Until relatively recently, geologists and other scientists were not entirely sure what caused earthquakes. Now with increasing technology, the resultant better monitoring, and an enhanced understanding of plate tectonics, they are able to give more solid reasons for them.
Earthquakes are defined as a vibration of the earth's surface that occurs after a release of energy in the earth's crust.
Because the earth's crust is made up of numerous segments or "plates" that are constantly moving slowly, vibrations can occur and result in small earthquakes. Most earthquakes are quite small but are not readily felt. Larger and more violent earthquakes are those that occur in a release of energy as the plates slide past or collide into one another.
Large earthquakes can focus on the boundaries where two plates meet, but they are not limited to these areas. As the plates move, fractures in the earth's crust develop and earthquakes are often located on them. These fractures are referred to as faults, of which there are three types and all generate earthquakes when they move.
The first type of fault is called a normal fault. These are described as being nearly vertical and occur in areas where earth's plates are pulled apart because of a divergent plate boundary nearby. On this fault, the hanging wall pushes down on the footwall. For reference, the hanging wall is the rock pushed above the fault plane and the footwall is the rock below the plane. The fault plane is the flat surface representing the fracture line of the fault.
Another nearly vertical fault is a reverse fault. These are created when the earth's crust is compressed when two plates collide. Here the hanging wall pushes up and the footwall pushes down.
Finally, the strike-slip fault is a horizontal fault where the areas of rock slide past one another. These occur in areas where there is a transform plate boundary. The San Andreas fault in California is an example of a strike-slip fault.
In all of these faults, the most damaging earthquakes happen when the blocks of rock become locked together due to the intense friction created when they move. As they continue attempting to move once they are locked, pressure continues to build until it has enough energy to move the rock and the blocks move, creating an earthquake.
The point where the energy is released is called the focus and the focal depth is the depth beneath the earth's surface where the energy release originates. The epicenter is another term used in studying earthquakes and this is the point on the earth's surface directly above the focus. From here, the energy released spreads out in rings moving across the surface - not unlike those caused when a rock hits still water.
In addition to the main shaking created by an earthquake, there are often foreshocks and aftershocks. Foreshocks generally increase in magnitude leading to the main earthquake, whereas aftershocks happen after the main event and decrease in strength.
Once an earthquake finishes its movement, its magnitude (strength) is determined with the Richter and/or Mercalli Scale. The Richter scale measures the energy released by an earthquake while the Mercalli scale measures the felt or observed intensity at a particular location.
The impacts of earthquakes vary based on their energy and intensity. The strongest earthquakes that occur can result in ground rupture, causing damage to bridges, dams, roads, railroad tracks, and the foundations of buildings. They can also cause landslides and avalanches as a result of the shaking. Intense shaking can also cause liquification of ground built on landfill when water mains break. The shaking of an earthquake is increased in areas of landfill.
Another major cause of damage is the fires that ignite when power lines fall and gas lines rupture. In addition, undersea earthquakes can generate tsunamis that are capable of traveling great distances from the epicenter and cause significant damage to coastal communities.
One of the strongest earthquakes to occur in the 20th Century was in San Francisco, California in 1906. It was measured to have a magnitude between 7.7 and 8.3 and caused $400 million in damage. Most of the damage was caused by the fires that resulted after the earthquake.
On March 27, 1964, Alaska (the U.S. state with the most earthquakes) experienced a quake measuring 9.2 which generated several tsunamis in the Pacific Ocean. Trees were said to have been snapped apart near the epicenter, but because Alaska is not heavily populated, fortunately only 114 people died.
Both of these earthquakes occurred on faults near plate boundaries, as do most. More recent events such as the 1989 Loma Preita, the 1994 Northridge, and the Pacific Northwest's Nisqually in 2001 all have a similar status and were equally damaging. Many other large earthquakes have struck similar areas around the globe.
In an effort to reduce the impacts of such earthquakes, areas prone to them have taken steps to retrofit buildings and educate citizens (in schools and the media for example) on what they should do when an earthquake occurs.
Scientists at universities around the world and organizations such as the United States Geological Survey (USGS) also work hard in monitoring such earthquakes in hopes of being able to one day predict them to alert people. However, until then, knowledge of what to do and always being prepared for an earthquake is the best way to reduce injuries, deaths, and damage to cities and towns when they occur.