March 9, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for February 2018

1 Major Seismic Activities

There was no remarkable activity.

• Seismic Activities for All of Japan in February 2018 (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 February 26, there was a magnitude (M) 5.8 earthquake at a depth of approximately 40 km, offshore Fukushima prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NNW-SSE direction.

• Tohoku Region
• Earthquake Offshore Fukushima Prefecture on February 26

to supplementary reports

(3) Kanto-Chubu Region

* On February 13, there was an M4.1 earthquake at a depth of approximately 5 km in the southern Nagano prefecture. The focal mechanism had a compression axis in a WNW-ESE direction. This event occurred within the crust.

• Kanto - Chubu Region
• Earthquake in the Southern Nagano Prefecture on February 13

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On February 19, there was an M5.0 earthquake at a depth of approximately 40 km under the Bungo Channel. The focal mechanism showed a normal fault type with a compression axis in an NW-SE direction. In addition there was another M5.0 earthquake at almost the same time and the place. Both earthquakes occurred within the Philippine Sea plate.

• Kinki-Chugoku-Shikoku Region
• Earthquake Under the Bungo Channel on February 19

to supplementary reports

(5) Kyushu-Okinawa Region

There was no remarkable activity.

• Kyushu Region
• Okinawa Region
• Earthquake near Iriomotejima on March 1

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 February 7 (JST time), there was an M6.7 earthquake at a depth of approximately 10 km near Taiwan. The focal mechanism had a compression axis in an N-S direction. In the vicinity of the recent event, the seismic activity has become high from February 4; On February 4, there was an M6.5 earthquake and there were 4 earthquakes of M6.0 or over until February 28. In addition, according to the results from Synthetic Aperture Radar Interferometry Analysis of data by the Advanced Land Observing Satellite-2 "DAICHI-2", a discontinuity in the surface displacement is found in the eastern Taiwan and the crustal deformation of the enormous rise is observed in its eastern side.

• Seismic Activity near Taiwan Since February 4
• Results of SAR Analysis for the earthquake at Hualian, Taiwan on February, 2018

to supplementary reports

Supplement

* On March 1, there was an M5.6 earthquake at a depth of approximately 15 km near Iriomotejima. The focal mechanism had a tension axis in an NE-SW direction. This event occurred within the continental crust.

• Seismic activity near Taiwan since February 4
• Past seismic activity and damage situation near Taiwan

Supplementary Information to the Evaluation of Seismic Activities for February 2018

March 9, 2018
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 February 2018.

• M4.0 or greater: 107 (90 events in January)
• M5.0 or greater: 18 (10 events in January)
• M6.0 or greater: 4 (1 event in January)

(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 February 2017 to late January 2018 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.

to the text

(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.

to the text

(4) Kinki-Chugoku-Shikoku Region

– Near the boundary between the Philippine Sea and the continental plates, from Ehime prefecture to the Bungo Channel, deep low-frequency earthquakes (tremors) are observed from February 21. Also due to the strain-inclination data, slight crustal deformation is observed in Ehime and Kochi prefectures from February 21. These phenomena are considered to be caused by the slow-slip in the short interval at the boundary between the Philippine Sea and the continental plates.

• Deep Low-Frequency Seismic (Tremor) Activity and Short-Term Slow Slip in the Area from Ehime Prefecture to the Bungo Channel
• Fig. 5. Observed Results of Strain, Tilt, and Underground Water in Shikoku (2018/2/5 00:00 – 2018/3/6 12:00)
• Fig. 6 Fault Model Explaining the Variation of Strain and Tilt (Fig.5[A]) 2018/2/21 21:00 – 2/28 AM
• Fig. 7 Fault Model Explaining the Variation of Strain, Tilt, and Underground Water (Fig.5[B]) 2018/2/28 PM – 3/2 AM
• Fig. 8 Fault Model Explaining the Variation of Strain and Tilt (Fig.5[C]) 2018/3/2 AM – 3/6 PM

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(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 February 26 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, February 29, 2018)
"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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