September 9, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for August 2021

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in August 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 August 22, there was a magnitude (M) 5.1 earthquake at a depth of approximately 60 km, offshore Fukushima prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred within the Pacific plate.

• Tohoku Region
• Earthquake Offshore Fukushima Prefecture on August 22

to supplementary reports

(3) Kanto-Chubu Region

* 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 were 14 earthquakes with seismic intensity 1 or over observed. In the neighborhood of the recent event there have been examples of active seismic activity: in 2008 there were M6.4 and M6.3 earthquakes on May 8 and then immediately there occurred an M7.0 earthquake.

* On August 27, there was an M5.1 earthquake at a depth of approximately 40 km, offshore Ibaraki prefecture. 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 Pacific and the continental plates.

• Kanto - Chubu Region
• Seismic Activity in the Noto Region, Ishikawa Prefecture
• Observed Data During the Seismic Activity in the Noto Region, Ishikawa Prefecture (provisional)
• Observed Data During the Seismic Activity in the Noto Region, Ishikawa Prefecture (provisional)
• Earthquake Offshore Ibaraki Prefecture on August 4
• Seismic Activity Offshore Ibaraki Prefecture (Comparison with that in 2008)
• Earthquake Offshore Ibaraki Prefecture on August 27

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On August 2, there was an M4.4 earthquake at a depth of approximately 5 km in the northern Hiroshima prefecture (#). Also, in the vicinity of the event, there was an M4.3 earthquake on August 18. The focal mechanisms of these earthquakes showed a strike-slip fault type with a compression axis in a WNW-ESE direction. These events occurred within the crust.

* On August 16 at 05:03 (JST), there was an M4.6 earthquake at a depth of approximately 15 km in the northern Shiga prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an E-W direction. This event occurred within the crust. Also, in the vicinity of the recent event, on the same day at 08:17 (JST) there was an M4.4 earthquake.

• Kinki-Chugoku-Shikoku Region
• Earthquakes in the Northern Hiroshima Prefecture on August 2 and 18
• Earthquake in the Northern Shiga Prefecture on August 16

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

(7) Other Regions

* On August 5, there was an M6.3 earthquake at a depth of approximately 10 km (CMT solution) near Taiwan. The focal mechanism showed a normal fault type with a tension axis in an N-S direction. This event occurred within the crust of the continental plate.

• Earthquake near Taiwan on August 5

to supplementary reports

 #: The region name [the eastern Shimane prefecture] was used in the Japan Meteorological Agency announcement.

Supplementary Information to the Evaluation of Seismic Activities for August 2021

September 9,, 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 August 2021.

• M4.0 or greater: 109 (99 events in July)
• M5.0 or greater: 17 (11 events in July)
• M6.0 or greater: 2 (1 event in July)

(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 August 2020 to late July 2021 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

to the text

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 crust of the Noto region, Ishikawa prefecture, the number of earthquakes is increasing since 2018 and the seismic activity becomes high since December 2020. On June 26, 2021 there occurred an M4.1 earthquake, on July 11 an M3.9, and on August 14 an M4.2. From December 2020 till the end of August, 2021 there occurred 30 earthquakes with seismic intensity 1 or over: during the last August, 14 earthquakes. According to the GNSS observation, the following crustal deformation is observed: the displacement of 1 cm or so in the south-southwest direction at the Noto observatory in Noto Town, Ishikawa prefecture, and the uplift of 2 cm or so at the Suzu observatory in Suzu City.

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, being 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 short-term slow-slip 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

to the text

(5) Kyushu-Okinawa Region

– According to the GNSS observation, the crustal deformation different from the ordinary trend in southern Kyushu, being 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 be stagnant 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)

to the text

(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 September 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, September 7, 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)"

to the text

(7) Other Regions

There is no supplementary information for other regions.

to the text