The original source of the Leaping Chinese Earthquake Doomsday Weapon was a whimsical proposal in Geotimes in 1969 by David Stone, now Professor Emeritus of Geophysics at the University of Alaska. It reached a much wider audience when it was reported in Time. I remember reading it as a teenager. The 19 December 1969 issue says:
If at a given moment, says Stone, all 750 million Chinese obeyed a command to jump from 6½-ft. platforms, they could constitute a "geophysical weapon." How? Assuming that the average Chinese weighs 110 lbs., he calculates, the energy released by this great leap downward would be equivalent to an earthquake of magnitude 4.5 on the Richter scale, causing extensive damage in China. But if the Chinese were organized to jump roughly every 54 minutes—just when the peak of a barely perceptible natural ripple that continually sweeps around the earth's surface passes through China—they might set up a world-girdling resonant ground wave that would cause even greater damage in distant lands. By properly aligning their millions and carefully timing the jump, for example, Peking could aim a ground wave along the Pacific-rim earthquake belt and possibly set off quakes in California far more devastating than the original shocks in China.
Would there be any defense? Certainly, says Stone. By having its population jump between the peaks of the ground waves stirred up by China, a threatened nation could damp them out before they grew intense enough to cause damage. There is one catch: the target nation would, of course, be less populous than China. Thus, to effectively counteract the massive Chinese geophysical aggression, its people would have to jump from higher platforms.
It has since been processed through the machinery of folklore, and turns up every now and then in various guises. Sometimes it's said to cause earthquakes, sometimes it's said to change the Earth's orbit, or the direction of its axis.
I wrote to Prof. Stone about this in 2006; he is amused to see the changes rung, like echoing seismic waves, on his idea:
"My favorite response to the great leap downward was in the London Economist where I was hailed as the saviour of the economy of SE Asia - who else could build 600 million step ladders?"
I have a paper copy of Stone's note, but if it's online, I'm not aware of it. I think I drove to the Wheaton College library, looked up Stone in the Reader's Guide to Periodical Literature, and photocopied the right issue of Geotimes.
Here's Phil Plait attempting to put out one of the folkloric fires.
Here's Cecil Adams putting out another. (He doesn't seem to be aware of Stone's original letter, nor of the concept of building up a resonant seismic wave through periodic jumps.)
In 2007, a form of Stone's idea was tested, Mythbusters style, for a German TV show.
Earlier notes from me: Making Light, August 2007. Autopope, April 2008.