Not sure about the setup for the punchline, or the purpose of the narration in the last panel. I think the joke is supposed to be that it wasn't an earthquake at all that caused the tsunami, but Willie's farting. Which could be funny, but it doesn't set it up that way. The title also gives away the punchline.
I'd do something like this:
Billie: "Man, that Asian tsunami was a real disaster."
Willie: "Yeah, can you believe they think an earthquake caused it?"
Billie: "It didn't?"
Willie: "Nope, it's all my fault."
Billie: "You mean...?"
Willie: "I had a craving for squid and it made me a little gassy. So sue me."
Not great, but an improvement.
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...Trot and Cap'n Bill were free from anxiety and care. Button-Bright never worried about anything. The Scarecrow, not being able to sleep, looked out of the window and tried to count the stars.