Maia, the ALT sitting next to me, was the first to vocalize it. As we sat there, the teachers around us began to take notice and comment, kind of amusedly, about it. The main quake lasted about 3 minutes here, and I still feel kind of queasy.
Twitter and Facebook are going crazy (if you happen to be following or friends with people living in Japan), and apparently there's coverage of the tsunami damage going on as we speak. I'll write more if anything major develops.
Update 15:51: It's been about an hour since the initial quake. I was just upstairs watching the news with some other teachers. At least one major fire broke out in Odaibai, Tokyo, but it seems to be under control. In Miyagi, there appears to be massive flooding, with cars being carried by the water. Also significant tsunami damage in Iwate. Tsunami warnings have also been issued along all of the Pacific Japanese coast line, although decreasing in severity further south. Around Hyogo the warning level is orange, on a scale of yellow-orange-red.
Update 16:04: According to NHK World, the SDF is being deployed to assist affected prefectures. This is one of the worst earthquakes in Japanese history, on par with the great Hanshin quake of 1995. Tsunami warnings have now been issued for parts of Russia, Guam, Taiwan, and other Asian countries.
Update 3/12/11 00:08: Things are still a mess up north. I've been in periodic contact with a friend of mine in Tokyo, and he's stuck spending the night at his office. Hasn't been able to get in touch with his girlfriend, so quite nerve-wracking. The cell phone networks still appear to be down. I can only imagine what people must be going through at the epicenter. If you're religious, pray for them.
There's a Wikipedia page up and running...apparently this is the biggest earthquake Japan has experienced. For more information about what's going on and for links and resources about how to find people, I recommend visiting Surviving in Japan blog. There's also a video up of the quake in Tokyo at This Japanese Life.
For those of you who understand Japanese or who just want to see what's going on in the news here, this is a live stream.