Global scale seismology experiments / observations disproving flat earth conspiracy?
As you rightly say, seismicity is one geological proof of an almost spherical planet, but surely it is far easier to prove your case by satellite data, such as scenes from http://weather.msfc.nasa.gov/GOES/ Irrespective of the position of the satellite, the Earth always appears as a disc, never 'edge on'. For that matter, if the Earth wasn't approximately spherical there wouldn't be any stable satellite orbits - so no satellite pictures in the first place. You could use your mathematical skills to demonstrate that the only stable orbital configuration is around a sphere. Also the consistency of gravity, at any known point on the planet's surface, points to a spherical Earth. Then there are commercial and military flights which have, over time, flown arcs along almost every conceivable great circle, without ever coming to an edge. Turning to astronomy, every stellar or planetary object large enough to form under gravity appears as a disc - never edge on - surely no coincidence..... etc. etc. 

But I'm curious - why bother?! You are never going to convince the likes of creationists, climate-change deniers, 'flat earthers', and the like. They have already made up their mind, and don't want to be confused by the facts. No matter how much evidence you supply, it is like water off a duck's back to the closed mind. They are comfortable with their own warped model of reality in their head - no matter how erroneous. Paradoxically, they view the discomfort of having to adjust their world view as a form of 'cognitive dissonance'. So trying to convince flat earthers is like the proverbial 'flogging a dead horse'.