April 9, 2019
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for March 2019

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in March 2019 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On March 2, there was a magnitude (M) 6.2 earthquake at a depth of approximately 30 km (due to CMT solution), offshore southeast of Nemuro Peninsula. The focal mechanism showed a strike-slip fault type with a tension axis in an N-S direction. This event occurred within the Pacific plate.

• Hokkaido Region
• Earthquake Offshore Southeast of Nemuro Peninsula on March 2

to supplementary reports

(2) Tohoku Region

* On March 7, there was an M4.6 earthquake at a depth of approximately 70 km, offshore Miyagi prefecture. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction. This event occurred within the Pacific plate.

* On March 11, there was an M6.0 earthquake at a depth of approximately 20 km (due to CMT solution), offshore Fukushima 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.

• Tohoku Region
• Earthquake Offshore Miyagi Prefecture on March 7
• Earthquake Offshore Fukushima Prefecture on March 11

to supplementary reports

(3) Kanto-Chubu Region

* On March 9, there was an M4.4 earthquake at a depth of approximately 40km under the central and western Mino region, Gifu prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.

* On March 11, there was an M6.1 earthquake under the ocean area near Ioto Island.

• Kanto - Chubu Region
• Earthquake Under the Central and Western Mino Region, Gifu Prefecture on March 9
• Earthquake Under the Ocean Area near Ioto Island on March 11

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On March 11, there was an M4.6 earthquake at a depth of approximately 40 km under the Nanyo region, Ehime prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an ENE-WSW direction. This event occurred within the Philippine Sea plate.

* On March 13, there was an M5.3 earthquake at a depth of approximately 45 km under the Kii Channel. The focal mechanism showed a strike-slip fault type with a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.

• Kinki-Chugoku-Shikoku Region
• Earthquake Under the Nanyo Region, Ehime Prefecture on March 11
• Earthquake Under the Kii Channel on March 13

to supplementary reports

(5) Kyushu-Okinawa Region

* On March 27 at 09:11 (JST), there was an M5.4 earthquake at a depth of approximately 15 km in the Hyuganada. Also on the same day at 15:38 (JST), an M5.4 earthquake occurred in the neighborhood. The focal mechanisms of these events showed a reverse fault type with a compression axis in a WNW-ESE direction. These events occurred at the boundary between the Philippine Sea and the continental plates.

* On March 30, there was an M5.0 earthquake at a depth of approximately 30 km in the ocean area near Tanegashima Island. 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 Philippine Sea and the continental plates.

• Kyushu Region
• Earthquake in the Hyuganada on March 27
• Earthquake in the Hyuganada on March 27, 2019 - the Relation Between the Origin and the Plate Boundary -
• Earthquake under the Ocean Area near Tanegashima Island on March 30
• 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 2019

April 9, 2019
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 2019..

• M4.0 or greater: 86 (79 events in February)
• M5.0 or greater: 17 (14 events in February)
• M6.0 or greater: 3 (0 event in February)

(reference)

• M4.0 or greater: 73 / month (Median number of earthquakes over the past 10 years from 1998 through 2007)
• M5.0 or greater: 9 / month (Median number of earthquakes over the past 35 years from 1973 through 2007)
• M6.0 or greater: 1.4 / month, approximately 17 / year (Average number of earthquakes over the past 84 years from 1924 through 2007)

The following events for the period from March 2018 to late February 2019 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

There is no supplementary information for Kanto-Chubu region.

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(4) Kinki-Chugoku-Shikoku Region

– According to the GNSS observation in the northern Kyushu from 2018 spring, and according to the GNSS and strain observation in the western Shikoku from 2018 autumn, crustal deformations different from the ordinary trend have been observed. These phenomena are considered to be caused by a slow slip at the deeper part of the boundary between the Philippine Sea and the continental plates, which began at the northern Hyuganada and still continues at the Bungo Channel.

– In the central and western Shikoku, for the period from March 1 until 22, a deep low-frequency seismic (tremor) activity was observed near the boundary between the Philippine Sea and the continental plates. According to the strain, tilt and GNSS data in the surrounding area, a slight crustal deformation was observed almost synchronously to the deep low-frequency seismic (tremor) activity. These phenomena are considered to be caused by a short-term slow slip at the boundary between the Philippine Sea and the continental plates.

• Non-Stationary Horizontal Crustal Deformation of Northern Kyushu and Western Shikoku
• Time Series of GNSS Continuous Observation: Northern Kyushu and Western Shikoku
• Long-Term Slow Slip in the Hyuganada and Bungo Channel Estimated from GNSS Data (provisional) (1) - Distribution of Estimated Slip -
• Long-Term Slow Slip in the Hyuganada and Bungo Channel Estimated from GNSS Data (provisional) (2) - Comparison Between Observed Values (black) and Calculated Values (white) -
• Activity Status of Deep Low-Frequency Tremors in Shikoku (March 2019)
• Deep Low-Frequency Seismic (Tremor) Activity and Slow-Slip in Shikoku
• Fig.6 Variation of Strain and Tilt (2019/02/15 00:00 - 2019/03/15 00:00 (JST)) (1)
• Fig.6 Variation of Strain and Tilt (2019/02/15 00:00 - 2019/03/15 00:00 (JST)) (2)
• Fig. 7 Fault Model Explaining the Variation of Strain and Tilt (Fig.6[A]) 2019/03/02-04AM
• Fig. 8 Fault Model Explaining the Variation of Strain and Tilt (Fig.6[B]) 2019/03/04PM-06
• Fig. 9 Fault Model Explaining the Variation of Strain and Tilt (Fig.6[C]) 2019/03/07-09
• Time Series of GNSS Continuous Observation: Around the Nankai Trough
• Slow-Slip in Shikoku Region Synchronizing with Deep Low-Frequency Tremor, Estimated from GNSS Data (provisional) - Distribution of Estimated Slip -

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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 5 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 5, 2019)
"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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