This is a cross-section of the plates, showing how the Juan de Fuca Plate is being pushed down under the North American Plate. Unfortunately, the JDF plate is not subducting smoothly like this. Instead, it is locked as shown below. The Juan de Fuca Plate is contnuing to subduct, but it is not sliding smoothly past the North America Plate.
The results of this locking are shown on the next page.