|MadSci Network: Earth Sciences|
The Richter scale is most commonly used for describing earthquakes. The Richter scale is a measurement of earthquake magnitude - the actual energy released during the quake. The Mercalli scale is a measure of earthquake intensity - the actual effects on the land surface as observed by people. Both scales have their merits and well, faults, if you will.
The Richter scale is preferred because its magnitude calculation uses the data obtained by seismographs. Seismographs record the shock waves generated by an earthquake. The size of the shock waves are a direct measure of the energy released during the earthquake. So, since everyone around the world uses seismographs to detect earthquakes, it makes a lot of sense to use a scale that is based upon the measurements made by seismographs. Scientists and the public all have a uniform way to understand how strong an earthquake in Turkey or California is.
The Mercalli scale was developed by Giuseppe Mercalli, an Italian seismologist, in 1902 so that he could compare the historical descriptions of earthquakes around the world with current earthquakes. The Mercalli scale does not involve any mathematical calculation, it simply ranks the observations of witnesses on a scale from I to XII (roman numerals) with a Magnitude I being not felt, except by a very few people, and a Magnitude XII being complete damage, most or all construction damaged or destroyed. (To see the Mercalli Scale you can visit http://vulcan.wr.usgs.gov/Glossary/Seismicity/description_mercalli.html)
The Mercalli scale is frequently used with the Richter scale to study how the same earthquake can cause different levels of destruction across a small area. For example, after each major quake, geologists in the San Francisco Bay area survey the amount and type of damage in different places. They found out that the worst shaking and damage often occurs in areas of loose, sandy soils or filled-in wetlands and the least damage is often in areas with firmer soils or solid rock. Many states conduct the same types of surveys and make the information available to the public in the form of earthquake intensity maps. In fact, my father lives in San Francisco and used an earthquake intensity map to decide where he should (and should not) buy a home!
Best of luck, David, and thanks for your question!
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