August 12, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for July 2020

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in July 2020 (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

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(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, and an M5.4 earthquake on May 19 and an M5.3 on 29th. This activity continues in July such as an M4.8 earthquake which occurred on 5th. There occurred 225 earthquakes with maximum seismic intensity 1 or over in the period from April 22 till July 31.
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.

* On July 9, there was an M4.7 earthquake at a depth of approximately 45 km under the southern 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 Philippine Sea and the continental plates.

* On July 30, there was an M6.0 earthquake at a depth of approximately 20 km (CMT solution) in the ocean area near Torishima Island. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction.

• Kanto - Chubu Region
• Seismic Activity near the Border Between Nagano and Gifu Prefectures (the Central Nagano Prefecture and the Hida Region, Gifu Prefecture) (1)
• Seismic Activity near the Border Between Nagano and Gifu Prefectures (the Central Nagano Prefecture and the Hida Region, Gifu Prefecture) (2)
• Earthquake Under the Southern Ibaraki Prefecture on July 9
• Earthquake Under the Ocean Area near Torishima Island on July 30

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.

• 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

Supplement

* On August 6, there was an M5.6 earthquake offshore Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction (provisional).

• Earthquake Offshore Ibaraki Prefecture on August 6

Supplementary Information to the Evaluation of Seismic Activities for July 2020

August 12, 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 July 2020.

• M4.0 or greater: 84 (159 events in June)
• M5.0 or greater: 11 (26 events in June)
• M6.0 or greater: 1 (2 events in June)

Note: The earthquake occurrence situation of June includes the seismic activity of the ocean area near Yonagunijima Island (75 and 18 for the number of earthquakes with M4.0 over and M5.0 over, respectively) which became active since June 14.

(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 2019 to late June 2020 have been listed as "Major Seismic Activities" in monthly "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

– 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.

– From June 29 to July 14, in the area from the Tokai region to the northern Kii Peninsula, 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.

• 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)
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area from Tokai Region to the Northern Kii Peninsula (1) Deep Low-Frequency Earthquake (Tremor) Activity
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area from Tokai Region to the Northern Kii Peninsula (2) Time Variation of Strain, Tilt and Underground water (2020/06/22 00:00 - 2020/07/25 00:00 (JST))
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area from Tokai Region to the Northern Kii Peninsula (3) Fault Model Explaining the Variation of Strain and Tilt (Fig.2[A]) 2020/06/30PM - 07/03
• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area from Tokai Region to the Northern Kii Peninsula (4) Fault Model Explaining the Variation of Strain and Tilt (Fig.2[B]) 2020/07/06 - 08AM
• Slow-Slip in the Northern Kii Peninsula Synchronizing with Deep Low-Frequency Tremor, 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.

• Non-Stationary Horizontal Crustal Deformation of the Central Shikoku (Linear Trend, Annual Component, Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Central Shikoku (1)
• Time Series of GNSS Continuous Observation: the Central Shikoku (2)
• Long-Term Slow Slip in the Central Shikoku Estimated from GNSS Data (provisional)

to the text

(5) Kyushu-Okinawa Region

There is no supplementary information for Kyushu-Okinawa region.

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 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, 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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