September 9, 2026
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On August 1, an earthquake with a magnitude (M) 4.7 occurred in Kumamoto region, Kumamoto Prefecture. The earthquake recorded a maximum seismic intensity 5 Lower in Kumamoto Prefecture.
* On August 23, an M5.9 earthquake occurred in southern Ibaraki Prefecture. The earthquake recorded a maximum seismic intensity 5 Lower in Ibaraki Prefecture, Saitama Prefecture, Chiba Prefecture, and Tokyo Metropolis, causing damage and injuries. Furthermore, long-period ground motion of class 2 was recorded in Saitama Prefecture, Chiba Prefecture, and Tokyo Metropolis.
* On August 2, an M4.9 earthquake occurred in the eastern Hidaka region at a depth of approximately 50 km. The focal mechanism indicated a reverse fault type with a pressure axis oriented in an NW-SE direction. It occurred at the boundary between the Pacific and continental plates.
* On August 23, an M6.0 earthquake occurred offshore of Urakawa at a depth of approximately 40 km. The focal mechanism indicated a reverse fault type with a pressure axis oriented in a NW-SE direction. It occurred at the boundary between the Pacific and continental plates.
* On August 1, an M5.7 earthquake occurred offshore east of Aomori Prefecture at a depth of approximately 75 km. The focal mechanism indicated a strike-slip fault type with a pressure axis oriented in an E-W direction. It occurred within the Pacific Plate.
* On August 9, an M5.6 earthquake occurred offshore of Iwate Prefecture at a depth of approximately 40 km. The focal mechanism indicated a reverse fault type with a pressure axis oriented in WNW-ESE direction. It occurred at the boundary between the Pacific and continental plates.
Following the M7.2 earthquake on June 25, continuous earthquake activity has been recorded near the epicenter.
To the southeast of this seismic activity zone lies the seismic activity of the earthquakes offshore of Sanriku on November 9, 2025 (M6.9) and April 20, 2026 (M7.7). In this series of seismic activity zones, 17 earthquakes of seismic intensity 1 or greater were recorded in August. To the east of these seismic activity zones, tremor activity has continued intermittently since the earthquake offshore of Sanriku on November 4, 2025 (M5.3).
GNSS observations indicate that approximately 4 months after the earthquake offshore of Sanriku on April 20 (M7.7), horizontal displacements, due to post-seismic deformations, were primarily recorded in Iwate Prefecture. For instance, Kuji station, a deformation of around 7cm in the east-southeast direction was recorded. The afterslip, inferred from these post-seismic deformations, is believed to continue in the seismic activity zone of the earthquake offshore of Sanriku on April 20 (M7.7), and to the west and east of this seismic activity zone. Furthermore, the analysis of repeating earthquakes indicates the ongoing occurrence of aseismic slip events in these regions.
Since 25 June, seismic activity has continued near the epicenter of the largest aftershock (M7.2 on January 7, 1995) on the western edge of the "1994 Sanriku-haruka-oki Earthquake" (M7.6) source region, including this earthquake. Moreover, this seismic activity, along with the series of seismic activity zones from offshore of Sanriku to offshore of Iwate Prefecture since November 2025, 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 respectively in the southern, central, and northern parts of the focal area of the "1968 Tokachi-oki Earthquake" (M7.9).
The M7.2 earthquake on June 25 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)(#1). This represents 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" of "offshore east of Aomori Prefecture and offshore north of Iwate Prefecture regions", 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).
* On August 21, an M5.4 earthquake occurred offshore east of Hachijojima Island at a depth of approximately 65 km. The focal mechanism indicated pressure axis oriented in an E-W direction. It occurred within the Pacific Plate.
* On August 23, an M5.9 earthquake occurred in southern Ibaraki Prefecture at a depth of approximately 70 km. The focal mechanism indicated a reverse fault type with a pressure axis oriented in WNW-ESE direction. It occurred at the boundary between the Pacific Plate and Philippine Sea Plate.
GNSS observation data indicates that there has been no significant crustal deformation associated with this event.
* On August 30, an M4.8 earthquake occurred offshore east of Chiba Prefecture at a depth of approximately 35 km. The focal mechanism indicated a tension axis oriented in an E-W direction.
There was no remarkable activity.
* Following the M7.1 earthquake that occurred in Kumamoto region, Kumamoto Prefecture on July 28 at 16:27, the number of earthquakes has remained higher than before the quake, though overall it has been gradually decreasing. From 16:00 on July 28 through August 31, a total of 735 earthquakes with a maximum seismic intensity 1 or greater were recorded (1 event with seismic intensity 7; 5 events with seismic intensity 5 Lower; 23 events with seismic intensity 4; 71 events with seismic intensity 3). The earthquake with the highest recorded maximum seismic intensity in August was an M4.7 earthquake on August 1 at 21:47 (depth: approximately 10 km; maximum seismic intensity 5 Lower).
This seismic activity occurred near the Futagawa and Hinaku Fault Zones.
GNSS observations indicate that following the M7.1 earthquake on July 28, crustal deformations believed to be post-seismic deformations have been recorded in Kumamoto Prefecture and its surrounding areas, primarily in the southwestern part of the Hinagu segment, Hinagu Fault Zone.
The Earthquake Research Committee assessed that, when active, the Takano-Shirahata and Hinagu segments of the Hinagu Fault Zone have the potential to generate earthquakes with magnitudes of approximately M6.8 and M7.5, respectively. The probability of such earthquakes occurring within 30 years was classified as Rank X (unknown) and Rank S* (high), respectively (#2). Furthermore, the probability of an M6.8 or greater earthquake occurring in the southern Kyushu area, including the Hinagu Fault Zone, was estimated to be between 7% and 18%.
Due to possible earthquakes with a maximum seismic intensity 5 Lower over next two weeks, vigilance is required. Furthermore, the likelihood of earthquakes with a maximum seismic intensity 5 Lower or greater remains higher compare to the background level of activity. As this seismic activity zone extends over a wide area and depending on the location and magnitude of subsequent earthquakes, shaking could be stronger than that caused by this one (M7.1). Moreover, if a large earthquake occurs on the seafloor, caution is required regarding tsunamis.
* On August 2, an M5.3 earthquake occurred in the Hyuganada Sea at a depth of approximately 20 km. The focal mechanism indicated a normal fault type with a tension axis oriented in an E-W direction. It occurred within the Philippine Sea Plate.
* On August 17, an M4.2 earthquake occurred in Fukuoka region, Fukuoka Prefecture at a depth of approximately 5 km. This earthquake occurred within the Earth's crust.
There have been no abnormal changes that would suggest that the likelihood of a great earthquake occurring along the Nankai Trough has increased compared to the normal background level of activity.
#1: 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 imminent earthquake cannot be ruled out).
#2: Active faults are classified as "S-rank" if the probability of an earthquake occurring within the next 30 years is 3% or higher, "A-rank" if the probability is between 0.1% and less than 3%, "Z-rank" if the probability is less than 0.1%, and "X-rank" if the probability is unknown (meaning that the evaluation is difficult due to insufficient historical earthquake data). An asterisk (*) is appended to the rank for active faults with an elapsed-time rate#3 of 0.7 or greater. Even for Z-rank, the simple existence of the active fault still indicates a possibility that a major earthquake could occur in the area.
#3: A value obtained by dividing the elapsed time from the most recent
seis
mic event by the average recurrence interval at the time of evaluation.
This value is 1.0 when the elapsed time reaches the average recurrence
interval.
Note: GNSS refers to a generic term for satellite positioning systems, including GPS.
September 9, 2026
Earthquake Research Committee
The numbers of earthquakes by magnitude (M) in and around Japan in August 2026 are as follows.
(Reference)
The following events for the period from August 2025 until the end of July 2026 have been listed as "Major Seismic Activity" in the monthly "Evaluation of Seismic Activity."
| – In the ocean near Tokara Islands | M4.7 on September 17, 2025 | ||
| – Offshore southeast of the Nemuro Peninsula | M5.8 on October 25, 2025 (Depth of approximately 40 km) |
||
| – Offshore of Sanriku | M6.9 on November 9, 2025 (Depth of approximately 15 km) | ||
| – In Aso region, Kumamoto Prefecture | M5.8 on November 25, 2025 (Depth of approximately 10 km) | ||
| – Offshore east of Aomori Prefecture | M7.5 on December 8, 2025 (Depth of approximately 55 km) | ||
| – In eastern Shimane Prefecture | M6.4 on January 6, 2026 (Depth of approximately 10 km) | ||
| – In southern Ibaraki Prefecture | M5.0 on April 1, 2026 (Depth of approximately 50 km) | ||
| – In northern Nagano Prefecture | M5.0 on April 18, 2026 (Depth of approximately 10 km) | ||
| – In northern Nagano Prefecture | M5.1 on April 18, 2026 (Depth of approximately 10 km) | ||
| – Offshore of Sanriku | M7.7 on April 20, 2026 (Depth of approximately 20 km) | ||
| – In the southern Tokachi region | M6.2 on April 27, 2026 (Depth of approximately 85 km) | ||
| – Offshore of Miyagi Prefecture | M6.4 on May 15, 2026 (Depth of approximately 45 km) | ||
| – In the ocean near Okinawa Main Island | M5.9 on May 20, 2026 (Depth of approximately 50 km) | ||
| – Offshore of Miyagi Prefecture | M6.4 on May 15, 2026 (Depth of approximately 45 km) | ||
| – In the ocean near Okinawa main island | M5.9 on May 20, 2026 (Depth of approximately 50 km) | ||
| – In Mindanao, the Philippines | Mw7.7 on June 8, 2026 | ||
| – In southern Ibaraki Prefecture | M5.5 on June 16, 2026 (Depth of approximately 50 km) | ||
| – Offshore of Iwate Prefecture | M7.2 on June 25, 2026 (Depth of approximately 45 km | ||
| – In eastern Yamanashi Prefecture and the Fuji Five Lakes | M5.6 on June 26, 2026 (Depth of approximately 20 km) | ||
| – The 2026 Kumamoto Earthquake | M7.1 on July 28, 2026 (Depth: approximately 15 km) | ||
There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
– GNSS observations suggest that crustal deformation in the area stretching from western Shizuoka to eastern Aichi Prefectures differs from the normal trend, a phenomenon that has been observed since early 2022. It is believed that this change is the result of long-term slow slip occurring deep within the boundary between the Philippine Sea Plate and the continental plate, extending from the Atsumi Peninsula to Lake Hamana. The slip center is gradually shifting from the Atsumi Peninsula towards the eastern side of Lake Hamana.
– Since February 2025, there have been continuous clusters of tremors occurring at depths of approximately 25 to 30 km in the lower crust of northern Yamaguchi Prefecture. These series of tremors have been accompanied by slight crustal deformations. Furthermore, since around mid-2025, there have been continuous clusters of tremors occurring at depths of approximately 5 to 20 km within the crust of northern Yamaguchi Prefecture.
– From July 29 to August 11, deep low-frequency earthquakes (tremors) were recorded near the boundary between the Philippine Sea Plate and the continental plate in western Shikoku. Strain and tilt data suggest that the slight crustal deformations recorded in the area were almost synchronized with the deep low-frequency earthquakes (tremors). It is believed that this phenomenon resulted from a short-term slow slip event occurring deep within the boundary between the two plates.
There is no supplementary information for the Kyushu-Okinawa region.
– "There have been no abnormal changes that would suggest that the likelihood of a great earthquake occurring along the Nankai Trough has increased compared to the normal background level of activity.":
(This evaluation corroborates with the views presented at the regular meeting
of the Nankai Trough Earthquake Assessment Committee on September 7 (reference
below).)
(Reference)
Recent Crustal Activity Around the Nankai Trough (Seismological and Volcanological Department, Japan Meteorological Agency, September 7, 2026)
"Currently, there have been no abnormal changes that would suggest that the likelihood of a great earthquake occurring along the Nankai Trough has increased compared to the normal background level of activity. (Excerpted)"
Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.
Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes: