The short answer is "none".

I plotted quakes per year from 1900 to 2011 using the Centennial catalog thru 1999 plus NEIC catalog from 2000 thru 2011.

A plot of mag 6+ quakes shows a dramatic increase from 1960 to 2000 or so. Interestingly, that's when I started working for the federal government but that isn't the reason. It's when the global network started being installed.

Plotting 7+ quakes shows a nearly level line for the entire period. Major quakes get noticed even when there are no seismographs to record them.

Roger