According to The Fresno Bee:

Digging without knowing where pipelines are buried is the top cause of gas pipeline damage in California over the last 20 years.

There have been 58 serious natural gas-pipeline breaches in California over the last 20 years, but last week’s Fresno blast was the first to cause injuries in California since the 2010 San Bruno disaster.

Thirteen people were hurt in the April 17 explosion at the Fresno County Sheriff’s Foundation gun range. In the week since, investigators have combed the site looking for answers. While the California Public Utilities Commission has not released any details yet about the cause, experts are pointing at earth-moving work that was happening at the time as a likely explanation — as it has been in most of the recent breaches in the state.

An analysis of data from the federal Pipeline and Hazardous Material Safety Administration indicates that of 58 significant incidents on natural-gas transmission lines in the state since 1995, 31 were the result of damage from excavation. The federal agency, a branch of the U.S. Department of Transportation, defines “significant incidents” as those in which people are killed or injured or that cause more than $50,000 in damage.

“If you look at the data, one of the top causes of pipeline failures (nationwide) continues to be excavation damage, like what happened in Fresno,” said Damon Hill, a spokesman for the pipeline safety office. “It’s especially those incidents that have an impact on people, where folks are injured or killed.”

Not every puncture of a pipeline ends in an explosion, but if there is a spark or an ignition source, the results can be catastrophic.

The resulting explosion sent flames up to 200 feet into the sky, and the flow of high-pressure gas raged with such ferocity that the intense heat scorched nearby trees, ignited a grass fire, and damaged the adjacent railroad tracks. The radiant heat could also be felt by motorists driving several hundred feet away on Highway 99 before the road was closed.