October 12, 2016
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for September 2016

1 Major Seismic Activities

* On September 26, there was a magnitude (M) 5.6 earthquake under the ocean area near Okinawa Main Island. This event had a maximum seismic intensity 5 Lower observed in Kagoshima prefecture (Okinoerabujima).

• Seismic Activities for All of Japan in September 2016 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On September 7, there was a M4.7 earthquake at a depth of approximately 110 km under the central Hidaka region. The focal mechanism had a tension axis in an NNE-SSW direction. This event occurred within the Pacific plate.

* On September 26, there was a M5.4 earthquake at a depth of approximately 30 km, offshore Urakawa. The focal mechanism showed a strike-slip fault type with a compression axis in an NE-SW direction.

• Hokkaido Region
• Earthquake under the Central Hidaka Region on September 7
• Earthquake Offshore Urakawa on September 26

to supplementary reports

(2) Tohoku Region

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* On September 7, there was a M4.9 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 September 9, there was a M5.4 earthquake at a depth of approximately 45 km offshore Ibaraki 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 September 13, there was a M4.9 earthquake at a depth of approximately 75 km under the southern Saitama prefecture. The focal mechanism had a tension axis in an NW-SE direction.

* On September 21, there was a M6.5 earthquake at a depth of approximately 10 km (due to CMT solution) under the ocean area near Torishima. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction.

* On September 23, there was a M6.7 earthquake at a depth of approximately 20 km (due to CMT solution) offshore east of Kanto region. The focal mechanism had a compression axis in an E-W direction. This event caused tsunami with height of 0.2 m at Hachijo-jima.

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

• Kanto - Chubu Region
• Earthquake under the Southern Ibaraki Prefecture on September 7
• Earthquake Offshore Ibaraki Prefecture on September 9
• Earthquake under the Southern Saitama Prefecture on September 13
• Earthquake under the Ocean Area near Torishima on September 21
• Earthquake Offshore East of Kanto Region on September 23
• Tsunami caused by the earthquake offshore east of Kanto region on September 23
• Continuous GNSS Monitoring in the Area from Kakegawa to Omaezaki

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 September 1, there was a M4.8 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. In addition, on September 7, there was a M3.9 earthquake at almost the same place as the previous one. The focal mechanisms showed a strike-slip fault type with a tension axis in an N-S direction. These events occurred within the crust.

* On September 26, there was a M5.6 earthquake at a depth of approximately 45 km under the ocean area near Okinawa Main Island. 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.

• Kyushu Region
• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• Number of Earthquakes of "the 2016 Kumamoto Earthquakes"
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (1)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (2)
• Earthquake under the Ocean Area near Okinawa Main Island (near Okinoerabujima) on September 26
• Okinawa Region

to supplementary reports

(6) Other Region

* On September 12, there was a M5.8 earthquake at a depth of approximately 20 km (due to CMT solution) in the southern Korean peninsula. The focal mechanism showed a strike-slip fault type with a compression axis in an ENE-WSW direction. This event occurred within the crust.

• Earthquake in the Southern Korean Peninsula on September 12

Note:GNSS (Global Navigation Satellite System) is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for September 2016

October 12, 2016
Earthquake Research Committee

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during September 2016.

• M4.0 or greater: 102 (75 events in August 2016)
• M5.0 or greater: 21 (16 events in August 2016)
• M6.0 or greater: 2 (4 events in August 2016)

(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 September 2015 to late August 2016 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

–"On September 23, there was a M6.7 earthquake at a depth of approximately 20 km, offshore east of Kanto region. (Excerpted) ":
In the vicinity of this event, seismic activity became high since September, and 9 earthquakes of M5.0 or over occurred until September 30.

–"In the GNSS data for the Tokai region, there has been no 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 September 26 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, September 26, 2016)
"No change of activity which is likely to be associated with the Tokai earthquake has been observed until now. (Excerpted)"

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(4) Kinki-Chugoku-Shikoku Region

– Non-stationary crustal deformation surrounding the Kii Channel observed from mid-2014 is now decaying. This deformation is thought to be due to a slow slip (slow-slip) at the boundary between the Philippine Sea and the continental plates.

– Non-stationary crustal deformation surrounding the Bungo Channel observed from around December 2015 is still continuing. This deformation is thought to be due to a slow slip (slow-slip) at the boundary between the Philippine Sea and the continental plates.

• Non-Stationary Crustal Deformation around the Kii Channel
• Non-Stationary Crustal Deformation around the Bungo Channel (1)
• Non-Stationary Crustal Deformation around the Bungo Channel (2)

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(5) Kyushu-Okinawa Region

–"On September 1, there was a M4.8 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. (Excerpted) ":
In September, 2 earthquakes with maximum seismic intensity 4 (2 in August) and 7 earthquakes with maximum seismic intensity 3 (3 in August) occurred in the Kumamoto region and the Aso region. A series of seismic activity of the 2016 Kumamoto Earthquakes, in the area extending from Kumamoto region to Aso region is still continuing, decaying as a whole. According to the GNSS observation, postseismic deformation is still observed in the area centered on the series of the seismic activity.

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