It'll be interesting to see what the chemical analysis shows, if anything. I'm no chemist or geologist, but when I hear that some natural phenomenon has a yellowish color, I immediately think sulfur, but not gaseous sulfur since no one is reporting any funny odors.
It's a subduction zone so things are being carried down, not up, right? Unless there is some kind of venting taking place, as perhaps, say, an underwater geyser. I doubt it's underwater volcanic activity of some long inactive volcano since volcanoes often are preceded with tremors. But everything I'm saying is useless! Just free associating, thinking out loud, stream of consciousness stuff. Useless.
Ummm, Japan's had a few 'tremors' recently...
Like others, I'd be alert if I lived around there. But not to the point of paranoia.