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Earthquake of March 28, 1964, Prince William Sound, Alaska. The magnitude 8.5 earthquake killed 131 people and caused 538 million in property damage. An area consisting of 120,700 km2 was shaken with damaging intensity.
Understanding Magnitude
Earthquakes are recorded by a seismographic network. Each seismic station in the network measures the movement of the ground at their site. When an earthquake occurs, it releases energy in the form of waves that radiate from the earthquake source in all directions. The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities.

The fastest wave, and therefore the first to arrive at a given location, is called the P wave. The P wave, or compressional wave, alternately compresses and expands material in the same direction it is traveling. The S wave is slower than the P wave and arrives next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling. Surface waves follow the P and S waves.

There are many ways to measure different aspects of an earthquake. Magnitude is the most common measure of an earthquake's size. It is a measure of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like. The Richter scale measures the largest wiggle on the recording, but other magnitude scales measure different parts of the earthquake.

Magnitude scales are logarithmic. That is, every time the magnitude goes up by one unit, the amplitude of waves recorded by a seismograph increases ten times. Generally, earthquakes must attain a 5.5 or greater magnitude before significant damage occurs near the source of an earthquake.

Earthquake size, as measured by the Richter Scale is a well known, but not well understood, concept. The idea of a logarithmic earthquake magnitude scale was first developed by Charles Richter in the 1930's for measuring the size of earthquakes occurring in southern California using relatively high-frequency data from nearby seismograph stations. As more seismograph stations were installed around the world, it became apparent that the method developed by Richter was strictly valid only for certain frequency and distance ranges. In order to take advantage of the growing number of globally distributed seismograph stations, new magnitude scales that are an extension of Richter's original idea were developed. Each being valid for a particular frequency range and type of seismic signal. Because of the limitations of all of the magnitude scales, a new, more uniformly applicable extension of the magnitude scale, known as moment magnitude, or Mw, was developed. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. New techniques that take advantage of modern telecommunications have recently been implemented, allowing reporting agencies to obtain rapid estimates of moment magnitude for significant earthquakes.</p align=">

Earthquakes release a tremendous amount of energy, which is why they can be so destructive. The table below shows magnitudes with the approximate amount of TNT needed to release the same amount of energy.

Magnitude-------------Approximate TNT Energy
4.0------------------------6 tons
5.0------------------------199 tons
6.0------------------------6,270 tons
7.0------------------------199,000 tons
8.0------------------------6,270,000 tons
9.0------------------------99,000,000 tons

Generally speaking, however, based on magnitude, earthquakes can be classified as follows

Great earthquake = M > 8
Major earthquake = 7 < M < 7.9
Strong earthquake = 6 < M < 6.9
Moderate earthquake = 5 < M < 5.9
Light earthquake = 4 < M < 4.9
Minor earthquake = 3 < M < 3.9
Move on to the "Intensity" page