December 29, 2016
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On December 28 at 21:38 (JST), there was a magnitude (M) 6.3 (provisional) earthquake at a depth of approximately 10 km in the northern Ibaraki prefecture. The maximum seismic intensity 6 Lower was observed in Ibaraki prefecture and damage was caused by this event. The largest earthquake which occurred afterwards, until December 29 at 16 o'clock, was a M4.7 (provisional) one on 28th at 21:53 (JST) which caused the maximum seismic intensity 4. The seismic activity around the origin of the M6.3 event is decaying, and confined in the region of about 15 km length extending in an NNW-SSE diction.
* The focal mechanism showed a normal fault type with a tension axis in an ENE-WSW direction. This event occurred at a shallow depth within the crust. The seismic fault estimated from the focal mechanism and the distribution of aftershocks is the fault plane dipping in a WSW direction.
* According to the GNSS observation, crustal deformation of about 2 cm (provisional) displacement is reported both for Satomi observatory, Ibaraki prefecture in a WSW direction and for Kita-Ibaraki observatory, Ibaraki prefecture in an ENE direction. Also the analysis of Synthetic Aperture Radar images observed by the Advanced Land Observing Satellite-2 "DAICHI-2" shows a subsidence or a westward crustal deformation of maximum 27 cm (provisional).
* In the area where strong shaking was felt, it is necessary to be prepared for an earthquake with maximum seismic intensity 6 Lower, for a week or so from the occurrence of the earthquake. Especially in a few days or so, an earthquake of a large magnitude often occurs.
* This earthquake occurred within the aftershock area of gthe 2011 off the Pacific Coast of Tohoku Earthquake.h Although the aftershock activity has been gradually decaying as time has passed, the seismic activity is about twice of that before the Tohoku Earthquake, and still in active state centered on coastal regions, where the stationary high activity is continuing. Within the aftershock area and its surroundings including in-lands, there is fear that large earthquakes will occur and there is a possibility of strong shaking and high tsunami for a long period from now on. It is necessary to pay attention continuously for it.
* 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 and Mw7.8 after about seven and a half years and about eleven years, respectively in the region of outer rise. Thus, there have been large earthquakes for a long period within the source area and its surroundings.