September 11, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for August 2018

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in August 2018 (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 August 8, there was a magnitude (M) 5.6 earthquake at a depth of approximately 30 km (due to CMT solution), offshore Sanriku. The focal mechanism showed a normal fault type with a tension axis in a WNW-ESE direction. This event occurred within the Pacific plate.

* On August 11, there was an M5.0 earthquake at a depth of approximately 20 km, offshore Fukushima prefecture. The focal mechanism showed a normal fault type with a tension axis in an NNW-SSE direction. This event occurred within the continental crust.

* On August 24, there was an M5.1 earthquake at a depth of approximately 30 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.

• Tohoku Region
• Earthquake Offshore Sanriku on August 8
• Earthquake Offshore Fukushima Prefecture on August 11
• Earthquake Offshore East of Aomori Prefecture on August 24

to supplementary reports

(3) Kanto-Chubu Region

* On August 17 at 03:23 (JST), there was an M6.6 earthquake at a depth of approximately 10 km (due to CMT solution) in the ocean area near Ioto Island. 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. In addition, on the same day at 03:21 (JST), there was an M6.3 earthquake in the neighborhood of the recent event.

• Kanto - Chubu Region
• Earthquake in the Ocean Area near Ioto Island on August 17 at 03:21 and 03:23 (JST)

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 August 22, there was an M4.1 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. The focal mechanism showed a normal fault type with a tension axis in an N-S direction. This event occurred within the crust.

• Kyushu Region
• Earthquake in the Kumamoto Region, Kumamoto Prefecture on August 22
• 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 September 5, there was an M5.5 earthquake offshore Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction.

* Refer to the other term for the 2018 Hokkaido Eastern Iburi Earthquake.

• Earthquake Offshore Ibaraki Prefecture on September 5

Supplementary Information to the Evaluation of Seismic Activities for August 2018

September 11, 2018
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 August 2018.

• M4.0 or greater: 108 (87 events in July)
• M5.0 or greater: 13 (15 events in July)
• M6.0 or greater: 2 (1 event in July)

(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 August 2017 to late July 2018 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

to the text

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 region.

to the text

(4) Kinki-Chugoku-Shikoku Region

There is no supplementary information for Kinki-Chugoku-Shikoku region.

to the text

(5) Kyushu-Okinawa Region

–"On August 22, there was an M4.1 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. (Excerpted)":
The serial activity of the 2016 Kumamoto Earthquakes in the Kumamoto and Aso regions, Kumamoto prefecture still remains, though it is continually decaying as a whole. The present activity will continue for the time beeing. GNSS observation result shows the slight aftershock deformation still continues.

• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (1)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (2)

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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 September 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, September 7, 2018)
"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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