Seismic Scales

Seismic Scales
A seismic scale is used to measure how severe earthquakes are.
There are two main scales to measure earthquake severity. The energy of an earthquake is measured on a magnitude scale, while the intensity of shaking on the Earth's surface is measured on an intensity scale.




Intensity Scales

The Rossi-Forel scale was one of the first seismic scales to measure the intensity of earthquakes. It was developed by Michele Stefano Conte de Rossi François-Alphonse Forel. This scale was used for about two decades until the introduction of the Mercalli intensity scale in 1902. Since then lots of intensity scales have been made. The scale currently used in the United States is the Modified Mercalli scale (MM), while the European Macroseismic Scale (EMS-98) is used in Europe, the Shindo scale is used in Japan, the Medvedev-Sponheuer-Karnik scale (MSK-64) scale is used in India, Israel and Russia. Most of these scales have twelve degrees of intensity, which are almost the same.


Magnitude Scales


The Richter Scale
The Richter scale, also known as the local magnitude scale (ML), is a well-known magnitude scale. In the 1930s, California seismologist Charles F. Richter came up with this scale. The name "Richter Scale" was coined by journalists who reported on the magnitude of earthquakes, and is not generally used by seismologists. Other improved magnitude scales are: body wave magnitude (Mb), which measure P-waves, surface wave magnitude (Ms), Which measure S-Waves and duration magnitude (MD), which measures how long the earthquake lasts. Each of these are made to give values similar to those given by the local Richter Scale; but because each is based on a measurement of one aspect of earthquake, they do not always capture the overall power of the source. The Richter Scale is the most widely used scale. This is the one you see on the telly and newspaper.


The Moment Magnitude Scale
The moment magnitude (MW) scale, to measure the magnitude of earthquakes, was invented by Thomas C. Hanks and Hiroo Kanamori in 1977. For very large earthquakes the moment magnitude scale gives the most reliable estimate of the magnitude of an earthquake. This is better than the Local Magnitude Scale because it can cope with stronger earthquakes and is more accurate. Magnitude 8+ on the Richter scale is inaccurate because seismographs can not cope with the shock waves. Therefore, the Moment Magnitude Scale is the scale with the most accurate magnitude.