December 25, 2025
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On December 8, at 11:15 PM, an earthquake with magnitude (M) 7.5 occurred
offshore east of Aomori Prefecture at a depth of approximately 55 km. The
earthquake registered maximum seismic intensity 6 Upper and caused damage
and injuries in Aomori Prefecture. Furthermore, it generated long-period
ground motion of class 3 in Sanpachi-Kamikita, Aomori Prefecture.
It triggered a tsunami along the Pacific coast, from Hokkaido to Fukushima Prefecture. The tsunami reached a height of 0.7 meters at the Kuji Port observation point in Iwate Prefecture and 0.5 meters at the Urakawa observation point in Hokkaido (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 indicated that this event caused crustal deformation, primarily in Aomori Prefecture. At the Higashidori 2 observation point in Higashidori Village, Shimokita County, Aomori Prefecture, an eastward shift of about 9 cm and subsidence of approximately 2 cm was recorded. Additionally, Synthetic Aperture Radar interferometric analysis from "Daichi 2" and "Daichi 4" also detected crustal deformation in eastern Aomori Prefecture, similar to that observed by GNSS measurements. Furthermore, according to GNSS observations, within approximately 10 days following the M7.5 quake, horizontal movement thought to be induced by postseismic activity was observed mainly in eastern Aomori Prefecture, such as at the Higashidori 2 observation point, where an eastward shift of approximately 1 cm was recorded.
* On December 12, seismic activity temporarily increased following an M6.9
earthquake that occurred approximately 50 km east of the M7.5 event's epicenter.
Although the number of earthquakes has decreased compared to immediately
after the M7.5 quake, seismic activity continues, with 47 events with maximum
seismic intensity 1 or greater recorded by December 22. The seismic activity
area expanded approximately 100 km eastward from the M7.5 event's epicenter.
Additionally, since the M7.5 event, shallow very-low-frequency earthquakes
and tremors have been occurring in the area east of the seismic activity
zone.
The M6.9 earthquake on December 12 occurred within an area where earthquakes of similar magnitudes are likely to occur successively (secondary active zone). Historical examples show that when a large earthquake occurs within such a zone, there is a probability of subsequent earthquakes of similar or greater magnitudes occurring.
The activity zone of the M7.5 earthquake is located in the northern part
of the focal area of the "1968 Tokachi-oki Earthquake" (M7.9),
whereas its southern part is adjacent to that of the 1994 Sanriku-haruka-oki
Earthquake (M7.6). Additionally, seismic activity from the November 9th
(M6.9) earthquake offshore of Sanriku was recorded in the southeast.
* 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." 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). Furthermore, to the southeast of its activity zone lies the epicenter area of the 1896 Meiji Sanriku Earthquake (tsunami magnitude 8.6-9.0). This event is classified as a "trench-adjacent plate boundary earthquake (tsunami earthquake, etc.)" that occurred in the "trench-adjacent zone extending from the offshore east of Aomori Prefecture to the offshore of Boso Peninsula." 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.
#: 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.