It lies where two tectonic plates meet. The plates are always rubbing against each other creating pressure. When the pressure is released you get earthquakes.

In a nutshell, that's it Yuna :)

There are actually 3 tectonic plates involved, right near Tokyo.

Last night's earthquake was the biggest we have felt in Tokyo since about 2006, when there was a slightly bigger one- that was the only time I have had stuff fall off shelves in an earthquake here, although I have felt many, many smaller ones. Luckily I was outside when the earthquake happened last night, in an area with no tall buildings around. Friends who were on upper floors of taller buildings got quite shaken up.

To the person above who said that Tokyo won't have an earthquake like the "Anshin" Earthquake (actually Hanshin), you are quite wrong. It has had earthquakes like that many times, most recently in 1923 (Tokyo was almost completely destroyed), and on average every 60 years of its history, so it is quite overdue. Tokyo is actually far more likely to have a large earthquake than the Kobe area.

It was a pretty good earthquake, and it picked a pretty bad time to hit. At the time I was developing film, and it hit just as I was pouring developer from my developing tank back into it's storage container. I couldn't stop or pause because the process is time sensitive, and I didn't want to lose the pictures I had taken earlier in the day. It's hard to pour liquids when the room is shaking and the pots and pans are swinging around on their hangers.

A friend of mine was on Skype from Tochigi and said it felt strong there, my gf was in Kunitachi and said it hit pretty hard there as well.

It seems there was no real damage, and no one was hurt. The rains today seem to be causing much more trouble. Many roads and train tracks are flooded, and JR train service to Northern Japan has been delayed.

wow. I never knew Japan was a part of the "ring of fire". I knew about california but not Japan. Well, *shrugs* Ya learn something new each day.

I know Japan has always been quite famous for its earthquakes.
But this thread got me thinking.
How serious is the earthquake situation there?
Basically I'm wondering: if I lived in Japan, should I really worry about them?

I used to live in Southern California and we got lots of earth quakes. I live in Florida now and we get hurricanes regularly, although you do get a little more warning before a hurricane hits. It's not that you shouldn't worry about those types of natural disasters it's that you can't do anything to stop them. You can be prepared for them though. I'm sure where ever you live has it's own natural disasters too and it doesn't stop you from living your life.

I thought high schools teach that Japan is part of the ring of fire. It's a very common fact when kids learn physical geography. Perhaps I'm wrong.

...ya... geography never was my strong suit. So that's probably why I don't know.

Well, you don't need to be paranoid about them, but if you live here they will happen, maybe even a large and devastating one, so it is important to be prepared and to know what to do if there is one.

How serious is the situation? I'm not sure what you mean by that. Do you mean are there often large earthquakes which cause damage and loss of life? The answer is yes, on average once every couple of years there is quite a serious earthquake, most recently in Miyagi and Niigata prefectures. Small ones happen all the time.