What limits the width of the downdip seismic zone?
The pattern of current deformation allows us to estimate which part of the thrust fault is locked. But what controls the limits of the locked seismogenic zone? Many factors have been suggested (e.g., discussion by Tichelaar and Ruff, 1993), but temperature appears to play a dominant role, at least for continental subduction zones such as Cascadia (Hyndman and Wang, 1993). The seismic zone is bounded seaward by a region that does not generate earthquakes. Free slip in the latter zone may be a consequence of the stable sliding clay minerals that are common in the region of subduction zone faults. With increasing temperature these clays become dehydrated and transform to stronger minerals. The fault becomes seismogenic where the temperature reaches about 150°C (see discussion by Hyndman and Wang, 1993). If this hypothesis is correct, the seaward limit of the Cascadia locked zone is beneath the lowermost continental slope. The downdip limit of the seismogenic zone may also be thermally con
