March 11, 2013
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On February 2, there was a M6.5 earthquake under the southern Tokachi region. This event had a maximum seismic intensity 5 Upper in Hokkaido. It caused damage with seriously injured persons.
* On February 25, there was a M6.3 earthquake in northern Tochigi prefecture. This event had a maximum seismic intensity 5 Upper in Tochigi prefecture. It caused damage.
* On February 2, there was a M6.5 earthquake at a depth of approximately 100 km under the southern Tokachi region [central Tokachi region]. The focal mechanism had a tension axis in a direction of the subsiding Pacific plate. This event occurred within the Pacific plate. According to the GNSS data, small crustal movements associated with this event were observed at the Erimo-1 station (Hokkaido).
* On February 17, there was a M5.3 earthquake at a depth of approximately 45 km, offshore east of Hokkaido. The focal mechanism showed a reverse fault with a compression axis in an NW-SE direction.
* On February 13, there was a M4.7 earthquake at a depth of approximately 55 km, offshore Miyagi prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate
* On February 24, there was a M4.5 earthquake at a depth of approximately 20 km in Hamadori, 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 February 9, there was a M5.2 earthquake offshore Ibaraki prefecture. The focal mechanism showed a normal fault with a tension axis in a WNW-ESE direction. This event occurred within the crust of the continental plate.
* On February 19, there was a M5.6 earthquake offshore east of Chiba prefecture. The focal mechanism had a tension axis in a WNW-ESE direction.
* On February 25, there was a M6.3 earthquake at a depth of approximately
5 km in northern Tochigi prefecture. The focal mechanism showed a strike-slip
fault with a compression axis in a WNW-ESE direction. This event occurred
within the crust. In the vicinity of this event, seismicity was seen after
about February 23. In addition, brisk aftershock activity was seen after
the recent event, but the activity has gradually been decaying since then.
The largest aftershock till now was a M4.7 event that occurred after approximately
10 minutes from the main event.
According to the GNSS data, small crustal movements associated with this event were observed at the Kuriyama station (Tochigi prefecture).
In the vicinity of the Gunma-Tochigi prefectural border where the recent seismicity was seen, seismicity increased after the 2011 off the Pacific Coast of Tohoku Earthquake.
* 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 February 6 at 10:12 (JST), there was an Mw7.9 earthquake at Santa Cruz Islands in the South Pacific. Tsunamis associated with this event were observed along the Pacific coast from the Hokkaido to Kyushu regions, Okinawa prefecture and Izu-Ogasawara Islands. The focal mechanism showed a reverse fault with a compression axis in an NE-SW direction. This event occurred on the boundary between the India-Australia and the Pacific plates
* On March 6, there was a M5.0 earthquake at a depth of approximately 50 km under the ocean near Okinawa Main Island (near Okinoerabujima). The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred on the boundary between the Philippine Sea and the continental plates.
Note: [ ] was 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.
March 11, 2013
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during February 2013.
(reference)
The following events for the period from February 2012 to late January 2013 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities"
| |
– Near Sado | M5.7 on February 8, 2012 (Depth of approximately 15 km) |
| – Northern Ibaraki Prefecture | M5.2 on February 19, 2012 (Depth of approximately 5 km) |
|
| – Offshore Ibaraki Prefecture | M5.3 on March 1, 2012 (Depth of approximately 55 km) |
|
| – Northern Ibaraki Prefecture | M5.4 on March 10, 2012 (Depth of approximately 5 km) |
|
| – Offshore Sanriku | M6.9 on March 14, 2012 | |
| – Offshore East of Chiba Prefecture | M6.1 on March 14, 2012 (Depth of approximately 15 km) |
|
| – Offshore Iwate Prefecture | M6.6 on March 27, 2012 (Depth of approximately 20 km) |
|
| – Offshore Fukushima Prefecture | M5.9 on April 1, 2012 (Depth of approximately 55 km) |
|
| – Northeastern Chiba Prefecture | M5.8 on April 29, 2012 (Depth of approximately 50 km) |
|
| – Offshore East of Aomori Prefecture | M6.1 on May 24, 2012 (Depth of approximately 60 km) |
|
| – Northern Nagano Prefecture | M5.2 on July 10, 2012 (Depth of approximately 10 km) |
|
| – Southern Tokachi Region | M6.1 on August 25, 2012 (Depth of approximately 50 km) |
|
| – Offshore Miyagi Prefecture | M5.6 on August 30, 2012 (Depth of approximately 60 km) |
|
| – 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) |
There is no supplementary information for the Hokkaido region.
There is no supplementary information for the Tohoku region.
- In the aftershock area of the 2011 off the Pacific Coast of Tohoku Earthquake,
the number of earthquakes of M4.0 or over during about one year after March
2012 became under one fifth smaller than the number during about one year
just after the 2011 off the Pacific Coast of Tohoku 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 five 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 upheaval was 5 cm. The movement was gradually decaying. Although the horizontal movement was slightly larger than 1 cm, and the vertical movement was less than 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 February 25 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, February 25)
"No changes of activity which are likely to be associated with the Tokai earthquake have been observed until now. (Excerpted)"
* In the vicinity of Hakonesan [western Kanagawa prefecture], there was a slight sequence of small earthquakes from mid-January. The seismicity has been subsiding since February 19. Although small crustal movements were recognized in tiltmeters around the seismic area, they have been subsiding since around mid- February.
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
* In the ocean near Suwanosejima (ocean near Tokara Rettou), there was a slight sequence of earthquakes with maximum M3.5 on February 19, and there were 8 earthquakes with a maximum seismic intensity 1 or over.
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: