Chicago to Milwaukee (center to center) is about 95 miles. Just over an hour driving time if the traffic isn't too thick.

The recent earthquake, according to reports I saw, was on a smaller "side branch" off the New Madrid; but the last big one on the main fault line, late in the past century as I recall, was near St Louis and rang the church bells in Boston - ~1,180 miles away. Reports nearer the center were that entire herds of Buffalo (Bison) were knocked off their feet. (A little more extreme than the ever-popular "cow tipping" exercise.) At the time, there were few building more than one or two stories tall near the center, so damage to "civilized objects" was probably minimal.

New Madrid is considered one of the most dangerous faults in the US, but in recent history (a few thousand years) apparently the main fault line has only slipped at about 1 or 2 hundred year intervals. (Since the data changes as new theories develop, I'm of course speaking in very general terms here, from prevous news reports a decade or two old.)

John