December 9, 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.
The earthquake 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 revealed that crustal deformation occurred after the earthquake, predominantly in Aomori Prefecture. At the Higashidori 2 observation point in Higashidori Village, Shimokita County, Aomori Prefecture, an eastward shift of approximately 9 cm was recorded.
* By 14:00 on December 9, seismic activity persisted, with 15 events recording maximum seismic intensity 1 or greater. From the occurrence of the M7.5 event until 14:00 on December 9, the largest earthquake was the M6.4 (preliminary value) event at 06:52 on December 9.
This seismic activity is taking place in the northern part of the epicenter area of the 1968 Tokachi-oki Earthquake (M7.9), whereas the southern part of the focal area is adjacent to the epicenter area of the 1994 Sanriku-haruka-oki Earthquake (M7.6). Additionally, seismic activity from the earthquake offshore of Sanriku on November 9th (M6.9) 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." 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 a week after a strong earthquake. For example, after the 1943 earthquake offshore east of Aomori Prefecture (M7.1), an M6.6 earthquake occurred approximately two days later. Thus, it is advisable to exercise caution for approximately a week after a strong earthquake, especially those with maximum seismic intensity 6 Upper. Furthermore, there is a possibility of earthquakes causing even stronger shaking. Notably, earthquakes that cause strong shaking often occur within two to three days after the initial shock.
* The Japan Meteorological Agency has 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, suggesting a higher likelihood of a mega-quake in this area than usual following this earthquake. 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 is a general name of satellite positioning system such as GPS.