Interseismic coupling maps and, especially, estimates of the location of the fully coupled (locked) zone relative to the trench, coastline, and slow slip events are crucial for determining megathrust earthquake hazard at subduction zones. We present a physically motivated interseismic coupling inversion that explicitly incorporates locked zones with boundaries bordering an updip transition zone creeping at constant stress and a downdip transition zone with creep rate distributions consistent with updip propagation of the creep front into the locked zone. We show that the locked zone at Cascadia is west of the coastline and 10 km updip of the slow slip zone along much of the margin, widest (25-125 km, extending to ~22 km depth) in northern Cascadia, narrowest (0-70 km) in central Cascadia, with moment accumulation rate equivalent to a Mw 8.78 and Mw 8.89 earthquake for 300- and 500-year earthquake cycles. We find a steep gradient in creep immediately below the locked zone, indicative of propagating creep, along the entire margin. At Nankai, we find three distinct zones of locking (offshore Shikoku, offshore southeast Kii peninsula, and offshore Shima peninsula) with a total moment accumulation rate equivalent to a Mw 8.73 earthquake for a 150-year earthquake cycle. The bottom of the locked zone is nearly under the coastline for all three locked regions at Nankai and is positioned 0-5 km updip of the the slow slip zone. In contrast with Cascadia, creep rate gradients below the locked zone at Nankai are generally gradual, consistent with stationary locking.