October 10, 2013
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On September 20, there was a magnitude (M) 5.9 earthquake in Hamadori, Fukushima prefecture. This event had a maximum seismic intensity 5 Upper in Fukushima prefecture and caused damage.
There was no remarkable activity.
* On September 15, there was a M3.5 earthquake at a depth of approximately 10 km in Aizu, Fukushima prefecture. The focal mechanism showed a strike-slip fault with a compression axis in an NW-SE direction. This event occurred within the crust.
* On September 20, there was a M5.9 earthquake at a depth of approximately 15 km in Hamadori, Fukushima prefecture. The focal mechanism showed a strike-slip fault with a compression axis in an NW-SE direction. This event occurred within the crust.
* 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.
* On September 9, there was a M4.4 earthquake at a depth of approximately 10 km in northern Ibaraki prefecture. The focal mechanism showed a normal fault with a tension axis in an E-W direction. This event occurred within the crust.
* On September 13 and 30, there were both M4.4 earthquakes at a depth of approximately 50 km, offshore Ibaraki prefecture. The focal mechanism of the former showed a compression axis in a WNW-ESE direction and that of the latter showed a reverse fault with a compression axis in an NW-SE direction. Both events occurred at the boundary between the Pacific and the continental plates.
* 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.
There was no remarkable activity.
There was no remarkable activity.
* On October 3, there was a M5.1 earthquake under the ocean near Amami Ohima.
Note: GNSS (Global Navigation Satellite System) is a general name of satellite positioning system such as GPS.
October 10, 2013
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during September 2013.
(reference)
The following events for the period from September 2012 to late August 2013 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities".
| – Offshore Miyagi Prefecture | M5.6 on October 25, 2012 (Depth of approximately 50 km) |
|
| – Offshore Sanriku | M7.3 on December 7, 2012 | |
| – Northern Ibaraki Prefecture | M4.8 on January 28, 2013 (Depth of approximately 75km) |
|
| – Northern Ibaraki Prefecture | M4.7 on January 31, 2013 (Depth of approximately 10km) |
|
| – Southern Tokachi Region | M6.5 on February 2, 2013 (Depth of approximately 100 km) |
|
| – Northern Tochigi Prefecture | M6.3 on February 25, 2013 (Depth of approximately 5 km) |
|
| – Near Awajishima | M6.3 on April 13, 2013 (Depth of approximately 15 km) |
|
| – Under the Ocean Near Miyakejima | M6.2 on April 17, 2013 (Depth of approximately 10 km) |
|
| – Offshore Miyagi Prefecture | M5.9 on April 17, 2013 (Depth of approximately 60 km) |
|
| – Offshore Fukushima Prefecture | M6.0 on May 18, 2013 (Depth of approximately 45 km) |
|
| – Offshore Miyagi Prefecture | M6.0 on August 4, 2013 (Depth of approximately 60 km) |
There is no supplementary information for the Hokkaido region.
–"On September 15, there was a M3.5 earthquake at a depth of approximately
10 km in Aizu, Fukushima prefecture. (Excerpted)":
Within the crust near the prefectural border extending from Aizu, Fukushima
prefecture to Okitama region, Yamagata prefecture, seismic activity has
been rather high after the middle March, 2011. Although the activity has
been decaying in strength, showing some up and down afterwards, still continues
now.
–"On September 20, there was a M5.9 earthquake at a depth of approximately
15 km in Hamadori, Fukushima prefecture. (Excerpted)":
Within the crust from Hamadori, Fukushima prefecture through northern Ibaraki
prefecture, the seismicity increased after the 2011 off the Pacific Coast
of Tohoku Earthquake. Although it has gradually been declining on the whole
with some up and down, it still keeps the higher level than that of prior
to the occurrence of the 2011 off the Pacific Coast of Tohoku Earthquake.
Most focal mechanisms of the earthquakes in this area show normal fault
type. Earthquakes of the focal mechanism with reverse fault or strike-slip
fault occur at relatively deep depth.
– 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 after that the aftershock activity has gradually 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.
–"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 caused rather large ground motions in the
Pacific side of Japan from Tohoku to Kanto regions (anomalous seismic intensity).
–"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 September 24 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, September 24)
"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.
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: