Today, scientists don't use the Richter scale the way he did because not all earthquakes of interest happen in California, and also because the type of seismometer he used is out of date now. This scale worked because he always used the same type of seismometer and all his earthquakes were in southern California, so there didn't have to be any extra corrections for different depth or rock type. Each integer increase represented a factor of 10 increase in ground motion amplitude. Then all the other earthquakes he had recorded could be compared to the reference, after correcting for distance. The way he did this was to pick a reference earthquake and measure its maximum ground motion. The Mercalli is not a very useful scale for science because it can't tell us much about big earthquakes that are not felt by people-earthquakes that occur at a great depth, or in the ocean, for example.Ĭharles Richter developed a magnitude scale in the 1930s because he wanted to be able to characterize the seismicity he had been measuring in California with some kind of numbering system that would encompass all the earthquakes, from ones that had hardly been felt at all, up to really big ones. Mercalli intensities are generally depicted on maps by several concentric rings around the epicenter of the earthquake that give some idea of the severity of the felt shaking at various distances away from the earthquake. For earthquakes in the historic record that happened before the advent of seismometers, Mercalli intensities are often assigned by checking out old newspaper reports and examining the foundations of old buildings. Sometimes the interviews can be supplemented by observing any earthquake damage to buildings. The amount of felt shaking is generally measured by interviewing witnesses to find out how much shaking they felt. This scale goes from I (not much of anything) to XII (total destruction). This scale is a qualitative measure of the amount of felt shaking caused by an earthquake. Here's an overview of the most common magnitude measurements. How big was the earthquake? Over the years, scientists have developed various ways to measure earthquake size and strength. When an earthquake happens, the main quantity of interest is its magnitude.
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