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

Evaluation of Seismic Activities for October 2018

1 Major Seismic Activities

* On October 5, there was a magnitude (M) 5.2 earthquake in the central and eastern Iburi regions, Hokkaido. Maximum seismic intensity 5 Lower was observed in Hokkaido.

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

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* Aftershock activity of the 2018 Hokkaido Eastern Iburi Earthquake is decaying but still continues: in October in addition to an M5.2 earthquake (maximum seismic intensity 5 Lower) on 5^th and there occurred 4 earthquakes with maximum seismic intensity 4 (16 earthquakes in September).

* On October 26, there was an M5.5 earthquake at a depth of approximately 20 km near Kunashiri Island. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred within the crust of the continental plate.

• Hokkaido Region
• Seismic activity of "the 2018 Hokkaido Eastern Iburi Earthquake" (1)
• Seismic activity of "the 2018 Hokkaido Eastern Iburi Earthquake" (2)
• Seismic activity of "the 2018 Hokkaido Eastern Iburi Earthquake" (3)
• Seismic activity of "the 2018 Hokkaido Eastern Iburi Earthquake" (4)
• Seismic activity of "the 2018 Hokkaido Eastern Iburi Earthquake" (5)
• Earthquakes near Kunashiri Island on October 26 and November 5

to supplementary reports

(2) Tohoku Region

* On October 22, there was an M5.0 earthquake at a depth of approximately 50 km, offshore Fukushima 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 October 26, there was an M5.7 earthquake at a depth of approximately 50 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 Fukushima Prefecture on October 22
• Earthquake Offshore Miyagi Prefecture on October 26

to supplementary reports

(3) Kanto-Chubu Region

* 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.

* On October 12, there was a M5.2 earthquake at a depth of approximately 50 km under the northeastern Chiba 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 Philippine Sea plates.

* On October 15, there was an M3.7 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.

* On October 27, there was an M5.0 earthquake at a depth of approximately 10 km (CMT solution) offshore Ibaraki prefecture. The focal mechanism had a tension axis in an E-W direction. This event occurred within the crust of the continental plate.

• Kanto - Chubu Region
• Earthquake Offshore East of Chiba Prefecture on October 4
• Earthquake Under the Eastern Aichi Prefecture on October 7
• Earthquake Under the Northeastern Chiba Prefecture on October 12
• Earthquake in the Northern Tochigi Prefecture on October 15
• Earthquake Offshore Ibaraki Prefecture on October 27

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 October 23, there was an M6.1 earthquake at a depth of approximately 25 km in the ocean near Yonagunijima Island. The focal mechanisms of this earthquake had a compression axis in an N-S direction. In the vicinity of the recent event, on October 24, there was an M6.3 earthquake. The focal mechanism of this earthquake showed a strike-slip fault type with a compression axis in an N-S direction.

• Kyushu Region
• Okinawa Region
• Earthquakes in the Ocean near Yonagunijima Island on October 23 and 24

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 November 2, there was an M5.4 earthquake at a depth of approximately 45 km under the Kii Channel. 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. In the vicinity of the recent event, on November 5, there was an M4.6 earthquake.

* On November 2, there was an M6.1 earthquake at a depth of approximately 490 km under the southern Okhotsk Sea. The focal mechanism had a compression axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

* On November 5, there was an M6.3 earthquake at a depth of approximately 20 km near Kunashiri Island. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred within the crust of the continental plate.

• Earthquakes Under the Kii Channel on November 2 and 5
• Earthquake Under the Southern Okhotsk Sea on November 2
• Earthquakes near Kunashiri Island on October 26 and November 5

Supplementary Information to the Evaluation of Seismic Activities for October 2018

November 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 October 2018.

• M4.0 or greater: 96 (137 events in September)
• M5.0 or greater: 16 (17 events in September)
• M6.0 or greater: 2 (3 events in September)

(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 October 2017 to late September 2018 have been listed as " 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

– In the area extending from northern Kii Peninsula to Tokai Region, for the period from September 18 until October 17, 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 and GNSS 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 and Short-Term Slow Slip in the Area from the Northern Kii Peninsula to Tokai Region
• Fig.2 Observed Records of Strain and Tilt (1) (2018/09/18 00:00 - 2018/10/18 00:00 (JST))
• Fig.3 Observed Records of Strain and Tilt (2) (2018/09/18 00:00 - 2018/10/18 00:00 (JST))
• Fig.4 Fault Model Explaining the Variation of Strain and Tilt (Fig.2,3[E]) 2018/10/02-05
• Fig.5 Fault Model Explaining the Variation of Strain and Tilt (Fig.2,3[F]) 2018/10/07-10
• Fig.6 Fault Model Explaining the Variation of Strain and Tilt (Fig.2,3[G]) 2018/10/13-15
• Slow Slip Synchronized with Deep Low-Frequency Tremor in the Area from Nara to Aichi Prefectures

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(4) Kinki-Chugoku-Shikoku Region

– In the western Shikoku, for the period from September 29 until October 15, 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 and GNSS 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.

– According to the GNSS-sonic ranging observation until September, a crustal deformation different from the ordinary trend has been observed on the sea bottom offshore the Kii Channel since about late 2017. This phenomenon is considered to be caused by a slow slip at the shallow part of the boundary between the Philippine Sea and the continental plates.

• Deep Low-Frequency Seismic (Tremor) Activity and Short-Term Slow Slip in Shikoku
• Fig.8 Observed Results of Strain and Tilt in Shikoku Region (2018/09/15 00:00-2018/10/15 00:00 (JST))
• Fig.9 Fault Model Explaining the Variation of Strain and Tilt (Fig.8[A]) 2018/09/29PM-10/01AM
• Fig.10 Fault Model Explaining the Variation of Strain and Tilt (Fig.8[B]) 2018/10/01PM-04AM
• Fig.11 Fault Model Explaining the Variation of Strain and Tilt (Fig.8[C]) 2018/10/04PM-06
• Slow Slip Synchronized with Deep Low-Frequency Tremor in the Western Shikoku
• Fault Model Explaining the Non-stationary Variation (Estimated Solution of Deep Inclination of Sound Velocity) Offshore Kii Channel

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(5) Kyushu-Okinawa Region

– According to the GNSS observation, a crustal deformation different from the ordinary trend has been observed in the northern Kyushu since about June. This phenomenon is considered to be caused by a long-term slow slip at the deep part of the boundary between the Philippine Sea and the continental plates.

• Time Series of GNSS Continuous Observation: Northern Kyushu
• Estimated Long-Term Slow Slip in the northern Hyuganada (provisional)

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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 November 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, November 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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