Earthquakes are the most destructive and powerful natural disasters. Within the time span of seconds they can flatten cities and kill thousands.

All of the land and water of the earth sit atop twenty major tectonic plates. These plates are large "chunks" that make up the earth's surface, the crust. These plates are like giant puzzle pieces that don't exactly fit together. The plates are always in motion, and every once in a while large pieces of rock snag and break (called elastic rebound), causing earthquakes. Faults are the places where these plates meet and where about 95% of all earthquakes happen. There are different types of faults. Some plates slide past each other in opposite directions horizontally, creating transform faults. One example of a transform fault is the well-known San Andreas fault of California. Subduction zones, or convergent fault boundaries, are the results of one piece of land sliding under another. These are the sites of the biggest earthquakes. A good example of a convergent fault is the boundary of the Nazca and South American plates (the Andes). The Himalayas were also created due to a convergent fault boundary. The majority of volcanoes also erupt at convergent fault boundaries. The other type of fault is called a divergent fault. These are the opposite of convergent faults, spreading apart instead of pushing together. One very good example lies at the bottom of the Atlantic Ocean, called the Mid-Atlantic Ridge.

Distant earthquakes are measured by seismographs. There are many types of seismographs, and they can cost from just a few hundred dollars to several hundred thousand. A simple seismograph can measure the motion of the ground in one direction, usually up and down. Most seismographs operated by professional geologists, however, can measure ground motion in all three directions, north-south, east-west, and vertically. These seismographs are linked to computers that are synchronized to the atomic clock which receive the data from the seismograph, amplify it, and store it. The United States Geological Survey in Golden, Colorado receive live data from all around the world. The data is also sent to a couple of other stations to ensure that it is not lost in the event of a malfunction at the main Earthquake Center Headquarters. Other instruments called Strong Motion Accelerometers measure the heavy shaking near fault boundaries.

Earthquakes can be measured in a variety of different ways. The most commonly used scale among the media is the Richter Scale (Mb). The Richter Scale is actually a measure of wave amplitude and is logarithmic. This means that a magnitude 2 earthquake has a ground motion of ten times that of a magnitude 1 earthquake and a magnitude 3 earthquake has the ground motion 100 times that of a magnitude 1 earthquake. Charles Richter came up with this scale. Since he chose zero arbitrarily, the scale (theoretically) has no upper or lower limits, although seismologists theorize that the rock of tectonic plates cannot withstand the tension of an earthquake greater than magnitude 9.5. A magnitude -1 earthquake is roughly equivalent to a 1-kilogram brick dropped from 200 meters. The energy released by an earthquake also rises exponentially. A magnitude 2 earthquake releases 32 times as much energy (not ground motion!) as a magnitude 1 earthquake. The scale by which the energy of earthquakes are measured is called the Moment Scale (Mw). This scale measures the amount of energy released from an earthquake, rather than the ground motion resulting from it. If you put your hands close together, face down, on a long table and push/pull them in opposite directions as hard as you can, the table will very likely not move very much. But if you put your hands at opposite ends of the table and then move them in opposite directions, the table will rotate with much greater ease. This is the way the Moment Scale works. It is the product of the energy and the area over which it is released. It is measured on a scale of 1 to 10, with 10 being the release of most energy. Another way in which earthquakes are measured is the intensity. Intensity is the measure of the effects that the earthquake has on common objects. The most commonly used Intensity scale is the Modified Mercalli Scale. The intensity is marked by a roman numeral from I - XII:

Abridged Modified Mercalli Intensity Scale *[1]

Relation Between Magnitude, Intensity, and Perceptability *[2]

The Largest Earthquakes of the 20th Century *[3]

Earthquake Prediction

This is similar to the method of deriving probability that is used at Zhonghao Shou's Short-Term Earthquake Prediction Website. On my Past Predictions page, the value displayed is the corrected probability, that is, the probability of the real earthquake occurring when and where it actually did (and its actual size), in relation to the predicted values.