It would be interesting to see if the graph if it is easy to link it for you. If aftershocks are not removed, I'd expect 1989 and 1990 to have as many M3s aas the rest of the years put together (Loma Prieta aftershocks). But, that would not explain the pattern you see from how you describe it.
If you don't link the graph, how big is the effect? 10%? 100% (factor of 2)?
Chris