Duh.
The LA basin is a huge oil and gas field, and methane gas seepage has been a known problem for many decades. J Paul Getty had many oil wells there, and made millions from them.
http://articles.latimes.com/1999/jul/30/local/me-61056
The Methane Down Below
By EDWARD J. BOYER, TIMES STAFF WRITERS and JANET WILSON, TIMES STAFF WRITERS
July 30, 1999
One witness thought it was “raining fire.” The earth was belching flames through cracks in the pavement, tongues of fire marching in a steady progression across a shopping center parking lot on 3rd Street near Fairfax Avenue in West Los Angeles.
The methane gas explosion that ripped through a Ross Dress for Less store in March 1985 left 24 people injured and forced the closure of stores in the center for several days.
[snip]
The region is dotted with working and abandoned oil wells from Santa Clarita in the north to Newport Beach in the south. And where there are oil wells, there is methane.
Does that mean neighborhoods and commercial developments built on old oil fields face the same risks that have caused so much concern at Belmont?
There is certainly risk whenever explosive gases are present, experts say, but building codes and state agencies have been able to keep that risk at manageable levels for the most part.
When builders follow proper procedures to vent methane, the risk of accidental explosions is virtually eliminated, experts say.
Having pumping oil wells on a site is better than having improperly abandoned wells because working wells vent methane harmlessly into the air, said Richard Baker, district deputy of the state Division of Oil, Gas and Geothermal Resources.
Add some offshore oil and gas seepage too. This is very big off Santa Barbara where globs of crude was up. My guess is LA has the same thing going on. Lots of gas bubbling up from under