From the Washington Post:
Japan’s shrinking labor force could constrain the country’s ability to rebuild — thus forcing politicians and the public to confront its misgivings about immigration. Japan has long exerted tight control of its borders and makes it difficult for foreigners to live and work in the country. Among leading industrial nations, only South Korea has a lower share of foreigners in its workplaces. The foreigners now in Japan fall into various niches: highly skilled white-collar expatriates; low-skilled, often illegal, laborers; imported rural brides. Economists have long argued that Japan needs to welcome more workers to remain economically competitive. The imperative to rebuild housing and infrastructure on a massive scale could force this immigration challenge into the open.
Clearly, Japan isn't densely populated enough. It needs more people living in tsunami zones.