May 14, 2026
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 20 at 16:52, a magnitude (M) 7.7 earthquake occurred offshore
of Sanriku at a depth of about 20 km. The earthquake registered a 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 79 cm at the Kuji Port observation point (Port and Harbor Bureau) in Iwate Prefecture and 39 cm at the Urakawa observation point (Port and Harbor Bureau) in Hokkaido (both provisional 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. For example, at the S-Fudai observation point located in Fudai Village, Iwate Prefecture, an eastward shift of approximately 9 cm and a subsidence of approximately 1 cm were recorded. In addition, approximately 10 days after the M7.7 earthquake, horizontal displacements believed to be post-seismic deformations were recorded, primarily in Iwate Prefecture. At the Taro-A observation point in Miyako City, Iwate Prefecture, approximately 2 cm of eastward deformation was recorded.
* Following this event, the seismic activity surged around the focal area.
This included an M5.6 event at 21:56 on the same day, an M5.3 event on
April 22, and an M5.1 event on April 27. Although the number of earthquakes
has declined, seismic activity continues. Between April 20 and 30, a total
of 38 events recording maximum seismic intensity 1 or greater were recorded.
The seismic activity zone extends approximately 150 km in the NW-SE direction.
GNSS observations and analyses of teleseismic waves suggest that the estimated
source fault of the M7.7 earthquake extended close to the rupture initiation
point.
To the southeast of the seismic activity zone lies the seismic activity 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 occurrence of an earthquake offshore of Sanriku on November 4, 2025 (M5.3), tremor activity has continued intermittently around the focal area. Following the M7.7 event, tremor activity surged to the east of the M7.7 epicenter. An increase in the aseismic slip events estimated from repeated earthquakes has also been observed in the focal area.
* 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.
* Intense seismic activity occurred in 1989, 1992, 2012, and 2015 in the area, including the seismic activity zone and the seismic activity zone that started in November 2025. Although the specific zones of activity differed somewhat, each episode included many earthquakes with M6 or greater. Furthermore, an analysis of GNSS observations indicated that the source fault of the current earthquake was very similar to that of the M7.4 earthquake that occurred on March 11, 2011.
* 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).
#: The probability of a subduction-zone earthquake occurring within the next 30 years is 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.