Earthquake magnitudes can be more accurately estimated using information about the extent of the rupture zone (the area of a fault over which the rocks slip when the earthquake happens) and the amount of displacement or slip. Based on the the map above (from Brian Atwater and others from the University of Washington) the rupture zone probably extended from central Vancouver Island to northern California - a length of between 650 and 850 km.
Work published by the Pacific Geoscience Centre supports this estimate, and shows that the average width of the current locked zone between theJuan de Fuca and North American plates is probably in the range of 50 to 80 km.
Based on the current subduction rate of 4.5 cm/year it can be estimated that the amount of displacement over a 500 year period should be close to 25 m. It is reasonable to suggest, therefore, that when the locked zone was released there could have been between 10 and 20 m of displacement on the fault - or even more.
This image used with the permission of Brian Atwater, University of Washington.