Matt Alt knows about the Japanese usage of cute mascot characters for all manner of applications. He wrote the book on it, actually. He posted a few examples of tsunami warning characters on his blog, and wrote:
Japan has a long history of using cute mascot characters in situations that can surprise Westerners. In fact, quite often the more terrifying or distressing a situation, the cuter the visual description of it becomes on official posters here.
This may sound counterintuitive or even a little condescending at first, but when you start thinking about it, it makes perfect sense. Whereas, say, photos of damage or devastation would probably cause people to avert their eyes, this sort of presentation lures people into actually reading the sign — particularly children, as tsunami education needs to start early.
The plush mascot above is Tsunami-man, “one of a series of regional mascots from Shikoku’s Kochi prefecture used to teach children about the dangers of earthquakes and tsunami. You can see him in action in a comic strip here.”