September 9, 2015
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
There was no remarkable activity.
* On August 14, there was a magnitude (M) 5.1 earthquake at a depth of approximately 80 km under the central Tokachi region. The focal mechanism of this event had a tension axis in an NNW-SSE direction. This event occurred within the Pacific plate.
* On August 3, there was a M5.1 earthquake at a depth of approximately 50 km, offshore Fukushima prefecture. In addition, on August 5 and 14, there were M5.0 and M5.1 earthquakes, respectively at almost the same depths. 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 24, there was a M5.1 earthquake at a depth of approximately 15 km, offshore Fukushima prefecture. The focal mechanism of this event showed a normal fault type with a tension axes in an N-S direction. This event occurred within the crust of the continental plate.
* 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.
* On August 17, there was a M6.1 earthquake under the ocean area near Iwotou Island. The focal mechanism of this event showed a normal fault type with a compression axis in an NE-SW direction. This event occurred within the Pacific plate.
* 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.
There was no remarkable activity.
* On August 17, there was a M5.0 earthquake at a depth of approximately 5 km in the ocean area near Tanegashima. The focal mechanism of this event showed a normal fault type with a tension axis in an NNE-SSW direction. This event occurred within the crust of the continental plate.
* On August 26, there was a M5.2 earthquake at a depth of approximately 35 km in Hyuganada. The focal mechanism of this event had a tension axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate.
* On August 13, there was a M5.7 earthquake near Taiwan (about 90 km south-west of Yonagunijima) [ocean area near Yonagunijima]. The focal mechanism had a compression axis in an N-S direction.
* On September 2, there was a M6.1 earthquake under the ocean area near Torishima. The focal mechanism of this event showed a reverse fault type with a compression axis in an E-W direction.
* On September 4, there was a M5.3 earthquake offshore north-east of Amami Oshima. The focal mechanism of this event showed a normal fault type with a tension axis in a WNW-ESE direction.
* On September 8, there was a M4.6 earthquake at a depth of approximately 25 km in the Suruga Bay. The focal mechanism of this event showed a strike-slip fault type with a tension axis in an NW-SE direction (preliminary). This event occurred within the Philippine Sea plate.
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.
September 9, 2015
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during August 2015.
(reference)
The following events for the period from August 2014 to late July, 2015 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – Offshore East of Aomori Prefecture | M6.1 on August 10, 2014 (Depth of approximately 50 km) |
|
| – Northern Tochigi Prefecture | M5.1 on September 3, 2014 (Depth of approximately 5 km) |
|
| – Southern Ibaraki Prefecture | M5.6 on September 16, 2014 (Depth of approximately 45 km) |
|
| – Northern Nagano Prefecture | M6.7 on November 22, 2014 (Depth of approximately 5 km) |
|
| – Southern Tokushima Prefecture | M5.1 on February 6, 2015 (Depth of approximately 10 km) |
|
| – Offshore Sanriku | M6.9 on February 17, 2015 | |
| – Offshore Iwate Prefecture | M5.7 on February 17, 2015 (Depth of approximately 50 km) |
|
| – Ocean Area near Yonagunijima | M6.8 on April 20 | |
| – Ocean Area near Torishima | M5.9 on May 3, 2015 | |
| – Offshore Miyagi Prefecture | M6.8 on May 13, 2015 (Depth of approximately 45 km) |
|
| – Ocean Area near Amami Oshima | M5.1 on May 22 (Depth of approximately 20 km) |
|
| – Northern Saitama Prefecture | M5.5 on May 25, 2015 (Depth of approximately 55 km) |
|
| – Offshore West of Ogasawara Islands | M8.1 on May 30 (Depth of approximately 680 km) |
|
| – Abashiri Region | M5.0 on June 4, 2015 (At a very shallow depth) |
|
| – Northern Inland of Iwate Prefecture | M5.7 on July 10, 2015 (Depth of approximately 90 km) |
|
| – Southern Oita Prefecture | M5.7 on July 13, 2015 (Depth of approximately 60 km) |
There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
– 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.
–"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 August 31 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 31, 2015)
"No change of activity which is likely to be associated with the Tokai earthquake has been observed until now. (Excerpted)"
There is no supplementary information for Kinki-Chugoku-Shikoku region.
There is no supplementary information for Kyushui-Okinawa region.
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: