Yo Cathryn. I remember reading somewhere a couple of years ago that the rupture of the entire zone was the exception rather then the rule however this recent article seems to contradict that. About the only thing that can be said is that sometimes it does and sometimes it doesn’t. Take Care…Don in creepy town

“Twelve earthquakes occurred in the last 6500–6720 cal yr B.P., recurring on average every 570–590 yr. Intervals between earthquakes varied from a few hundred years to over 1000. Comparisons of the Coquille record to earthquake histories from adjacent sites in Oregon, southwestern Washington, and northwestern California suggest that at least two earthquakes in the last 4000 yr did not rupture the entire length of the subduction zone. An earthquake 760–1140 cal yr B.P. in southwestern Washington may have ruptured as far south as Coos Bay but probably stopped before it reached the Coquille estuary because no buried soil records
the event, and tidal marsh conditions were set to record an earthquake. An earthquake limited to a southern segment of the Cascadia margin 1940–2130 cal yr B.P. probably did not rupture north of the Coquille estuary.”

http://walrus.wr.usgs.gov/cascadia/

• Cascadia Fault Ruptures