Ross Stein from the US Geological Survey (USGS) is one of a US/Japanese team that has modelled the region around the triple junction to help gauge future risks.

They used seismic signals from 300,000 tremors of various sizes to build a three-dimensional view of what was going on deep in the Earth, much like a doctor might use X-ray tomography to scan tumours in the brain.

They found a 25km-thick fragment

broken off one of the plates that they now believe plays a significant role in shaping seismic activity in the Kanto region – and by implication, the outlook for Tokyo.