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

Evaluation of Seismic Activities for June 2020

1 Major Seismic Activities

* On June 25, there was a magnitude (M) 6.1 earthquake offshore east of Chiba prefecture. This event caused maximum seismic intensity 5 Lower observed in Chiba prefecture, and damage with injured persons.

• Seismic Activities for All of Japan in June 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

* On June 1, there was an M5.2 earthquake at a depth of approximately 95 km under the northern Ibaraki prefecture. The focal mechanism had a tension axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

* On June 4, 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 an E-W direction. This event occurred at the boundary between the Pacific and the continental plates.

* On June 17, there was an M4.4 earthquake at a depth of approximately 5 km in the central and western Mino regions, Gifu prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred within the crust.

* On June 25, there was an M6.1 earthquake at a depth of approximately 35 km, offshore east of Chiba prefecture. The focal mechanism showed a reverse fault type with a compression axis in an N-S direction. According to the GNSS observation, no significant crustal deformation was observed.

* 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 M3.0 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 29^th. The maximum magnitude earthquake in the period from April 22 till July 9 at 09 o'clock was the M4.8 one which occurred on July 5 at 15:09 (JST). There occurred 210 earthquakes with maximum seismic intensity 1 or over in the period from April 22 till July 9 at 09 o'clock.
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 16^th, and earthquakes of M3.0 over in December.

• Kanto - Chubu Region
• Earthquake Under the Northern Ibaraki Prefecture on June 1
• Earthquake Offshore Ibaraki Prefecture on June 4
• Earthquake in the Central and Western Mino Regions, Gifu Prefecture on June 17
• Earthquake Offshore East of Chiba Prefecture on June 25

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On June 10, there was an M4.7 earthquake at a depth of approximately 20 km in the Tosa Bay. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction. This event occurred within the crust of the continental plate.

• Kinki-Chugoku-Shikoku Region
• Earthquake in Tosa Bay on June 10

to supplementary reports

(5) Kyushu-Okinawa Region

* On June 1, there was an M4.3 earthquake at a depth of approximately 10 km, offshore west of the Satsuma Peninsula. The focal mechanism showed a strike-slip fault type with a tension axis in an NW-SE direction. This event occurred within the crust.

* On June 14, there was an M6.3 earthquake at a depth of approximately 165 km, offshore northwest of Amami Oshima. The focal mechanism had a compression axis in the subducting direction of the Philippine Sea plate. This event occurred within the Philippine Sea plate.

• Kyushu Region
• Earthquake Offshore West of the Satsuma Peninsula on June 1
• Earthquake Offshore Northwest of Amami Oshima on June 14
• Okinawa Region
• The Seismic Activity in the Ocean Area Near Yonagunijima Island Since June 14

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

Note: GNSS is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for June 2020

July 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 June 2020.

• M4.0 or greater:159 (91 events in May)
• M5.0 or greater: 26 (16 events in May)
• M6.0 or greater: 2 (1 event in May)

Note: The earthquake occurrence situation of June includes the seismic activity of the ocean area near Yonagunijima Island 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 June 2019 to late May 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.

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(2) Tohoku Region

There is no supplementary information for Tohoku region.

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(3) Kanto-Chubu Region

– According to the GNSS observation, a slight 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.

• Observed Data Before and After Earthquake Offshore East of Chiba Prefecture (June 25, M6.1) (provisional)
• 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)
• Seismic Activity near the Border Between Nagano and Gifu Prefectures - Table of Numbers of Earthquakes
• 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 western Kii Peninsula and the eastern Shikoku observed from spring 2019 seems to be finished. The deformation 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. The long-term slow slip seems to have stopped.

– 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 Western Kii Peninsula and Eastern Shikoku (Linear Trend, Annual Component, Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and Eastern Shikoku (1)
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and Eastern Shikoku (2)
• Long-Term Slow Slip in the Kii Channel Estimated from GNSS Data (provisional)
• on-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)

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(5) Kyushu-Okinawa Region

– The seismic activity in the ocean area near Yonagunijima Island became high since June 14, and the numbers of earthquakes of M4.0 or over and M5.0 or over until 30^th are 75 and 18, respectively. The maximum magnitude earthquake was an M5.9 on June 15. Also, there were 9 earthquakes with seismic intensity 1 or over.

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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 July 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, July 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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