There were no codes in place during the rebuilding after 1945. My folks were stationed there in 1951-1961 with the US Army.
http://www.nytimes.com/2011/03/12/world/asia/12codes.html
Hidden inside the skeletons of high-rise towers, extra steel bracing, giant rubber pads and embedded hydraulic shock absorbers make modern Japanese buildings among the sturdiest in the world during a major earthquake. And all along the Japanese coast, tsunami warning signs, towering seawalls and well-marked escape routes offer some protection from walls of water.
In Japan, where earthquakes are far more common than they are in the United States, the building codes have long been much more stringent on specific matters like how much a building may sway during a quake.
Japan has gone much further than the United States in outfitting new buildings with advanced devices called base isolation pads and energy dissipation units to dampen the ground’s shaking during an earthquake.
The isolation devices are essentially giant rubber-and-steel pads that are installed at the very bottom of the excavation for a building, which then simply sits on top of the pads. The dissipation units are built into a sound building’s structural skeleton. They are hydraulic cylinders that elongate and contract as the building sways, sapping the motion of energy.