August 11, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for July 2021

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in July 2021 (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 July 18 at 18:50 (JST), there was a magnitude (M) 4.7 earthquake at a depth of approximately 5 km in Aizu, Fukushima prefecture. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred within the crust. In the vicinity of the recent event, in addition to an M4.6 earthquake occurring at 19:11 (JST) on the same day, from July 18 till 24 there occurred 10 earthquakes with seismic intensity 1 or over.

* On July 26, there was an M5.2 earthquake at a depth of approximately 50 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 July 27, there was an M4.5 earthquake at a depth of approximately 80 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 Pacific plate.

• Tohoku Region
• Earthquake in Aizu, Fukushima Prefecture on July 18
• Earthquake Offshore East of Aomori Prefecture on July 26
• Earthquake Offshore Fukushima Prefecture on July 27

to supplementary reports

(3) Kanto-Chubu Region

* On July 11, there was an M3.9 earthquake at a depth of approximately 15 km, in the Noto region, Ishikawa prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred within the crust. In the vicinity of the recent event, the number of earthquakes increased since about 2018, and seismic activity has been high since December 2020. The largest was the M4.1 earthquake on June 26, 2021. From December 2020 till July 2021 there occurred 16 earthquakes with seismic intensity 1 or over. According to the GNSS observation, since December 2020, crustal deformation has been observed in the Noto Peninsula, such as about 1 cm displacement in south-south-west direction at Noto observatory, Noto town, Ishikawa prefecture and about 2 cm rise at Suzu observatory, Suzu City.

* On July 16, there was an M5.4 earthquake at a depth of approximately 10 km (CMT solution) in the ocean area near Hachijojima Island. The focal mechanism had a tension axis in an E-W direction. This event occurred within the crust of the Philippine Sea plate. In the vicinity of the recent event, from July 15 till 17 there were 14 earthquakes with seismic intensity 1 or over observed.

• Kanto - Chubu Region
• Earthquake in the Noto Region, Ishikawa Prefecture on July 11
• Observed Data Before and After the Seismic Activity (the Largest Earthquake on June 26, M4.1) in the Noto Region, Ishikawa Prefecture (provisional)
• Observed Data Before and After the Seismic Activity (the Largest Earthquake on June 26, M4.1) in the Noto Region, Ishikawa Prefecture (provisional)
• Earthquake in the Ocean Area near Hachijojima Island on July 16

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On July 31, there was an M4.5 earthquake at a depth of approximately 45 km in the southern Tokushima prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.

• Kinki-Chugoku-Shikoku Region
• Earthquake Under the Southern Tokushima Prefecture on July 31

to supplementary reports

(5) Kyushu-Okinawa Region

* On July 17, there was an M5.1 earthquake at a depth of approximately 75 km under the Iyonada. The focal mechanism had a tension axis in an ENE-WSW direction. This event occurred within the Philippine Sea plate.

• Kyushu Region
• Seismic Activity offshore Northwest of Amami Oshima (Seismic Activity about 100 km West of Amami Oshima)
• Earthquake under the Iyonada on July 17
• 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.

• Secular Variation of Benchmark 2595 (Omaezaki City)
• Vertical Movement of GNSS-Based Control Stations - Omaezaki
• Vertical Movements of GNSS-Based Control Stations - Kii Peninsula and Around Muroto Cape

to supplementary reports

(7) Other Regions

* On July 13, there was an M6.2 earthquake at a depth of approximately 50 km (CMT solution) under the Kurile Islands. The focal mechanism had a compression axis in an ENE-WSW direction.

• Earthquake under the Kurile Islands on July 13

to supplementary reports

Supplement (Seismic Activity Since August 1)

* On August 2, there was an M4.4 earthquake at a depth of approximately 5 km in the northern Hiroshima prefecture (#). The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction. This event occurred within the crust.

* On August 4, there was an M6.0 earthquake at a depth of approximately 20 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. In the vicinity of the recent event, from August 3 till 4, there occurred 14 earthquakes with seismic intensity 1 or over. In the neighborhood there were some examples of high seismic activities: during the activity in 2008, on May 8 there were M6.4 and M6.3 earthquakes, and immediately after these there occurred an M7.0 earthquake.

* On August 5, there was an M6.3 earthquake near Taiwan.

• Earthquake in the Northern Hiroshima Prefecture on August 2
• Earthquake Offshore Ibaraki Prefecture on August 4
• Seismic Activity Offshore Ibaraki Prefecture (Comparison with that in 2008)
• Earthquake near Taiwan on August 5

 #: The region name [the eastern Shimane prefecture] was used in the Japan Meteorological Agency announcement.
 Note: GNSS is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for July 2021

August 11, 2021
Earthquake Research Committee

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during July 2021.

• M4.0 or greater: 99 (76 events in June)
• M5.0 or greater: 11 (9 events in June)
• M6.0 or greater: 1 (0 event 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 2020 to late June 2021 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

There is no supplementary information for Kanto region.

to the text

(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 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.

– According to the GNSS observation, the crustal deformation different from the ordinary trend in the western Kii Peninsula and the eastern Shikoku, observed from summer 2020, is considered to be caused by a long-term slow slip around the Kii Channel at the deeper part of the boundary between the Philippine Sea and the continental plates. This crustal deformation seems to have slowed down recently.

– In the area from the central to the western Shikoku, from July 16 till August 1, near the boundary between the Philippine Sea and the continental plates, deep low-frequency earthquakes (tremors) have been observed. According to the strain-tilt data, in the surrounding area, almost synchronous with deep low-frequency earthquakes (tremors), slight crustal deformation is observed. These phenomena are considered to be caused by the slow-slip in the short interval at the boundary between the Philippine Sea and the continental plates.

• Non-Stationary Horizontal Crustal Deformation of the Central Shikoku (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Central Shikoku (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Central Shikoku (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Central Shikoku Estimated from GNSS Data (provisional)
• Non-Stationary Horizontal Crustal Deformation of the Western Kii Peninsula and the Eastern Shikoku (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and the Eastern Shikoku (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and the Eastern Shikoku (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Kii Channel Estimated from GNSS Data (provisional)
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from the Central to the Western Shikoku
• Fig.2 Observed results of Strain and Tilt in the Central Shikoku (2021/07/11 00:00 - 2021/08/04 00:00 (JST))
• Fig.3 Observed results of Strain and Tilt in the Western Shikoku (2021/07/11 00:00 - 2021/08/03 00:00 (JST))
• Fig.4 Fault Model Explaining the Variation of Strain and Tilt (Fig.2,3[A]) during 2021/07/18 - 19AM
• Fig.5 Fault Model Explaining the Variation of Strain and Tilt (Fig.2,3[B]) during 2021/07/19PM - 21AM
• Fig.6 Fault Model Explaining the Variation of Strain and Tilt (Fig.2,3[C]) during 2021/07/21PM - 27
• Fig.7 Fault Model Explaining the Variation of Strain and Tilt (Fig.2,3[D]) of 2021/07/28
• Fig.8 Fault Model Explaining the Variation of Strain and Tilt (Fig.2,3[E]) during 2021/07/29 - 08/01AM

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(5) Kyushu-Okinawa Region

– Since July 4, offshore northwest of Amami-Oshima (about 100 km west of Amami-Oshima), there was a high seismic activity and till 31, there were 10 earthquakes with seismic intensity 1 or over. The largest was the M4.8 earthquakes on July 7. In the neighborhood of the recent seismic activity, there was a high seismic activity in 2014.

– According to the GNSS observation, the crustal deformation different from the ordinary trend in southern Kyushu, observed from summer 2020, 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. This crustal deformation seems to have slowed down recently.

• Non-Stationary Horizontal Crustal Deformation of the Kyushu Region (After Linear Trend Removed)
• Time Series of GNSS Continuous Observation: the Kyushu Region
• 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 6 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 6, 2021)
"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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