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

Evaluation of Seismic Activities for November 2018

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in November 2018 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On November 2, there was a magnitude (M) 6.1 earthquake at a depth of approximately 490 km under the southern Okhotsk Sea. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

* On November 5, there was an M6.3 earthquake at a depth of approximately 20 km near Kunashiri Island. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred within the crust of the continental plate. In the vicinity of the recent event, on October 26, there was an M5.5 earthquake.

* On November 14, there was an M4.7 earthquake at a depth of approximately 30 km in the middle and the eastern Iburi regions. The focal mechanism showed a reverse fault type with a compression axis in an N-S direction. This event occurred within the continental plate.

• Hokkaido Region
• Earthquake Under the Southern Okhotsk Sea on November 2
• Earthquake near Kunashiri Island on November 5

to supplementary reports

(2) Tohoku Region

* On November 23, there was an M5.0 earthquake at a depth of approximately 50 km, offshore Fukushima 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 November 28, there was an M5.7 earthquake at a depth of approximately 20 km (due to CMT solution), offshore east of Aomori prefecture. The focal mechanism had a compression axis in an N-S direction. This event occurred within the crust of the continental plate.

• Tohoku Region
• Earthquake Offshore Fukushima Prefecture on November 23
• Earthquake Offshore East of Aomori Prefecture on November 28

to supplementary reports

(3) Kanto-Chubu Region

* On November 27, there was an M5.0 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 Pacific and the Philippine Sea plates.

• Kanto - Chubu Region
• Earthquake Under the Southern Ibaraki Prefecture on November 27

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On November 2, there was an M5.4 earthquake at a depth of approximately 45 km under the Kii Channel. The focal mechanism showed a strike-slip fault type with a tension axis in an ENE-WSW direction. This event occurred within the Philippine Sea plate. In the vicinity of the recent event, on November 5, there was an M4.6 earthquake.

• Kinki-Chugoku-Shikoku Region
• Earthquakes Under the Kii Channel on November 2 and 5

to supplementary reports

(5) Kyushu-Okinawa Region

* On November 21, there was an M5.2 earthquake at a depth of approximately 120 km under the ocean area near Tanegashima Island. The focal mechanisms of this earthquake had a tension axis in the subducting direction of the Philippine Sea plate. This event occurred within the Philippine Sea plate.

• Kyushu Region
• Earthquake Under the Ocean Area near Tanegashima Island on November 21
• 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

Supplementary Information to the Evaluation of Seismic Activities for November 2018

December 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 November 2018.

• M4.0 or greater: 88 (96 events in October)
• M5.0 or greater: 11 (16 events in October)
• M6.0 or greater: 2 (2 events in October)

(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 November 2017 to late October 2018 have been listed as " in monthly "Evaluation of Seismic Activities."

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2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

–"On November 14, there was an M4.7 earthquake at a depth of approximately 30 km in the middle and the eastern Iburi regions. (Excerpted)
The seismic activity of the 2018 Hokkaido Eastern Iburi Earthquake still continues decaying as a whole, such that in November there occurring 1 earthquake with maximum seismic intensity 4 and 13 earthquakes with seismic intensity 1 or over.

• Seismic activity of "the 2018 Hokkaido Eastern Iburi Earthquake" (1)
• Seismic activity of "the 2018 Hokkaido Eastern Iburi Earthquake" (2)
• Number of earthquakes of "the 2018 Hokkaido Eastern Iburi Earthquake" by the maximum seismic intensity

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(2) Tohoku Region

There is no supplementary information for Tohoku region.

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(3) Kanto-Chubu Region

– On November 23, the seismicity became active in the Hida region, Gifu prefecture [the middle Nagano prefecture or the Hida region, Gufu prefecture] (near the boundary between Gifu and Nagano prefectures) and there occurred 51 earthquakes with seismic intensity 1 or over until December 9. The largest earthquake was the M3.1 one occurring on November 25.

  Note: [ ] is the regional name of the epicenter used in the Japan Meteorological Agency announcement.

• Seismic Activity in the Hida Region, Gifu Prefecture (near the Boundary between Gifu and Nagano Prefectures) Beginning on November 23

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(4) Kinki-Chugoku-Shikoku Region

– According to the GNSS-sonic ranging observation until September, a crustal deformation different from the so far trend has been observed on the sea bottom offshore the Kii Channel since about the end of 2017. This phenomenon is considered to be caused by a slow slip at the shallow part of the boundary between the Philippine Sea and the continental plates.

• Fault Model Explaining the Non-stationary Variation (Estimated Solution of Deep Inclination of Sound Velocity) Offshore Kii Channel

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(5) Kyushu-Okinawa Region

– According to the GNSS observation, a crustal deformation different from the so far trend has been observed in the northern Kyushu since about the spring of 2018. This phenomenon is considered to be caused by a long-term slow slip at the deep part of the boundary between the Philippine Sea and the continental plates.

• Time Series of GNSS Continuous Observation: Northern Kyushu
• Estimated Long-Term Slow Slip in the Northern Hyuganada (provisional)

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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 December 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, December 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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