September 9, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for August 2020

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in August 2020 (Magnitude 4.0 or over)

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2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There was no remarkable activity.

• Hokkaido Region

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(2) Tohoku Region

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* On August 6, there was a magnitude (M) 5.6 earthquake 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.

• Kanto - Chubu Region
• Earthquake Offshore Ibaraki Prefecture on August 6

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

Supplement

* On September 2, there was an M4.6 earthquake at a depth of approximately 10 km in the Kaga region, Ishikawa 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 September 3, there was an M4.8 earthquake in the ocean area near Chichijima Island.

* On September 4, there was an M5.0 earthquake at a depth of approximately 5 km in Reihoku, Fukui prefecture. Maximum seismic intensity 5 Lower was observed in Fukui prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred within the crust.

• Earthquake in the Kaga Region, Ishikawa Prefecture on September 2
• Earthquake in the Ocean Area near Chichijima Island on September 3
• Earthquake in Reihoku, Fukui Prefecture on September 4

# "The western Toyama prefecture" is the region name used in the announcement of Japan Meteorological Agency.

Supplementary Information to the Evaluation of Seismic Activities for August 2020

September 9, 2020
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 2020.

• M4.0 or greater: 79 (84 events in July)
• M5.0 or greater: 6 (11 events in July)
• M6.0 or greater: 0 (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 2019 to late July 2020 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

– Near the border between Nagano and Gifu prefectures (the central Nagano prefecture and the Hida region, Gifu prefecture) within the crust, there was an active seismic activity such as the increase of earthquakes of magnitude (M) 3.0 over beginning on April 22 and an M5.5 earthquake on April 23, an M5.4 earthquake on May 19 and an M5.3 on 29th. This activity continues in August, but there occurred no earthquake of M3.0 over.
In the neighborhood, also in 1998, the seismic activity became high: there occurred an M5.0 earthquake on August 12, and an M5.6 on 16th, and earthquakes of M3.0 over in December.

– According to the GNSS observation, the crustal deformation different from the ordinary trend observed in the Shima Peninsula from mid-2019 seems to be decaying recently. This phenomenon is considered to be caused by a long-term slow slip around the Shima Peninsula at the deeper part of the boundary between the Philippine Sea and the continental plates.

• Seismic Activity near the Border Between Nagano and Gifu Prefectures (Comparison Between 1998 and 2020: for Half a Year)
• Nonstationary Horizontal Crustal Deformation in the Shima Peninsula (After Linear Trend, Annual Component, Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Shima Peninsula (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Shima Peninsula (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Shima Peninsula 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 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.

– From July 22 to August 23, in the area from the eastern to the western Shikoku, a deep low-frequency earthquake (tremor) activity was observed near the boundary between the Philippine Sea and the continental plates. According to the strain, tilt and GNSS data, synchronously to the deep low-frequency earthquake (tremor) activity, a 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.

• Non-Stationary Horizontal Crustal Deformation of the Central Shikoku (After Linear Trend, Annual Component, 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)
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (1) Deep Low-Frequency Earthquake (Tremor) Activity
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (2) Fig.5 Time Variation of Strain and Tilt (2020/07/13 00:00-2020/08/25 00:00 (JST))
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (3) Fig.6 Time Variation of Strain and Tilt (2020/07/13 00:00-2020/08/25 00:00 (JST))
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (4) Fig.7 Fault Model Explaining the Variation of Strain, and Tilt During 2020/07/27PM-29AM (Fig.5-6[A])
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (5) Fig.8 Fault Model Explaining the Variation of Strain, and Tilt During 2020/07/29PM-31AM (Fig.5-6[B]: East Side Observation)
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (6) Fig. 9 Fault Model Explaining the Variation of Strain, and Tilt During 2020/07/29PM-31AM (Fig.5-6[B]: West Side Observation)
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (7) Fig. 10 Fault Model Explaining the Variation of Strain, and Tilt During 2020/07/31PM-08/03 (Fig.5-6[C]: East Side Observation)
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (8) Fig. 11 Fault Model Explaining the Variation of Strain, and Tilt During 2020/07/31PM-08/03 (Fig.5-6[C]: West Side Observation)
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (9) Fig. 12 Fault Model Explaining the Variation of Strain, and Tilt During 2020/08/04-05 (Fig.5-6[D])
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area Extending from Eastern to Western Shikoku (10) Fig. 13 Fault Model Explaining the Variation of Strain, and Tilt During 2020/08/06-08AM (Fig.5-6[E])
• Slow Slip Synchronous with Deep Low-Frequency Tremor in the Western Shikoku Estimated from GNSS Data (provisional)

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(5) Kyushu-Okinawa Region

There is no supplementary information for Kyushu-Okinawa region.

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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 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, 2020)
"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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