Making sense of Nankai earthquake risk

Two timescales of risk

On 2 November, JMA’s latest listed monthly assessment was 7 October 2026: no special increase above baseline, with high long-term M8–9 risk. Check new announcements for updates.[1]

Plates lock, bend and slip

From Suruga Bay to Hyuga-nada, the Philippine Sea Plate sinks beneath the landward plate by several cm yearly. Locked areas deform the upper plate; slip releases seismic waves.

A bent ruler pictures stored deformation and release. Real faults have complex rock and friction.[2] [5]

History shows several patterns

A broad rupture occurred in 1707. The 1854 pair was 32 hours apart; the 1944–1946 pair, two years. December 2026 marks 80 years since 1946. Future timing and extent remain uncertain. Follow instructions after a major event.[2]

What does “within 30 years” mean?

As of 1 January 2026, HERP gives 30-year M8–9 probabilities of “around 60–90% or higher” using intervals and uplift, and “20–50%” using intervals alone. Both rank high.

A 15-year-old reaches 45 over this window. A hypothetical 70% means about 70 of 100 comparable 30-year histories contain such an event. The official estimates are the ranges above. The models use three uplift records or six intervals. Data and assumptions matter; HERP advises preparing with the higher estimate in mind.[3] [4]

From seafloor movement to waves

Seafloor movement displaces water and sends waves outward, like lifting a panel beneath a bath. Waves travel faster in deep water; the coast and seabed shape local height. Wave height, land inundation depth and elevation reached use different reference points.[5]

More time to react

From 12 March 2026, JMA uses 18 N-net coastal stations. Warning gains for Shikoku–Hyuga-nada reach about six seconds, or 20 with the full network. Faster detection after onset gives variable time to lower yourself and protect your head.[6]

Read the action instructions

Extra attention calls for checking preparations and evacuation readiness. Alert may require a week of advance evacuation in designated areas where escape after onset could be too late. Follow local instructions.[7]

Know your route

Check hazard maps and higher evacuation places and routes near home or your hotel. Plan family contact; charge phones and backup batteries. Near coasts or rivers, strong or prolonged shaking calls for immediate evacuation. Act before seeing waves; remain until warnings and advisories end and follow local orders.[8] [9]