October 9, 2019
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for September 2019

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 September 7, there was an M5.0 earthquake at a depth of approximately 65 km, offshore east of Aomori 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.

(3) Kanto-Chubu Region

There was no remarkable activity.

(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.

# : [Offshore Urakawa] was the regional name of the epicenter used in the Japan Meteorological Agency announcement.



Supplementary Information to the Evaluation of Seismic Activities for September 2019

October 9, 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 September 2019.

(reference)

The following events for the period from September 2018 to late August 2019 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

  – Middle and Eastern Iburi Regions *  M6.7 on September 6, 2018
(Depth of approximately 35 km)
  – Middle and Eastern Iburi Regions *  M5.2 on October 5, 2018
(Depth of approximately 30 km)
  – Kumamoto Region, Kumamoto Prefecture  M5.1 on January 3, 2019
(Depth of approximately 10 km)
   M4.3 on January 26, 2019
(Depth of approximately 10 km)
  – Middle and Eastern Iburi Regions *  M5.8 on February 21, 2019
(Depth of approximately 35 km)
  – Northern Hyuganada  M6.3 on May 10, 2019
(Depth of approximately 25 km)
  – Northeastern Chiba Prefecture  M5.1 on May 25, 2019
(Depth of approximately 40 km)
  – Offshore Yamagata Prefecture  M6.7 on June 18, 2019
(Depth of approximately 15 km)
  – Offshore Fukushima Prefecture  M6.4 on August 4, 2019
(Depth of approximately 45 km)
  * Seismic activity of the 2018 Hokkaido Eastern Iburi Earthquake

   

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

– According to the GNSS observation in the northern Kyushu from spring 2018, and according to the GNSS and strain observations in the western Shikoku from autumn 2018, crustal deformations different from the ordinary trend have been observed. These phenomena are considered to be caused by a slow slip at the deeper part of the boundary between the Philippine Sea and the continental plates, which continues at the northern Hyuganada and around the Bungo Channel. These crustal deformations and the long-term slow slip seem to be stagnant since June 2019.

(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 October 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, October 7, 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)"

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 or greater, or events that have maximum seismic intensity 4 or greater
  2. Onshore events of M4.5 or 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 the 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
  4. A sequence of slow-slip events of estimated M6.0 or greater and accompanying low-frequency earthquakes (tremors)