Tell her that if we detonated every single nuclear weapon extant in the world today in a fault line in the ocean it wouldn't raise a ripple in comparison. The amount of energy required to create such a tsunami is simply astronomical in magnitude. Far beyond the capabilities of any evil SUV driving human beings.
That can't be right. The geophysics training I got in 1978 from Superman: The Movie clearly showed that nuclear weapons can trigger earthquakes. Lex Luthor, Superman's nemesis, wanted to make the West Coast of California "more valuable between the time you buy it and the time you sell it." To do this, he caused a major earthquake using two missiles he programmed to hit the San Andreas Fault with the intent of causing most of the state to slide into the ocean. As much as I hate to admit it, this science clearly supports your cousin's argument.
Forget it. Your suggestion requires thought and a degree of intelligence in order to achieve comprehension. The first is something a liberal is not capable of and the latter is something totally alien to them.
It's all about the emotion, man.