I'm not really sure why there should be tsunami walls etc built, since the howevermany scores of nukes it takes probably will leave large parts of the pacific northwest uninhabitable (either de facto or by choice) for a while.
So, it would be easier, cheaper and safer (but equally infeasible) to simply force resettlement of the population somewhere without a crack in the crust.
Underground nuclear explosions are fairly well contained; done at the depth required to deal with a fault, they'd probably be unnoticeable. Also, perhaps impossible, but that's a different story.
"Cannikin was detonated on November 6, 1971 51°28′13.20″N 179°6′40.75″E, as the thirteenth test of the Operation Grommet (1971–1972) underground nuclear test series. The announced yield was 5 megatons (21 PJ) – the largest underground nuclear test in US history.[24] (Estimates for the precise yield range from 4.4[36] to 5.2[37] megatons or 18 to 22 PJ). The ground lifted 20 feet (6 m), caused by an explosive force almost 400 times the power of the Hiroshima bomb.[38] Subsidence and faulting at the site created a new lake, over a mile wide.[3] The explosion caused a seismic shock of 7.0 on the Richter scale, causing rockfalls and turf slides of a total of 35,000 square feet (3,300 m2).[24] Though earthquakes and tsunamis predicted by environmentalists did not occur,[33] a number of small tectonic events did occur in the following weeks, (some registering as high as 4.0 on the richter scale) thought to be due to the interaction of the explosion with local tectonic stresses.[39]"
So, it would be easier, cheaper and safer (but equally infeasible) to simply force resettlement of the population somewhere without a crack in the crust.