The first report from federal investigators on September 9th's San Bruno explosion came out last night, and though it gives no clear answers about the cause of the blast, it does give some telling details. As the Chron reports, an expert reading of the report shows no definitive answer about whether the section of pipe that ruptured was weakened or corroded. Judging from measurements taken of all the pieces of the pipe that were found, none were significantly thinner or more corroded than normal.
"If you had significant internal corrosion, you would expect that wall thickness to be significantly decreased," said Brigham McCown, a Texas lawyer who once headed the federal Pipeline and Hazardous Materials Safety Administration... He cautioned, though, that corrosion could not be ruled out as a factor because the damage may be small and isolated to an area where the pipe failed.
The other big clue is that there was an equipment failure at PG&E's Milpitas terminal, the starting point of the gas line, just before the explosion. The failure caused workers to lose control of the pressure in the line, causing it to spike what they thought was not a significant amount. But experts say that such a spike "could have pushed a weakened line over the edge." See the full NTSB report here, and SF Weekly's take here.