Today, on May 14, 2012, I studied English for IT II, in this course I learned about Charles Francis Richter and the following is a summary :
Seismologists have found the two scales of measurement that allows them to describe and record information about the earthquake in quantitative terms. Measurements of the most widely known is the Richter scale, logarithmic numerical scale was developed and introduced by the American seismologist Charles F. Richter in 1935. The purpose of this scale is to measure the amplitude of the biggest tracks recorded by seismographs standard one hundred kilometers from the epicenter.
Other earthquake-scale assessment, introduced by the Italian seismologist Giuseppe Mercalli, measures the intensity of shaking, using a gradient of 1 to 12. Because shock effects such as fade with distance from the epicenter of the earthquake, the Mercalli ratings depending on where the measurements.
Mercalli earthquake of 2 or 3 on the Richter essentially the same as 3 or 4; measuring 11 or 12 on the Mercalli scale can be roughly correlated with the magnitude of 8 or 9 on the Richter scale. It is estimated that almost one million earthquakes occur each year, but some of them so small that they pass undetected. In fact, more than a thousand earthquakes measuring 2 or lower on the Richter scale happen every day.