Why wouldn't NK just set off another underwater nuke like we did last time to cause the tsunami?

Tough to detect who set it off.

In fact, why wouldn't they do the same stealthy thing offshore LA ? Better yet - New York

Check that.... probably much more likely that the US would do that - or at least allow it - like we did for 9/11, for the casus belli for yet another trillion dollars for war. Nothing like a war to kick the economy in the pants.