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.

• Seismic Activities for All of Japan in October 2020 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There was no remarkable activity.

• Hokkaido Region

to supplementary reports

(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.

• Tohoku Region
• Earthquake under the southern coast of Iwate Prefecture on October 6

to supplementary reports

(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.

• Kanto - Chubu Region
• Earthquake Offshore East of Hachijojima Island on October 3

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

# "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.

• M4.0 or greater: 64 (90 events in September)
• M5.0 or greater: 4 (9 events in September)
• M6.0 or greater: 0 (1 event in September)

(reference)

• M4.0 or greater: 81 / month (69-104)
• (Median monthly number from 1998 to 2017, half of data are included in the interval shown in parentheses)
• M5.0 or greater: 10 / month (7-14)
• (Median monthly number from 1973 to 2017, half of data are included in the interval shown in parentheses)
• M6.0 or greater: 1 / month (0-2)
• (Median monthly number from 1919 to 2017, half of data are included in the interval shown in parentheses)
• M6.0 or greater: 16 / year (12-21)
• (Median annual number from 1919 to 2017, half of data are included in the interval shown in parentheses)

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."

to the text

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

to the text

(2) Tohoku Region

There is no supplementary information for Tohoku region.

to the text

(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.

• Nonstationary Horizontal Crustal Deformation in the Shima Peninsula (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Shima Peninsula (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Shima Peninsula (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Shima Peninsula Estimated from GNSS Data (provisional)

to the text

(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.

• Non-Stationary Horizontal Crustal Deformation of the Central Shikoku (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Central Shikoku (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Central Shikoku (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Central Shikoku Estimated from GNSS Data (provisional)

to the text

(5) Kyushu-Okinawa Region

There is no supplementary information for Kyushu-Okinawa region.

to the text

(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)"

to the text