The Juan de Fuca subduction zone represents one small segment of the so called "Ring of Fire" around the Pacific Ocean. All of the other subductions zones - shown as red lines on the map below - have experienced major earthquakes within the past 50 years (such as the huge 1960 earthquake in Chile, the 1964 "Good Friday" earthquake in Alaska, the devasting 1985 earthquake in Mexico, and the 1999 earthquake in Taiwan). In contrast, there has been no major earthquake on the Juan de Fuca subduction zone for 300 years.
Over the past couple of decades geologists have wondered why this is the case. Some have suggested that the plates are not locked, or that subduction is not taking place at a significant rate. There is lots of evidence that subduction is taking place - such as the Cascadia volcanic arc extending from British Columbia to California, but how do we know that the plates are locked?