January 15, 2019
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for December 2018

1 Major Seismic Activities

There was no remarkable activity.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On December 30, there was a magnitude (M) 5.3 earthquake at a depth of approximately 50 km under the southern Tokachi region. 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 continental plates.

• Hokkaido Region
• Earthquake Under the Southern Tokachi Region on December 30

to supplementary reports

(2) Tohoku Region

* On December 8, there was an M5.0 earthquake at a depth of approximately 50 km, offshore Fukushima prefecture. The focal mechanism had a compression axis in an NW-SE direction.

* On December 16, there was an M4.6 earthquake at a depth of approximately 60 km under the southern coast of Iwate prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an E-W direction. This event occurred within the Pacific plate.

* On December 31, there was an M5.0 earthquake at a depth of approximately 45 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.

• Tohoku Region
• Earthquake Offshore Fukushima Prefecture on December 8
• Earthquake Under the Southern Coast of Iwate Prefecture on December 16
• Earthquake Offshore Fukushima Prefecture on December 31

to supplementary reports

(3) Kanto-Chubu Region

There was no remarkable activity

• Kanto - Chubu Region

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

There was no remarkable activity

• Kyushu Region
• 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

* Refer to the other article (Evaluation of the Earthquake in the Kumamoto region, Kumamoto prefecture on January 3, 2019) for the earthquake in the Kumamoto region, Kumamoto prefecture on January 3.

* On January 6, there was an M4.5 earthquake at a depth of approximately 100 km under the northern Miyagi prefecture. The focal mechanism had a tension axis in the subducting direction of the plate. This event occurred within the Pacific plate.

* On January 8, there was an M4.0 earthquake at a depth of approximately 10 km in the ocean area near Amami Oshima. This event occurred within the crust of the continental plate.

* On January 8, there was an M6.0 earthquake at a depth of approximately 30 km in the ocean area near Tanegashima 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. The result of the GNSS observation showed the crustal movement in Tanegashima Island accompanying the event: about 1 cm displacement in south-east direction at Nishinoomote observatory and about 1 cm displacement in east-south-east direction at Nakatane observatory.

• Earthquake under the Northern Miyagi Prefecture on January 6
• Earthquake in the Ocean Area near Amami Oshima on January 8
• Earthquake in the Ocean Area near Tanegashima Island on January 8
• Observed Data Before and After the Earthquake in the Ocean Area near Tanegashima Island (January 8, M6.0) (provisional)

Supplementary Information to the Evaluation of Seismic Activities for December 2018

January 15, 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 December 2018.

• M4.0 or greater: 79 (88 events in November)
• M5.0 or greater: 13 (11 events in November)
• M6.0 or greater: 0 (2 events in November)

(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 December 2017 to late November 2018 have been listed as " 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

There is no supplementary information for Kanto-Chubu region.

to the text

(4) Kinki-Chugoku-Shikoku Region

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

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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 spring of 2018, and also in the western Shikoku since about autumn of 2018. These phenomena are 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 in the neighborhood of the northern Hyuganada and 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)

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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 January 10 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, January 10, 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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