August 11, 2015
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for July 2015

1 Major Seismic Activities

* On July 10, there was a magnitude (M) 5.7 earthquake under the northern inland of Iwate prefecture [northern coast of Iwate prefecture]. The maximum seismic intensity 5 Lower was observed in Iwate prefecture and some damage with injured persons was caused.

* On July 13, there was a M5.7 earthquake under the southern Oita prefecture. The maximum seismic intensity 5 Upper was observed in Oita prefecture and some damage with injured persons was caused.

• Seismic Activities for All of Japan in July 2015 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On July 7, there was a M6.3 earthquake offshore east of Hokkaido. The focal mechanism of this event showed a reverse fault type with a compression axis in a WNW-ESE direction.

• Hokkaido Region
• Earthquake Offshore East of Hokkaido on July 7

to supplementary reports

(2) Tohoku Region

* On July 9, there was a M5.1 earthquake at a depth of approximately 35 km, offshore Iwate prefecture. The focal mechanism of this event 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 continental plates.

* On July 10, there was a M5.7 earthquake at a depth of approximately 90 km under the northern inland of Iwate prefecture [northern coast of Iwate prefecture]. The focal mechanism of this event had a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate.

• Tohoku Region
• Aftershock Activities of the 2011 off the Pacific Coast of Tohoku Earthquake
• Crustal Movements After the 2011 off the Pacific Coast of Tohoku Earthquake (M9.0) (Horizontal) -One Month-
• Crustal Movements After the 2011 off the Pacific Coast of Tohoku Earthquake (M9.0) (Vertical) -One Month-
• Crustal Movements After the 2011 off the Pacific Coast of Tohoku Earthquake (M9.0) (Time Series)
• Crustal Movements Before and After the 2011 off the Pacific Coast of Tohoku Earthquake
• Earthquakes Offshore Iwate Prefecture on July 9
• Earthquake Under the Northern Inland of Iwate Prefecture on July 10

to supplementary reports

(3) Kanto-Chubu Region

* On July 10, there was a M4.4 earthquake at a depth of approximately 45 km under the southern Ibaraki prefecture. The focal mechanism of this event showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Philippine Sea 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 under the Southern Ibaraki Prefecture on July 10
• Continuous GNSS Monitoring in the Area from Kakegawa to Omaezaki

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* 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.

• Kinki-Chugoku-Shikoku Region
• Earthquake under the Nanyo Region, Ehime Prefecture on July 24

to supplementary reports

(5) Kyushu-Okinawa Region

* On July 13, there was a M5.7 earthquake at a depth of approximately 60 km under the southern Oita prefecture. The focal mechanism of this event had a tension axis in an NW-SE direction. This event occurred within the Philippine Sea plate.

• Kyushu Region
• Earthquake under the Southern Oita Prefecture on July 13
• Earthquake under the Southern Oita Prefecture on July 13, 2015
• Okinawa Region

to supplementary reports

Supplement

* On August 3, there was a M5.1 earthquake at a depth of approximately 50 km, offshore Fukushima prefecture. In addition, on August 5, there was a M5.0 earthquake. The focal mechanisms of these events showed a reverse fault type with compression axes in a WNW-ESE direction. These events occurred at the boundary between the Pacific and the continental plates.

* On August 6, there was a M5.2 earthquake at a depth of approximately 55 km, offshore Ibaraki prefecture [the northern Ibaraki prefecture]. The focal mechanism of this event 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.

• Earthquakes Offshore Fukushima Prefecture on August 3 and 5
• Earthquake Offshore Ibaraki Prefecture on August 6

to supplementary reports

Note:  [ ] is the regional name of the epicenter used in the Japan Meteorological Agency announcement.
     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 2015

Supplementary Information to the Evaluation of Seismic Activities for July 2015
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 2015.

• M4.0 or greater: 92 (92 events in June, 2015)
• M5.0 or greater: 6 (14 events in June, 2015)
• M6.0 or greater: 1 (1 event in June, 2015)

(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 2014 to late June, 2015 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.

to the text

(3) Seismic activities and crustal deformations associated with the 2011 off the Pacific Coast of Tohoku Earthquake

– Within the aftershock area of the Tohoku-Oki Earthquake, the number of earthquakes of M4.0 or over, during one year from March 2012 (March 2012~February 2013) became less than one fifth, during one year from March 2013 (March 2013~February 2014) became less than one tenth, and during one year from March 2014 (March 2014~February 2015) became less than one fifteenth, of the number during about one year just after the Tohoku-Oki Earthquake, and thus the aftershock activity has been decaying.
According to the GNSS data, crustal deformation which is thought to be postseismic deformation has still been observed in a wide area from the Tohoku through the Kanto-Chubu regions. During about one month just after the Tohoku-Oki Earthquake, at the maximum value, the horizontal movement was 30 cm, and the vertical subsidence and upheaval were 6 cm and 5 cm, respectively. Although both the horizontal and the vertical deformations were almost 1cm during the recent one month, the deformation has not yet returned to the state before the Tohoku-Oki Earthquake.
In case of the earthquake (Mw 9.1) offshore west of the northern Sumatra Island in 2004, there were Mw8.6 after four months, Mw8.5 after about two and a half years, Mw7.5 after about five and a half years, and Mw8.6 after about seven and a half years in the region of outer rise. Thus, there have been large earthquakes for long period within the source area and its surroundings.
Although the aftershock activity is gradually decaying as a whole, the activity is still in higher state than the one before the Tohoku-Oki Earthquake. In addition, there are overseas examples for gigantic earthquakes. Synthetically judging from these facts, within the aftershock area and its surroundings, there is fear that large earthquakes will occur from now on, and there is a possibility of strong shaking and high tsunami. It is necessary to pay attention continuously for it.

(4) Kanto-Chubu Region

–"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 27 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 27, 2015)
"No change of activity which is likely to be associated with the Tokai earthquake has been observed until now. (Excerpted)"

– From July 14 to 17, there was a cluster of earthquakes, the maximum one being M4.7 (maximum seismic intensity 2), under the ocean area near Hachijojima. There were 11 earthquakes observed with seismic intensity 1 or over.

• Seismic Activity under the Ocean Area near Hachijojima from July 14

– In Mt. Hakone, seismic activity with clusters of small magnitude earthquakes beginning from April 26 (the maximum is M3.3 as of July 31) became rather low in July. According to the GNSS data, crustal deformation was observed at the same period.

• Seismic Activity near Mt. Hakone
• GNSS Observation Data around Mt. Hakone (1)
• GNSS Observation Data around Mt. Hakone (2)

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(5) Kinki-Chugoku-Shikoku Region

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

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(6) Kyushu-Okinawa Region

There is no supplementary information for Kyushui-Okinawa region.

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