April 21, 2026
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 20 at 16:52, an earthquake with a magnitude (M) 7.7 (provisional) occurred offshore of Sanriku at an approximate depth of 20 km. The earthquake registered maximum seismic intensity 5 Upper in Aomori Prefecture. This event resulted in damage and injuries. Additionally, it generated long-period ground motion of class 3 in the southern inland of Akita Prefecture and northern Miyagi Prefecture.
The earthquake triggered a tsunami along the Pacific coast, from Hokkaido to the Tohoku region. The tsunami waves reached a height of 0.8 meters at the Kuji Port observation point in Iwate Prefecture and 0.4 meters at the Urakawa observation point in Hokkaido (both preliminary results).
* The focal mechanism involved reverse faulting, with the pressure axis oriented in the WNW-ESE direction. The focal mechanism and hypocenter depth suggest that the earthquake occurred at the boundary between the continental and Pacific Plates.
* GNSS observations revealed that the earthquake was accompanied by crustal deformation, predominantly in Iwate Prefecture. At the S-Fudai observation point located in Fudai Village, Iwate Prefecture, an eastward shift of approximately 8 cm was recorded.
* As of 15:00 on April 21, seismic activity persists, with 12 events registering maximum seismic intensity 1 or greater. The seismic activity zone extends approximately 100 km in a NW-SE direction. GNSS observations and analysis using far-field seismic waves suggest that the estimated source fault of the M7.7 earthquake extends near the rupture initiation point.
To the southeast of the seismic activity zone lies the seismic activity zone of the earthquake offshore of Sanriku (M6.9) on November 9, 2025. To the north lies the seismic activity of the earthquake offshore east of Aomori Prefecture (M7.5) on December 8, 2025.
Since the M5.3 earthquake offshore of Sanriku on November 4, 2025, tremor activity has continued intermittently around the focal area. An increase in aseismic slip, as estimated from repeated earthquakes, has also been observed.
* This seismic activity occurred near the southern edge of the focal area of the "1968 Tokachi-oki Earthquake" (M7.9) and in a region adjacent to the southern side of the focal area of the "1994 Sanriku-haruka-oki Earthquake" (M7.6). Furthermore, the focal area of the "1994 Sanriku-haruka-oki Earthquake" and the seismic activity zone of the earthquake offshore east of Aomori Prefecture (M7.5) on December 8, 2025, are located in the central and northern parts of the focal area of the "1968 Tokachi-oki Earthquake," respectively.
* This earthquake occurred in the area anticipated by the Earthquake Research Committee as likely to experience "slightly smaller interplate earthquakes" (approximately M7.0 to M7.5) in its "Long-Term Evaluation of Seismic Activity Along the Japan Trench" (published February 26, 2019) for "offshore east of Aomori Prefecture and offshore north of Iwate Prefecture regions," and "offshore south of Iwate Prefecture." The probability of such an earthquake occurring within 30 years is classified as Rank III (high probability) (#), representing the highest probability among subduction-zone earthquakes. Note that the "1994 Sanriku-haruka-oki Earthquake," which occurred in the same area, is categorized as a "slightly smaller interplate earthquake," whereas the "1968 Tokachi-oki Earthquake," also in the same area, is considered as a "large interplate earthquake" (approximately M7.9). Both have a 30-year occurrence probability classified as Rank III (high).
* Around this seismic activity, there have been past instances where earthquakes of similar magnitudes have successively occurred within approximately 1 week after a strong earthquake. For example, after the M6.9 earthquake offshore of Sanriku on February 17, 2015, an M6.5 and an M6.4 earthquake occurred approximately 3 and 5 days later, respectively. Therefore, it is advisable to exercise caution for approximately 1 week after a strong earthquake, especially those with maximum seismic intensity 5 Upper. Notably, earthquakes causing even stronger shaking often occur within 2 to 3 days after the initial shock. For instance, the mainshock of the "1994 Sanriku-haruka-oki Earthquake" occurred on December 28, and its largest aftershock (M7.2) occurred on January 7 of the following year.
* Following this earthquake, the Japan Meteorological Agency issued an "Off the Coast of Hokkaido and Sanriku Subsequent Earthquake Advisory" for the region stretching from offshore of Nemuro in Hokkaido to offshore of Sanriku in Iwate Prefecture, owing to a higher likelihood of a mega-quake in the region than usual. The Cabinet Office has also urged people to take appropriate disaster prevention measures. Residents in areas susceptible to strong shaking and large tsunamis should follow the guidance provided by the central government and local authorities.
#: The probability of a subduction-zone earthquake occurring within the next 30 years is ranked as follows: Rank III: greater than 26%, Rank II: 3% ~ 26%, Rank I: less than 3%, Rank X: unknown (an immediate earthquake cannot be ruled out).
Note: GNSS refers to a generic term for satellite positioning systems, including GPS.