Over the past decade a great deal of information has been uncovered to show that there have indeed been large earthquakes in the Juan de Fuca area. Much of this work has been done by researchers at the U.S. Geological Survey and the University of Washington in Seattle. The following diagram, and several others, are from that source.
Earthquake effects are particularly noticeable in coastal areas, especially in areas which experience subsidence during an earthquake, because the biological environment can change dramatically. The land drops by a metre or two, water ruhses in and kills the existing vegetation. In some cases that water is associated with a tsunami and with sand deposits. Eventually a new soil layer forms.
The information described in these diagrams forms the basis of a 1997 paper: Yamaguchi, D.K. et al., Tree-ring dating the 1700 Cascadia Earthquake, Nature, V. 398, p. 922-23.
This image used with the permission of Brian Atwater, University of Washington.