Coastal Geomorphology and Tsunami Disaster by the 2011 off the Pacific Coast of Tohoku Earthquake

Abstract

This study investigated the tsunami deposits accumulated at the Pacific coast of Tohoku district by the tsunami associated with the 2011 off the Pacific coast of Tohoku Earthquake. Influence of coastal landforms on the tsunami deposits and run-up heights was also discussed. Pleistocene terraces, rias, strand plain, and valley plain surrounded by terraces or hills occur generally along the coast from north to south. Tsunami deposits up to several tens of centimeters in thickness accumulated broadly in strand plain and valley plain. Although thick tsunami deposits also formed at small bay-head deltas in rias, tsunami deposits were not clear and erosional landforms caused by tsunami backwash were common in a very small, steep valley plain along the ria coasts. Sandy deposits show landward-thinning trend at wide strand plain and valley plain. However, the similar trend was not clear at narrow strand plain. The difference is probably related to decrease in sediment transport capacity of the tsunami and sediment supply. Sediment distribution and run-up height are strongly influenced by onshore topography. In high-lying topography, run-up height may relate to onshore slope and shape of lowland. On the other hand, run-up height is possibly controlled mainly by the characteristics of incident wave (wave period and height) in low-lying topography. Further detailed investigation about submarine topography is also required in the near feature.