Japan's megaquake advisories explained: Nankai Trough Extra Information and the Hokkaido-Sanriku advisory

In August 2024, Japan issued its first "Nankai Trough Earthquake Extra Information (Megathrust Earthquake Attention)," and many residents and visitors were unsure what it meant. Japan has two systems of this kind: one for the Nankai Trough off the Pacific coast of western and central Japan, and one for the Japan and Kuril trenches off Hokkaido and northeastern Japan. Neither is an earthquake prediction. Both tell people that, after a large earthquake, the chance of an even larger one has risen above normal for a while, and ask them to get ready. This article explains both, using the Japan Meteorological Agency (JMA) and Japan's Cabinet Office (disaster management).

Nankai Trough Earthquake Extra Information

JMA issues this information with one of four keywords. The English keywords below are from the Cabinet Office's multilingual leaflet (August 2025); the conditions and actions are from JMA.

• How much more likely? JMA says that, in worldwide data, an earthquake of Mw 8 class or larger follows within seven days in roughly 1 in 100 cases after an Mw 7.0 or larger quake, and roughly 1 in 10 cases after an Mw 8.0 or larger quake.
• No warning is guaranteed. JMA states that current science cannot predict the time, place and size of a Nankai Trough earthquake with high confidence, and that one may strike suddenly without this information ever being issued. As JMA's English page puts it, information indicating elevated potential "does not necessarily mean that one will actually strike."
• Why one week? JMA explains that the one-week period reflects how long people can realistically keep up special measures; the risk does not end when the week does. JMA does not issue an "all clear."

The first one: August 8, 2024

At about 4:43 p.m. on August 8, 2024, a magnitude 7.1 earthquake struck off Hyuganada (off Miyazaki, Kyushu). JMA assessed it as Mw 7.0 and, at 7:15 p.m., issued the first-ever Megathrust Earthquake Attention. The government's call for special caution ended one week after the earthquake, at 5 p.m. on August 15. In January 2025, another Hyuganada earthquake triggered "Under Analysis," but it was Mw 6.7, below the threshold, so JMA issued "Analysis Complete" the same evening.

After that first experience, the Cabinet Office revised its response guidelines in August 2025. They note that during a Megathrust Earthquake Attention, schools and public transport are expected to run normally and daily life to continue much as usual, and they ask people not to stockpile food and daily necessities beyond what they need.

Hokkaido and Sanriku Subsequent Earthquake Advisory

For the Japan and Kuril trenches, JMA issues this advisory when an earthquake of Mw 7.0 or more occurs where it could affect the expected source area of a megaquake stretching from off Nemuro in Hokkaido to off Sanriku in northeastern Honshu. It started on December 16, 2022. The areas asked to take action are on the Pacific side from Hokkaido to Chiba.

• When it is issued, the government asks people in those areas to take "special preparedness" measures and re-check their everyday preparations for one week after the earthquake.
• JMA notes the probability is low, about 1 in 100 based on worldwide cases, but also that in 2011 an Mw 7.3 earthquake off Sanriku was followed two days later by the Mw 9.0 Tohoku earthquake.
• It has been issued twice: on December 9, 2025, after an M7.5 earthquake east off Aomori, and at 7:30 p.m. on April 20, 2026, after an M7.7 earthquake off Sanriku.

What "special preparedness" looks like (our suggestions)

• Check that heavy furniture in your bedroom is secured, and keep shoes and a flashlight by your bed.
• Confirm your evacuation site and how your family will contact each other.
• If you are near the coast, know your tsunami evacuation route and leave immediately after strong or long shaking or a tsunami warning.
• Carry a small pouch with water, a light, a phone charger and a portable toilet when you go out.
• Don't panic-buy. Keep a rolling stock of food and water instead (see Rolling stock).

Related: Earthquakes in Japan, Japan's tsunami warnings explained. Our Japanese edition has the same guide for residents: 南海トラフ地震臨時情報と北海道・三陸沖後発地震注意情報.