Statment between the magnitude of a earthquake and how often earthquakes of the magnitude occur?

One statement you could write is that "Earthquakes of higher magnitudes are much rarer than those of lower magnitudes".

The magnitude of earthquakes is a logarithmic scale, so a magnitude of 8 is TEN TIMES more powerful than a magnitude 7. This is why earthquakes of higher magnitudes are so much rarer than those of lower magnitudes.