October 12, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for September 2018

1 Major Seismic Activities

* On September 6, there was a magnitude (M) 6.7 earthquake* in the central and eastern Iburi regions, Hokkaido. Maximum seismic intensity 7 was observed and damage with fatalities and so on was caused.

* Japan Meteorological Agency named this earthquake as "The 2018 Hokkaido Eastern Iburi Earthquake."

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* Refer to the other term for the 2018 Hokkaido Eastern Iburi Earthquake.

• Hokkaido Region

to supplementary reports

(2) Tohoku Region

* On September 11, there was an M5.0 earthquake at a depth of approximately 45 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 September 19, there was an M5.0 earthquake at a depth of approximately 55 km, offshore Miyagi 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 East of Aomori Prefecture on September 11
• Earthquake Offshore Miyagi Prefecture on September 19

to supplementary reports

(3) Kanto-Chubu Region

* On September 5, there was an M5.5 earthquake at a depth of approximately 60 km, offshore Ibaraki prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred within the Pacific plate.

* On September 10, there was an M4.7 earthquake at a depth of approximately 35 km, offshore southeast of Chiba prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an NW-SE direction. This event occurred within the Philippine Sea plate.

* On September 14, there was an M4.9 earthquake at a depth of approximately 5 km in the northern Ibaraki prefecture. The focal mechanism showed a normal fault type with a tension axis in an ENE-WSW direction. This event occurred within the crust.

* On September, 18 there was a M4.3 earthquake at a depth of approximately 75 km under the southern Saitama prefecture [the southern Ibaraki prefecture.] 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 Pacific and the Philippine Sea plates.

• Kanto - Chubu Region
• Earthquake offshore Ibaraki Prefecture on September 5
• Earthquake Offshore Southeast of Chiba Prefecture on September 10
• Earthquake in the Northern Ibaraki Prefecture on September 14
• Earthquake under the Southern Saitama Prefecture on September 18

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 September 15, there was an M6.2 earthquake at a depth of approximately 10 km (CMT solution) in the ocean area near Okinawa Main Island. In the area around the recent event, on September 16, there was an M6.0 earthquake. The focal mechanisms of these earthquakes showed a normal fault type with a tension axis in an NW-SE direction.

• Kyushu Region
• Okinawa Region
• Seismic Activity in the Ocean Area near Okinawa Main Island Beginning on September 15

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 October 4, there was an M4.7 earthquake at a depth of approximately 30 km, offshore east of Chiba prefecture. The focal mechanism had a compression axis in an N-S direction.

* On October 7, there was an M5.0 earthquake at a depth of approximately 40 km under the eastern Aichi prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an ENE-WSW direction. This event occurred within the Philippine Sea plate.

• Earthquake Offshore East of Chiba Prefecture on October 4
• Earthquake under the Eastern Aichi Prefecture on October 7

Comment:[ ] is the regional name of the epicenter used in the Japan Meteorological Agency announcement.

Supplementary Information to the Evaluation of Seismic Activities for September 2018

October 12, 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 September 2018.

• M4.0 or greater: 137 (109 events in August)
• M5.0 or greater: 17 (13 events in August)
• M6.0 or greater: 3 (2 events in August)

(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 September 2017 to late August 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

– In the area extending from Nara prefecture to Ise Bay, for the period from September 18 until October 1, a deep low-frequency seismic (tremor) activity was observed near the boundary between the Philippine Sea and the continental plates. According to the strain and tilt data in the surrounding area, a slight crustal deformation was observed almost synchronously to the deep low-frequency seismic (tremor) activity. These phenomena are considered to be caused by a 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 Nara to Aichi Prefectures and Short-Term Slow Slip
• Fig.2 Observed Results of Strain and Tilt in the area extending from Kii Peninsula to Tokai Region (2018/09/04 00:00 - 2018/10/04 00:00 (JST))
• Fig.3 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[A]) 2018/09/19PM-22AM
• Fig.4 Fault Model Explaining the Variation of Strain, Tilt and Underground Water (Fig.2[B]) 2018/09/22PM-25
• Fig.5 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[C]) 2018/09/26-28AM
• Fig.6 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[D]) 2018/09/28PM-30AM
• Fig.7 Fault Model Explaining the Variation of Strain and Tilt (Fig.2[E]) 2018/10/02-03

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(4) Kinki-Chugoku-Shikoku Region

– In the area extending from the Bungo Channel to the central Iyo, Ehime prefecture, for the period from September 29 until October 7, a deep low-frequency seismic (tremor) activity was observed near the boundary between the Philippine Sea and the continental plates. According to the strain and tilt data in the surrounding area, a slight crustal deformation was observed almost synchronously to the deep low-frequency seismic (tremor) activity. These phenomena are considered to be caused by a 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 the Bungo Channel to the Central Iyo, Ehime Prefecture and Short-Term Slow Slip
• Fig.9 Observed Results of Strain and Tilt in Shikoku Region (2018/09/15 00:00-2018/10/04 00:00 (JST))
• Fig.10 Fault Model Explaining the Variation of Strain and Tilt (Fig.9[A]) 2018/09/29PM-10/01
• Fig.11 Fault Model Explaining the Variation of Strain, Tilt and Underground Water (Fig.9[B]) 2018/10/02-03

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 October 5 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, October 5, 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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