Nice site! however! As an FYI, if everyone in China jumped at once, it would not cause an earthquake, in fact, we wouldn't even feel it.
Even if you were to gather everyone on earth into one spot, and they all jumped and landed simultaneously... there would be no impact.
Since I am new, it will not let me post a link, but if you google it, I am sure you can find A LOT of internet space dedicated to this topic! The guy who wrote the book "What If? Serious Scientific Answers to Absurd Hypothetical Questions" answered it on his site.
Thanks, and again, useful site. Maybe I can win a league now! Lol