Nankai Trough Earthquake: What It Is and How to Read JMA Information
What the Nankai Trough is, how to understand long-term probability and what JMA’s special information actually means.
A JISHINDESU editorial guide based on the cited official and primary materials. It does not replace official safety information.
On this page
The Nankai Trough is a broad zone, not one point
The Nankai Trough extends from the Suruga Bay area along the Pacific side of central and western Japan towards Hyūga-nada. Here the Philippine Sea Plate subducts beneath the plate on the archipelago side.
A “Nankai Trough earthquake” is not one fault, city or predetermined scenario. Different segments may rupture, and the earthquake’s size, epicentre, shaking and tsunami depend on which sections move and how.
What does the historical record tell us?
Magnitude 8 and larger earthquakes have repeatedly occurred along this boundary. JMA describes intervals of roughly 100–150 years, but this is not a regular clock: both the gaps between events and the rupture areas have varied.
Segments may rupture over a wider area at nearly the same time or in successive events. History helps define hazard scenarios, but it cannot calculate the day or year of the next earthquake.
How should a 30-year probability be understood?
Japan’s Headquarters for Earthquake Research Promotion publishes a long-term Nankai evaluation and places the zone in its highest probability rank. Revised methodology presents results from more than one calculation, so always check the date and explanation in the current official table instead of repeating one percentage without context.
A probability over the next 30 years is not an expiry date, countdown or forecast that the event will happen near the end of that period. It may happen earlier, later or not within that window. Science cannot currently predict a precise date, place and magnitude for such an event.
What is Nankai Trough Earthquake Extra Information?
JMA may begin an assessment after detecting an unusual phenomenon and issue special Nankai information. It does not mean a megathrust earthquake has been predicted. It says experts are investigating or have judged the relative likelihood to be higher than usual.
Keywords cover an investigation in progress, a megathrust earthquake alert, a megathrust earthquake advisory and completion of the investigation. Actions depend on the category and local plan. If a tsunami warning follows an actual earthquake, evacuation from the tsunami takes priority.
- 「調査中」 — an observed phenomenon is being investigated,
- 「巨大地震警戒」 — Megathrust Earthquake Alert; follow official instructions,
- 「巨大地震注意」 — Megathrust Earthquake Advisory; review preparedness,
- 「調査終了」 — investigation completed; long-term hazard has not disappeared.
What should you do instead of following sensational predictions?
Prepare before any notice: check tsunami hazard maps and evacuation routes, secure furniture, plan family contact and keep water, food, medicine and power supplies. No special information does not mean no risk—a major earthquake can occur without a detectable precursor.
JISHINDESU displays events that have already been reported and can monitor selected places. It does not predict Nankai, and its web push notifications are not Japan’s Earthquake Early Warning. In danger, rely first on JMA, local authorities and official warning apps.