> and deduce that every X amount of years the Cascadia Subduction Zone ruptures with some regularity.

I guess this depends on your definition of "regular." If you look a the intervals between events reconstructed from the turbidite record (Table 12 on page 115 of Goldfinger et al., 2012), you'll see that these aren't exactly evenly spaced. E.g., the spacing in years between events is 232, 316, 446, 311, 982, 492, 415, 665, 661, 1189, 508, 715, 443, 548, 733, 195, 117, 577. From this you can calculate an average and it tells you that generally you'd expect an event every few hundred years, but after a given event, there's not necessarily anything to indicate whether the next one is going to be in ~100 years or ~1000 years. I would not describe that as having a particularly "regular" pattern of strain release.