June 11, 2019
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for May 2019

1 Major Seismic Activities

* On May 10, there was a magnitude (M) 6.3 earthquake in the Hyuganada. Maximum seismic intensity 5 Lower was observed in Miyazaki prefecture.

* On May 25, there was an M5.1 earthquake under the northeastern Chiba prefecture^#. Maximum seismic intensity 5 Lower was observed in Chiba prefecture.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* 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.

• Hokkaido Region
• Earthquake Offshore Southeast of Nemuro Peninsula on May 5

to supplementary reports

(2) Tohoku Region

* 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 18, there was an M4.6 earthquake at a depth of approximately 80 km under the northern Miyagi prefecture. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

• Tohoku Region
• Earthquake Offshore Iwate Prefecture on May 8
• Earthquake Under the Northern Miyagi Prefecture on May 18

to supplementary reports

(3) Kanto-Chubu Region

* On May 25, there was an M5.1 earthquake at a depth of approximately 40 km under the northeastern Chiba prefecture^#. The focal mechanism showed a normal fault type with a tension axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate. In the vicinity of the event, there was an M4.7 earthquake on June 1.

* On May 27, there was an M4.3 earthquake at a depth of approximately 10 km in the northern Ibaraki prefecture. The focal mechanism had a tension axis in an NE-SW direction. This event occurred within the crust.

• Kanto - Chubu Region
• Earthquakes Under the Northeastern Chiba Prefecture on May 25 and June 1
• Earthquake in the Northern Ibaraki Prefecture on May 27

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 May 10 at 07:43 (JST), there was an M5.6 earthquake at a depth of approximately 25 km in the Hyuganada. Also on the same day at 08:48 (JST), there was an M6.3 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 these events, crustal deformation of about 1 cm displacement in an S-E 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 showed a normal fault type with a tension axis in an E-W direction. This event occurred within the Philippine Sea plate.

* On May 15, there was an M5.7 earthquake offshore northeast of Amami Oshima. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction.

• Kyushu Region
• Earthquakes in the Hyuganada on May 10
• Observed Data Before and After the Earthquakes in the Hyuganada (May 10, M5.6, M6.3)
• Earthquake Under the Hyuganada on May 11
• Earthquake Offshore Northeast of Amami Oshima on May 15
• 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

Supplement

* On June 4, there was an M6.2 earthquake at a depth of approximately 450 km under the ocean area near Torishima Island. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

• Earthquake Under the Ocean Area near Torishima Island on June 4

# : The region name of the epicenter used in the JMA announcement is "the southern Chiba prefecture."

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

Supplementary Information to the Evaluation of Seismic Activities for May 2019

June 11, 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 May 2019.

• M4.0 or greater:78 (95 events in April)
• M5.0 or greater:8 (16 events in April)
• M6.0 or greater:1 (2 events in April)

(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 May 2018 to late April 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 (Linear trend, annual component, semi-annual component removed)
• 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) - 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 June 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, June 7, 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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