Scientists have discovered why the 2011 Japan earthquake and tsunami was so much bigger than had been thought possible.
A team of researchers, including some from Otago University, had previously discovered the fault line released much more energy than is normal in a quake and at a shallower depth.
They have now concluded that's because the fault line is thin and largely made up of weak clay sediments.
The scientists say the clay acted as a lubricant, allowing the sea floor to slip up to 50 metres in places, the largest movement ever recorded from a single quake.
Otago University geologist Dr Virginia Toy says the results were surprising, and will help set guidelines for what scientists should look for in other faults.
She says the study helps define the composition and fabric they should be looking for in other subduction zones, such as the Hikurangi margin off the east coast of the North Island.
Dr Toy says if these are similar to the Japan Trench then New Zealand should think about its tsunami hazard estimates.