Great post, Mary, and for me this is a very interesting subject. Seismic imaging has essentially picked-up traces of the Farallon Plate still actively descending into the mantle beneath the continental portion of the North American Plate. The rippling effect of NorAmer sliding across the descending Farallon is thought to have produced the Laramide Orogeny and the Black Hills of the Dakotas, as well as the basin and range country farther west - among other features. Friction from the collision has undoubtedly altered the North American landscape into the mid-continent, and possibly as far east as the New Madrid Seismic Zone, suggesting some role for the Farallon in the events of 1811-12; however, my take is that the subducting Farallon is probably not THE major player. About the same time as NorAmer contacted the Farallon, NorAmer also rammed into Eurasia. If you'll dig out a map of the world's tectonic plates you'll find that eastern Siberia, including the Kamchatka Peninsula, is really part of the North American Plate. When two continents collide, the inevitable result is orogeny and uplifting, and this is quite prominent throughout eastern Siberia and northwestern North America. Collision with Eurasia, in fact, elevated western North America just after the Farallon began to subduct along the west coast. As NorAmer elevated, the underlying asthenosphere billowed-upwards. Coeval lithospheric and asthenospheric uplifting thus facilitated the "shallow subduction" of the Farallon. By roughly 50 M.A., North America and Eurasia were locked in collision along their Siberian interface. This was a collision kept in motion and augmented by spreading along the North Atlantic M.O.R.. To the south of the Alaskan Peninsula, however, there was no collision-zone, only open sea. Continued spreading along the North Atlantic M.O.R. thus eventually pushed the portion of North America south of the Alaska Peninsula to the west. In time, this sub-Alaskan portion of our continent was rotated clockwise and resulting flexure created extension stresses along the mid-continent. Extension then created the Mississippi Valley and "reactivated" an old rift zone near what is now New Madrid Missouri. This was completed by perhaps 40 M.A.+_ 15 M.A.. The great quakes of 1811-12 are thus most appropriately viewed as the byproducts of these rather large and grandiose plate tectonic events, but the most important of those events was probably the collision along the North American-Eurasian interface, which facilitated rotation and extension of the continent and ultimately the reactivation of the New Madrid Seismic Zone.