Hello Roger...if a strong magnetic storm can trigger M3s, but may not trigger M7s, then you need to look at the smaller quakes. You would plot number of M3+s as one curve and AA index as another curve (or set of bars). If it is, as Lowell W. suggested, volcanic areas that are more sensitive...then for one volcanic region you would not have enough large quakes to come up with a significant pattern.....
Chris