Is the Cascadia megathrust active? Or are there associated strike/slip faults which are the problem?
It is still active, although the last great event was 1700.
http://www.pnsn.org/HAZARDS/CASCADIA/cascadia_event.html
There was a very interesting show on PBS talking about it. One of the main questions was "minute of terror" or "decade of terror".
The idea being, does the fault give a little here, then a little there, then a little more somewhere else, or
DOES IT ALL GO AT ONCE?
By checking the sediments in the deep ocean valleys offshore, they are able to map eqs. (sediment falls down from the sides of the chasms during a quake that are not normal annual depositions).
The answer is: at least the last time, in 1700, it went ALL AT ONCE. A 9.0+ quake stretching from north Vancouver island to sothern Oregon, almost the Bay area...
Rock 'n roll!!