July 10, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for June 2018

1 Major Seismic Activities

* On June 17, there was a magnitude (M) 4.6 earthquake in the southern Gunma prefecture. Maximum seismic intensity 5 Lower was observed in Gunma prefecture and was caused damage with partially destroyed houses and so on.

* On June 18, there was an M6.1 earthquake in the northern Osaka prefecture. Maximum seismic intensity 6 Lower was observed in Osaka prefecture and was caused damage with fatalities and so on.

• Seismic Activities for All of Japan in June 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

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* Beginning on June 3, there has been a high seismic activity in the area extending from offshore east of Chiba prefecture to the northeastern and the southern Chiba prefecture: there occurred an M4.9 earthquake on June 12, an M4.4 on June 16, and an M4.3 on June 26. The focal mechanisms of these events showed a reverse fault type with a compression axis roughly in an NNW-SSE direction. All these events occurred at the boundary between the Philippine Sea and the continental plates.

* On June 17, there was an M4.6 earthquake at a depth of approximately 15 km in the southern Gunma prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred within the crust.

• Kanto - Chubu Region
• Seismic Activity in the Area Extending from Offshore East of Chiba Prefecture to the Northeastern and the Southern Chiba Prefecture
• Earthquake in the Southern Gunma Prefecture on June 17

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* Refer to another article on the earthquake in the northern Osaka prefecture on June 18 and the aftershocks activity.

* On June 26, there was an M5.0 earthquake at a depth of approximately 10 km in the northern Hiroshima prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an E-W direction. This event occurred within the crust.

• Kinki-Chugoku-Shikoku Region
• Earthquake in the Northern Hiroshima Prefecture on June 26

to supplementary reports

(5) Kyushu-Okinawa Region

* On June 12, there was an M5.6 earthquake at a depth of approximately 30 km, offshore east of Osumi peninsula. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred at the boundary between the Philippine Sea and the continental plates.

• Kyushu Region
• Earthquake Offshore East of Osumi Peninsula on June 12
• 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 July 2 at 02:27 (JST), there was an M4.9 earthquake at a depth of approximately 65 km, 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.

* On July 2 at 20:53 (JST), there was an M5.0 earthquake at a depth of approximately 40 km, 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.

* On July 7, there was an M6.0 earthquake at a depth of approximately 65 km, offshore east of Chiba prefecture. Maximum seismic intensity 5 Lower was observed in Chiba prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction (preliminary).

• Earthquake Offshore East of Aomori Prefecture on July 2 at 02:27
• Earthquake Offshore East of Aomori Prefecture on July 2 at 20:53
• Earthquake Offshore east of Chiba Prefecture on July 7

Supplementary Information to the Evaluation of Seismic Activities for June 2018

July 10, 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 June 2018.

• M4.0 or greater: 74 (67 events in May)
• M5.0 or greater: 6 (11 events in May)
• M6.0 or greater: 1 (0 event in May)

(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 June 2017 to late May 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

– "Beginning on June 3, there has been a high seismic activity in the area extending from offshore east of Chiba prefecture to the northeastern and the southern Chiba prefecture: there occurred an M4.9 earthquake on June 12, an M4.4 on June 16, and an M4.3 on June 26. (Excerpted)":
Although the seismic activity is decaying, it still continues: until June 30 there have occurred 8 earthquakes with maximum seismic intensity 3 or over, and 26 earthquakes with maximum seismic intensity 1 or over. Also according to the tilt, GNSS and strain data, a slight crustal deformation was observed in Boso peninsula in early June, and now it has decayed. These phenomena are considered to be caused by a short-term slow-slip at the boundary between the Philippine Sea and the continental plates. In this neighborhood the similar phenomena have been observed once for several years. In the 2011 case, the maximum magnitude earthquake (M5.2, maximum seismic intensity 4) occurred more than 1 month after the beginning of the phenomena.

• Non-stationary Crustal Deformation in Boso Peninsula (provisional)
• Distribution of Slip on the Pate Boundary Estimated by the Time-Dependent Inversion (provisional)
• Slow-Slip Event Offshore Boso Peninsula (June 2018)
• Change of Strain Accompanying the Slow-Slip Event Offshore Boso Peninsula
• Past Seismic Activity of the Offshore East of Chiba Prefecture Region (Transition of each activity in 3 months) (1)
• Past Seismic Activity of the Offshore East of Chiba Prefecture Region (Transition of each activity in 3 months) (2)

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(4) Kinki-Chugoku-Shikoku Region

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

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 July 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, July 6, 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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