September 10, 2013
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On August 4, there was a magnitude (M) 6.0 earthquake offshore Miyagi prefecture. This event had a maximum seismic intensity 5 Upper in Miyagi prefecture and caused damage.
* On August 22, there was a M4.8 earthquake at a depth of approximately 55 km under the eastern Hidaka region. The focal mechanism showed a reverse fault with a compression axis in an NW-SE direction. This event occurred on the boundary between the Pacific and the continental plates.
* On August 4, there was a M6.0 earthquake at a depth of approximately 60 km, offshore Miyagi prefecture. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred within the Pacific plate. In the vicinity of this event, there was a M7.2 earthquake (maximum seismic intensity 6 Upper) on April 7, 2011
* On August 30, there was a M3.9 earthquake at a depth of approximately 10 km in Aizu, Fukushima prefecture. The focal mechanism showed a compression axis in an NW-SE direction. This event occurred within the crust.
* On August 3, there was a M4.9 earthquake at a depth of approximately 35 km in Enshu-Nada. The focal mechanism showed a normal fault with a tension axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate.
* In the GNSS data for the Tokai region, there have been no changes of activity which are likely to be associated with the Tokai earthquake.
* On August 30, there was a M4.4 earthquake at a depth of approximately 10 km, offshore south of Wakayama prefecture. The focal mechanism showed a strike-slip fault with a compression axis in an NNW-SSE direction. This event occurred within the crust of the continental plate.
There was no remarkable activity.
* On September 4, there was a M6.8 earthquake at a depth of approximately 450 km under the ocean near Torishima. The focal mechanism showed a compression axis in the dipping direction of the Pacific plate. This event occurred within the Pacific plate.
Note: GNSS (Global Navigation Satellite System) is a general name of satellite positioning system such as GPS.
September 10, 2013
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during August 2013.
(reference)
The following events for the period from August 2012 to late July 2013 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities"
| – Southern Tokachi Region | M6.1 on August 25, 2012 (Depth of approximately 50 km) |
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| – Offshore Miyagi Prefecture | M5.6 on August 30, 2012 (Depth of approximately 60 km) |
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| – Offshore Miyagi Prefecture | M5.6 on October 25, 2012 (Depth of approximately 50 km) |
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| – Offshore Sanriku | M7.3 on December 7, 2012 | |
| – Northern Ibaraki Prefecture | M4.8 on January 28, 2013 (Depth of approximately 75km) |
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| – Northern Ibaraki Prefecture | M4.7 on January 31, 2013 (Depth of approximately 10km) |
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| – Southern Tokachi Region | M6.5 on February 2, 2013 (Depth of approximately 100 km) |
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| – Northern Tochigi Prefecture | M6.3 on February 25, 2013 (Depth of approximately 5 km) |
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| – Near Awajishima | M6.3 on April 13, 2013 (Depth of approximately 15 km) |
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| – Under the Ocean Near Miyakejima | M6.2 on April 17, 2013 (Depth of approximately 10 km) |
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| – Offshore Miyagi Prefecture | M5.9 on April 17, 2013 (Depth of approximately 60 km) |
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| – Offshore Fukushima Prefecture | M6.0 on May 18, 2013 (Depth of approximately 45 km) |
There is no supplementary information for the Hokkaido region.
–"On August 4, there was a M6.0 earthquake at a depth of approximately 60 km, offshore Miyagi prefecture. (Excerpted)":
This earthquake occurred in the aftershock area of the earthquake of April 7, 2011, (M7.2, maximum seismic intensity 6 Upper), which was an aftershock of the 2011 off the Pacific Coast of Tohoku Earthquake. This region shows high seismic activity and a M5.6 earthquake occurred on August 30, 2012 and M5.2 one on November 24, 2012.
–"On August 30, there was a M3.9 earthquake at a depth of approximately
10 km in Aizu, Fukushima prefecture. (Excerpted)":
Within the crust extending from Aizu, Fukushima prefecture to border area
in Okitama region, Yamagata prefecture, seismic activity has been rather
high after the middle March, 2011. Although the activity is decaying in
strength, showing some up and down afterwards, still continues now.
– During about one year (March 2012 - February, 2013), the number
of earthquakes of M4.0 or over in the aftershock area of the 2011 off the
Pacific Coast of Tohoku Earthquake became under one fifth smaller than
the one during about one year (March 11, 2011 - February, 2012) just after
the Earthquake, and thus the aftershock activity has been decaying. However,
in comparison with the mean activity before the 2011 off the Pacific Coast
of Tohoku Earthquake, the number of earthquakes of M4.0 or over in the
aftershock area was more than four times, and the aftershock activity still
remains in brisk state.
According to the GNSS data during about one month just after the 2011 off the Pacific Coast of Tohoku Earthquake, at the maximum value, the horizontal movement was 30 cm, and the vertical subsidence and upheaval were 6 cm and 5 cm respectively. The movement was gradually decaying. Although the horizontal movement was up to around 1cm, and the vertical movement was up to around 1cm during the recent one month, crustal movements which are thought to be the postseismic deformation have still been observed in a wide area from the Tohoku through the Kanto-Chubu regions.
In case of the earthquake (moment magnitude (Mw) 9.1) offshore west of the northern Sumatra Island in December 2004, within the source area and its surroundings, there have been large earthquakes for long period. As to the 2011 off the Pacific Coast of Tohoku Earthquake, within the aftershock area and its surroundings, there is fear that large earthquakes will occur from now on, and there is a possibility that it will be hit by strong shaking and high tsunami. It is necessary to do care continuously for it.
–"In the GNSS 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 August 29 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, August 29)
"No changes of activity which are likely to be associated with the Tokai earthquake have been observed until now. (Excerpted)"
There is no supplementary information for Kinki-Chugoku-Shikoku region.
There is no supplementary information for Kyushu-Okinawa region.
–"On September 4, there was a M6.8 earthquake at a depth of approximately 450 km under the ocean near Torishima. (Excerpted).":
Seismic waves passing through the descending Pacific plate caused rather
large ground motions in the Pacific side (of Japan) from Tohoku to Kanto
regions.
Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.
Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes: