Paradoxical effects of the record-high tsunamis caused by the 2011 Tohoku earthquake on public judgments of danger

The 2011 Tohoku earthquake caused about 19,000 casualties, dead and missing, of which most resulted from the enormous subsequent tsunami. Those who survived the tsunami had evacuated to higher ground, their action reflecting the importance of public judgment in mitigating the disaster. In this study, we examined the change in the risk assessment of tsunami heights since the 2011 Tohoku earthquake among residents in western Japan, an area that was not affected by the earthquake but that has long been at risk for such natural disasters. While existing research argues that the experience of natural disasters increases the public's alertness, the results of our survey (N=733 at pre-quake and N=1036 at post-quake) showed that, after the earthquake, those who live in this area had augmented their estimations of what tsunami heights are dangerous enough to warrant evacuation. Laypersons are highly inclined to make judgments based on the numbers first presented to them. This adjustment and anchoring heuristic well explains these paradoxical results. The vast amount of media coverage of the record-breaking tsunami established an anchor and prompted people to elevate the evacuation height, altering their judgments in the direction of greater vulnerability. Considering the difficulty of subsequent adjustments, our findings indicate the importance of conveying basic information about the risks of lesser intensity natural disasters in the initial stages of reporting a record-breaking disaster to those who have not directly experienced one.

► The tsunami height perceived dangerous increased after the 2011 Tohoku earthquake.
► The record-breaking tsunamis led people to underrate the risk of small tsunamis.
► The anchoring heuristic explains how people change their risk judgment on tsunami.