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.
U.S. Navy personnel unload food supplies donated by residents of Ebina City at the Port of Sendai, Japan, in March 2011. The 1 500 lb (680 kg) shipment was delivered as part of humanitarian relief operations following the M9.0 Tōhoku earthquake and tsunami. Credit: U.S. Navy
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.
The evaluation was based on both the magnitude and the location of the rupture zone, which overlapped with regions capable of generating a trench-type megathrust event.
JMA cited global statistical data covering 1 477 earthquakes of Mw7.0 or higher between 1904 and 2017. Within this dataset, only 17 earthquakes of Mw8.0 or greater occurred within 500 km (310 miles) of a previous Mw7-class event within seven days, representing a frequency of about one in one hundred.
This includes the sequence in March 2011, when an Mw7.3 earthquake off the coast of Sanriku preceded the Mw9.0 2011 Tohoku Earthquake by two days, and the 1963 Etorofu sequence, where an Mw8.5 earthquake followed an Mw7.0 event 18 hours later.
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.
Past geological records compiled by the Central Disaster Prevention Council show that the largest tsunamis along this corridor recur every 300–400 years, and the elapsed time since the last 17th-century event suggests that the region is within an imminent phase of its long-term seismic cycle.
JMA repeated that the advisory does not predict an imminent major earthquake but indicates a relative increase compared with normal background levels.
Residents of Hokkaido, Aomori, Iwate, and Miyagi prefectures, as well as other Pacific coastal areas, are urged to reconfirm evacuation routes and tsunami-safe areas, be prepared to evacuate immediately if strong shaking is felt or a tsunami warning is issued, and follow instructions from national and local disaster management authorities.
The probability of a subsequent earthquake generally decreases with time following a major event and with increasing distance from the original epicenter. Nonetheless, JMA reaffirmed that large-scale trench earthquakes can occur suddenly, often without warning.
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.
References:
1 Hokkaido/Sanriku Offshore Earthquake Warning – JMA – 02:00 JST on December 9, 2025
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