The observation that motivates this study is the difference in c values in moment-magnitude relations of the form log M0=cML+d between central and southern California. This difference is not at all related to geographical area; rather, it results from positive curvature in the log M0-MM plane and the relatively large number of ML2 1/2) California earthquakes whose spectral corner frequency lies in the ''visible'' bandwidth, f0≤fmas.