May 14, 2015
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 20, there was a magnitude (M) 6.8 earthquake under the ocean area near Yonagunijima.
There was no remarkable activity.
* On April 23, there was a M3.8 earthquake at a depth of approximately 5 km in the Shimokita region, Aomori prefecture. This event occurred within the crust.
* On April 30, there was a M5.4 earthquake at a depth of approximately 25 km, offshore Iwate 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.
* 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.
There was no remarkable activity.
* On April 20 at 10:42, there was a M6.8 earthquake under the ocean area near Yonagunijima. The focal mechanism showed a reverse fault type with a compressional axis in an N-S direction. GNSS observation showed a slight crustal deformation with this event at Yonaguni observatory. After this event, on the same day, there were a M6.0 earthquake at 20:45 and a M6.4 one at 20:59.
* On May 3, there was a M5.9 earthquake under the ocean area near Torishima. Tsunami was observed along the Izu Islands, such as height of 0.5 m (preliminary) at Yayene, Hachijojima.
* On May 3, there was a M5.0 earthquake at a depth of approximately 45 km, offshore Fukushima 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 May 11, there was a M6.3 earthquake under the ocean area near Torishima. The focal mechanism of this event showed a reverse fault type with a compression axis in an ENE-WSW direction.
* On May 13, there was a M6.8 earthquake at a depth of approximately 45 km, offshore Miyagi prefecture. The focal mechanism of this event showed a reverse fault type with a compression axis in an E-W direction (preliminary). This event occurred at the boundary between the Pacific 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.
May 14, 2015
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during April 2015.
(reference)
The following events for the period from April 2014 to late March, 2015 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities".
| – Along the Northern Coast of Chile | Mw8.1 on April 2, 2014 | |
| – Ocean Area Near the Izu-Oshima | M6.0 on May 5, 2014 (Depth of approximately 160 km) |
|
| – Rat Islands, Aleutian Islands | Mw7.9 on June 24, 2014 | |
| – Offshore IwatePrefecture | M5.9 on July 5, 2014 (Depth of approximately 50 km) |
|
| – Middle Eastern Iburi Region | M5.6 on July 8, 2014 (Depth of approximately 5 km) |
|
| – Offshore Fukushima Prefecture | M7.0 on July 12, 2014 | |
| – 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) |
There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
– 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 less than 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.
–"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 April 27 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 27, 2015)
"No change of activity which is likely to be associated with the Tokai earthquake has been observed until now. (Excerpted)"
– In Mt. Hakone, there have been clusters of small magnitude earthquakes from April 26 (the maximum is M3.0 as of May 11). Tilt-meters and volumetric-strain-meters around the area and GNSS observations show a slight crustal deformation at the same period. Also the satellite-ward (upward) crustal deformation within a very small area at Owakudani was observed by the analysis of data of the Advanced Land Observing Satellite-2 "DAICHI-2".
There is no supplementary information for Kinki-Chugoku-Shikoku region.
–"On April 20 at 10:42, there was a M6.8 earthquake under the ocean area near Yonagunijima. (Excerpted)":
As a result of this event, Tsunami Advisory was announced, however, tsunami was not observed.
–"On May 3, there was a M5.9 earthquake under the ocean area near Torishima.(Excerpted)":
In the area near the epicenter, there were a M5.9 earthquake on June 13, 1984, and a M6.2 one on September 5. 1996, and a M 5.9 one on January 1, 2006 and tsunami was observed as well as this time, in spite of rather small M6.0 or so.
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: