If all 1.5 billion people in China jumped at the same time would it cause a tsunami?
No. It's just a common bit of trivia that emerged out of who-knows-where, but is patently untrue in every possible scientific way.
The Straight Dope actually did the calculations of this question a while back, and here's what they came up with:
He calculated that the resulting impact would be about the equivalent of 500 tons of TNT - a pretty decent explosion, but nowhere near enough to shake the earth or cause an earthquake, tsunami, or any other cataclysmic event.