May 13, 2013
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 13, there was a magnitude (M) 6.3 earthquake near Awajishima. This event had a maximum seismic intensity 6 Lower in Hyogo prefecture and caused damage with seriously injured persons.
* On April 17, there was a M6.2 earthquake under the ocean near Miyakejima. This event had a maximum seismic intensity 5 Upper in Miyakejima and caused damage with injured persons. In addition, small tsunamis were observed in Miyakejima.
* On April 17, there was a M5.9 earthquake offshore Miyagi prefecture. This event had a maximum seismic intensity 5 Lower in Miyagi prefecture and caused damage with injured persons.
There was no remarkable activity.
* 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 there were 23 earthquakes of M4.0 or over by April 30.
* On April 14, there was a M5.3 earthquake at a depth of approximately 50 km, offshore Fukushima prefecture. The focal mechanism showed a reverse fault with a compression axis in an NW-SE direction. This event occurred on the boundary between the Pacific and the continental plates.
* On April 17, there was a M5.9 earthquake at a depth of approximately 60 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 April 21, there was a M4.3 earthquake at a depth of approximately 10 km in Aizu, Fukushima prefecture [Nakadori, Fukushima prefecture]. The focal mechanism showed a reverse fault with a compression axis in an E-W direction. This event occurred within the crust.
* On April 4, there was a M4.2 earthquake at a depth of approximately 15 km in the Kaga region, Ishikawa prefecture. The focal mechanism had a compression axis in an E-W direction. 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. In the vicinity of this event, there was a M4.6 earthquake at a depth of approximately 15 km on April 19. The focal mechanism showed a strike-slip fault with a tension axis in an E-W direction. Both of these events 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.
* On April 14, there was a M4.6 earthquake at a depth of approximately 95 km under southern Saitama prefecture [northern Saitama prefecture]. The focal mechanism had a compression axis in an NW-SE direction.
* From April 17, seismicity became brisk under the ocean near Miyakejima.
The seismicity began from about 10 o’clock on April 17 and at 17:57 (JST)
on April 17 there was a M6.2 earthquake at a depth of approximately 10
km. It had a maximum seismic intensity 5 Upper in Miyakejima and caused
damage. The focal mechanism showed a strike-slip fault with a compression
axis in an NNW-SSE direction. This event occurred within the crust. According
to the GNSS data, small crustal deformations associated with this event
were observed in Miyakejima. In addition, small tsunamis associated with
this event were observed in Miyakejima.
Since then, the seismicity has been declining.
* On April 21, there was a M6.4 earthquake at a depth of approximately 450 km under the ocean near Torishima. The focal mechanism had a compression axis in a dip direction of the Pacific plate. This event occurred within the Pacific plate.
* On April 29, there was a M5.6 earthquake at a depth of approximately 35 km, offshore east of Chiba prefecture [offshore 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.
* 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.
* (See a separate paper about the earthquake near Awajishima and its aftershock activity)
* From around April 15, seismic activities began under the ocean near Yonagunijima.
There was a sequence of earthquakes of M5 or so from April 17 and activities
became brisk. There was a M6.1 earthquake at 23:08 on April 18. The focal
mechanism of this event showed a normal fault with a tension axis in an
N-S direction. There were mainly earthquakes of a normal fault including
this event.
Although the brisk seismicity has been continuing since then, it has gradually been declining.
According to the GNSS data, crustal deformations were observed at the Yonaguni observation point and others (Okinawa prefecture) from around April 17 when the seismicity became brisker.
* On April 5, there was a M6.3 earthquake at a depth of approximately 590 km under the vicinity of Vladivostok. The focal mechanism had a compression axis in a dip direction of the Pacific plate. This event occurred within the Pacific plate.
* On April 11, there was a M6.3 earthquake near Taiwan. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction.
* On April 19, there was a M7.0 earthquake at a depth of approximately 130 km under the Kurile Islands (offshore southeast of Urup Island). The focal mechanism had a tension axis in an N-S direction. This event occurred within the Pacific plate.
* On May 2, there was a M4.1 earthquake at a depth of approximately 5 km in southern Gunma prefecture. The focal mechanism showed a reverse fault with a compression axis in an NW-SE direction. This event occurred within the crust. In the vicinity of this event, from around April 30, a sequence of earthquakes, to some extent, such as a M4.2 earthquake at a depth of approximately 5 km on April 30 was seen.
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.
May 13, 2013
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during April 2013.
(reference)
The following events for the period from April 2012 to late March 2013 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities"
| – 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.
–"On April 2, there was a M6.2 earthquake offshore Sanriku. (Excerpted)":
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.
– 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 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.
– "From April 17, seismicity became brisk under the ocean near Miyakejima.
(Excerpted)":
There were 53 earthquakes with a maximum seismic intensity 1 or over from April 17 to 30.
It was the first time that an earthquake of M6.0 or over occurred under the ocean near Miyakejima since July 30, 2000 (M6.5).
In the region from Miyakejima through Niijima-Kozujima, seismicity was brisk in 2000, and in February 2003 there was a sequence of seismicity with the maximum M5.1 earthquake (maximum seismic intensity 4).
– "On April 21, there was a M6.4 earthquake at a depth of approximately
450 km under the ocean near Torishima. (Excerpted)":
This event gave stronger shaking in the Pacific coast side from the Tohoku to the Kanto regions, due to a transmission of seismic waves within the subducting Pacific plate (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 April 22 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, April 22)
"No changes of activity which are likely to be associated with the Tokai earthquake have been observed until now. (Excerpted))"
–"On April 13, there was a M6.3 earthquake at a depth of approximately 15 km near Awajishima. (Excerpted)":
There were 22 earthquakes with a maximum seismic intensity 1 or over including
this event from April 13 to 30.
–"From around April 15, seismic activities began under the ocean
near Yonagunijima. There was a sequence of earthquakes of M5 or so from
April 17 and activities became brisk. (Excerpted)":
There were 13 earthquakes with a maximum seismic intensity 1 or over from
April 15 to 30. In the region where the recent seismicity was seen, there
was a sequence of seismicity once or so every some years.
–"On April 5, there was a M6.3 earthquake at a depth of approximately
590 km under the vicinity of Vladivostok. (Excerpted)":
This event had a maximum seismic intensity 1 in the Kanto region by transmission of seismic waves within the subducting Pacific plate (anomalous seismic intensity).
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: