Crustal deformation processes and the stability of the Gutenberg-Richter relationship

Global and regional surveys of earthquakes show that empirically determined b-values of the Gutenberg-Richter distribution are remarkably stable, gene rally limited to values of -1 +/- 0.2. Here I interpret observations from C alifornia, New Zealand, and Japan to suggest that the stability of the b-va lue is a manifestation of a physical process; specifically, the tendency of crustal strains to organize along relatively discrete zones. Given a porti on of the earth's crust subject to a displacement field, displacement is ac commodated by a system of fault lengths that obey a power law distribution. With continued displacement, longer faults develop at the expense of short er faults and take up an increasingly greater portion of the displacement b udget. Shifts in regional displacement directions lead to reversals of thes e trends. The changes in the ratio of long faults to short faults in a regi on is thus accompanied by changes in the relationship between fault length and fault slip rate. Because the recurrence of earthquakes on faults is a f unction of both fault length and fault slip rate, the deformation process m ay result in a system of feedback that inhibits changes in the b-value as f ault populations change. A corollary to the idea is that the magnitude-freq uency statistics of seismicity may be attributed to the same physical proce sses that are responsible for the development of plate tectonic boundaries and the gross spatial distribution of seismicity around the globe.