I found data for large earthquakes before the 1900's. You can check out the earthquake graphs I made here and their sources:
Amy Van Gerpen
Tdd Nelson (19 Apr 2011)
"EARTHQUAKES - HISTORY - RESULTS - AVERAGES"
Please look at this link http://earthquake.usgs.gov/earthquakes/eqarchives/year/mag8/magnitude8_1900_date.php
8.0 and above quakes look to be pretty evenly distributed from 1900 to 2010.
It’s hard to find data for 7 – 8 magnitude quakes going back from 1990, but that does not mean there were only a few of them.
In the short term, in the last year or two, quakes do seem to be going up, but no long term trend can be discerned from the data. It remains to be seen if we are on a medium or long term uptick.