He based his scale on an analogy with the stellar brightness scale commonly used in astronomy which is also similar to the pH scale used to measure acidity (pH is a logarithmic measure of the Hydrogen ion concentration in a solution). In 1935 Charles Richter constructed a similar diagram of peak ground motion versus distance and used it to create the first earthquake magnitude scale (a logarithmic relationship between earthquake size and observed peak ground motion). The fact that earthquakes of different size generated curves that were roughly parallel suggested that a single number could quantify the relative size of different earthquakes. In 1931 a Japanese seismologist named Kiyoo Wadati constructed a chart of maximum ground motion versus distance for a number of earthquakes and noted that the plots for different earthquakes formed parallel, curved lines (the larger earthquakes produced larger amplitudes). It was natural for these instrumental measures to be used to compare earthquakes, and one of the first ways of quantifying earthquakes using seismograms was the magnitude. In the example above the surface wave has the largest deflection, so it determines the peak amplitude. The maximum or "peak" ground motion is defined as the largest absolute value of ground motion recorded on a seismogram. The development and deployment of seismometers lead to many changes in earthquake studies, magnitude was the first quantitative measure of earthquake size based on seismograms. With the invention and deployment of seismometers it became possible to accurately locate earthquakes and measure the ground motion produced by seismic waves. Even for earthquakes close enough to population centers values such as maximum intensity and the area experiencing a particular level of shaking did not correlate well. The damage and devastation produced by an earthquake will depend on its location, depth, proximity to populated regions, as well as its "true" size.
The problem with these measures is that they don't correlate well. For example, we could use values such as the number of fatalities or injuries, the maximum value of shaking intensity, or the area of intense shaking. Early estimates of earthquake size were based on non-instrumental measures of the earthquakes effects. The magnitude is the most often cited measure of an earthquake's size, but it is not the only measure, and in fact, there are different types of earthquake magnitude.
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