A Moment Magnitude Scale
It is well known that the most widely used earthquake magnitude scales, ML (local magnitude), M, (surface wave magnitude), and mb (body wave magnitude), are, in principle, unbounded from above. It is equally well known that, in fact, they are so bounded, and the reasons for this are understood in terms of the operation of finite bandwidth instrumentation on the magnitude-dependent frequency characteristics of the elastic radiation excited by earthquake sources.
Hanks, T.C. and Kanamori, H. 1979, A moment magnitude scale: Journal of Geophysical Research, v. 84, p. 2348-2350.