It is just over 15 years since the devastating Tohoku earthquake and tsunami struck Japan, killing almost 20,000 people and triggering the Fukushima Daiichi nuclear disaster. Fresh analysis of video footage of the wave has revealed that the mud-rich coastline made the tsunami far more destructive than it might otherwise have been.

Patrick Sharrocks, from the University of Leeds, and colleagues studied helicopter video footage, along with before and after images from Google Earth, to estimate the speed, shape and power of the tsunami flow front. They found that as the wave travelled over mud-rich rice paddies it changed from a fast-moving, clear-water flow into a thick, gloopy, mud-laden one.

Writing in the Journal of the Geological Society they explain how a dense and fast-moving mud-rich flow would have exerted considerably more force than its clear-water equivalent, resulting in greater damage to the buildings in its path and far higher numbers of injuries and fatalities.

This type of flow is similar to the destructive mud flows that sometimes form on the side of volcanoes when water mixes with sediment. The authors recommend that tsunami hazard assessments are updated to reflect the amplified risk posed by mud-rich coastal settings and used to inform coastal land use decisions in tsunami-prone regions.