The Cascadia subduction zone is a young subduction zone with associated volcanic activity but with no history of large thrust earthquakes associated with it as is characteristic of other subduction zones. This has led to questions about the potential for great earthquakes associated with this subduction. The seismic potential associated with this subduction has been investigated in this study by examining and comparing the number of shallow earthquakes with magnitude >6 which have occurred in different subduction zones since 1910. This examination shows that all subduction zones exhibit a higher level of activity than Cascadia. Furthermore, two of these zones with similar rates of plate motion and age at subduction, the Rivera plate and southwest Japan, have not only experienced great earthquakes but show a higher rate of activity than the Cascadia zone. The activity rate in subduction zones with predominantly aseismic slip is also higher than the activity rate in the Cascadia zone. Earthquake activity in the rupture zones of great earthquakes both before and after the occurrence of great earthquakes is higher than the activity rate in the Cascadia zone. These results therefore suggest that the low rate of activity in the Cascadia zone is unusual and supports the idea that the subduction here is different and that perhaps the potential for great earthquakes cannot be extrapolated simply on the basis of certain similarities between the Cascadia zone and other subduction zones with a history of great earthquakes.

The low rate of earthquake activity in the Cascadia zone can, however, be explained on the basis of the combined effect of low rate of plate motion, significant aseismic subduction, and young age at subduction. The activity rate in the Cascadia zone is consistent with the activity rates in other subduction zones if it is assumed that much of the plate motion is accommodated aseismically. The seismic slip rate associated with this subduction zone is a small fraction of the rate of plate motion, and therefore even if the potential for great earthquakes may exist, the return period for great earthquakes will be much longer (2 or 3 times) than in other zones with similar characteristics such as the Rivera plate or southwest Japan. ¿ American Geophysical Union 1992