my strongest earthquake yet

So far the tally on earthquakes that I have felt since coming to Japan, lies at a measly 5. Two of them were felt last night, the first happened around 130am and was just a gentle rocking of our apartment, whilst the second was the longest and largest that I had felt to date. It lasted about 10-15secs to my reckoning and shook the apartment pretty well, mainly by rattling the doors. At one point I was a little worried that it would get stronger and I did start contemplating getting out of bed to hide under the table or something, but it wasn't necessary.

Anyway, it turns out that these two tremors were caused by quite large earthquakes off Ibaraki prefecture [茨城県沖], JMA magnitudes 6.3 and 6.7 respectively.
The above diagram of seismic intensity (different from magnitude) - the level of shaking that different places felt/experienced is taken from the JMA website for the 6.7 magnitude quake, direct link here.