September 9, 2015
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for August 2015

1 Major Seismic Activities

There was no remarkable activity.

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* 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.

(2) Tohoku Region

* 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.

(3) Kanto-Chubu Region

* 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.

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

(5) Kyushu-Okinawa Region

* 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.

(6) Other Region

* 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.

Supplement

* 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.



Supplementary Information to the Evaluation of Seismic Activities for August 2015

September 9, 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 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)

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

(2) Tohoku Region

There is no supplementary information for Tohoku region.

(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 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)"

(5) Kinki-Chugoku-Shikoku Region

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

(6) Kyushu-Okinawa 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.

  1. Events of M6.0 and greater, or events that have maximum seismic intensity 4 or greater
  2. Onshore events of M4.5 and greater that have maximum seismic intensity 3 or greater
  3. Events under the ocean areas of M5.0 or greater that have maximum seismic intensity 3 or greater

Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes:

  1. Reference information related to seismic activity described in the "Evaluation of Seismic Activity"
  2. Recent earthquake activity over the past year related to the events described in "Major Seismic Activity" in the "Evaluation of Seismic Activity"
  3. Seismic activity that has been evaluated, but not included in the "Evaluation of Seismic Activity" because it is within normal bounds and not prominent