August 9, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for July 2023

1 Major Seismic Activities

There was no remarkable activity.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On July 17, there was a magnitude (M) 4.5 earthquake at a depth of approximately 110 km under the central Tokachi region. The focal mechanism had a tension axis in the dipping direction of the Pacific plate. This event occurred within the Pacific plate.

• Hokkaido Region
• Earthquake Under the Central Tokachi Region on July 17

to supplementary reports

(2) Tohoku Region

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* In the crust of the Noto region, Ishikawa prefecture, the number of earthquakes is increasing since about 2018, then the seismic activity became high since December 2020, and it has become higher since about July 2021. From December 1, 2020 till August 8, 2023 at 08 o�'clock (JST), there occurred 464 earthquakes with seismic intensity 1 or over observed: the state of high seismic activity continues.
In the series of the seismic activity, the largest earthquake is the M6.5 earthquake on May 5, 2023 at 14:42 (JST) offshore the Noto Peninsula (#1). The series of the seismic activity before the occurrence of the M6.5 earthquake, was mainly observed in the land area and the coastal area of the northeastern Noto Peninsula. After the occurrence of the M6.5 earthquake, the seismic activity then extends further to the northern and eastern sea area. Since July 1 till August 8 at 08 o�'clock (JST), there occurred 15 earthquakes with seismic intensity 1 or over observed: the largest is the M4.0 earthquake occurring on July 8. The seismic activity is decreasing as time passes, and as a whole the number of earthquakes returns in general to that of the stage before the occurrence of the M6.5 earthquake.
According to the result of the GNSS observation, since about December 2020 till the occurrence of the M6.5 earthquake, the following crustal deformation had been observed: the cumulative displacement of 1 cm over in horizontal direction and the uplift of 4 cm or so at the Suzu observatory, Suzu City, Ishikawa prefecture and so on. The GNSS observation and the analysis of the synthetic aperture radar images by Advanced Land Observing Satellite �gthe Daichi-2�h showed that accompanying the M6.5 earthquake, the crustal deformations of the order maximum 20 cm was observed at the surrounding area of the epicenter. The deformation in different tendency from the pre-earthquake stage observed at several observatories after the M6.5 event has recently decayed and it seems that the tendency in the pre-earthquake stage has returned.
Based on the situation of the seismic activity and the crustal deformation, the series of the seismic activity will continue for the time being. We should continue to pay attention for strong shaking and tsunami.

* On July 22, there was an M4.8 earthquake at a depth of approximately 50 km, offshore Ibaraki 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 July 29, there was an M4.6 earthquake at a depth of approximately 75 km under the southern Ibaraki prefecture (#2). The focal mechanism had a compression axis in an NW-SE direction.

• Kanto - Chubu Region
• Seismic Activity in the Noto Region, Ishikawa Prefecture
• Seismic Activity in the Noto Region, Ishikawa Prefecture (Comparison of Numbers of Earthquakes with M1.6 or over)
• Crustal Deformation in the Noto Region, Ishikawa Prefecture (Provisional)
• Observed Data After the Earthquake (May 5, 2023) in the Noto Region, Ishikawa Prefecture (provisional)
• Crustal Deformation in the Noto Region, Ishikawa Prefecture (Provisional) (1)
• Crustal Deformation in the Noto Region, Ishikawa Prefecture (Provisional) (2)
• Comprehensive Analysis Result of the GNSS Observing Stations of Universities and GSI
• Time Variations of Daily Coordinate Values of Stations Corrected for the Pre-Event Trend (for all Stations) and for the Seasonal Change (only for Station 0253) (Precise Ephemeris Used) (1)
• Time Variations of Daily Coordinate Values of Stations Corrected for the Pre-Event Trend (for all Stations) and for the Seasonal Change (only for Station 0253) (Precise Ephemeris Used) (2)
• Earthquake Offshore Ibaraki Prefecture on July 22
• Earthquake Under the Southern Ibaraki Prefecture on July 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

* On July 22, there was an M5.0 earthquake at a depth of approximately 35km in the Hyuganada. The focal mechanism showed a normal fault type with a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.

• Kyushu Region
• Earthquake in the Hyuganada on July 22
• 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

   #1: [In the Noto region, Ishikawa prefecture] was the regional name of the epicenter used in the JMA announcement.

   #2: [The southern Tochigi prefecture] was the regional name of the epicenter 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 July 2023

August 9, 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 July 2023.

• M4.0 or greater: 79 (93 events in June)
• M5.0 or greater: 13 (14 events in June)
• M6.0 or greater: 0 (2 events in June)

(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 July 2022 to late June 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.

to the text

(3) Kanto-Chubu Region

– In the northern and central Kii Peninsula, from July 15 till 23, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the tilt observation, synchronous with the deep low-frequency seismic (tremor) activity, slight crustal deformation was observed in the surrounding area. These phenomena are considered to be caused by the short-term slow slip at the boundary between the Philippine Sea and the continental plates.

• The Deep Low-Frequency Seismic (Tremor) Activity and the Short-Term Slow Slip in the Northern and Central Kii Peninsula
• Situation of the Deep Low-Frequency Tremor Activity in the Kii Peninsula and the Tokai Region (July 2023)
• Fig.8 Observed Result of Strain, Tilt and Underground Water in the Kii Peninsula (2023/07/02 00:00 - 2023/07/30 00:00 (JST))
• Fig.9 Fault Model Explaining the Variation of Strain and Tilt (Fig.8[A]) During 2023/07/16-18AM
• Fig.10 Fault Model Explaining the Variation of Strain (Fig.8[Bsw]) During 2023/07/18PM-20AM
• Fig.11 Fault Model Explaining the Variation of Strain (Fig.8[Csw]) During 2023/07/20PM-22AM
• Fig.12 Fault Model Explaining the Variation of Strain, Tilt and Underground Water Level (Fig.8[Bne]) During 2023/07/19-22AM

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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.

– In the period from June 18 till July 10, in the area extending from the eastern to the central Shikoku, 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 seismic (tremor) activity, slight crustal deformations was observed in the surrounding area. These phenomena are considered to be caused by the short-term slow slip at 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)
• The Deep Low-Frequency Seismic (Tremor) Activity and the Short-Term Slow Slip in the Eastern and Central Shikoku
• Fig.2 Observed Result of Strain and Tilt in the Shikoku Region (2023/06/07 00:00 - 2023/07/17 00:00 (JST))
• Fig.3 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[A]) During 2023/06/21PM-24AM
• Fig.4 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[B]) During 2023/06/24PM-30AM
• Fig.5 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[C]) During 2023/07/01PM-05AM
• Fig.6 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[D]) During 2023/07/05PM-10AM

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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 early 2023. 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. Recently this crustal deformation seems to be slowing down.

• 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 August 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, August 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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