Seismic Activity from Hamadori, Fukushima Prefecture through Northern Ibaraki Prefecture
Within the crust from Hamadori, Fukushima prefecture through northern Ibaraki prefecture (Area a), the seismicity increased after "the 2011 off the Pacific Coast of Tohoku Earthquake". Although it has gradually been declining on the whole with some up and down, as of the end of September, 2013, it still keeps the higher level than that of prior to the occurrence of the 2011 off the Pacific Coast of Tohoku Earthquake. In September 2013, for example, on 9th 09:04, at a depth of 10 km under northern Ibaraki prefecture, there was a M4.4 earthquake (maximum seismic intensity 4, normal fault type with a tension axis in an E-W direction, ①), and on 20th 02:25, at a depth of 17 km in Hamadori, Fukushima prefecture, there was a M5.9 earthquake (maximum seismic intensity 5 Upper, strike-slip fault type with a compression axis in an NW-SE direction, ②), in total there were 15 earthquakes of M3.0 or over, and 20 earthquakes with seismic intensity 1 or over in Area a.
The seismic activity of M3.0 or over in Area a are divided into northern-, middle-, and southern-part, then in the northern part the activity has been low since October, 2011. In the middle and the southern parts, the activity has been rather high, as earthquakes of M4.0 or over have sometimes occurred even in 2013.
Most of the focal mechanisms of this seismic activity have normal fault type. The directions of the tension axis are different in parts: mostly NW-SE directions in northern part, and mostly ENE-WSW directions in southern part. And in middle part there are earthquakes with the focal mechanism of reverse or strike-slip fault centered at rather deep depth from 15 to 20 km.
Japan Meteorological Agency
[Evaluation of Seismic Activities for September 2013 (October 10, 2013)]