Roger....Kp is probably what you should use. I don't know what AA* is. Someone posted (maybe Canie, maybe me) where you can find the Kp data for the last few decades...on this page, a few months ago....
Kp is the magnetic field at earth's surface in North America....measured at several locations. You may want to look not only at M7+, but also at smaller (say. M3+) quakes for, say, California, or some other location in North America. Could try, for example, the province of Quebec and NE U.S....

Chris