April 9, 2013
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
There was no remarkable activity.
* On March 9, there was a magnitude (M) 5.0 earthquake at a depth of approximately 100 km under the southern middle-Kushiro region. The focal mechanism had a tension axis in a dip direction of the Pacific plate. This event occurred within the Pacific plate.
* On March 10, there was a M5.1 earthquake at a depth of approximately 50 km, offshore Kushiro. The focal mechanism showed a normal fault with a tension axis in an N-S direction. This event occurred within the Pacific plate.
* On March 13, there was a M5.0 earthquake at a depth of approximately 50 km, offshore Iwate prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred on the boundary between the Pacific and the continental plates.
* On March 31, there was a M5.3 earthquake at a depth of approximately 45 km, offshore Miyagi prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred on the boundary between the Pacific and the continental plates.
* On March 18, there was a M4.4 earthquake at a depth of approximately 55 km under northern Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred on the boundary between the Pacific and the continental plates.
* On March 21, there was a M4.7 earthquake at a depth of approximately 70 km under northern Ibaraki prefecture. The focal mechanism had a compression axis in a dip direction of the Pacific plate. This event occurred within the Pacific 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.
There was no remarkable activity.
* On March 6, there was a M5.0 earthquake at a depth of approximately 50 km near Okinoerabujima [under the ocean near Okinawa Main Island]. The focal mechanism showed a reverse fault with a compression axis in an NW-SE direction. This event occurred on the boundary between the Philippine Sea and the continental plates.
* On March 11, there was a M5.2 earthquake at a depth of approximately 25 km in Hyuganada. 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.
* On March 27, there was a M6.1 earthquake at a depth of approximately 10 km near Taiwan (central Taiwan). The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction.
* On April 2, there was a M6.2 earthquake offshore Sanriku. The focal mechanism
showed a reverse fault with a compression axis in a WNW-ESE direction.
This event occurred on the boundary between the Pacific and the continental
plates. In the vicinity of this event, there was a M5.7 earthquake about
7 hours after, and a sequence of seismicity with 19 earthquakes of M4.0
or over from April 2 to 4 was seen.
In the vicinity of this event, there was sometimes a sequence of seismicity
with the maximum M6-M7 class earthquake. In addition, the seismicity increased
after the 2011 off the Pacific Coast of Tohoku Earthquake, and in May 2012
a sequence of seismicity with the maximum M6.5 earthquake was seen.
* On April 4, there was a M4.2 earthquake at a depth of approximately 15 km in the Kaga region, Ishikawa prefecture. This event occurred within the crust.
* On April 4, there was a M5.1 earthquake at a depth of approximately 10 km, offshore east of Chiba 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 April 6, there was a M5.2 earthquake offshore Ibaraki prefecture. The focal mechanism showed a normal fault with a tension axis in an NW-SE direction.
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.
April 9, 2013
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during March 2013.
(reference)
The following events for the period from March 2012 to late February 2013 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities"
| – 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) |
|
| – 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) |
There is no supplementary information for the Hokkaido region.
There is no supplementary information for the Tohoku region.
- During about one year after March 2012, 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 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 subsidence and upheaval were 6 cm and 5 cm respectively. 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 March 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, March 25)
"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 the Kinki-Chugoku-Shikoku region.
There is no supplementary information for the 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: