At first glance there may only appear to be a few insignificant variances between the Ricther Scale and Moment Scale, but somewhere beneath the surface lies a much more complex explanation.
Now, before we start diving headlong into these unchartered waters, we should take a moment to expand a bit on each group.
The most common method of measuring earthquakes is known as the local magnitude (ML) or coined as the Richter scale, developed in 1935 by Charles Richter. Although now almost antiquated, the Richter scale's purpose was to quantify medium sized earthquakes, which was rated between magnitudes of 3.0 to 7.0. The Richter scale was based upon the data collected by the seismographs and the distance from the actual epicenter (point of origin). Basically, the outcome was determined by a simple equation based upon given variables collected.
Next we have the latest way to measure an earthquake magnitude; the moment magnitude scale (MMS), denoted as (Mw). Once again used by seismologists, in the process of measuring the size of earthquakes in terms of the amount of energy released. Introduced in 1979, by Harvard University seismologists Thomas C. Hanks and Hiroo Kanamori, they planed to address the missing gaps found in the Richter scale, while maintaining a solid consistency. The result was the now standard model used to measure all earthquakes both medium and large.
So, where are the differences? I can remember shrugging my shoulders to that same question, but luckily my professor was a little forthcoming with the explanation.
Lets go back to the Richter, which has no upper nor lower limits, and is based on measurements received by seismographs and uses distance to the epicenter.
Now, the more recently defined measurement, which is completely independent of the type of instrument. So, we can now say the moment magnitude scale comes from the actual seismic moment. Confused? OK, lets go back to a more rudimentary subject; physics. If you remember anything about torque, you might already know it is the force that changes the angular momentum, also known as the moment of force. This is defined by the force times the distance from the center of rotation. Since earthquakes are caused by internal torque, such as the pressure of opposing shifting plates, we have now established the seismic motion. This basically translates into rather complex mathematical equation, which literally translates into a series of formulas that even today I would have a difficult time deciphering.
However, without going to a drawing board, we can basically state that the major difference is limitations. Unfortunately the Richter scale faced a few problems, since there was an upper limit to the highest measurable magnitude, which meant all larger earthquakes, would always fall under a local magnitude of 7.0, regardless of the severity. This also meant that the estimate of the earthquake size was quite unreliable for any measurements taken at a distance that exceeded anything over 350 miles from the actual epicenter.
So, in short both scales have the same goal, but are use differently during specific circumstances and data collected. Opposed to the Richter scale that the moment of magnitude number does not become lost as it enters into the higher magnitude earthquakes, whereas before the scale became useless in the upper limits. Keeping in mind, there are few earthquakes that range from medium to large, which means a majority of earthquakes are not rated using the Moment magnitude rating. Almost all quakes ranging 3.5 or less in magnitude are still measured using other scales similar to the Richter scale.
On a final note there is another measure not mentioned, which I though might be important, which is the actual earthquake intensity. Much like the Fujita or F-Scale, the Mercalli intensity scale is used to measure severity. The scale is defined by how an earthquake will affect the Earth's surface, humans, objects of nature, and man-made structures. The scale rates from one, meaning barely noticeable; all the way to twelve, dubbed catastrophic, which can purport instrumental damage to almost anything within proximity to the epicenter, and severe to disastrous damage even miles away.
The field of earthquake study is a taxing and precarious subject that is filled with revelations and moments of frustration alike. It would be ideal to discover a way to predict an earthquake and provide an early warning system, but unfortunately all they can do is pinpoint sensitive areas where an earthquake is likely to occur.