The nation honored its dead last week from the earthquake and tsunami that devastated Tohoku's Pacific coastline on March 11 four years ago.

For those who have spent those years helping survivors in Tohoku, the experience has thrown up the challenge of how to knit back together communities built over centuries, then shattered in the space of minutes on that Friday afternoon in 2011.

Doctors and others say the work in Tohoku has also offered a fast-forward glimpse into Japan's own future and perhaps what awaits other industrialized societies trying to understand how communities will function as their youth populations are slowly eclipsed by the elderly.