People are confused about the various scales used in the earthquake biz, and rightly so. A scale is an attempt to put numbers on the effects of earthquakes.
First, there is the earthquake magnitude. This was an attempt by Richter to quantify the energy of the earthquakes at the point of origin, irrespective of distance. He wanted it to resemble the brightness magnitude of stars. Thus, he roughly shoved the total range of expected energy into a scale that would go up to 10. In order to do that, the scale turned out to be logarithmic, or that each unit in the scale was a factor of 30 in total energy.
The system worked well in California where it was calibrated, but then they had to shoe-horn it for the rest of the world. Thus, there evolved many ways to calculate magnitude in a way that was consistent with the original principles. There is a large jitter in magnitudes as it is calculated in many ways. I always round to the nearest whole digit, since I consider the error and jitter to be quite large. The usgs calculates to the nearest decimal point, but then is always changing it.
Thus, this is a rough scale of some interest to scientists, but generally useless to people. People want to know the effects, and once again we have many scales. Following the trend to force everything to a scale of 10, we have the Modified Mercalli scale of intensity. This is always reported along with the magnitude in Oklahoma.
Each notch in the scale denotes a significant increase in earthquake damage. Thus, you go from 'felt' at Intensity 4 or so, to 'total destruction' at 9 or 10. This is a very rough scale with huge error bars, so it is forced to be a single digit when reporting the value. The methodology hasn't changed since the beginning of time. There is a questionnaire, asking things like ' Did the Royal Dalton fall?', or 'How's your chimney?'. For each large OK earthquake, there are thousands that respond, giving us a nice map of intensities. The intensity is always the largest right on top of the earthquake, unless we have bizarre soil conditions.
- to be continued
ps. underrated m5 in Nebraska. Everybody tries to inject the 'liquid gold' of gas frack waste, and then suddenly stop it with horror in their eyes. :)