After feeling a small aftershock about 20 minutes ago, I looked it up.
Looks like it wasn't just a small tremor after all.
It was apparently a 5.4 magnitude and was just on the outskirts of northern Tokyo (I'm on the south where the cluster of whitish dots are to the left of the bay), with
about 5 aftershocks (so far) reaching up to 4.2.
(the map is of the Tokyo bay area, with the city on the western side. Each dot is a measuring station, and the ones near me registered 4... the aftershock registered a 2 near me, so that means I can feel a 2 and know what difference a 4 feels like).
Apparently this is the third mid-to-large one to hit Japan in the last three weeks, so I'm lucky it wasn't as intense as the one three weeks ago (which was a 7.1 earthquake).
If that had hit here where this one did, I think I'd be in a bit of trouble, as this 5.4 didn't do any damage, but did suspend high-speed trains (hopefully not for long) to check the tracks. A 7 earthquake is 100 times more powerful than a 5... and that would have done a lot of damage all across the city.
Going back over previous earthquake reports (from the first two links
and this link), it looks like a number of 4 to 5 magnitude earthquakes had occurred in the last 36 hours, along the fault-line from the north of Japan.