Revealing complex tsunami evacuation process patterns induced by social interactions: A case study in Ishinomaki

Abstract

Abstract Compared to evacuation behaviours, behaviours preceding evacuation including milling during tsunamis remain unclear. This study developed an analysis approach that evaluates the similarities in behaviour sequences throughout the evacuation process and investigated both the preceding behaviours and evacuation behaviours in Ishinomaki during the 2011 tsunami. The developed method was first applied to 101 detailed evacuation interview data and identified 16 major evacuation trip patterns in the study area during the 2011 tsunami. Then, we characterised the major patterns based on narratives and found key behavioural characteristics during tsunamis that both expanded and mitigated human loss. The analysis only considered the different destinations visited by people during evacuation processes; however, the proposed method successfully distilled evacuation processes with not only similar behavioural patterns but also similar thoughts or intentions. The collected behaviours revealed the complex evacuation processes during the tsunami, i.e., nearly half of the interviewees had two or more destinations in their evacuation trips. Evacuating alone was rarely observed, and many local residents first attempted to perform safety or damage confirmations, which caused evacuation delays. On the other hand, the results also indicated that this tendency of unification along with immediate school evacuation promoted evacuations in the study area by triggering household movements towards a hill, which unintentionally worked as a tsunami evacuation trip. We inferred that such secondary household movements potentially triggered other evacuation movements as tertiary effects by being sighted by other people. The findings of this study provide evidence suggesting that evacuation processes are largely affected by social structures or ties and highlight the importance of evacuation preparedness considering the revealed behaviour characteristics.