Edit for the [dead]: sewer gas isn't gasoline in the sewer. Sewage produces hydrogen sulfide, an explosive gas. There are many videos online of sewers blowing up after people drop lit cigarettes into sewers and in those videos the street is often blown apart like is shown in the OP video.
Much more likely that natural gas leaked into an existing void containing air... such as a sewer line.
It would be natural gas that leaked in.
Normally it would not be the sewer since it should be sealed, unlike storm water, but many cities still mix sewer and stormwater. You can see the pipe is actually square in some pics, which is not normally sewage in developed places.
Gasoline in the sewer occasionally happens, but there's no reason to think that happened here.
I'm surprised no terrorists have tried gasoline in the sewer. Or maybe they have and it doesn't work and just like lazy universities they don't write up the null hypothesis.