Residents told me that when the seawalls were completed (and the town nearly bankrupt) in 1958, a new spate of construction began immediately. But physical infrastructure created new norms and altered risk behaviors that set them up for calamity.

On March 11, 2011, a massive 9.0 earthquake struck off Tohoku’s coast. Residences and commercial buildings, by and large, suffered little damage. But that earthquake set off a tsunami as high as 65 feet, which surged toward the coast about 40 minutes after the tremors stopped. The government issued tsunami warnings and evacuation orders, but not everyone left vulnerable, low-lying areas along the coast.

Many residents in Taro believed that the wall would save them. They didn’t evacuate to higher ground. Others climbed to and stood on top of the seawall to see what risks they faced. Much of the wall crumbled and was swept away, and 144 people from the community — about 6 percent — died. Across the Tohoku region some 18,400 people died, almost all of them in the black waves.

Since my own home was destroyed by flooding after Hurricane Katrina in New Orleans in 2005, I have learned that what keeps people safe during disaster and accelerates their recovery afterwards is actually social, not physical, infrastructure. In communities in Japan, strong social connections motivated young people to carry the elderly and infirm to safety ahead of the tsunami. In the 40 minutes between the earthquake and the arrival of the waves, healthy and younger residents could easily walk the mile or less uphill to higher ground. But those in bed or facing disabilities needed help to get to safety. In an academic study of more than 140 communities along the coast, my Japanese colleague and I found that those with more social infrastructure had lower levels of mortality than similar ones that had division and a lack of trust.

Social ties matter after a crisis. In the recovery process, individuals with more connections to neighbors better handled the immense stress of evacuation. Some who fled homes near the Fukushima nuclear power plants had to move six times over the course of a year.

My Japanese colleagues and I interviewed members of the city of Futaba, which had to evacuate in March 2011 as the plants melted down. Through conversations and surveys with more than 500 people, we learned that having neighbors whom they knew reduced their stress and post-trauma symptoms.

Finally, our study of more than 40 tsunami-affected cities and towns found those communities with more vertical ties to decision makers and politicians rebuilt infrastructure, homes, and schools more effectively. Towns and villages in Tohoku that had only a single powerful advocate in the central government — think of former Massachusetts senator Ted Kennedy or former Senate Majority Leader Harry Reid — did measurably worse than those with four or more such power brokers.

While decision makers may move quickly to construct large-scale physical infrastructure, as has been proposed here in Boston to handle rising seas, they should instead look to the power of social infrastructure.