JMA issues advisory of elevated megathrust earthquake risk along Japan Trench after December 8 M7.6 Sanriku quake

JMA issues advisory of elevated megathrust earthquake risk along Japan Trench after December 8 M7.6 Sanriku quake

Rishav Kothari

Following the M7.6 Sanriku offshore earthquake on December 8, 2025, the Japan Meteorological Agency (JMA) warned that the likelihood of a new large-scale earthquake of Mw8 or higher occurring along the Japan Trench–Kuril Trench has increased relative to normal. While the probability remains low, JMA urged residents of Hokkaido and Tohoku’s Pacific coast to review disaster preparedness and evacuation plans.

Around 90 000 people were evacuated, and between 30 and 35 were injured in northeastern Japan on December 8, after major M7.6 earthquake struck near the coast of Hokkaido.

The earthquake, recorded at 23:15 JST (14:15 UTC), generated small tsunami waves along the Sanriku coast and caused temporary power outages, minor fires, and light structural damage in parts of Aomori, Iwate, and Hokkaido prefectures

At 02:00 JST on December 9, the Japan Meteorological Agency issued a statement assessing the likelihood of a new large-scale earthquake in northern Japan as relatively higher than normal.

According to the agency, the December 8 earthquake occurred within a seismically active zone influencing the hypothetical source area of a megathrust earthquake along the Japan Trench and Kuril Trench system, which extends from offshore Nemuro in Hokkaido to offshore Sanriku in Tohoku.

The earthquake met the criteria outlined in the national Basic Plan for Promoting Disaster Prevention Measures for Trench-Type Earthquakes Around the Japan Trench and the Kuril Trench, which governs the issuance of information regarding the likelihood of subsequent earthquakes.

While the probability remains low in absolute terms, JMA says that the current conditions statistically correspond to a period of temporarily elevated risk within the broader Japan Trench–Kuril Trench region.

If a major Mw8+ earthquake were to occur along this trench system, it could produce a large tsunami affecting the Pacific coast from Hokkaido to Chiba Prefecture, with strong shaking across wide areas.

The Japan Trench–Kuril Trench forms part of the boundary where the Pacific Plate subducts beneath the Okhotsk microplate. It is one of the world’s most seismically active subduction systems, responsible for multiple Mw8–9 earthquakes over the past centuries.

Monitoring of this region is a national priority under Japan’s trench-type earthquake countermeasures program, combining GPS crustal deformation data, ocean-bottom pressure sensors, and long-term geophysical observation networks.