The 2011 Tohoku-oki tsunami — Three years on.

The 2011 Tohoku-oki tsunami that devastated the Pacific coast of Tohoku, Japan was a turning point for modern research. As a result of this event it was recognized that paleotsunami research is vital to help understand the size and recurrence interval of low-frequency large tsunamis. This paper reviews the progress of geological research on the 2011 Tohoku-oki tsunami and summarizes new questions that are arising out of this work. For example, recent work suggests that the landward extent and thickness of the sandy deposit, as well as the presence or absence of marine microfossils in the sediment are most likely to be mainly controlled by the initial wave properties, sediment source, offshore bathymetry and onshore topography. This in turn implies that there are certain relationships between the characteristics of a tsunami deposit and the wave properties and it may be possible to reconstruct the latter from the deposits. Offshore tsunami deposits related to the 2011 Tohoku-oki tsunami have also been well described. This recent research indicates that sedimentation and erosion in inner bay and open ocean (~ 20 m water depth) locations can be in the order of several meters, suggesting that the tsunami shear force was strong in the nearshore zone. On the other hand, sandy to muddy deposits a few centimeters thick were observed at about 100 to 6000 m water depth. It is likely that the tsunami resulted in resuspension of sea bottom sediments and that suspended material flowed downslope as a turbidity current or suspended flow, although many authors recognize the possibility that strong earthquake groundshaking might have also generated turbidity currents. Studies of the 2011 Tohoku-oki event have led researchers back to two of the fundamental issues of tsunami geology: understanding the linkage between onshore and offshore sedimentation and erosion, and establishing identification criteria for tsunami deposits. Moreover though, beyond the issue of simple tsunami geology, it is important for all researchers to communicate with governments and the general public in order to reduce future casualties by using risk assessments based on our understanding of infrequent large tsunamis. [ABSTRACT FROM AUTHOR]