March 11, 2013
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* Aftershocks of the 2011 off the Pacific Coast of Tohoku Earthquake (hereinafter referred to as the Tohoku-Oki Earthquake) occurred in a wide area from the region offshore Iwate prefecture through the region offshore east of Chiba prefecture, and there were 108 aftershocks of magnitude (M) 6.0 or over up to now. There were 7 earthquakes of M7.0 or over. Among them, 3 earthquakes occurred just after the main shock. The largest aftershock was a M7.6 earthquake offshore Ibaraki prefecture about 30minutes later after the main shock.
* Within the aftershock area of the Tohoku-Oki Earthquake, the number of
earthquakes of M4.0 or over during about one year from March 2012 became
under one fifth smaller than the number during about one year just after
the Tohoku-Oki Earthquake, and thus the aftershock activity has been decaying.
However, in comparison with the mean activity before the Tohoku-Oki Earthquake, the number of earthquakes of M4.0 or over within the aftershock area was more than five times, and the aftershock activity still remains in brisk state.
* After the Tohoku-Oki Earthquake, seismicity increased in the regions such as the southern inland, Akita prefecture, offshore of Akita prefecture, the vicinity of the prefectural border of Aizu, Fukushima prefecture and the Okitama region, Yamagata prefecture, the vicinity of the prefectural border of Gunma-Tochigi prefectures, the vicinity of the prefectural border of Nagano-Niigata prefectures, southern Ibaraki prefecture, and eastern Shizuoka prefecture, other than the aftershock area of the Tohoku-Oki Earthquake from the Tohoku to the Kanto-Chubu regions.
* According to the GNSS data during about one month just after the Tohoku-Oki Earthquake, the maximum displacement was 30 cm in the horizontal deformation, 6 cm in the vertical subsidence, 5 cm in and the vertical upheaval. The deformation was gradually decaying. Although the horizontal deformation was slightly larger than 1 cm, and the vertical deformation was less than 1cm during the recent one month, 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.
* In case of the earthquake (moment magnitude (Mw) 9.1) offshore west of the northern Sumatra Island in December 2004, there were Mw8.6 in March 2005 after four months, Mw8.5 in September 2007 after about two and a half years, Mw7.5 in June 2010 after about five and a half years, and Mw8.6 in April 2012 after more than seven years in the region of outer rise. Thus, there have been large earthquakes for long period within the source area and its surroundings.
* Even in case of the Tohoku-Oki Earthquake, there was a M7.3 aftershock in December 2012 after about one year and nine months in the vicinity of the Japan Trench offshore of Sanriku. Like the oversea example, 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.
* Offshore Sanriku, there was a M7.3 earthquake within the plate near the
Japan Trench on December 7, 2012. In the vicinity of this event, there
was an estimated M7-class earthquake just before this event.
In addition, in May 2012, there was a sequence of seismicity with the maximum M6.5 earthquake on the boundary between the Pacific and the continental plates. In this region, the seismicity increased after the Tohoku-Oki Earthquake. Although the seismicity was gradually declining on the whole, brisk seismicity has been continuing as compared with the activity before the Tohoku-Oki Earthquake.
* Offshore Iwate prefecture, there was a M6.6 earthquake within the crust of the continental plate on March 27, 2012. Even after that, a sequence of the seismicity was seen until around May 2012. Since then, the seismicity has been low in this region.
* Offshore Miyagi prefecture, there was a M7.2 earthquake within the Pacific plate on April 7, 2011. Within the aftershock area of this event, brisk seismicity has been continuing. Even after 2012, there were a M5.6 earthquake on August 30 and a M5.2 earthquake on November 24.
* Within the crust from Hamadori, Fukushima prefecture through northern Ibaraki prefecture, seismicity has been brisk since the Tohoku-Oki Earthquake. There were mainly normal-fault earthquakes. There were a M7.0 earthquake on April 11, 2011 and a M6.4 earthquake on April 12, 2011. Even after that, the brisk activity has been continuing as compared with activity before the Tohoku-Oki Earthquake.
* In the vicinity of Choshi [offshore east of Chiba prefecture], seismicity has been brisk since the Tohoku-Oki Earthquake. There were mainly normal-fault earthquakes. Even after 2012, there was a M6.1 earthquake on March 14 and there have sometimes been earthquakes of M4.0 or over. The brisk activity has been continuing.
* Offshore Akita prefecture, there was a M6.4 earthquake on March 12, 2011. Even after that, there have sometimes been earthquakes of M3.0 or over. This event occurred within the aftershock area of the 1983 Nihonkai-Chubu Earthquake
* In the vicinity of the prefectural border of Nagano-Niigata prefectures, there was a M6.7 earthquake on March 12, 2011. This event had a maximum seismic intensity 6 Upper in Nagano prefecture. After that, a sequence of seismicity was seen such as two earthquakes with a maximum seismic intensity 6 Lower. In the surroundings around the aftershock area of this event (northern Nagano prefecture), there was seismicity including a M5.6 earthquake on April 12, 2011. After that, the aftershock activities decayed and nearly subsided.
* In eastern Shizuoka prefecture, there was a M6.4 earthquake on March 15, 2011. This event had a maximum seismic intensity 6 Upper in Shizuoka prefecture. After that, the aftershock activities decayed and nearly subsided after 2012.
* In northern Tochigi prefecture, there was a M6.3 earthquake on February 25, 2013. After that, brisk aftershock activities were seen and gradually decaying. Within the crust near the prefectural border of Gunma-Tochigi prefectures, the seismicity increased after the Tohoku-Oki Earthquake.
* Although the boundary between the continental and the Philippine Sea plates under southern Ibaraki prefecture was the region where seismicity was usually brisk, much brisker seismicity has been continuing as compared with the activity before the Tohoku-Oki Earthquake
* Within near the vicinity of the prefectural border of Aizu, Fukushima prefecture and the Okitama region, Yamagata prefecture, a sequence of seismicity was seen from mid-March 2011. There were a few earthquakes in a month even now, and relatively brisk seismicity has been continuing.
* For cumulative crustal deformation during about two years after the Tohoku-Oki Earthquake, the maximum of horizontal displacement amounts to 96 cm or so (east-southeastward) at Yamada station (Iwate prefecture) and the maximum of vertical displacement amounts to 24 cm or so (upheaval) at M-Oshika station (Miyagi prefecture).
* According to the data of postseismic deformation after the Tohoku-Oki Earthquake, slip is inferred on the plate boundary under the coast of the Tohoku region. The maximum amount of the inferred slip was over 1.2 m during one month just after the Tohoku-Oki Earthquake. It was gradually decaying to 0.1 m or so during one month after July 2012. In addition, the slip has been inferred on the plate boundary deeper than the main shock, and remarkable change of the spatial distribution has not been seen.