In simple terms a seismic bump occurs when the stresses caused by mining aren't properly compensated. It's one of the reasons we need well enforced safety regulations in this hazardous line of work.
Bumps occur when too much stress builds on mine support structures. Often the mine's roof and floor are strong, so what gives under the pressure is the coal, usually on the support pillars. It can cause the floor and roof to buckle, too.
It's a benign name for something that explodes like a hand grenade. Mine "bumps'' shoot high-speed coal and rock at anyone in the way as support pillars buckle.
This is not related to earthquakes or natural seismicity, despite the efforts of some (like Utah Senator Orrin Hatch) to claim otherwise:
"You use the best technology you can, you do best you possibly can. You hope natural disasters don't occur, like did occur. I don't think anybody could say it's anything but a natural disaster," Hatch said.Actually, people who know earthquakes are saying it wasn't a natural disaster.
It'd be nice if the Senator was as concerned about the safety of mine workers as he is about the welfare of mine owners.