[fidelis]

“As a contractor started to weld, an explosion ensued. The tank separated from the ground, lifted approximately 70 feet vertically before a tsunami occurred, causing major damage to well site 7,” Lemoore City Manager Nathan Olson said.

My guess is that this welding was going on near or below the tank in a confined space that had not been properly purged of some gas that had settled there, be it methane or some other explosive gas. Open flame meets gas in confined space = KABLOOEY.

[fireman15]

These events are fairly rare but definitely not unheard of. Here is a thread discussing some similar water tank explosions from scienceforums.net. Hydrogen sulfide is mentioned as a gas that can accumulate in large tanks formed by the action of bacteria in sulfur rich water with deficient oxygen content.

https://www.scienceforums.net/topic/75448-rare-event-mystery-of-exploding-massive-municipal-water-tanks/