The US has a major presence in Japan and its society as a culture will come together during a major event to support the community. Unless many people at all levels in the government die I don't see them collapsing. It would do a ton of damage to the economy though. The US would immediately jump in to help using its military. Those stationed in Japan would move to humanitarian relief and ships in Hawaii and San Diego would be immediately dispatched. you could have 3 air craft carries, 8 large amphibious ships (including 3 help carriers) and a whole range of smaller vessels there within 2 weeks. They would spread out and start support every majorly affect community.