November 11, 2020
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for October 2020

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 October 6, there was a magnitude (M) 4.7 earthquake at a depth of approximately 95 km, under the southern coast of Iwate prefecture (#). The focal mechanism had a tension axis in an NW-SE direction. This event occurred within the Pacific plate.

(3) Kanto-Chubu Region

* On October 3, there was an M5.8 earthquake at a depth of approximately 40 km (due to CMT solution), offshore east of Hachijojima Island. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction.

(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.

# "The northern coast of Iwate prefecture" is the region name used in the announcement of Japan Meteorological Agency.



Supplementary Information to the Evaluation of Seismic Activities for October 2020

November 11, 2020
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 October 2020.

(reference)

The following events for the period from October 2019 to late September 2020 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

  – Offshore East of Aomori Prefecture  M5.5 on December 19, 2019
(Depth of approximately 50 km)
  – Noto Region, Ishikawa Prefecture  M5.5 on March 13, 2020
(Depth of approximately 10 km)
  – Offshore East of Chiba Prefecture  M6.1 on June 25, 2020
(Depth of approximately 35 km)
  – Reihoku, Fukui Prefecture  M5.0 on September 4, 2020
(Depth of approximately 5 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

– According to the GNSS observation, the crustal deformation different from the ordinary trend observed in the Shima Peninsula from mid-2019 seems to have decayed. This phenomenon is considered to be caused by a long-term slow slip around the Shima Peninsula at the deeper part of the boundary between the Philippine Sea and the continental plates.

(4) Kinki-Chugoku-Shikoku Region

– According to the GNSS observation, the crustal deformation different from the ordinary trend in the central Shikoku has been observed from spring 2019. This phenomenon is considered to be caused by a long-term slow slip around the central Shikoku at the deeper part of the boundary between the Philippine Sea and the continental plates.

(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 November 9 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, November 9, 2020)
"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)