Don,

You might just plot date (x-axis) vs magnitude (y-axis) for Mammoth and St. Helens to see visually how similar is the seismicity in the two areas. A correlation is unlikely, but who knows?

Recently we are seeing correlations due to unsteady fault creep at depth across tens of km with variations in activity on scales from weeks to months (Nadeau and Burgmann, Science, for central California, and the group following tremor and slow slip in Cascadia).

There is triggered seismicity after large events due to the shaking on the scale of hundreds of km, but that is only after the biggest earthquakes, like Landers and the recent on in Alaska.

John