May 11, 2011
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 1, there was a M5.0 earthquake in the northern inland, Akita prefecture. This event had a maximum seismic intensity 5 Upper in Akita prefecture, and caused damage.
* On April 2, there was a M5.0 earthquake in southern Ibaraki prefecture. This event had a maximum seismic intensity 5 Lower in Ibaraki prefecture.
* On April 7, there was a M7.1 earthquake off-shore of Miyagi prefecture. This event had a maximum seismic intensity 6 Upper in Miyagi prefecture, and caused damage with fatalities.
* On April 11, there was a M7.0 earthquake in Hamadori, Fukushima prefecture. This event had a maximum seismic intensity 6 Lower in Fukushima prefecture, and caused damage with fatalities.
* On April 12, there was a M5.6 earthquake in northern Nagano prefecture. This event had a maximum seismic intensity 5 Lower in Nagano prefecture.
* On April 12, there was a M6.4 earthquake off-shore east of Chiba prefecture. This event had a maximum seismic intensity 5 Lower in Chiba prefecture..
* On April 16, there was a M5.9 earthquake in southern Ibaraki prefecture. This event had a maximum seismic intensity 5 Upper in Ibaraki prefecture, and caused damage with injured persons.
* On April 17, there was a M4.9 earthquake near the border between Nagano and Niigata prefectures. This event had a maximum seismic intensity 5 Lower in Niigata prefecture, and caused damage.
* On April 19, there was a M4.9 earthquake in the southern inland, Akita prefecture. This event had a maximum seismic intensity 5 Lower in Akita prefecture, and caused damage.
* On April 21, there was a M6.0 earthquake off-shore east of Chiba prefecture. This event had a maximum seismic intensity 5 Lower in Chiba prefecture, and caused damage with a heavily injured person.
There was no remarkable activity.
* On April 1, there was a M5.0 earthquake at a depth of approximately 10 km in the northern inland, Akita prefecture. The focal mechanism showed a strike-slip fault with a compression axis in an ENE-WSW direction. This event occurred within the crust.
* On April 7, there was a M7.1 earthquake at a depth of approximately 65 km, off-shore of Miyagi prefecture. This event occurred within the Pacific plate. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. According to the GPS data, crustal movements associated with this event were observed and there was a crustal movement with a horizontal displacement of approximately 3 cm toward the west and an upheaval of approximately 5 cm at the M-Oshika station (Miyagi prefecture).
* On April 11, there was a M7.0 earthquake at a depth of approximately 5 km in Hamadori, Fukushima prefecture. The focal mechanism showed a normal fault with a tension axis in an ENE-WSW direction. This event occurred within the crust. According to the GPS data, crustal movements associated with this event were observed and there was a crustal movement with a horizontal displacement of approximately 29 cm toward the north-northwest and a subsidence of approximately 50 cm at the Iwaki-4 station (Fukushima prefecture). According to the data of Synthetic Aperture Radar (SAR) from the advanced land observing satellite, "Daichi", crustal movements were observed in the vicinity of the epicenter. According to the field survey, surface fault displacements were observed.
* On April 19, there was a M4.9 earthquake at a depth of approximately 5 km in the southern inland, Akita prefecture. The focal mechanism showed a strike-slip fault with a tension axis in a NW-SE direction. This event occurred within the crust.
* On April 2, there was a M5.0 earthquake at a depth of approximately 55 km under southern Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred on the boundary between the Philippine Sea and the continental plates. In the vicinity of this event, there were M5.0 earthquakes on April 19 and 26.
* On April 12, there was a M5.6 earthquake at a very shallow depth in northern Nagano prefecture. The focal mechanism showed a strike-slip fault with a compression axis in a NNW-SSE direction. This event occurred within the crust. According to the GPS data, crustal movements associated with this event were observed and there was a movement of approximately 3 cm toward the northeast at the Nagano-Sakae station (Nagano prefecture).
* On April 12, there was a M6.4 earthquake at a depth of approximately 25 km, off-shore east of Chiba prefecture. This event occurred within the Philippine Sea plate. The focal mechanism showed a strike-slip fault with a compression axis in an S-N direction. According to the GPS data, crustal movements associated with this event were observed and there was a movement of approximately 1 cm toward the south at the Choshi station (Chiba prefecture).
* On April 16, there was a M5.9 earthquake at a depth of approximately 80 km under southern Ibaraki prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction.
* On April 17, there was a M4.9 earthquake at a depth of approximately 10 km near the border between Nagano and Niigata prefectures. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred within the crust.
* On April 17, there were M4.5 and M4.6 earthquakes at a depth of approximately 35 km under northeastern Chiba prefecture.
* On April 21, there was a M6.0 earthquake at a depth of approximately 45 km, off-shore east of Chiba 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 Philippine Sea plates.
* In the GPS 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 April 9, there was a M5.8 earthquake off-shore southeast of Tanegashima. The focal mechanism showed a normal fault with a tension axis in an E-W direction.
* On March 11, there was a M4.6 earthquake at a depth of approximately 5 km in the Izu region, Shizuoka prefecture. This event had a maximum seismic intensity 5 Lower in Shizuoka prefecture. Since this event, there was brisk seismicity such as a M4.2 earthquake on March 21.
* On May 4, there was a M4.6 earthquake at a depth of approximately 90 km under the Sanpachi-Kamikita region, Aomori prefecture.
* On May 7, there was a M4.6 earthquake at a depth of approximately 10 km in the Aizu region, Fukushima prefecture.
May 11, 2011
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during April 2011.
(reference)
The following events for the period from March 2010 to the end of February 2011 have been listed as "Major Seismic Activity" in monthly "Evaluation of Seismic Activity"
| – Off-shore of Fukushima Prefecture | M6.2 on June 13, 2010 (Depth of approximately 40km) |
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| – Northeastern Chiba Prefecture | M5.0 on July 23, 2010 (Depth of approximately 35km) |
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– Joetsu Region, Niigata Prefecture | M4.7 on October 3, 2010 (Depth of approximately 20km) |
| – Under the Ocean near Miyakojima | M6.4 on October 4, 2010 | |
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– Under the Ocean near Chichijima |
M7.4 on December 22, 2010 |
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– Off-shore of Sanriku |
M7.3 on March 9, 2011 |
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– The 2011 off the Pacific Coast of Tohoku Earthquake | M9.0 on March 11, 2011 (Depth of approximately 25 km) |
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– Izu Region, Shizuoka Prefecture | M4.6 on March 11, 2011 (Depth of approximately 5 km) |
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– Near the Border between Nagano and Niigata Prefectures | M6.7 on March 12, 2011 (Depth of approximately 10 km) |
| – Eastern Shizuoka Prefecture | M6.4 on March 15, 2011 (Depth of approximately 15 km) |
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| – Northern Ibaraki Prefecture | M6.1 on March 19, 2011 (Depth of approximately 5 km) |
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– Hamadori, Fukushima Prefecture | M6.0 on March 23, 2011 (Depth of approximately 10 km) |
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– Southern Ibaraki Prefecture | M4.8 on March 24, 2011 (Depth of approximately 50 km) |
There is no supplementary information for the Hokkaido region.
"On April 11, there was a M7.0 earthquake at a depth of approximately
5 km in Hamadori, Fukushima prefecture. (The rest is omitted.)"
Since this event, there was a M6.4 earthquake with a maximum seismic intensity 6 Lower, and brisk seismicity has been seen.
– In the aftershock area of the 2011 off the Pacific Coast of Tohoku Earthquake, there have been earthquakes with a maximum seismic intensity 5 Lower apart from the earthquakes which were individually evaluated, brisk aftershock activities have been seen. There is fear that large aftershocks will occur from now on, and there is a possibility that it will be hit by strong shaking and high tsunami. Crustal movements to the east which are thought to be the post seismic deformation have still been observed in a wide area from the Tohoku through the Kanto-Chubu regions.
"On April 12, there was a M5.6 earthquake at a very shallow depth in northern Nagano prefecture. (The rest is omitted.)"
Since this event, there was brisk seismicity such as three earthquakes of M4.0 or over.
"On April 17, there was a M4.9 earthquake at a depth of approximately 10 km near the border between Nagano and Niigata prefectures. (The rest is omitted.)"
In the vicinity of the recent event, there was a M6.7 earthquake on March 12.
"In the GPS 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 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, April 25, 2011)
"So far, there have been no changes of activity which are likely to be associated with the Tokai earthquake.
1. Status of seismicity
General seismicity has been slightly brisk since mid-2005 within the crust under mid-western Shizuoka prefecture.
There continue to be fewer events within the Philippine Sea plate under the region around Hamanako.
In other regions, seismic activity is generally at a normal level.
2. Status of crustal movements
There have been no remarkable changes observed over the entire region.
According to the GPS data and leveling results, the trend of long-term subsidence at Omaezaki has been continuing. In view of the observation data of tilt, strain meters and others as a whole, the data showing remarkable changes associated with coupling state of the Philippine Sea and the continental plates in the expected focal region of the Tokai earthquake and its surrounding area have not been obtained.
According to the GPS data, post seismic movements associated with the 2011 off the Pacific Coast of Tohoku Earthquake have also been seen in the Tokai region."
– Since the 2011 off the Pacific Coast of Tohoku Earthquake, in inland from Tohoku through Kanto-Chubu regions, there are some places where brisk activities have still been seen.
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
– In the Kumamoto region, Kumamoto prefecture, there were brisk seismic activities such as 6 earthquakes of M3.0 or over. A M4.1 earthquake on April 25 was the largest.
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: