The google map below shows the spot where the pipeline exploded.  It also shows the asphalt patches placed on the street surface by a San Francisco firm (D’Arcy and Harty) after it completed sewer work in May 2008.

Before a contractor digs in the street, PG&E is supposed to mark on the street with spray paint the location of its gas lines so that the contractor can avoid them. Interestingly, those paint marks aren’t visible. Maybe they were worn away, or perhaps the contractor just paved over them after it finished the work.  Maybe the photo just isn’t of high enough resolution.  But certainly the situation brings back memories of the last underground gas explosion in the bay area —  the one on Spencer Avenue in Santa Rosa.

In the Santa Rosa explosion, PG&E failed to properly mark the location of its gas line. A contractor hit the line with a backhoe and damaged it. The line leaked months later. Residents smelled gas in the days leading to the blast, but nothing was done to find or repair the leak.  

One thing might be different here.  The contractor may have damaged the gas line not with a backhoe, as in Santa Rosa, but with the vibration resulting from its unique method of sewer line replacement.  According to the San Jose Mercury News

To avoid the disruption of digging trenches in the street, the contractor used a method called "pipe bursting." Crews pulled a large, cone-shaped device through the aging, 6-inch sewer pipes, shattering them, and replaced them by pulling a new, 10-inch, polyethylene sewer pipe in behind them. The technique can cause ground shaking and disruption of adjacent soil and rock.

Local residents report that there was certainly lots of shaking and pounding in connection with the D’Arcy and Harty project.

Of course, to a large extent, how the damage to the gas line was done isn’t nearly as relevant as why PG&E didn’t find and fix it.

San Bruno Excavation