Geology 40, 887–890 (2012)

Credit: © STOCKTREK IMAGES / THINKSTOCK

Layers of thick sediment typically deposited on land by a tsunami are thought to originate largely from the beach and shoreface. An analysis of the sediment deposited on the Sendai Plain, Japan, by the Tohoku-oki tsunami, however, reveals that much of the material consists of terrestrial sand and soil liberated by the preceding earthquake.

Kazuhisa Goto at the Chiba Institute of Technology, Japan, and colleagues used field surveys and satellite and digital elevation data to analyse changes in the topography of the Sendai Plain, before and after the Tohoku-oki earthquake and tsunami in March 2011. The Sendai Plain is a flat-lying coastal plain that hosts extensive rice paddies and is located immediately onshore from the earthquake's epicentre. The volume of sediment deposited on the plain by the tsunami was found to be four times greater than the amount eroded from the beaches. The tsunami deposits were particularly thick in the rice paddies, and a closer inspection of the sands revealed ripped-up clasts of soil. The researchers suggest that sand from beneath the rice paddies was liberated and expulsed by a process called liquefaction, whereby unconsolidated and saturated sediments are transformed into a liquid-like substance during ground shaking caused by an earthquake.

Some marine sediments are probably also transported onshore during tsunami events. However, the results imply that the terrestrial environment can contribute significantly to tsunami deposits.