An earthquake (also known as a quake, tremor or temblor) is the perceptible shaking of the surface of the Earth. Earthquakes can be violent enough to toss people around and destroy whole cities. Such earthquakes being caused by characters are a method useable to determine Attack Potency. For the...
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For meteor impacts, the formula for energy in Joules is: 10^((Richter Magnitude + 5.87)/0.67)
So in this case it would be 5.2E+17 J, which is 124.4 Megatons as stated in the original post.
If the magnitude 6 is felt a very long distance away from the site of the impact, then the actual magnitude of the meteor can be higher.