Japan Times is reporting that the Japan Meteorological Agency said the 5:18 p.m. quake was likely an aftershock of the 2011 Tōhoku earthquake and tsunami [東北地方太平洋沖地震], the magnitude-9 quake that devastated the region on 11 March 2011 and warned of an aftershock from Friday’s temblor of up to magnitude 6 within a week.
A tsunami of up to 1 metre was detected around the coast of Miyagi, Iwate and Fukushima; the same prefectures affected last year.
Reports from friends in Japan were that the quake was a bit unusual; the motion was up and down and buildings swayed for some 3 minutes.
To cast your mind back to March 2011, here’s a visualisation of every tremblor above M3 during 2011. Quakes with a Shindo damage rating of 5 or more [explained here] are also listed. In the bottom left is a counter that shows that there were some 850 M3 or above tremblors from 01/01/2011 to 28/02/2011.
You’ll also note that there was a big quake on 9 March 2011. When I first heard the reports of 11 March on Australian TV, I thought that they had only just caught up with old news.