Nankai megathrust earthquakes

[1] The megathrust boundary extends about 700 km from the southern end of Kyūshū to the triple junction with the Okhotsk microplate near Mount Fuji.

The presence of this 'weak' zone may lead to shallow coseismic rupture along the megasplay faults during megathrust earthquakes, explaining the large tsunamis created by these events.

The pattern of historical seismicity reveals that the megathrust surface is segmented, with five separate zones of rupturing identified, conventionally labelled A–E, from west to east.

This behaviour has been reproduced by modelling the viscoelastic response of the megathrust fault plane with lateral variations in both convergence rate and frictional properties.

A future great earthquake involving rupture along this and possibly other segments has been proposed as a major risk for the southern coast of Honshu.

[7] Despite the uncertainty of when such an earthquake will occur, local authorities are already taking action to prepare residents for what they regard as an inevitability.

A study by the Japan Society of Civil Engineers in 2018 estimated that the long-term damage from the earthquake could result in ¥1.240 trillion in economic losses over a 20-year period.