May 12, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for April 2023

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in April 2023 (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 April 17, there was a magnitude (M) 4.8 earthquake at a depth of approximately 45 km, offshore Fukushima 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 Fukushima Prefecture on April 17

to supplementary reports

(3) Kanto-Chubu Region

* With regard to the seismic activity in the Noto region, Ishikawa prefecture, refer to another report (Evaluation of the Seismic Activity in the Noto Region, Ishikawa Prefecture).

* 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 30, there occurred 20 earthquakes with seismic intensity 1 or over observed.

• Kanto - Chubu Region
• Seismic Activity in the Ocean Area 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

* On April 10, there was an M5.0 earthquake at a depth of approximately 50 km under the ocean area near Yonagunijima Island. The focal mechanism showed a reverse fault type with a compression axis in an NNW-SSE direction. This event occurred at the boundary between the Philippine Sea and the continental plates.

* On April 27, there began a cluster activity in the ocean area near Okinawa Main Island: there occurred an M6.4 earthquake on May 1. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Philippine Sea and the continental plates. Till May 7, there occurred 8 earthquakes with seismic intensity 1 or over observed.

• Kyushu Region
• Okinawa Region
• Earthquake Under the Ocean Area near Yonagunikima Island on April 10
• Seismic Activity in the Ocean Area near Okinawa Main Island

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 May 1)

* With regard to the earthquakes offshore the Noto Peninsula (*) on May 5, refer to another report (Evaluation of Seismic Activity in the Noto Region, Ishikawa Prefecture).

* On May 6, there was an M5.7 earthquake at a depth of approximately 55 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.

* On May 11, there was an M5.2 earthquake at a depth of approximately 40 km under the southern Chiba prefecture. Maximum seismic activity 5 Upper was observed in Chiba prefecture. The focal mechanism (preliminary) had a tension axis in an NW-SE direction. This event occurred within the Philippine Sea plate.
Note that according to the GNSS observation, no significant crustal deformation was observed so far.
In the area of strong shaking, we should pay attention for a week or so, to the possibility of an earthquake which causes maximum seismic intensity 5 Upper or so. Especially for 2 or 3 days after the earthquake, there is a possibility that a large earthquake occurs.
In the vicinity of the recent event there have sometimes been earthquakes of M5 or over: recently on May 25, 2019, there was an M5.1 earthquake. Note that on December 17, 1987, there was an M6.7 earthquake.

• Earthquake Offshore East of Aomori Prefecture on May 6
• Earthquake Under the Southern Chiba Prefecture on May 11
• Observed Data Before and After the Earthquake Under the Southern Chiba Prefecture (May 11, M5.2) (provisional)

    * : [The Noto region, Ishikawa prefecture] was the regional name of the epicenters used in the JMA announcement.

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

Supplementary Information to the Evaluation of Seismic Activities for April 2023

May 12, 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 April 2023.

• M4.0 or greater: 67 (89 events in March)
• M5.0 or greater: 7 (8 events in March)
• M6.0 or greater: 0 (1 event in March)

(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 April 2022 to late March 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.

to the text

(2) Tohoku Region

There is no supplementary information for Tohoku region.

to the text

(3) Kanto-Chubu Region

– In the area extending from the northern Kii Peninsula to Tokai region, since March 25 till April 9, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the strain, tilt, and GNSS data, almost synchronous with the deep low-frequency earthquakes (tremors), 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 (April 2023)
• Fig.2 Time Variation of Strain,Tilt and Underground Water Level (1) (2023/03/18 00:00 - 2023/04/15 00:00 (JST))
• Fig.2 Time Variation of Strain,Tilt and Underground Water Level (2) (2023/03/18 00:00 - 2023/04/15 00:00 (JST))
• Fig.2 Time Variation of Strain,Tilt and Underground Water Level (3) (2023/03/18 00:00 - 2023/04/15 00:00 (JST))
• Fig.3 Fault Model Explaining the Variation of Strain,Tilt and Underground Water Level (Fig.2[A]) During 2023/03/26-28
• Fig.4 Fault Model Explaining the Variation of Strain,Tilt and Underground Water Level (Fig.2[B]) During 2023/03/29-31
• Fig.5 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[C]) During 2023/04/01-04
• Fig.6 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[D]) During 2023/04/05-07
• Slow Slip Synchronized with Deep Low-Frequency Earthquakes (Tremors) in the Tokai Region Estimated from GNSS Data (provisional)

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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 (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 May 10 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, May 10, 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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