December 10, 2019
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for November 2019

1 Major Seismic Activities

There was no remarkable activity.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On November 23, there was a magnitude (M) 5.4 earthquake, offshore east of Hokkaido. The focal mechanism had a compression axis in an ENE-WSE direction.

• Hokkaido Region
• Earthquake Offshore East of Hokkaido on November 23

to supplementary reports

(2) Tohoku Region

* On November 3, there was an M5.0 earthquake at a depth of approximately 40 km, offshore Fukushima prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction.

* On November 29, there was an M5.6 earthquake at a depth of approximately 30 km (CMT solution), offshore Sanriku. 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 November 3
• Earthquake Offshore Sanriku on November 29

to supplementary reports

(3) Kanto-Chubu Region

* On November 8, there was an M4.4 earthquake at a depth of approximately 50 km, offshore Ibaraki prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an NE-SW direction. This event occurred within the Pacific plate.

* On November 17, there was an M4.7 earthquake at a depth of approximately 15 km in the ocean area near Izu-Oshima. The focal mechanism showed a strike-slip fault type with a compression axis in an N-S direction. This event occurred within the Philippine Sea plate.

* On November 22, there was an M4.5 earthquake at a depth of approximately 45 km, offshore 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.

• Kanto - Chubu Region
• Earthquake Offshore Ibaraki Prefecture on November 8
• Earthquake in the Ocean Area near Izu-Oshima on November 17
• Earthquake Under the Southern Ibaraki Prefecture on November 22

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On November 26, there was an M4.6 earthquake at a depth of approximately 15 km in the central Setonaikai. The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. This event occurred within the crust.

• Kinki-Chugoku-Shikoku Region
• Earthquake in the Central Setonaikai on November 26

to supplementary reports

(5) Kyushu-Okinawa Region

* On November 22, there was an M5.2 earthquake at a depth of approximately 25 km in the Hyuganada. 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 Philippine Sea and the continental plates.

• Kyushu Region
• Earthquake in the Hyuganada on November 22
• 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

* On December 3, there was an M4.7 earthquake at a depth of approximately 50 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 4, there was an M4.9 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 NNE-SSW direction. This event occurred within the crust. In addition, on the 5th, there was an M4.6 earthquake at almost the same place.

* On December 4, there was an M4.8 earthquake at a depth of approximately 10 km in the northern Tochigi prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. This event occurred within the crust.

• Earthquake Under the Southern Ibaraki Prefecture on December 3
• Earthquakes in the Northern Ibaraki Prefecture on December 4 and 5
• Earthquake in the Northern Tochigi Prefecture on December 4

Supplementary Information to the Evaluation of Seismic Activities for November 2019

December 10, 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 November 2019.

• M4.0 or greater: 63 (79 events in October)
• M5.0 or greater: 6 (7 events in October)
• M6.0 or greater: 0 (0 event in October)

(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 November 2018 to late October 2019 have been listed as "Major Seismic Activities" 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

– In the northern Kii Peninsula, from November 10 to 20, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates.
According to the strain, tilt and GNSS data, slight crustal deformations were observed in the surrounding area synchronously to the deep low-frequency earthquakes (tremors).
These phenomena are considered to be caused by the short-term slow slip at the boundary between the Philippine Sea and the continental plates

• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Area from the Northern Kii Peninsula to Tokai Region
• Time Variation of Strain, Tilt, Underground Water (2019/11/04 00:00 - 2019/11/25 00:00 (JST)) (1)
• Time Variation of Strain, Tilt, Underground Water (2019/11/04 00:00 - 2019/11/25 00:00 (JST)) (2)
• Time Variation of Strain, Tilt, Underground Water (2019/11/04 00:00 - 2019/11/25 00:00 (JST)) (3)
• Fault Model Explaining the Variation During 2019/11/11-13AM (Fig.2-4[A])
• Fault Model Explaining the Variation During 2019/11/13PM-14AM (Fig.2-4[B])
• Fault Model Explaining the Variation During 2019/11/14PM-18AM (Fig.2-4[C-1])
• Fault Model Explaining the Variation During 2019/11/14PM-20 (Fig.2-4[C-2])
• Slow-Slip in the Northern Kii Peninsula Synchronizing with Deep Low-Frequency Tremor, Estimated from GNSS Data (provisional).

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(4) Kinki-Chugoku-Shikoku Region

– The crustal deformation different from the ordinary trend, found in GNSS Observation, in the northern Kyushu beginning in spring 2018 seems to have converged in autumn 2019. Also the similar crustal deformation, in GNSS and strain observation, in the western Shikoku beginning in autumn 2018 seems to be stagnant in June 2019. 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. The long-term slow slip seems to have converged in the northern Hyuganada in autumn 2019 and to be stagnant around the Bungo Channel since June 2019.

• Non-Stationary Horizontal Crustal Deformation of Western Shikoku (Linear trend, annual component, semi-annual component removed)
• Time Series of GNSS Continuous Observation: Western Shikoku
• Long-Term Slow Slip in the Hyuganada and Bungo Channel Estimated from GNSS Data (provisional) (1) - Distribution of Estimated Slip -
• Long-Term Slow Slip in the Hyuganada and Bungo Channel Estimated from GNSS Data (provisional) (2) - Comparison Between Observed Values (black) and Calculated Values (white) -

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(5) Kyushu-Okinawa Region

There is no supplementary information for Kyushu-Okinawa region.

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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 December 6 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 6, 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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