November 10, 2022
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for October 2022

1 Major Seismic Activities

* On October 2, there was a magnitude (M) 5.9 earthquake offshore east of the Osumi Peninsula and maximum seismic intensity 5 Lower was observed in Miyazaki prefecture. Long-period ground motion class 2 was observed in Miyazaki prefecture.

* On October 21, there was an M5.0 earthquake offshore Fukushima prefecture. Maximum seismic intensity 5 Lower was observed in Fukushima prefecture.

• Seismic Activities for All of Japan in October 2022 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There was no remarkable activity.

• Hokkaido Region

to supplementary reports

(2) Tohoku Region

* On October 21, there was an M5.0 earthquake at a depth of approximately 30 km, offshore Fukushima prefecture. The focal mechanism showed a normal fault type with a tension axis in a WNW-ESE direction. This event occurred within the continental plate. According to the result of the GNSS observation, no significant crustal deformation was observed associated with this earthquake.

• Tohoku Region
• Earthquakes Offshore Fukushima Prefecture on October 21
• Observed Data Before and After the Earthquake Offshore Fukushima Prefecture (October 21, M5.0) (provisional)

to supplementary reports

(3) Kanto-Chubu Region

* In the crust of the Noto region, Ishikawa prefecture, where the number of earthquakes is increasing since about 2018, the seismic activity became high since December 2020, and it has become higher since about July 2021. The largest is an M5.4 earthquake which occurred on June 19, 2022. Beside this there occurred an M5.1 earthquake on September 16, 2021 and an M5.0 earthquake on June 20, 2022. From December 1, 2020 till November 9, 2022 at 08 o'clock (JST), there occurred 231 earthquakes with seismic intensity 1 or over observed: of these 32 earthquakes with seismic intensity 3 or over observed.
From October 2022 till November 9 at 08 o'clock (JST), there occurred 14 earthquakes with seismic intensity 1 or over observed. The serial seismic activity still continues in an active state, without any sign of decline at present.
According to the result of the GNSS observation, since about December 2020, the following crustal deformation has continued: the cumulative displacement of 1 cm over in the south-southeast direction and the uplift of 4 cm or so at the Suzu observatory, Suzu City, Ishikawa prefecture and the cumulative displacement of 1 cm over in the south-southwest direction at the Noto observatory, Noto Town and so on are observed.
Based on the situation of the seismic activity and the crustal deformation, the series of the seismic activity will continue for the time being and we should pay attention for a strong shaking.

• Kanto - Chubu Region
• Seismic Activity in the Noto Region, Ishikawa Prefecture
• Crustal Deformation of the Noto Region, Ishikawa Prefecture (provisional) – Vector Diagram –
• Crustal Deformation of the Noto Region, Ishikawa Prefecture (provisional) – Graphs After Removal of the Linear Trend, Year-Periodic Component, and Half-Year Periodic Component
• Crustal Deformation of the Noto Region, Ishikawa Prefecture (provisional) – Portable GNSS Continuous Observing Device (REGMOS) –

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

* On October 2, there was an M5.9 earthquake at a depth of approximately 30 km, offshore east of Osumi Peninsula. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Philippine Sea and the continental plates. According to the result of the GNSS observation, no significant crustal deformation was observed associated with this earthquake.

• Kyushu Region
• Earthquake Offshore East of the Osumi Peninsula on October 2
• Observed Data Before and After the Earthquake Offshore East of the Osumi Peninsula (October 2, M5.9) (provisional) (1)
• Observed Data Before and After the Earthquake Offshore East of the Osumi Peninsula (October 2, M5.9) (provisional) (2)
• Okinawa Region

to supplementary reports

(6) Around the Nankai Trough

* There has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time.

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

to supplementary reports

Supplement (Seismic Activity Since October 1)

* On November 3, there was an M4.9 earthquake at a depth of approximately 70 km, under the northwestern Chiba prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the Philippine Sea plates.

* On November 6, there was an M5.4 earthquake at a depth of approximately 50 km, offshore east of Hokkaido. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction.

* On November 9, there was an M4.9 earthquake at a depth of approximately 50 km, under the southern Ibaraki prefecture. Maximum seismic intensity 5 Upper was observed in Ibaraki prefecture. The focal mechanism had a compression axis in an NNW-SSE direction. This event occurred at the boundary between the Philippine Sea and the continental plates. Note that according to the result of the GNSS observation, no significant crustal deformation accompanying this event has been observed.

• Earthquake Under the Northwestern Chiba Prefecture on November 3
• Earthquake Offshore East of Hokkaido on November 6
• Earthquake Under the Southern Ibaraki Prefecture on November 9
• Observed Data Before and After the Earthquake Under the Southern Ibaraki Prefecture (November 9, M4.9) (provisional)

Note: GNSS is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for October 2022

November 10, 2022
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during October 2022.

• M4.0 or greater: 86 (126 events in September)
  M5.0 or greater: 12 (22 events in September)
  M6.0 or greater: 0 (4 events in September)
  

(Reference)

• M4.0 or greater: 81 / month (69-104)
• (Median monthly number from 1998 to 2017, half of data 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 data 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 data 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 data are included in the interval shown in parentheses)

The following events for the period from October 2021 to late September 2022 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

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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

There is no supplementary information for Tohoku region.

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(3) Kanto-Chubu Region

– In the Tokai region, during the period from October 14 till October 26, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the strain and tilt data, almost synchronized with the deep low-frequency earthquake (tremor) activity, slight crustal deformations were observed in the surrounding area. These phenomena are considered to be caused by the short-term slow slip at the deeper part of the boundary between the Philippine Sea and the continental plates. Note that preceding the current activity, during the period from September 30 till October 5, deep low-frequency earthquakes (tremors) were observed under the northern Kii Peninsula, near the boundary between the Philippine Sea and the continental plates.

• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Northern and the Western Kii Peninsula
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Tokai Region
• Situation of Deep Low-Frequency Tremor Activity in the Kii Peninsula and Tokai Region (October 2022)
• Fig.10 Time Variation of Strain and Tilt in the Area Extending from the Kii Peninsula to the Tokai Region (1)
• Fig.11 Time Variation of Strain and Tilt in the Area Extending from the Kii Peninsula to the Tokai Region (2)
• Fig.12 Time Variation of Strain and Tilt in the Area Extending from the Kii Peninsula to the Tokai Region (3)
• Fig.13 Fault Model Explaining the Variation of Strain and Tilt (Fig.10, 11, 12[A]) During 2022/10/11PM - 15
• Fig.14 Fault Model Explaining the Variation of Strain and Tilt (Fig.10, 11, 12[B]) During 2022/10/16 - 18
• Fig.15 Fault Model Explaining the Variation of Strain and Tilt (Fig.10, 11, 12[C]) During 2022/10/19 - 23AM

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(4) Kinki-Chugoku-Shikoku Region

– According to the GNSS observation, the crustal deformation different from the ordinary trend in the central Shikoku has been observed from about spring 2019. This phenomenon is considered to be caused by a long-term slow-slip around the central Shikoku at the deeper part of the boundary between the Philippine Sea and the continental plates.

• Time Series Representing Non-Stationary Crustal Deformation of the Observatories in the Central Shikoku and Calculated Values
• Long-Term Slow Slip in the Central Shikoku Estimated from GNSS Data (provisional)

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(5) Kyushu-Okinawa Region

– Within the continental plate offshore northwest of Okinawa Main Island, an active seismic activity is observed from January 30, and till October 31, there occurred 77 earthquakes with seismic intensity 1 or over observed. In the series of seismic activity, the largest is an M6.0 earthquake on September 18. Note that the seismic activity has become low in October.
According to the result of the GNSS observation, crustal deformation was observed from about February 2022 such as a southeastward displacement of 1 cm over of Gushikawa observatory, Kumejima Town, Okinawa prefecture.

– In the ocean near the Tokara Islands (near Suwanosejima Island), in October 2022 there occurred 11 earthquakes, and in November till 9^th at 08 o'clock (JST) there occurred 1 earthquake, with seismic intensity 1 or over observed. The seismic activity was within the continental plate, and is considered to be related to the volcanic activity of Suwanosejima Island. Note that in September, there occurred 16 earthquakes with seismic intensity 1 or over observed.

– According to the GNSS observation, the crustal deformation different from the ordinary trend in the southern Kyushu, was observed from about summer 2020, became stagnant about summer 2021 and afterwards completely converged. This crustal deformation is considered to be caused by a long-term slow-slip around the southern Hyuganada at the deeper part of the boundary between the Philippine Sea and the continental plates. The long-term slow-slip seems to be stagnant since summer 2021, but now seems to have stopped.

* Revision of the analytic method was performed and the representation was altered on the observed crustal deformation different from the ordinary trend in the southern Kyushu, which caused estimated long-term slow-slip around the southern Hyuganada at the deeper part of the plate boundary.

• Seismic Activity Offshore Northwest of Okinawa Main Island
• Crustal Deformation Around Kumejima Island (provisional)
• Seismic Activity in the Ocean near the Tokara Islands (near Suwanosejima Island) (1)
• Seismic Activity in the Ocean near Tokara Islands (near Suwanosejima Island) (2)
• Time Series Representing Non-Stationary Crustal Deformation of the Observatories in the Kyushu Area and Calculated Values
• Long-Term Slow Slip Around the Southern Hyuganada Estimated from GNSS Data (provisional)
• Long-Term Slow Slip Around the Southern Hyuganada Estimated from GNSS Data (provisional) (2) - Reference -
• Long-Term Slow Slip Around the Southern Hyuganada Estimated from GNSS Data (provisional) (3) - Reference -

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(6) Around the Nankai Trough

– "There has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time.":
(This evaluation is in agreement with the views presented at the regular meeting on November 8 of the Assessment Committee for Earthquakes Along the Nankai Trough (reference below).)

(Reference)

Recent Crustal Activity Around the Nankai Trough (Seismological and Volcanological Department, Japan Meteorological Agency, November 8, 2022)
"In the present, there has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time. (Excerpted)"

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