October 9, 2026
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for September 2026

1 Major Seismic Activities

There was no remarkable activity

• Seismic Activity Across Japan in September 2026 (Magnitude 4.0 or Greater)

to supplementary reports

2 Seismic Activities by Region

(1) Hokkaido Region

There was no remarkable activity

• Hokkaido Region

to supplementary reports

(2) Tohoku Region

There was no remarkable activity

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* On September 17, an earthquake with a magnitude (M) 4.9 occurred in southern Ibaraki Prefecture at a depth of approximately 65 km. The focal mechanism indicated a strike-slip fault type with a pressure axis oriented in WNW-ESE direction. It occurred at the boundary within the Pacific Plate.

* On September 29, an earthquake with M4.9 occurred in southern Ibaraki Prefecture at a depth of approximately 45 km. The focal mechanism indicated a reverse fault type with a pressure axis oriented in NW-SE direction. It occurred at the boundary between the Philippine Sea Plate and the continental plate.

• Kanto - Chubu Region
• Earthquake in Southern Ibaraki Prefecture on September 17
• Earthquake in Southern Ibaraki Prefecture on September 29

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

* 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. A total of 79 earthquakes with a maximum seismic intensity 1 or greater were recorded in September (2 events with seismic intensity 4; 4 events with seismic intensity 3; 19 events with seismic intensity 2; 54 events with seismic intensity 1). The earthquakes that recorded a maximum seismic intensity 4 were the M4.0 event on September 16 in Amakusa and Ashikita region, Kumamoto Prefecture, and the M4.6 event on September 26 in Kumamoto region, Kumamoto Prefecture.
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.
Based on this figure, the number of earthquakes is expected to remain higher than the normal background level of activity for the time being.

* On September 20, an earthquake with M5.0 occurred in the ocean near Amami Oshima Island at a depth of approximately 65 km. The focal mechanism indicated a strike-slip fault type with a pressure axis oriented in an NNW-SSE direction. It occurred within the Philippine Sea Plate.

* On September 30, an earthquake with M5.9 occurred in the ocean near Yonagunijima Island. The focal mechanism indicated a pressure axis oriented in the NNW-SSE direction.

• Kyushu Region
• Seismic Activity of the 2026 Kumamoto Earthquake
• Seismic Activity of the 2026 Kumamoto Earthquake (continued)
• Crustal Deformation After the 2026 Kumamoto Earthquake (July 28, M7.1) (provisional)
• Crustal Deformation Before and After the 2026 Kumamoto Earthquake (July 28, M7.1) (provisional)
• Crustal Deformation Before and After the 2026 Kumamoto Earthquake (July 28, M7.1) (provisional) (continued)
• Crustal Deformation Before and After the 2026 Kumamoto Earthquake (July 28, M7.1) (provisional) (continued)
• Crustal Deformation Before and After the 2026 Kumamoto Earthquake (July 28, M7.1) (provisional) (continued)
• Crustal Deformation Before and After the 2026 Kumamoto Earthquake (July 28, M7.1) (provisional) (continued)
• Crustal Deformation Associated with the 2026 Kumamoto Earthquake Based on Dense GNSS Observations (September 2026)
• Crustal Deformation Associated with the 2026 Kumamoto Earthquake Based on Dense GNSS Observations (September 2026) (continued)
• Disaster Prevention Precautions and Future Outlook
• Earthquake in the Ocean Near Amami Oshima Island on September 20
• Okinawa Region
• Earthquake in the Ocean Near Yonagunijima Island on September 30

to supplementary reports

(6) Around the Nankai Trough

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.

• Vertical Movement of GNSS-Based Control Stations - Omaezaki
• Vertical Displacement of GNSS-Based Control Stations - the Kii Peninsula and Around Cape Muroto -

to supplementary reports

(7) Other Regions

* On September 23, an earthquake with M6.0 occurred in the Mariana Islands at a depth of approximately 310 km.

• Earthquake in the Mariana Islands on September 23

Note: GNSS refers to a generic term for satellite positioning systems, including GPS.

Supplementary Information to the Evaluation of Seismic Activities for September 2026

October 9, 2026
Earthquake Research Committee

1 Major Seismic Activities

The numbers of earthquakes by magnitude (M) in and around Japan in September 2026 are as follows.
There were 91 earthquakes with M4.0 or greater (compared to 106 in August) and 9 earthquakes with M5.0 or greater (compared to 13 in August). Furthermore, there was one earthquake with M6.0 or greater (compared to one in August).

(Reference)

• M4.0 or over: 81 / month (69-104)
• (Median monthly number from 1998 to 2017, half of the numbers are included in the interval shown in parentheses)
• M5.0 or greater: 10 / month (7-14)
• (Median monthly number from 1973 to 2017, half of the numbers are included in the interval shown in parentheses)
• M6.0 or greater: 1 / month (0-2)
• (Median monthly number from 1919 to 2017, half of the numbers are included in the interval shown in parentheses)
• M6.0 or greater: 16 / year (12-21)
• (Median annual number from 1919 to 2017, half of the numbers are included in the interval shown in parentheses)

The following events have been listed as "Major Seismic Activity" in the monthly "Evaluation of Seismic Activity" from September 2025 through the end of August 2026.

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2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

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(2) Tohoku Region

– Since November 2025, seismic activity has been recorded from offshore of Sanriku to offshore of Iwate Prefecture. This includes the earthquakes that occurred offshore of Sanriku on November 9, 2025 (M6.9) and on April 20, 2026 (M7.7), as well as the earthquake offshore of Iwate Prefecture on June 25, 2026 (M7.2). Within these series of seismic activity zones, 7 earthquakes with a seismic intensity 1 or greater were recorded in September (compared to 17 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 5 months following the M7.7 earthquake offshore of Sanriku on April 20, horizontal displacements, attributed to post-seismic deformations, were primarily recorded in Iwate Prefecture. For example, at the Kuji observation point in Iwate Prefecture, a deformation of approximately 8 cm in 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 west and east of the seismic activity zone of this earthquake. Furthermore, the analysis of repeating earthquakes indicates the ongoing occurrence of aseismic slip events in these regions.
Since the M7.2 earthquake on June 25, the seismic activity has been occurring near the epicenter of the largest aftershock (M7.2 on January 7, 1995) on the western edge of the source region of the "1994 Sanriku-haruka-oki Earthquake" (M7.6). Furthermore, this seismic activity zones, which have been active since November 2025, include areas offshore of Sanriku to offshore of Iwate Prefecture, 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).

• Seismic Activity from Offshore of Sanriku to Offshore of Iwate Prefecture Since November 2025 (Chronology of Seismic Activity Since December 1994)
• Seismic Activity from Offshore of Sanriku to Offshore of Iwate Prefecture Since November 2025 (Chronology of Seismic Activity Since November 2025)
• Tremor Activity in the Northern Japan Trench in September 2026
• GNSS Observation Data After the Earthquake Offshore of Sanriku (April 20, M7.7) (provisional)
• GNSS Observation Data After the Earthquake Offshore of Sanriku (April 20, M7.7) (provisional) (continued)
• GNSS Observation Data After the Earthquake Offshore of Sanriku (April 20, M7.7) (provisional) (continued)
• GNSS Observation Data After the Earthquake Offshore of Sanriku (April 20, M7.7) (provisional) (continued)
• GNSS Observation Data After the Earthquake Offshore of Sanriku (April 20, M7.7) (provisional) (continued)
• GNSS Observation Data After the Earthquake Offshore of Sanriku (April 20, M7.7) (provisional) (continued)
• GNSS Observation Data After the Earthquake Offshore of Sanriku (April 20, M7.7) (provisional) (continued)
• Slow Slip in the Tohoku Region Estimated from GNSS Data (provisional)
• The Progression of Aseismic Slip Estimated from Repeating Earthquakes
• Comparison of Seismic Activity Zones from Offshore East of Aomori Prefecture to Offshore of Sanriku

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(3) Kanto-Chubu 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.

– From September 3 to 4, deep low-frequency earthquakes (tremors) were recorded near the boundary between the Philippine Sea Plate and the continental plate in the Tokai region. 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.

• Coordinate Time Series (Black Circles) and Calculated Values (Red Lines) of Observing Stations in the Tokai Region
• Long-Term Slow Slip in Tokai Region Estimated from GNSS Data (provisional)
• Deep low-frequency Earthquake Activity (Tremor) and Short-Term Slow Slip in the Tokai Region
• Short-term Slow Slip Recorded in the Tokai Region (September 4-8)
• Results of Short-Term SSE Analysis in the Tokai Region (September 2026)

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(4) Kinki-Chugoku-Shikoku Region

– Since February 2025, there have been continuous clusters of tremors occurring at depths of approximately 25 to 35 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. On September 29, an earthquake with M3.0 (maximum seismic intensity 1) occurred at a depth of approximately 15 km.

• Seismic Activity in Northern Yamaguchi Prefecture
• Crustal Deformation Around Seismic Activity Zone in Northern Yamaguchi Prefecture (provisional)
• Crustal Deformation Around Seismic Activity Zone in Northern Yamaguchi Prefecture (provisional) (continued)
• Crustal Deformation Around Seismic Activity Zone in Northern Yamaguchi Prefecture (provisional) (continued)
• Crustal Deformation Around Seismic Activity Zone in Northern Yamaguchi Prefecture (provisional) (continued)
• Crustal Deformation Around Seismic Activity Zone in Northern Yamaguchi Prefecture (provisional) (continued)
• Crustal Deformation Around Seismic Activity Zone in Northern Yamaguchi Prefecture (provisional) (continued)

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(5) Kyushu-Okinawa Region

There is no supplementary information for the Kyushu-Okinawa region.

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(6) Around the Nankai Trough

– "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 October 7(reference below).)

(Reference)

Recent Crustal Activity Around the Nankai Trough (Seismological and Volcanological Department, Japan Meteorological Agency, October 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."

(Note) Even under"normal conditions,"it is estimated that there is a high probability of a major earthquake (M8 to M9 class) occurring along the Nankai Trough within the next 30 years (for details, see"Long-Term Evaluation of Seismic Activity in the Nankai Trough [Second Edition, Partially Revised]"). Since approximately 80 years have passed since the Showa Tonankai and Showa Nankai earthquakes, the risk is considered very high.

(Excerpted)

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