About a mile from my place ocurred that San Bruno underground gas pipe explosion, got me wondering...

 

As it left a crater and ejected a pipe section, the explosion was underground... I think.

How the needed air/oxygen for an explosion to occur got into the packed pipe sorroundings?

 

If the pipe was leaking before the event, less chance for air to get underground, as would be flushed out by the outgoing gas. I think.

 

The fire in the gas filled pipe did not propagate inside the pipe further lenghtwise to other served neighborhoods, as there was no oxygen.

The spark... on highly doubtful conditions, the spark had to happen underground... perhaps metal-to-metal grinding at the moment of rupture.

 

Any guesses until experts say something ?

In the pipe, the methane is probably pure: no air, no explosion.

In the air, the methane is probably too dilute to explode.

 

The lower explosion limit of methane is 5%vol, the upper explosion limit is 15%vol.

 

Somewhere in between those two, it goes through its explosive concentration range. And that is probably under ground. All you need then is a spark, or enough heat, and it explodes. So, there must have been a leak... that's all you need for an explosion, or at least a fire.

 

I found this link which may be an interesting read.