Stanron; "So why don't they burn the gas to drive a steam turbine to generate electricity?"

It's a utility company's underground storage facility meant for that and general distribution. The well casing is fractured and the stored, odorized product is escaping below ground then venting to atmosphere. The evacuations are because of the heavier-than-air methyl mercaptan odorant stinking out the nearby suburbs.

The base gas is the same as used for buoyancy by late 19th century captive balloonists so the direct effects will be mostly in the upper atmosphere (global warming.) The drilling and rework will cause more environmental damage at and below ground level than the leak itself. Not much really.

It's not fracking though there is "tertiary" tech working the same field. No matter anyway, it's California. The 1971 San Fernando and 1994 Northridge faults are just a few miles away. The "Big One" San Andreas maybe fifty or sixty.