The Nankai and Cascadia subduction zones have low-stress forearcs, where the margin-normal horizontal compressive stress is similar to or less than the vertical stress except in the accretionary prisms. In the absence of active back-arc spreading, the forearc margin-normal stress is controlled mainly by the total shear force along the subduction fault (or the plate coupling force) and the gravitational force. The plate coupling force creates compression, and the gravitational force, in the presence of margin topography, creates lateral tension. We give a simple expression that relates the forearc stress to topography and average fault strength, with the latter represented by an effective coefficient of friction μ'. We also use a finite element model of two converging plates in frictional contact to model the forearc stresses. The analyses indicate that a low-stress forearc requires not only the subduction fault to be very weak but also the mechanically coupled fault area to be narrow in the downdip direction and shallow in depth. For Nankai and Cascadia, where the coupled fault areas are narrow and shallow because of the young and hot subducting plates, μ' is about 0.05 or less and in any case less than 0.09. The low fault strength is probably not a unique property of low-stress forearcs, since the same low μ' values combined with wider and deeper coupled areas can be shown to create large margin-normal compression. In addition, there does not seem to be any relation between subduction earthquake magnitudes and plate coupling forces. Great earthquakes occur at subduction zones having low-stress forearcs such as Nankai and Cascadia. ¿ 1999 American Geophysical Union