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

Evaluation of Seismic Activities for August 2019

1 Major Seismic Activities

* On August 4, there was a magnitude (M) 6.4 earthquake offshore Fukushima prefecture. Maximum seismic intensity 5 Lower was observed in Miyagi and Fukushima prefectures.

• Seismic Activities for All of Japan in August 2019 (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 August 4, there was an M6.4 earthquake at a depth of approximately 45 km, offshore Fukushima prefecture. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.

* On August 15 at 14:32 (JST), there was an M5.5 earthquake at a depth of approximately 95 km under the Sanpachi-Kamikita region, Aomori prefecture. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate. Also, at 15:06 (JST) on the same day, there was an M4.5 earthquake at almost the same place as the recent event.

* On August 24, there was an M5.6 earthquake offshore Fukushima prefecture. The focal mechanism had a tension axis in a WNW-ESE direction.

* On August 29, there was an M6.1 earthquake at a depth of approximately 30 km (CMT solution), 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.

• Tohoku Region
• Earthquake Offshore Fukushima Prefecture on August 4
• Earthquake Under the Sanpachi-Kamioka Region, Aomori Prefecture on August 15
• Earthquake Offshore Fukushima Prefecture on August 24
• Earthquake Offshore East of Aomori Prefecture on August 29

to supplementary reports

(3) Kanto-Chubu Region

There was no remarkable activity.

• Kanto - Chubu Region

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

* On August 24, there was an M5.2 earthquake near the ocean area of Ishigakijima Island. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction.

• Kyushu Region
• Okinawa Region
• Earthquake near the Ocean Area of Ishigakijima Island on August 24

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

(7) Other Regions

* On August 8, there was an M6.4 earthquake near Taiwan. The focal mechanism showed a strike-slip fault type with a compression axis in an NE-SW direction.

• Earthquake near Taiwan on August 8

to supplementary reports

Supplementary Information to the Evaluation of Seismic Activities for August 2019

September 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 August 2019.

• M4.0 or greater: 83 (97 events in July)
• M5.0 or greater: 12 (15 events in July)
• M6.0 or greater: 3 (3 events in July)

(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 August 2018 to late July 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.

to the text

(3) Kanto-Chubu Region

There is no supplementary information for Kanto-Chubu region.

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(4) Kinki-Chugoku-Shikoku Region

– In the area from the western to central Shikoku, deep low-frequency earthquakes (tremors) were observed from August 1 to 20, near the boundary between the Philippine Sea and the continental plates. According to the strain, tilt, and GNSS data, a slight crustal deformation was observed in the vicinity, almost synchronized with the deep low-frequency earthquakes (tremors). It is presumed that these are due to the short-term slow slip at the boundary between the Philippine Sea and the continental plates.

– According to the GNSS observation in the northern Kyushu from 2018 spring, and according to the GNSS and strain observations in the western Shikoku from 2018 autumn, 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.

• Deep Low-Frequency Seismic (Tremor) Activity and Slow Slip in Shikoku (1)
• Deep Low-Frequency Seismic (Tremor) Activity and Slow Slip in Shikoku (2)
• Deep Low-Frequency Seismic (Tremor) Activity and Slow Slip in Shikoku (3)
• Deep Low-Frequency Seismic (Tremor) Activity and Slow Slip in Shikoku (4)
• Deep Low-Frequency Seismic (Tremor) Activity and Slow Slip in Shikoku (5)
• Deep Low-Frequency Seismic (Tremor) Activity and Slow Slip in Shikoku (6)
• Deep Low-Frequency Seismic (Tremor) Activity and Slow Slip in Shikoku (7)
• Slow Slip Synchronized with Deep Low-Frequency Tremor in Shikoku Estimated from GNSS Data (provisional)
• Non-Stationary Horizontal Crustal Deformation of Northern Kyushu and Western Shikoku (Linear Trend, Annual Component, Semi-annual Component Removed)
• Time Series of GNSS Continuous Observation: Northern Kyushu and 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 September 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, September 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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(7) Other Regions

There is no supplementary information for other regions.

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