April 10, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for March 2020

1 Major Seismic Activities

* On March 13, there was a magnitude (M) 5.5 earthquake in the Noto region, Ishikawa prefecture. The maximum seismic intensity 5 Upper was observed in Ishikawa prefecture.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On March 28, there was an M5.0 earthquake at a depth of approximately 70 km, offshore Urakawa. 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.

• Hokkaido Region
• Earthquake Offshore Urakawa on March 28

to supplementary reports

(2) Tohoku Region

* On March 11, there was an M4.4 earthquake at a depth of approximately 10 km, in the southern inland region, Akita prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred within the crust.

• Tohoku Region
• Earthquake in the Southern Inland, Akita Prefecture on March 11

to supplementary reports

(3) Kanto-Chubu Region

* On March 13, there was an M5.5 earthquake at a depth of approximately 10 km in the Noto region, Ishikawa prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred within the crust. This event occurred at the eastern end of the activity area of the 2007 Noto-Hanto Earthquake. According to the GNSS observation, any significant crustal deformation associated with the recent event was not observed.

• Kanto - Chubu Region
• Earthquake in the Noto Region, Ishikawa Prefecture on March 13
• Observed Data Before and After the Earthquake in the Noto Region, Ishikawa Prefecture (March 13, M5.5) (provisional)

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

* On March 9, there was an M4.7 earthquake at a depth of approximately 60 km under the Bungo Channel. The focal mechanism had a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.

• Kyushu Region
• Earthquake under the Bungo Channel on March 9
• 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

Supplementary Information to the Evaluation of Seismic Activities for March 2020

April 10, 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 March 2020.

• M4.0 or greater: 55 (78 events in February)
• M5.0 or greater: 8 (10 events in February)
• M6.0 or greater: 0 (1 event in February)

(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 March 2019 to late February 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

– From March 7 to 23, in the northern and western Kii Peninsula, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the strain and tilt data, slight crustal deformations almost synchronous with the deep low-frequency earthquakes (tremors) were 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.

• Activity Status of Deep Low-Frequency Tremors in Kii Peninsula and Tokai Region (March 2020)
• Observed Results of Strain and Underground Water in the Area from Kii Peninsula to Tokai Region (2020/03/01 00:00-2020/03/30 00:00 (JST))
• Observed Results of Strain and Underground Water in Kii Peninsula (2020/03/01 00:00-2020/03/30 00:00 (JST))
• Observed Results of Tilt in Kii Peninsula (2020/03/01 00:00-2020/03/30 00:00 (JST))
• Fault Model Explaining the Variation of Strain, Tilt, Underground Water During 2020/03/10PM-12 (Fig.12-14[A])
• Fault Model Explaining the Variation of Strain, Tilt, Underground Water During 2020/03/13-14 (Fig.12-14[B])
• Fault Model Explaining the Variation of Strain, Tilt, Underground Water During 2020/03/16PM-18 (Fig.12-14[C])
• Fault Model Explaining the Variation of Strain, Tilt, Underground Water During 2020/03/19PM-21AM (Fig.12-14[D])
• Fault Model Explaining the Variation of Strain, Tilt, Underground Water During 2020/03/21PM-22PM (Fig.12-14[E])
• Fault Model Explaining the Variation of Strain, Tilt, Underground Water During 2020/03/22PM-23AM (Fig.12-14[F])

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(4) Kinki-Chugoku-Shikoku Region

– In the central and the western to Shikoku, since February 10 till March 9, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the strain, tilt, and GNSS data, slight crustal deformations were observed in the surrounding area, almost synchronous with the deep low-frequency earthquakes (tremors). These phenomena are considered to be caused by the short-term slow slip at the boundary between the Philippine Sea and the continental plates.

– According to the GNSS observation in the western Kii Peninsula and the eastern Shikoku, the crustal deformation different from the ordinary trend observed since the spring 2019 is decaying from the beginning of this year. This crustal 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.

• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Central and Western Shikoku
• Observed Result of Strain in Shikoku Region (2020/02/01 00:00-2020/03/03 00:00 (JST))
• Observed Result of Tilt in Shikoku Region (2020/02/01 00:00-2020/03/03 00:00 (JST)) (1)
• Observed Result of Tilt in Shikoku Region (2020/02/01 00:00-2020/03/03 00:00 (JST)) (2)
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[A]) 2020/02/08-11AM
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[B]) 2020/02/11PM-14
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[C]) 2020/02/15-18
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[D]) 2020/02/19-20AM
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[E]) 2020/02/20PM-21AM
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2-4[F]) 2020/02/21PM-22
• Slow Slip Synchronous with Deep Low-Frequency Tremor in the Western Shikoku Estimated from GNSS Data (provisional)
• 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)

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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 April 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, April 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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