July 11, 2011
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Seismic Activities for June 2011

1 Major Seismic Activities

* On June 2, there was a M4.7 earthquake in the Chuetsu region, Niigata prefecture. This event had a maximum seismic intensity 5 Upper in Niigata prefecture and caused damage.

* On June 23, there was a M6.9 earthquake offshore Iwate prefecture. This event had a maximum seismic intensity 5 Lower in Aomori and Iwate prefectures and caused damage.

* On June 30, there was a M5.4 earthquake in central Nagano prefecture. This event had a maximum seismic intensity 5 Upper in Nagano prefecture and caused damage with severely injured persons.

• Seismic Activities for All of Japan in June 2011 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On June 14, there was a M5.1 earthquake at a depth of approximately 75 km, offshore Kushiro. This event occurred within the Pacific plate. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction.

* On June 25, there was a M5.4 earthquake at a depth of approximately 55 km, offshore Urakawa. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred on the boundary between the Pacific and the continental plates.

• Hokkaido Region
• Earthquake Offshore Kushiro on June 14
• Earthquake Offshore Urakawa on June 25

to supplementary reports

(2) Tohoku Region

* On June 18, there was a M4.6 earthquake at a depth of approximately 15 km in Nakadori, Fukushima prefecture. The focal mechanism had a tension axis in a N-S direction. This event occurred within the crust.

* On June 23, there was a M6.9 earthquake at a depth of approximately 35 km, offshore Iwate prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred on the boundary between the Pacific and the continental plates. According to the GPS data, crustal movements associated with this event were observed and there was a movement of approximately 2 cm toward the east at the S-Fudai station (Iwate prefecture).

• 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-
• Earthquake Offshore Iwate Prefecture on June 23
• Crustal Movements Associated with Earthquake (June 23, M6.9) Offshore Iwate Prefecture
• Earthquake in the Southern Inland, Akita Prefecture on June 26

to supplementary reports

(3) Kanto-Chubu Region

* On June 2 at 11:33 (JST), there was a M4.7 earthquake at a depth of approximately 5 km in the Chuetsu region, Niigata prefecture. The focal mechanism had a compression axis in a NW-SE direction. This event occurred within the crust. There was brisk seismicity, such as a M4.1 earthquake at 12:51 on the same day.

* On June 3, there was a M4.5 earthquake at a depth of approximately 60 km under southern Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in an ENE-WSW direction. This event occurred on the boundary between the Pacific and the Philippine Sea plates.

* On June 30, there was a M4.6 earthquake at a depth of approximately 50 km under northeastern Chiba prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred on the boundary between the Pacific and the Philippine Sea plates.

* On June 30, there was a M5.4 earthquake at a depth of approximately 5 km in central Nagano prefecture. The focal mechanism showed a strike-slip fault with a compression axis in a WNW-ESE direction. This event occurred within the crust. According to the GPS data, small crustal movements associated with this event were observed in the vicinity of the epicenter.

* In the GPS data for the Tokai region, there have been no changes of activity which are likely to be associated with the Tokai earthquake.

• Kanto - Chubu Region
• Earthquake in the Chuetsu Region, Niigata Prefectures on June 2
• Earthquakes in Southern Ibaraki Prefecture on June 3
• Earthquakes under Northeastern Chiba Prefecture on June 30
• Earthquake in Central Nagano Prefecture on June 3
• Earthquake in Central Nagano Prefecture on June 30 (Past Activities)
• Earthquake in Central Nagano Prefecture on June 30
• Crustal Movements Associated with Earthquake (June 30, M5.4) in Central Nagano Prefecture
• Continuous GPS Monitoring in the Area from Kakegawa to Omaezaki

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

* On June 4, there was a M5.2 earthquake at a depth of approximately 10 km in eastern Shimane prefecture. The focal mechanism showed a strike-slip fault with a compression axis in a NW-SE direction. This event occurred within the crust.

• Kinki-Chugoku-Shikoku Region
• Earthquake in Eastern Shimane Prefecture on June 4

to supplementary reports

(5) Kyushu-Okinawa Region

* On June 28, there was a M4.2 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. The focal mechanism showed a strike-slip fault with a tension axis in a N-S direction. This event occurred within the crust.

• Kyushu Region
• Earthquake in the Kumamoto Region, Kumamoto Prefecture on June 28
• Okinawa Region

to supplementary reports

Supplements

* On July 3, there was a M3.9 earthquake at a depth of approximately 10 km in the Aizu region, Fukushima prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred within the crust.

* On July 5, there was a M5.5 earthquake at a depth of approximately 5 km in northern Wakayama prefecture. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred within the crust.

* On July 10, there was a M7.3 earthquake offshore Sanriku. This event occurred within the Pacific plate. The focal mechanism showed a strike-slip fault with a tension axis in a WNW-ESE direction (preliminary). Tsunamis with a height of 0.1 m associated with this event were observed at Ofunato (Iwate prefecture) and Soma (Fukushima prefecture).

• Earthquake in the Aizu Region, Fukushima Prefecture on July 3
• Earthquake in Northern Wakayama Prefecture on July 5
• Earthquake in Northern Wakayama Prefecture on July 5
• Aftershock Activities for the 2011 off the Pacific Coast of Tohoku Earthquake
• Focal Mechanism (CMT Solution) for the Earthquake Offshore Sanriku on July 10, 2011 (Preliminary)
• Tsunami Observation for the Earthquake Offshore Sanriku on July 10, 2011

Supplementary Information to the Evaluation of Seismic Activities for June 2011

July 11, 2011
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 June 2011

• M4.0 or greater: 276 (377 events in May)
• M5.0 or greater: 28 (37 events in May)
• M6.0 or greater: 4 (a total of 99 events in 2011, as of the end of June 2011.)

(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 May 2010 to the end of April 2011 have been listed as "Major Seismic Activity" in monthly "Evaluation of Seismic Activity"

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2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for the Hokkaido region.

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(2) Tohoku Region

"On June 18, there was a M4.6 earthquake at a depth of approximately 15 km in Nakadori, Fukushima prefecture. (Excerpted)":
Apart from this event, there were 8 earthquakes of M4.0 or over from Hamadori, Fukushima prefecture through northern Ibaraki prefecture, and brisk seismicity has been seen since the 2011 off the Pacific Coast of Tohoku Earthquake.

– Brisk seismicity has been seen in the southern inland, Akita prefecture and Aizu, Fukushima prefecture from mid-March.

• Seismic Activities from Hamadori, Fukushima Prefecture to Northern Ibaraki Prefecture

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

– In the aftershock area of the 2011 off the Pacific Coast of Tohoku Earthquake that occurred on March 11, brisk seismic activities have bee seen, such as earthquakes with a maximum seismic intensity 5 Lower, except for the earthquakes which were evaluated in Nationwide Seismic Activities by Region. There is fear that large aftershocks will occur from now on, and there is a possibility that it will be hit by strong shaking and high tsunami. Crustal movements to the east which are thought to be the postseismic deformation have still been observed in a wide area from the Tohoku through the Kanto-Chubu regions.

(4) Kanto-Chubu Region

"On June 30, there was a M5.4 earthquake at a depth of approximately 5 km in central Nagano prefecture. (Excerpted)":
Since this event, brisk seismicity has been seen, such as a M5.1 earthquake at 08:21 on the same day. To the east of the recent event, there is the Itoigawa-Shizuoka-kozosen fault zone (an interval including the Gofukuji fault), and some aftershocks occurred in the vicinity of the Gofukuji fault.

"In the GPS data for the Tokai region, there have been no changes of activity which are likely to be associated with the Tokai earthquake.":
(This evaluation is in agreement with the views presented at the regular meeting on June 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, June 27, 2011)

"So far, there have been no changes of activity which are likely to be associated with the Tokai earthquake.

1. Status of seismicity
General seismicity has been slightly brisk since mid-2005 within the crust under mid-western Shizuoka prefecture.
There continue to be fewer events within the Philippine Sea plate under the region around Hamanako.
In other regions, seismic activity is generally at a normal level.

2. Status of crustal movements
There have been no remarkable changes observed over the entire region.
According to the GPS measurement results, the trend of long-term subsidence at Omaezaki has been continuing. In view of the observation data of tilt, strain meters and others as a whole, the data showing remarkable changes associated with coupling state of the Philippine Sea and the continental plates in the expected focal region of the Tokai earthquake and its surrounding area have not been obtained.
According to the GPS data, the postseismic deformation associated with the 2011 off the Pacific Coast of Tohoku Earthquake has also been seen in the Tokai region." 

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(5) Kinki-Chugoku-Shikoku Region

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

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(6) Kyushu-Okinawa Region

"On June 28, there was a M4.2 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. (Excerpted)":
In the vicinity, brisk seismicity has been continuing since mid-April.

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