January 15, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for December 2017

1 Major Seismic Activities

There was no remarkable activity.

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There was no remarkable activity.

(2) Tohoku Region

* On December 16, there was a magnitude (M) 5.5 earthquake at a depth of approximately 50 km, offshore Iwate prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an ENE-WSW direction. This event occurred within the Pacific plate.

(3) Kanto-Chubu Region

* On December 2, there was an M4.4 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 December 6, there was an M5.3 earthquake at a depth of approximately 10 km in the middle Nagano prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction. This event occurred within the crust.

* On December 6, there was an M4.5 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. This event occurred within the crust.

* On December 21, there was an M6.0 earthquake at a depth of approximately 10 km (CMT solution) in the ocean area near Torishima. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction.

* On December 27, there was an M4.5 earthquake at a depth of approximately 70 km under the Tokyo Bay [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.

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

(5) Kyushu-Okinawa Region

There was no remarkable activity.

(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.

Supplement

* On January 6, there was an M4.7 earthquake at a depth of approximately 70 km under the Tokyo Bay [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 January 6, there was an M4.5 earthquake at a depth of approximately 10 km, offshore east of Izu peninsula [the ocean area near Izu Oshima]. The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. This event occurred within the crust of the Philippine Sea plate.


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



Supplementary Information to the Evaluation of Seismic Activities for December 2017

January 15, 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 December 2017.

(reference)

The following events for the period from December 2016 to late November 2017 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

  – Northern Ibaraki Prefecture  M6.3 on December 28, 2016
(Depth of approximately 10 km)
  – Offshore Fukushima Prefecture  M5.7 on February 28, 2017
(Depth of approximately 50 km)
  – Bungo Channel  M5.0 on June 20, 2017
(Depth of approximately 40 km)
  – Southern Nagano Prefecture  M5.6 on June 25, 2017
(Depth of approximately 5 km)
  – Central and Eastern Iburi Region  M5.1 on July 1, 2017
(Depth of approximately 25 km)
  – Aso Region, Kumamoto Prefecture  M4.5 on July 2, 2017
(Depth of approximately 10 km)
  – Kagoshima Bay  M5.3 on July 11, 2017
(Depth of approximately 10 km)
  – Southern Inland, Akita Prefecture  M5.2 on September 8, 2017
(Depth of approximately 10 km)
  – Offshore Fukushima Prefecture  M5.9 on October 6, 2017
(Depth of approximately 55 km)

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

(2) Tohoku Region

There is no supplementary information for Tohoku region.

(3) Kanto-Chubu Region

There is no supplementary information for Kanto-Chubu region.

(4) Kinki-Chugoku-Shikoku Region

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

(5) Kyushu-Okinawa Region

There is no supplementary information for Kyushu-Okinawa region.

(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 25 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 25, 2017)
"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)"

Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.

  1. Events of M6.0 and greater, or events that have maximum seismic intensity 4 or greater
  2. Onshore events of M4.5 and greater that have maximum seismic intensity 3 or greater
  3. Events under the ocean areas of M5.0 or greater that have maximum seismic intensity 3 or greater

Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes:

  1. Reference information related to seismic activity described in the "Evaluation of Seismic Activity"
  2. Recent earthquake activity over the past year related to the events described in "Major Seismic Activity" in the "Evaluation of Seismic Activity"
  3. Seismic activity that has been evaluated, but not included in the "Evaluation of Seismic Activity" because it is within normal bounds and not prominent