Ten years after the 2011 Tohoku-oki earthquake and tsunami: Geological and environmental effects and implications for disaster policy changes

Although the 2011 Tohoku-oki earthquake and tsunami were devastating to coastal environments, many of their effects have not lasted: over several years they have generally recovered naturally and artificially. Some environments have not fully recovered or have become stable with some different status. By contrast to the short-lasting tsunami impact, long-lasting crustal movement and reconstruction projects, together with the nuclear power plant accident in the area have strongly affected human society and local coastal environments in a long-term. Investigation reveals that damage outlooks adopted before the 2011 event differed at the prefectural level based on limited knowledge and awareness of the prior 400 years (mainly 120 years). Knowledge, awareness, and reconstruction work undertaken after the 1896, 1933, and 1960 tsunamis were effective to a certain degree during the 2011 tsunami, especially in northern Tohoku. However, past memories of large tsunami disasters in southern Tohoku such as the 869 and 1611 events were lost to the public to a degree that tsunami awareness and countermeasures were insufficient. Although post-2011 governmental policy has been designed to prepare for maximum possible earthquake and tsunami occurrence in the future, no consensus exists of how to forecast such events. Remarkable post-seismic crustal movement has continued. Seismologic and paleotsunami studies have projected a high risk of future large earthquakes and tsunamis surrounding the 2011 source region along the Japan Trench. Considering that low-frequency but large tsunamis have caused extraordinarily intensive and extensive damage, studies based on geology, archaeology, and history are invaluable for elucidating tsunami phenomena and forming accurate risk assessments. In fact, although many historical and geological studies have been conducted along the many coasts of Japan after the 2011 event, no evidence has been provided about extremely large tsunami waves, which greatly exceeded the knowledge available before 2011. Ten years is an extremely short period on a geologic timescale, but it is sufficiently long for the general public to lose their disaster memories gradually. How can we continuously remind public people about such low-frequency tsunami events? How can we foster a social consensus to prepare for them? These issues, which were crucially important before the 2011 event, remain unsolved today.