Hi Cathryn. I asked Dr. Brian Atwater your question about the Cascadia Subduction Zone. He has probably studied it more then any other person I know of. Here is his reply. Take Care…Don in creepy town

Interesting question Don. It's one that Earth scientists have been trying to answer for the past twenty years. Thus far your hunch is among the possibilities. Other subduction zones are known to vary, from one earthquake to the next, in rupture mode: sometimes the whole length, other times a series of shorter ruptures, or even isolated ruptures that
leave part of the zone unbroken. -- Brian

Brian F. Atwater
U.S. Geological Survey at Department of Earth and Space Sciences
University of Washington, Box 351310
Seattle, WA 98195-1310
206-553-2927 voice, 206-553-8350 fax
atwater@u.washington.edu