May 11, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for April 2018

1 Major Seismic Activities

* On April 9, there was a magnitude (M) 6.1 earthquake in the western Shimane prefecture. Maximum seismic intensity 5 Upper was observed in Shimane prefecture, and damage with injured persons and so on was caused.

* On April 14, there was an M5.4 earthquake offshore southeast of Nemuro peninsula. Maximum seismic intensity 5 Lower was observed in Hokkaido.

• Seismic Activities for All of Japan in April 2018 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On April 14, there was an M5.4 earthquake at a depth of approximately 55 km, offshore southeast of Nemuro peninsula. The focal mechanism showed a normal fault type with a tension axis in an NW-SE direction. This event occurred within the Pacific plate.

* On April 24, there was an M5.4 earthquake at a depth of approximately 85 km, offshore southeast of Nemuro peninsula. The focal mechanism had a tension axis in a subsiding direction of the Pacific plate. This event occurred within the Pacific plate.

• Hokkaido Region
• Earthquakes Offshore Southeast of Nemuro Peninsula on April 14 and April 24

to supplementary reports

(2) Tohoku Region

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* On April 14, there was an M4.5 earthquake at a depth of approximately 5 km in the western Aichi prefecture. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction. This event occurred within the crust.

* On April 21, there was an M5.1 earthquake at a depth of approximately 35 km offshore east of Chiba prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NNE-SSW direction.

• Kanto - Chubu Region
• Earthquake in the Western Aichi Prefecture on April 14
• Earthquake Offshore East of Chiba Prefecture on April 21

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On April 9 at 01:32 (JST), there was an M6.1 earthquake at a depth of approximately 10 km in the western Shimane prefecture. This event occurred within the crust. The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction. The seismic fault is estimated to be a left-lateral strike-slip one extending in an NNW-SSE direction. By the GNSS observation, the crustal deformation of about 1 cm movement in east-south-east direction, accompanying this event, was observed at Ota observatory, Shimane prefecture. In the vicinity of this event, there were 4 earthquakes with maximum seismic intensity 4 until April 30, including an M4.9 earthquake which occurred on April 9 at 02:10 (JST).

• Kinki-Chugoku-Shikoku Region
• Earthquake under the Western Shimane Prefecture on April 9
• Earthquake under the Western Shimane Prefecture on April 9 (Hypocenter Re-determination by the DD Method)
• Observed Data Before and After the Earthquake under the Western Shimane Prefecture (April 9, M6.1)

to supplementary reports

(5) Kyushu-Okinawa Region

There was no remarkable activity.

• Kyushu Region
• Seismic Activity near the Ocean Area of Tokara Islands Beginning on April 15 (near Kodakarajima and Takarajima)
• 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 May 6, there was an M5.7 earthquake in the ocean area near Torishima [offshore east of Hachijojima]. This event occurred within the crust of the Philippine Sea plate. Tsunami with height of 0.2 m (preliminary) was observed at Hachijojima, Tokyo Metropolis.

* On May 6, there was an M3.9 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. The focal mechanism had a tension axis in an N-S direction. This event occurred within the crust.

* On May 7, there was an M5.1 earthquake at a depth of approximately 100 km under the northern inland, Iwate prefecture. The focal mechanism had a compression axis in an NNE-SSW direction. This event occurred within the Pacific plate.

• Earthquake Near the Ocean Area of Torishima on May 6
• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• Earthquake under the Northern Inland, Iwate Prefecture on May 7

to supplementary reports

Note:GNSS is a general name of satellite positioning system such as GPS.
 [ ] is the regional name of the epicenter used in the Japan Meteorological Agency announcement.

Supplementary Information to the Evaluation of Seismic Activities for April 2018

May 11, 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 April 2018.

• M4.0 or greater: 84 (75 events in March)
• M5.0 or greater: 12 (11 events in March)
• M6.0 or greater: 1 (0 event in March)

(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 March 2017 to late February 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.

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(2) Tohoku Region

There is no supplementary information for Tohoku region.

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(3) Kanto-Chubu Region

– Near the boundary between the Philippine Sea and the continental plates, from Ise Bay to Mie prefecture, deep low-frequency earthquakes (tremors) were observed in the period from April 13 to 21. Also due to the strain-inclination data, slight crustal deformation was observed in Aichi and Mie prefectures from about April 13 to 20. 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 Seismic (Tremor) Activity and Short-Term Slow Slip in the Area from Ise Bay to Kii Peninsula
• Fig. 2 Observed Results of Strain and Tilt in Kii Peninsula and Aichi Prefecture (2018/04/01 00:00 – 2018/04/26 00:00 (JST)).
• Fig. 3 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[A]) 2018/04/13PM – 04/17
• Fig. 4 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[B]) 2018/04/18 – 04/20

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(4) Kinki-Chugoku-Shikoku Region

There is no supplementary information for Kinki-Chugoku-Shikoku region.

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(5) Kyushu-Okinawa Region

– A seismic activity is observed near the ocean area of Tokara Islands (near Kodakarajima and Takarajima) from April 15. 19 earthquakes with seismic intensity 1 or over occurred until April 16. The largest was an M3.7 one on April 16.

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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 May 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, May 9, 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) Supplement

–"On May 6, there was an M5.7 earthquake in the ocean area near Torishima [offshore east of Hachijojima]. (Excerpted)"
In the vicinity of this event, there occurred an M5.9 earthquake on June 13, 1984, an M6.2 on September 5, 1996, an M5.9 on January 1, 2006, and an M5.9 on May 3, 2015 which all caused tsunami observed, in spite of the magnitude of M6.0 or so.

–"On May 6, there was an M3.9 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. (Excerpted)"
The seismic activity series in the Kumamoto and Aso regions of the 2016 Kumamoto Earthquakes, decaying as a whole, still continues and the activity at the present level is expected for the time being. According to the GNSS observation result, a slight aftereffect deformation is continuing.

• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (1)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (2)

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