That's not precisely accurate. The cascadian subductions zone uas produced magnitude 9 quakes in the past. The results I've seen (from a decade and a half ago)

That's not precisely accurate. The cascadian subductions zone uas produced magnitude 9 quakes in the past. The results I've seen (from a decade and a half ago) pegged the probability of a magnitude > 9 in the next 50 years at 1/10 and a magnitude > 8 at 1/3. The 500 years average is if you look at the last 7 events. 7 is a horribly low sample size and if you extend the timescale, that average drops by half and indicates we are past the 250 year average.