Yeah, I agree with you. It's an analysis problem ... Here is SCSN's take on it ...

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ML works well for local earthquakes (within a few hundred km from stations) between about 2.0 and 5.0. If they are too small, the Wood-Anderson does not record them & if they are too large, the pen falls off the paper & gets stuck. We actually do still use ML for most of our day-to-day small quakes, except that, since we don’t use the Wood-Andersons any more, we use synthetic Wood-Anderson records generated in the computer from our digital, broadband seismometers.

Various other magnitude scales have been developed over the years to cover small, larger, more distant & deeper earthquakes which are outside of ML’s useful range. Mb uses teleseismic (distant) body waves (P and S waves). MS uses teleseismic surface waves with a 20 sec period. Md (duration) & Mc (coda) magnitudes address the tiny earthquakes. The most reliable & physically sensible magnitude, which can be computed for earthquakes over about 4.0, is called moment magnitude Mw & is based on a physical quantity called the seismic moment. Seismic moment can be thought of as an indicator of how much rock moved how far during the quake. To compute seismic moment, we match the whole broadband seismogram at several stations to a computer model of the earthquake. The seismic moment is converted to Mw based on a formula that is intended to match ML, should one be available. Mw differs from the older MS and ML magnitudes significantly for very large earthquakes, since only Mw truly represents the prodigious long period energy these earthquakes produce. For example, the 1964 Alaska earthquake had a MS of 8.4, but a Mw of 9.2, making it the second largest earthquake ever recorded instrumentally.

Note that all this sometimes results in multiple magnitudes for the same earthquake. Although the calibrations are such that these numbers should be roughly the same, they often do not agree because they are computed entirely different ways. It has become customary for the news media to select the largest one & use that as the “Richter scale” reading. Our practice is to use Mw when it appears to be reliable & ML or Md for smaller events.

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Michael