April 11, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for March 2023

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in March 2023 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On March 7, there was a magnitude (M) M5.0 earthquake at a depth of approximately 20 km, offshore Kushiro. The focal mechanism showed a reverse fault type with a compression axis in an NNW-SSE direction.

* On March 11, there was an M4.8 earthquake at a depth of approximately 50 km under the eastern Hidaka region. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.

• Hokkaido Region
• Earthquake Offshore Kushiro on March 7
• Earthquake Under the Eastern Hidaka Region on March 11

to supplementary reports

(2) Tohoku Region

* On March 27, there was an M5.3 earthquake at a depth of approximately 60 km, offshore Miyagi prefecture. The focal mechanism had a compression axis in an NW-SE direction. This event occurred within the Pacific plate.

* On March 28, there was an M6.2 earthquake at a depth of approximately 30 km, offshore east of Aomori prefecture. 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 Pacific and the continental plates.

• Tohoku Region
• Earthquake Offshore Miyagi Prefecture on March 27
• Earthquake Offshore East of Aomori Prefecture on March 28

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. In March 2023, there was an M4.1 earthquake on 29^th: the serial seismic activity still continues in an active state, without any sign of decline at present.
The largest so far is the 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 April 10, 2023 at 08 o'clock (JST), there occurred 306 earthquakes with seismic intensity 1 or over observed: of these, 45 earthquakes with seismic intensity 3 or over observed. From March 1, 2023 till April 10, 2023 at 08 o'clock (JST), there occurred 20 earthquakes with seismic intensity 1 or over observed: of these, 3 earthquakes with seismic intensity 3 or over observed.
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. Adding data of the more surrounding GNSS observing stations, after the M5.4 earthquake on June 19, 2022, the crustal deformation is generally decaying, but it still continues in the north part where the seismic activity is high.
Judging from the variation of the crustal deformation area, upward movement of the seismic activity, the distribution of the electric conductivity and so on, the recent activity may involve fluid movement. 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.

* On March 2, there was an M5.0 earthquake in the ocean area near Hachijojima Island. This event occurred within the Philippine Sea plate.

* On March 24, there was an M4.7 earthquake at a depth of approximately 85 km under the northern Ibaraki prefecture. The focal mechanism had a tension axis in an NW-SE direction. This event occurred within the Pacific plate.

* On March 31, there began a cluster activity in the ocean area near Chichijima Island: there occurred an M5.7 earthquake on March 31 and an M5.1 earthquake on April 4. Till April 10 at 08 o'clock (JST), there occurred 14 earthquakes with seismic intensity 1 or over observed.

• Kanto - Chubu Region
• Seismic Activity in the Noto Region, Ishikawa Prefectur
• 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) –
• Source Region, in Each Cluster, Expands from the Deeper Part to the Shallower Part as a Whole.
• Snap Shots of the Crustal Deformation (1)
• Snap Shots of the Crustal Deformation (2)
• Underground Structure Obtained from Observation in 2021 Fiscal Year
• Earthquake in the Ocean near Hachijojima Island on March 2
• Earthquake Under the Northern Ibaraki Prefecture on March 24
• Seismic Activity in the Ocean near Chichijima Islands

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

There was no remarkable activity.

• Kyushu Region
• 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

• Earthquake in the Ocean near Hachijojima Island on March 2

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

Supplementary Information to the Evaluation of Seismic Activities for March 2023

April 11, 2023
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 March 2023.

• M4.0 or greater: 89 (69 events in February)
• M5.0 or greater: 8 (7 events in February)
• M6.0 or greater: 1 (1 event in February)

(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 March 2022 to late February 2023 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 area extending from the northern Kii Peninsula to Tokai region, since March 25 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 synchronous 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.

• Deep Low-Frequency Seismic (Tremor) Activity and Short-Term Slow Slip in the Area from the Northern Kii Peninsula to Tokai Region
• Situation of Deep Low-Frequency Tremor Activity in the Northern Kii Peninsula and Tokai Region (March 2023)
• Fig.5 Time Variation of Strain,Tilt and Underground Water Level (1) (2023/03/18 00:00 - 2023/04/04 00:00 (JST))
• Fig.5 Time Variation of Strain,Tilt and Underground Water Level (2) (2023/03/18 00:00 - 2023/04/04 00:00 (JST))
• Fig.6 Fault Model Explaining the Variation of Strain,Tilt and Underground Water Level (Fig.5[A]) During 2023/03/26 - 04/01

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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 aboutspring 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 (Black Points) Representing the Coordinates of the Observatories in the Central Shikoku and Calculated Values (Red Lines) – Time-Dependent Inversion –
• Long-Term Slow Slip in the Central Shikoku Estimated from GNSS Data (provisional)

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(5) Kyushu-Okinawa Region

– According to the GNSS observation, the crustal deformation different from the ordinary trend in the southern Kyushu has been observed from about spring 2019. This phenomenon 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.

• Time Series (Black Points) Representing the Coordinates of the Observatories in Kyushu Region and Calculated Values (Red Lines) – Time-Dependent Inversion –
• Long-Term Slow Slip Around the Southern Hyuganada Estimated from GNSS Data (provisional)

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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 April 7 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, April 7, 2023)
"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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