January 13, 2017
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On December 28 at 21:38 (JST), there was a magnitude (M) 6.3 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 January 13 at 16 o'clock, was a M4.7 one on December 28 at 21:53 (JST) which caused the maximum seismic intensity 4. The seismic activity around the hypocenter of the M6.3 event which is confined in the region of about 15 km length extending in an NNW-SSE direction, is decaying but still continues.
* 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.
* According to the GNSS observation, crustal deformation such as about 3 cm displacement in a WSW direction for Satomi observatory, Ibaraki prefecture is reported. 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.
* The seismic fault estimated from the focal mechanism, the distribution of aftershocks, the GNSS observation, and the analysis of SAR images is the normal fault of about 10 km length which extends in an NNW-SSE direction and dips to WSW direction.
* This earthquake occurred within the aftershock area of the 2011 off the Pacific Coast of Tohoku Earthquake. 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 a 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 to 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.