May 15, 2019
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for April 2019

1 Major Seismic Activities

There was no remarkable activity.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On April 15, there was a magnitude (M) 5.1 earthquake at a depth of approximately 45 km, offshore Kushiro. 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 Pacific and the continental plates.

* On April 28, there was an M5.6 earthquake at a depth of approximately 100 km under the southern Tokachi region. The focal mechanism had a tension axis in the subducting direction of the plate. This event occurred within the Pacific plate.

• Hokkaido Region
• Earthquake Offshore Kushiro on April 15
• Earthquake Under the Southern Tokachi Region on April 28

to supplementary reports

(2) Tohoku Region

* On April 11, there was an M6.2 earthquake at a depth of approximately 20 km (due to CMT solution), offshore Sanriku. 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 Sanriku on April 11

to supplementary reports

(3) Kanto-Chubu Region

There was no remarkable activity.

• Kanto - Chubu Region

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 18, there was an M6.5 earthquake at a depth of approximately 20 km near Taiwan. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction.

• Earthquake near Taiwan on April 18

to supplementary reports

Supplement

* On May 5, there was an M5.3 earthquake at a depth of approximately 50 km, offshore southeast of Nemuro Peninsula. The focal mechanism had a tension axis in an NW-SE direction. This event occurred within the Pacific plate.

* On May 8, there was an M4.4 earthquake at a depth of approximately 50 km, offshore Iwate 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 May 10 at 08:48 (JST), there was an M6.3 earthquake at a depth of approximately 25 km in the Hyuganada. Maximum seismic intensity 5 Lower was observed in Miyazaki prefecture. Also at 07:43 (JST) on the same day, there was an M5.6 earthquake at almost the same place. The focal mechanisms of these earthquakes 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. According to the GNSS observation, accompanied with the M6.3 event, crustal deformation of about 1 cm displacement (provisional) in an ESE direction was observed at Sadowara observatory, Miyazaki city.

* On May 11, there was an M5.0 earthquake at a depth of approximately 35km under the Hyuganada. The focal mechanism had a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.

• Earthquake Offshore Southeast of Nemuro Peninsula on May 5
• Earthquake Offshore Iwate Prefecture on May 8
• Earthquakes in the Hyuganada on May 10
• Observed data before and after the Earthquake in the Hyuganada (May 10, M6.3)
• Earthquake under the Hyuganada 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 2019

May 15, 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 April 2019.

• M4.0 or greater:95 (86 events in March)
• M5.0 or greater:16 (17 events in March)
• M6.0 or greater:2 (3 events in March)

(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 April 2018 to late March 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.

to the text

(2) Tohoku Region

There is no supplementary information for Tohoku region.

to the text

(3) Kanto-Chubu Region

There is no supplementary information for Kanto-Chubu region.

to the text

(4) Kinki-Chugoku-Shikoku Region

– According to the GNSS observation in the northern Kyushu from 2018 spring, and according to the GNSS and strain observations 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 continues at the northern Hyuganada and around the Bungo Channel.

• 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) -

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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 May 13 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 13, 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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(7) Other Regions

There is no supplementary information for other regions.

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