The Farallon plate was the huge plate that abutted the great continent of Pangaea. As it broke apart what is now the North American Plate rode on top of it and as it did the Farallon subducted so far east what is left of of it lies between the Rockies(which were created by this subduction) and the Atlantic coast. The Juan de Fuca and the Cocos plates are the last remaining visible vestiges of this one huge plate. Anyways it is in my belief that sitting on top of the subducted Farallon not only acted as an anchor for the North American Plate but indeed in relation to other areas provided a nice shield from earth's mantle.
In the yearly heat maps of North America you can clearly see the outline and extent of the Farallon drawn by the chilling of the lithosphere temps. Over the last few years one can also see that area of the continental USA are warming east of the Rockies.
If I am correct in thinking that the Farallon is now finally succumbing to the mantle in masse it would explain much. Not the least of which would be small to moderate earthquakes and in time larger ones.