What the vulcanism of the Cascade Mountains tells you is that this range not only has active subduction beneath it, but that the rock layer riding on top is thin. If the rock layer were thick or heavily baffled, volcanism would be suppressed. Imagine the process of subduction over the eons, repeated subduction. At first it is two plates abutting each other or sliding past each other. Then one dives under and the very edge of the plate riding on top is rumpled, mountains building by this rumpling. Then the next instance of subduction occurs years later, and the lip of the subducting plate pushes further, but what happens to the rumpling that occurred the first time? This has hardened, any melting due to friction long cooled, and the very fact that it is rumpled prevents the entire mass from melting again. This process continues, with the original rumpling repeatedly being rumpled until the point arrives where it is above any sliding of one plate over another, riding like foam on a wave. This is indeed a safer zone than your Cascade Mountains for this reason. However, to avoid the possibility of being where any melted rock might occur, do not be in the river valley itself, but rather up in ravines.
ZetaTalk July, 2010