Earthquake!

So, last night I was engaged in a bout of light sleep when the building started moving of its own accord. Nothing particularly unusual about that, but normally I sleep through such events, and this one was not only the first I've experienced in this building (only moved in a couple of weeks ago) but it seemed to go on for a fairly long time. Not sure whether to be alarmed (the building is very new and might have just ridden out a dramatic seismic event) I put the TV on, and sure enough it seemed to have been quite a biggy: within 60 - 120 seconds of a significant earth-moving event, information on the affected areas pops up on the screen, and here we see that northern Miyagi, parts of Yamagata and northern Ibaraki prefectures have just felt a "3":

Now, these areas are a couple of hours train ride north of Tokyo, and it felt like a "3" here too, so it wasn't presumably a minor tremor. Sure enough, here we have Gumma and Chiba prefectures as well as Tokyo also registering a 3:

Some specifics: "The earthquake was off the coast of Ibaraki Prefecture, depth 40km, magnitude 6.7" (note: the map on the screen in these shots is entirely coincidental):

"This earthquake may cause minor changes to tidal patterns but no damage is expected":

A couple of minutes later NHK (the Japanese national broadcaster) interrupted their programming with a hasty news bulletin, complete with a map showing the relevant numbers:

For the uninitiated, Tokyo is where the "3" is at the bottom middle of the map, and Ibaraki Prefecture, off whose coast the earthquake occurred, is where the rightmost "5-" is.

These numbers aren't the magnitude of the earthquake (this is the "M6.7" shown at the top of the screen, next to the time which was 1:45am), but reflect the "shindo" (震度, or in English "Japan Meteorological Agency seismic intensity scale"), or perceived intensity of the earthquake in the areas where it could be felt. This information I have just gleaned from the internet, as all these years I haven't been exactly sure what the numbers represent, apart from the fact that a "3" is a noticeable if undamaging event. This handy diagram from the Japan Meteorological Agency gives a nice overview of the differing degrees of fear one should go through while experiencing an earthquake:

See this page: http://www.jma.go.jp/jma/kishou/know/shindo/explane.html for a detailed description of the effects to be expected at each level on the scale.

The Meteorlogical Agency also provides near-real time information on quakes here (and in Japanese here).