Beginning in the mid-1960's, seismologists developed a fairly complete understanding of how a slipping fault generates ground vibrations. An important quantity that characterizes the strength of the faulting is the seismic moment, the algebraic product of the fault area, the fault slip and the stiffness of the surrounding rock. Generally speaking, an earthquake with large magnitude corresponds to faulting with a large moment, with an increase in one magnitude unit corresponding to an increase of moment by about a factor of 30. But the relationship is inexact, and many cases occur where small faulting causes an unexpectedly large magnitude earthquake or vice versa.�

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