate convergence can be found in the Andes Mountain Range of South America as the Nazca plate subsides beneath it.
An earthquake causes two major types of seismic waves that move through the earth and cause damage. The first of these are body waves, which travel directly through rock and cause the vertical and horizontal displacement of the surface. This form of wave is essentially divided between the primary waves and the secondary waves. They can be measured from around the planet and help scientists to pinpoint the epicenter, or point of friction, that caused the earthquake where the most significant movement has been achieved. The primary waves can move through liquid and rock, but the secondary waves are not able to move through water. The reason for this is because these types of waves depend upon refraction to keep their energy. As either of these types of waves move through the rock, they are most affected by the type of material they are moving through. The second type of seismic wave is the surface wave. Like the body waves, these can also be divided into two main subgroups. In this case, they are called the Love waves and the Rayleigh waves. These move more like the ripples in water and exist only on the surface. They can also cause vertical and horizontal displacement but lose strength the further they are from directly above the epicenter. Again, one form is able to affect bodies of water, the Rayleigh waves which contain an element of vertical movement, and the other has little effect as the Love waves can only influence the horizontal placement of water boundaries.
When earthquakes happen, people have figured out different ways of measuring the intensity of the movement using these various waves. Seismometers are sensitive instruments that are able to record when the earth moves. Through a long history of complicated calculations and adjustments, the seismograph has been developed to illustrate the magnitude of an earthquake based on the strength and speed of the body waves and surface waves created. A scale developed in the 1930s to indicate earthquake strength is still referred to as the Richter scale on which an earthquake of 3 or lower is relatively unnoticeable by the average individual while a rating of 7 indicates a highly destructive and violent quake. By measuring these waves at various points on the earth and comparing them, scientists can pinpoint just where the earthquake originated and be able to determine with some degree of accuracy whether another quake is immediately eminent.
By understanding the various reasons why earthquakes happen and how they affect the planet, it is easier to understand complicated instruments such as the seismometer. As the plates of the world shift against each other in their constant movement between the magnetic poles or through other geologic forces, they have several options of how to adjust either passing each other by, destroying landmasses or creating new ones. These conflicts cause seismic motions to travel through the planet to greater or lesser intensities that can be measured by the seismometers and give scientists a clearer picture of the planet and the changes they might still anticipate.