May 14, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for April 2020

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in April 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

* On April 20, there was a magnitude (M) 6.2 earthquake at a depth of approximately 45 km, offshore Miyagi 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. According to the GNSS observation a slight crustal deformation was observed accompanied with this event.

* On April 24, there was an M5.2 earthquake at a depth of approximately 65 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 April 30, there was an M5.3 earthquake offshore east of Aomori prefecture. The focal mechanism had a tension axis in an N-S direction.

• Tohoku Region
• Earthquake Offshore Miyagi Prefecture on April 20
• Observed Data Before and After the Earthquake Offshore Miyagi Prefecture (April 20, M6.2) (provisional)
• Earthquake Offshore East of Aomori Prefecture on April 24
• Earthquake Offshore East of Aomori Prefecture on April 30

to supplementary reports

(3) Kanto-Chubu Region

* On April 12, there was an M5.0 earthquake at a depth of approximately 55 km under the southern Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NNW-SSE direction. This event occurred at the boundary between the Philippine Sea and the continental plates.

* On April 13, there was an M4.2 earthquake at a depth of approximately 5 km in the northern Nagano prefecture. The focal mechanism had a compression axis in an E-W direction. This event occurred within the crust.

* On April 18, there was an M6.8 earthquake at a depth of approximately 480 km, offshore west of the Ogasawara Islands. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

* On April 23 at 13:44 (JST), there was an M5.5 earthquake at a depth of approximately 5 km in the central Nagano prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. This event occurred within the crust. After the occurrence of this event, the seismic activity became high: there occurred an M5.0 earthquake on the same day at 13:57(JST) and another M5.0 one on 26^th at 02:22 (JST). Note that there occurred 94 earthquakes with the maximum seismic intensity 1 or over in the period from April 22 till May 14 at 15 o'clock. Also, in 1998, the seismic activity became high: there occurred an M5.0 earthquake on August 12, and an M5.6 one on 16^th.

* On April 26, there was an M4.8 earthquake at a depth of approximately 65 km under the southern 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 Philippine Sea plates.

• Kanto - Chubu Region
• Earthquake Under the Southern Ibaraki Prefecture on April 12
• Earthquake in the Northern Nagano Prefecture on April 13
• Earthquake Offshore West of the Ogasawara Islands on April 18
• Earthquake in the Central Nagano Prefecture on April 23
• Seismic Activity in the Central Nagano Prefecture (Comparison of 1998 and 2020)
• Table of Number of Earthquakes in the Central Nagano Prefecture
• Earthquake Under the Southern Ibaraki Prefecture on April 26

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

(7) Other Regions

* On April 11, there was an M6.1 earthquake near the Philippine Islands.

• Earthquake near the Philippine Islands on April 11

Supplement

* On May 3, there was an M6.2 earthquake 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 of the continental plate.

* On May 4, there was an M5.6 earthquake at a depth of approximately 50 km under the northeastern Chiba 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 Philippine Sea plates.

* On May 6, there was an M5.0 earthquake at a depth of approximately 70 km under the northwestern Chiba prefecture. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction.

* On May 11, there was an M5.8 earthquake at a depth of approximately 45 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.

• Earthquake Offshore West of the Satsuma Peninsula on May 3
• Earthquake Under the Northeastern Chiba Prefecture on May 4, Earthquake Under the Northwestern Chiba Prefecture on May 6
• Earthquake Offshore Ibaraki Prefecture on May 11

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

Supplementary Information to the Evaluation of Seismic Activities for April 2020

May 14, 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 April 2020

• M4.0 or greater: 100 (55 events in March)
• M5.0 or greater: 14 (8 events in March)
• M6.0 or greater: 3 (0 event in March)

(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 April 2019 to late March 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.

to the text

(2) Tohoku Region

There is no supplementary information for Tohoku region.

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(3) Kanto-Chubu Region

– According to the GNSS observation in the Shima Peninsula, a slight crustal deformation different from the ordinary trend has been observed from mid-2019. 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.

• Nonstationary Horizontal Crustal Deformation in the Shima Peninsula (Linear Trend, Annual Component, Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Shima Peninsula
• Long-Term Slow Slip in the Shima Peninsula Estimated from GNSS Data (provisional)

to the text

(4) Kinki-Chugoku-Shikoku Region

– According to the GNSS observation in the western Kii Peninsula and the eastern Shikoku, the crustal deformation different from the ordinary trend observed from spring 2019 seems to be stagnant recently. This phenomenon is considered to be caused by a slow slip around the Kii Channel 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
• Long-Term Slow Slip in the Kii Channel 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 May 12 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, May 12, 2020)
"n 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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