But the release of methane is not uniform over time; it can appear in puffs, creating a potentially explosive concentration.
...said that one frequent source of methane reaching the working areas of mines, where workers could accidentally create a spark and ignite it, are the “gob areas” of the mine, which have been mined out and shut down. These are supposed to be sealed off, but sometimes the seal is not effective.
Mr. Pilcher said that most machinery used in coal mines comes equipped with methane sensors that shut everything down if the concentration in the air gets too high. One key to safety, he said, is whether miners inform their managers when equipment shuts itself down in this way. The incentives to do so are mixed, because some miners are paid bonuses for higher production.
This is interesting. There is always seismic activity around mining, but regional seismometers could never catch it. There was an M3.4 just days before the accident, but that's probably the smallest they can record. The US is very cheap on seismic monitoring! All these sealed pockets of methane can burst out with the slightest seismic motion, or a 'slow earthquake' (strain event).
I don't think they do enough to look at the natural variations. Those big fans carry out a huge amount of methane gas in this mine. If they had the ability to look at a time history, I'm sure they would have a very wobbly line. If you plotted these bursts as a standard earthquake frequency plot (binomial distribution), you would see there is a remote chance for a very big burst, just like an M8 sits on top of a very large number of smaller earthquakes. So, it was inevitable, just like a big earthquake for California!