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

Evaluation of Seismic Activities for July 2018

1 Major Seismic Activities

* On July 7, there was a magnitude (M) 6.0 earthquake, offshore east of Chiba prefecture. Maximum seismic intensity 5 Lower was observed in Chiba prefecture.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On July 16, there was an M4.9 earthquake at a depth of approximately 55 km under the eastern Hidaka region. 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 Pacific and the continental plates.

• Hokkaido Region
• Earthquake under the Eastern Hidaka Region on July 16

to supplementary reports

(2) Tohoku Region

* 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 31, there was an M5.8 earthquake at a depth of approximately 20 km, offshore Fukushima prefecture. The focal mechanism showed a normal fault type with a tension axis in an NNW-SSE direction. This event occurred within the crust of the continental plate.

• Tohoku Region
• 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 Fukushima Prefecture on July 31

to supplementary reports

(3) Kanto-Chubu Region

* On July 7, there was an M6.0 earthquake at a depth of approximately 55 km, offshore east of Chiba prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate.

* On July 17, there was an M4.8 earthquake at a depth of approximately 50 km, offshore Ibaraki 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 29, there was an M4.5 earthquake at a depth of approximately 5 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.

• Kanto - Chubu Region
• Earthquake Offshore east of Chiba Prefecture on July 7
• Earthquake Offshore Ibaraki Prefecture on July 17
• Earthquake in the Northern Tochigi Prefecture on July 29

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 July 25, there was an M4.4 earthquake at a depth of approximately 5 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.

• Kyushu Region
• Earthquake in the Kumamoto Region, Kumamoto Prefecture on July 25
• 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.

• Secular variation of benchmark 2595 (Omaezaki City)
• Vertical Movement of GNSS-Based Control Stations - Omaezaki
• Vertical Movements of GNSS-Based Control Stations - Kii Peninsula and Around Muroto Cape

to supplementary reports

Supplementary Information to the Evaluation of Seismic Activities for July 2018

August 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 July 2018.

• M4.0 or greater: 87 (74 events in June)
• M5.0 or greater: 15 (6 events in June)
• M6.0 or greater: 1 (1 event in June)

(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 July 2017 to late June 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

There is no supplementary information for Kanto region.

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(4) Kinki-Chugoku-Shikoku Region

– From July 10 until 28 in the area extending from the central to southern Iyo, Ehime prefecture, and also from July 18 until 20 in the Bungo Channel, a deep low-frequency seismic (tremor) activity was observed near the boundary between the Philippine Sea and the continental plates. According to the strain-tilt data, slight crustal deformation in the surrounding areas was observed almost synchronously to the deep low-frequency seismic (tremor) activity. 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 in the Area from Tokushima Prefecture to the Bungo Channel
• Fig.2 Observed Results of Strain, Tilt and Underground Water in Shikoku Region (2018/06/25 00:00 – 2018/07/28 00:00 (JST)).
• Fig.3 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[A]) 2018/07/10 - 17
• Fig.4 Fault Model Explaining the Variation of Strain, Tilt, and Underground Water (Fig.2[B]) 2018/07/18 - 21
• Fig.4 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[C]) 2018/07/22 - 25

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(5) Kyushu-Okinawa Region

–"On July 25, there was an M4.4 earthquake at a depth of approximately 5 km, in the Kumamoto region, Kumamoto prefecture. (Excerpted)":
The serial activity of Kumamoto Earthquakes in the Kumamoto and Aso regions, Kumamoto prefecture still remains, although it is continually decaying as a whole. The present activity will continue for several months. GNSS observation result shows the slight aftershock deformation still continues.

• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• 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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(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 August 7 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, August 7, 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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