The GutenbergRichter Law is an empirical relation 
log(N) = b⋅m + a 
between the magnitude m and the number N of earthquakes of magnitude
exceeding m. 
Straightforward calculations
(pgs 178179 of Turcotte) convert this to a
power law. 
N = B⋅r^{2b} 
where the length of the fault break is
r = √A and A is the area of the
fault break. Then 2b = D, the dimension of seismic
activity for the region from which
the data were collected. 
For example, the graph below (inspired by page
179 of Turcotte)
illustrates the number N of earthquakes per year of magnitude exceeding m in
southern California for the period 19321972. The circle represents the expected
rate of great earthquakes in southern California. The data predict about one per
century. 
