August 9, 2016
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for July 2016

1 Major Seismic Activities

* On July 27, there was a magnitude (M) 5.4 earthquake in the northern Ibaraki prefecture. This event had a maximum seismic intensity 5 Lower observed in Ibaraki prefecture.

• Seismic Activities for All of Japan in July 2016 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On July 24, there was a M4.9 earthquake at a depth of approximately 95 km under the central Tokachi region. The focal mechanism had a tension axis in an NNE-SSW direction. This event occurred within the Pacific plate.

• Hokkaido Region
• Earthquake under the Central Tokachi Region on July 24

to supplementary reports

(2) Tohoku Region

* On July 16, there was a M4.6 earthquake at a depth of approximately 10 km in the northern inland of Akita prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an NW-SE direction. This event occurred within the crust.

• Tohoku Region
• Earthquake in the Northern Inland of Akita Prefecture on July 16

to supplementary reports

(3) Kanto-Chubu Region

* On July 1, there was a M4.6 earthquake at a very shallow depth in the Joetsu region, Niigata prefecture (near the boundary between Niigata and Nagano prefectures). The focal mechanism showed a strike-slip fault type with a compression axis in a WNW-ESE direction. This event occurred within the crust. In the vicinity of the recent event, there was a high seismic activity, beginning on June 25 with a M4.8 earthquake on the same day, and 13 earthquakes with seismic intensity 1 or over occurred until July 1.

* On July 17, there was a M5.0 earthquake at a depth of approximately 40 km under the southern Ibaraki prefecture. Also on July 20, there was an earthquake of the same order magnitude occurring at almost the same place as the previous one. The focal mechanisms of these events showed a reverse fault type with a compression axis in an NW-SE direction. These events occurred at the boundary between the Philippine Sea and the continental plates.

* On July 19, there was a M5.2 earthquake at a depth of approximately 35 km under the northeastern 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 July 27, there was a M5.4 earthquake at a depth of approximately 55 km under the northern 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.

* In the GNSS data for the Tokai region, there has been no change of activity which is likely to be associated with the Tokai earthquake.

• Kanto - Chubu Region
• Earthquake in the Joetsu Region, Niigata Prefecture (Near the Boundary between Niigata and Nagano Prefectures) on July 1
• Earthquakes under the Southern Ibaraki Prefecture on July 17 and 20
• Earthquake under the Northeastern Chiba Prefecture on July 19
• Earthquake under Northern Ibaraki Prefecture on July 27
• Continuous GNSS Monitoring along the Line from Kakegawa to Omaezaki (Baseline Length and Relative Height)
• Secular Variation of Benchmark 2595 (Omaezaki City)

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

* A series of seismic activity of the 2016 Kumamoto Earthquakes, in the area extending from Kumamoto region, Kumamoto prefecture to the central Oita prefecture, including a M6.5 earthquake on April 14 and a M7.3 one on April 16, is continually decaying as a whole. The activity in the Kumamoto region and the Aso region is still continuing in July: There was a M4.5 earthquake on July 9 which was the only earthquake of M4.0 or over. The activity in the central Oita prefecture has decayed. According to the GNSS observation, crustal deformation is still observed in the area centered on the series of the seismic activity.

• Kyushu Region
• "The 2016 Kumamoto Earthquakes" (1)
• "The 2016 Kumamoto Earthquakes" (2)
• Number of Earthquakes of "the 2016 Kumamoto Earthquakes"
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (1)
• Postseismic Deformation of the 2016 Kumamoto Earthquake (April 16, M7.3) (2)
• Okinawa Region

to supplementary reports

(6) Other Region

* On July 30, there was a M7.7 earthquake at a depth of approximately 230 km under the Mariana Islands. The focal mechanism had a tension axis in the subducting direction of the Pacific plate. This event occurred within the Pacific plate.

• Earthquake under the Mariana Islands on July 30

Note:GNSS (Global Navigation Satellite System) is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for July 2016

August 9, 2016
Earthquake Research Committee

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during July 2016.

• M4.0 or greater: 100 (97 events in June 2016)
• M5.0 or greater: 12 (11 events in June 2016)
• M6.0 or greater: 0 (1 event in June 2016)

(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 2015 to late June 2016 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

– Beginning on July 17, there was a high seismic activity in the ocean area near Izu- Oshima, the maximum being a M3.7 earthquake. 18 earthquakes occurred until July 25.
In the vicinity of this seismic activity, there were similar activities in 2003 and 2011.

• Seismic Activity under the Ocean Area near Izu-Oshima beginning on July 17

–"In the GNSS data for the Tokai region, there has been no change of activity which is likely to be associated with the Tokai earthquake.":
(This evaluation is in agreement with the views presented at the regular meeting on July 25 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster (reference below).)

(Reference)

Recent seismic and crustal activity in the Tokai region and its vicinity (Seismological and Volcanological Department, Japan Meteorological Agency, July 25, 2016)
"No change of activity which is likely to be associated with the Tokai earthquake has been observed until now. (Excerpted)"

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(4) Kinki-Chugoku-Shikoku Region

– Non-stationary crustal deformation surrounding the Kii Channel observed from mid-2014 is now decaying. This deformation is thought to be due to a slow slip (slow-slip) at the boundary between the Philippine Sea and the continental plates.

– Non-stationary crustal deformation surrounding the Bungo Channel observed from around December 2015 is still continuing. This deformation is thought to be due to a slow slip (slow-slip) at the boundary between the Philippine Sea and the continental plates.

• Non-Stationary Crustal Deformation around the Kii Channel
• Non-Stationary Crustal Deformation around the Bungo Channel (1)
• Non-Stationary Crustal Deformation around the Bungo Channel (2)

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(5) Kyushu-Okinawa Region

– There was a seismic activity under the ocean area near Tokara Islands, beginning on July 4, and there were 22 earthquakes with maximum seismic intensity 1 or over, until August 3.

• Seismic Activity under the Ocean Area near Tokara Islands (near Takarajima - Kodakarajima)

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