Nankai Trough Earthquake Extra Information: what "warning", "advisory" and "under investigation" mean, and what to do

The short answer

The Nankai Trough Earthquake Extra Information is issued by the Japan Meteorological Agency (JMA) when something unusual is observed along the Nankai Trough. It does not predict a megaquake. It tells you that the chance of one is relatively higher than usual.

When it is issued, review your everyday preparations, such as furniture fixing and evacuation routes, and spend about a week ready to leave at once if you feel shaking or a tsunami warning is issued. Work and school basically carry on. Only people living in a "pre-evacuation area" set by their municipality are asked to evacuate in advance for a week, and only under a "megaquake warning".

The four keywords

A keyword is added to the name of the information. Here is a summary of JMA's explanation.

Mw (moment magnitude) is a value calculated by examining how far the fault slipped, and it can differ from the magnitude announced right after a quake. A "warning" or "advisory" comes out about two hours after the event at the earliest. While the status is "under investigation", it is enough to wait for updates on TV, radio and from your local government.

What to do at home and at work

The government’s disaster information pages divide the response into three parts.

1. Review everyday preparations: shelters and evacuation routes, how to contact family, furniture fixing, and stocks of emergency food
2. Special preparations: staying ready to leave at once and carrying an emergency bag at all times, among other things
3. Advance evacuation: residents of areas where a tsunami would arrive so fast that leaving after the shaking may be too late evacuate on their municipality's instructions

Under a "megaquake warning", people in pre-evacuation areas evacuate for a week, then keep up special preparations for a further week. People elsewhere in the designated areas continue special preparations and reviews for two weeks while carrying on with work and daily life. Under a "megaquake advisory", the main response is a week of special preparations and reviews.

At home, being ready to leave might mean keeping shoes and a torch by your bed and putting the emergency bag by the front door. At work, check the evacuation guidance procedure and how you will tell everyone in the building.

The first issuance on 8 August 2024

At about 16:43 on 8 August 2024, a magnitude 7.1 earthquake struck off Hyuganada, with intensity 6-lower observed in Nichinan, Miyazaki Prefecture. JMA issued "under investigation" at 17:00 and, after the evaluation committee met, a "megaquake advisory" at 19:15. It was the first issuance since the system began in May 2019.

The government asked people to review their everyday preparations and to be ready to evacuate as soon as a quake struck. It did not ask for advance evacuation. This special call for caution ended at 17:00 on 15 August, one week after the earthquake. The Cabinet Office's public relations magazine later noted some confusion and misunderstanding, and explained again that the information does not say a quake will happen within a set period.

How it differs from the Hokkaido/Sanriku subsequent earthquake advisory

Along the Japan Trench and the Kuril Trench there is a similar system, the "Hokkaido/Sanriku subsequent earthquake advisory". It began in December 2022 and is issued after a quake of Mw7.0 or larger in a place that can affect the source area. It was issued for the first time at 2:00 on 9 December 2025, after the quake off eastern Aomori on 8 December (M7.5, maximum intensity 6-upper), and again after the quake off Sanriku on 20 April 2026.

The difference is that it comes in one form only, an advisory, with no stage like the Nankai "megaquake warning" that assumes advance evacuation. It covers municipalities mainly along the Pacific side from Hokkaido to Chiba, and what people are asked to do is much the same as under a Nankai "megaquake advisory": a week of reviewing preparations and staying ready to leave. The December 2025 materials say that, worldwide, a quake of Mw7.8 or larger followed a quake of Mw7.0 or larger within seven days in about one case in a hundred.

Things to keep in mind

JMA notes that a Nankai Trough earthquake may also strike suddenly, without any extra information beforehand. Everyday preparation is the foundation. The government never issues predictions naming a date for an earthquake, so do not share posts claiming "a big quake on such-and-such a day". People are also asked not to stockpile food and other goods beyond what they need. To make sure your phone receives earthquake early warnings, see our article on the earthquake early warning drill.