Earthquake Offshore of Sanriku on April 20
On April 20, 2026, at 16:52, an M7.7 earthquake occurred offshore of Sanriku at a depth of 19 km. It registered a seismic intensity 5 Upper in Hashikami Town, Aomori Prefecture, and seismic intensities between 5 Lower and 1 were recorded from Hokkaido to the Kinki region. Furthermore, a long-period ground motion class 3 was registered in the southern inland region of Akita Prefecture and northern Miyagi Prefecture, whereas long-period ground motion between class 2 and 1 was recorded from Hokkaido to the Chubu region, including Niigata Prefecture. The earthquake triggered a tsunami that primarily affected the Pacific coast from Hokkaido to the Tohoku region. Tsunami heights of 0.8 m(Note 2) were recorded at Kuji Port in Iwate Prefecture(Note 1), and 0.4 m(Note 2) at Urakawa in Hokkaido(Note 1).
At 16:53:23.3, just 19.8 seconds after detecting the first seismic waves, the Japan Meteorological Agency issued an Earthquake Early Warning (Warnings). Later, at 16:55 on the same day, it issued a Tsunami Warning for the central Pacific coast of Hokkaido and Iwate Prefectures. At 17:08, the advisory for the Pacific coast of Aomori Prefecture was upgraded to a Tsunami Warning. At 20:15, the Tsunami Warning was downgraded to an advisory, and at 23:45, the Tsunami Advisory was lifted.
At 19:30 on April 20, the Japan Meteorological Agency issued an "Off the Coast of Hokkaido and Sanriku Subsequent Earthquake Advisory." This decision was based on a detailed analysis of the earthquake's epicenter and magnitude, which met the criteria for issuing such advisory information urging caution for subsequent earthquakes, as outlined in the national basic plan and regulations.
The focal mechanism (CMT solution) involved reverse faulting, with the pressure axis oriented in the WNW-ESE direction, and occurred at the boundary between the continental and Pacific Plates.
From November 4, 2025, a cluster of seismic activity was detected near the epicenter (Area b). This includes an M6.9 event (maximum seismic intensity 4) on November 9, 2025, and an M6.7 event (maximum seismic intensity 4) on March 26, 2026. In Area b, from November 4, 2025, until the occurrence of this earthquake, 66 earthquakes with seismic intensity 1 or greater were recorded (2 events with seismic intensity 4, 12 events with seismic intensity 3, 24 events with seismic intensity 2, and 28 events with seismic intensity 1)(Note 3).
The quake caused damage, including 6 injuries (as of 8:00 on April 21, 2026, according to the Fire and Disaster Management Agency, Ministry of Internal Affairs and Communications).
A review of seismic activity since September 2020 reveals that earthquakes of M5.0 or greater have often occurred near the epicenter (Area c).
North of the epicenter is the location of the "1994 Sanriku-haruka-oki Earthquake" (M7.6, maximum seismic intensity 6) that occurred on December 28, 1994. This earthquake resulted in 3 deaths, 788 injuries, and the destruction of 72 homes (data sourced from the "Materials for Comprehensive List of Destructive Earthquakes in Japan").
(Note 1) Observation facilities of the Ports and Harbours Bureau, the Ministry of Land, Infrastructure, Transport, and Tourism.
(Note 2) Observation values may change after further review.
(Note 3) The number of earthquakes with seismic intensity 1 or higher may change based on future investigations.
Japan Meteorological Agency
[Evaluation of Earthquake Offshore of Sanriku on April 20, 2026 (April 21, 2026)]