Did you correct for the changing definition of magnitude?
No, I'm using data from a couple of USGS catalogs. Besides, wasn't most of the changes in magnitude definition made in the 70's? Also, it seems to be almost a "wash" from some of the charts I've seen comparing earthquake magnitudes before and after the change in magnitude computations? Finally, I don't see any indication of such in the chart.
Not defending, just trying to cover the bases. I appreciate the question.