September 11, 2017
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for August 2017

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in August 2017 (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 26, there was a magnitude (M) 4.9 earthquake at a depth of approximately 10 km, Nakadori, Fukushima prefecture [Hamadori, Fukushima prefecture]. The focal mechanism showed a normal fault type with a tension axis in an N-S direction. This event occurred within the crust.

• Tohoku Region
• Earthquake in Nakadori, Fukushima Prefecture on August 26

to supplementary reports

(3) Kanto-Chubu Region

* On August 2, there was an M5.5 earthquake at a depth of approximately 10 km in the northern Ibaraki prefecture. The focal mechanism showed a normal fault type with a tension axis in an NE-SW direction. In addition on August 27, there was an M4.8 earthquake in the vicinity of this event. These events occurred in the crust.

* On August 2, there was an M4.6 earthquake at a depth of approximately 50 km under the southern Ibaraki prefecture. 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 Philippine Sea and the continental plates.

* On August 3, there was an M4.6 earthquake at a depth of approximately 45 km under the southern Ibaraki prefecture. 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 Philippine Sea and the continental plates.

* On August 10, there was an M5.0 earthquake at a depth of approximately 65 km under the northwestern Chiba prefecture. 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.

* On August 14, there was an M4.5 earthquake at a depth of approximately 100 km under the northwestern Chiba prefecture. The focal mechanism had a tension axis in a WNW-ESE direction. This event occurred within the Pacific plate.

* In the GNSS data for the Tokai region, there has been no unusual change of activity which is likely to be associated with the Tokai earthquake.

• Kanto - Chubu Region
• Earthquakes in the Northern Ibaraki Prefecture on August 2 and August 27
• Earthquake under the Southern Ibaraki Prefecture on August 2
• Earthquake under the Southern Ibaraki Prefecture on August 3
• Earthquake under the Northwestern Chiba Prefecture on August 10
• Earthquake under the Northwestern Chiba Prefecture on August 14
• Continuous GNSS Monitoring along the Line from Kakegawa to Omaezaki (Baseline Length and Relative Height)

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 4, there was an M5.3 earthquake at a depth of approximately 65 km under near Iriomotejima. The focal mechanism showed a reverse fault type with a compression axis in an NE-SW direction. This event occurred within the Philippine Sea plate.

* On August 8, there was an M5.2 earthquake at a depth of approximately 15 km in the ocean area near Okinawa Main Island. The focal mechanism showed a normal fault type with a tension axis in an NW-SE direction. This event occurred within the crust of the continental plate.

* On August 24, there was an M4.4 earthquake at a depth of approximately 5 km in the Kagoshima Bay. The focal mechanism showed a strike-slip fault type with a tension axis in a WNW-ESE direction. This event occurred within the crust.

• Kyushu Region
• Seismic Activity in the Kagoshima Bay
• Seismic Activity in the Kagoshima Bay since December 2016
• Situations Before and After Earthquakes in Satsuma Region, Kagoshima prefecture in 1893, 1894
• Okinawa Region
• Earthquake near Iriomotejima on August 4
• Earthquake in the ocean area near Okinawa Main Island on August 8

to supplementary reports

Supplement

* On September 8, there was an M4.1 earthquake at a depth of approximately 15 km in the Kumamoto region, Kumamoto prefecture. The focal mechanism showed a normal fault type (preliminary) with a tension axis in an N-S direction. This event occurred within the crust.

* On September 8, there was an M5.2 earthquake at a depth of approximately 10 km in the southern inland, Akita prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an NW-SE direction. This event occurred within the crust.

• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• Earthquake in Southern Inland, Akita Prefecture on September 8, 2017 (Details of Generating Places)
• Earthquake in Southern Inland, Akita Prefecture on September 8, 2017 (Past Seismicity in the Surrounding Area)

Note:[ ] is the regional name of the epicenter used in the Japan Meteorological Agency announcement.
      (Global Navigation Satellite System) is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for August 2017

September 11, 2017
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 2017.

• M4.0 or greater: 62 (78 events in July)
• M5.0 or greater: 8 (13 events in July)
• M6.0 or greater: 0 (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 2016 to late July 2017 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

–"In the GNSS data for the Tokai region, there has been no unusual change of activity which is likely to be associated with the Tokai earthquake.":
(This evaluation is in agreement with the views presented at the regular meeting on August 28 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster (reference below).)

(Reference)

Recent seismic and crustal activity in the Tokai region and its vicinity (Seismological and Volcanological Department, Japan Meteorological Agency, August 28, 2017)
"There has been no unusual change of activity which is likely to be associated with the Tokai earthquake until now. (Excerpted)"

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(4) Kinki-Chugoku-Shikoku Region

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

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(5) Kyushu-Okinawa Region

– In August, no earthquake (1 in July) with maximum seismic intensity 4 or over and 1 earthquake (1 in July) with maximum seismic intensity 3 occurred in the Kumamoto and Aso regions, Kumamoto prefecture. Note that in early stage of September there was 1 earthquake with maximum seismic intensity 4. A series of seismic activity of the 2016 Kumamoto Earthquakes in the Kumamoto and Aso regions still continues, decaying as a whole, and the seismic activity at present will continue for a while. According to the GNSS observation, postseismic deformation in the area centered on the series of the seismic activity is subsiding as a whole.

–"On August 24, there was an M4.4 earthquake at a depth of approximately 5 km in the Kagoshima Bay. (Excerpted.) ": 
After the occurrence of an M5.3 earthquake on July 11, a rather high seismic activity continues. In August there were 28 earthquakes with maximum seismic intensity 1 or over. In addition, in the Satsuma region, Kagoshima prefecture, in the south of this activity, seismic activity has become a little higher.

• "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)
• Seismic Activity in the Kagoshima Bay
• Seismic Activity in the Kagoshima Bay since December 2016
• Situations Before and After Earthquakes in Satsuma Region, Kagoshima prefecture in 1893, 1894

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