On December 18, 1968, a blowout and fire occurred at one of the wells when an operator attempted to remove two gas-lift valves, destroying equipment but causing no injuries.

As opposed to current practice, older wells were not sealed to the surrounding rock formation, including their often more than one mile of steel casing.

According to KPCC's website, some residents posted complaints of earlier symptoms on a Porter Ranch gas leak Facebook page.

[19][20][21][22][23][24] For instance, a February 2016 article in the Orange County Register called "Gas facility that had blowout over Porter Ranch will have to play by new rules."

"[27] The source of the leak was a metal pipe in a breached 7-inch (180 mm) casing of injection well "Standard Sesnon 25" (SS 25) that lies 8,750 feet (2,670 m) deep.

Multiple safety valves had been removed from the Aliso Canyon/Porter Ranch 1950s-era pipes in 1979, and were never replaced, a fact that was also confirmed by a special investigation into the leak by Congressman Brad Sherman's office.

[32] On May 17, 2019, the California Public Utilities Commission announced that Blade Energy Partners had completed their root cause analysis of the gas leak that occurred on October 23, 2015.

The principal findings from the root cause analysis include:[33] On May 17 2019, the California Public Utilities Commission also published a video describing the details of the leak.

Blade Energy also determined that SoCal Gas' updated practices in 2019 and new state regulations address most, if not all, of the causes identified in the report.

[37] As of January 2016, the latest methane measurement per the California Air Resources Board (Carb) was from December 22, 2015, and had decreased from a peak of 58,000±12,000 kg/h[16] to 30,300 kg/h,[30] the equivalent of more than 1.4 million cars by a different calculation using EPA estimates.

[11] Steve Conley, an atmospheric scientist at UC Davis and owner of Scientific Aviation, independently measured the gas emissions.

[47] On December 4, 2015, SoCal Gas started drilling a relief well to the caprock, 8,000 feet (2,400 m) down, with the help of Boots & Coots, a subsidiary of Halliburton.

[50] After the seventh effort to plug the leak with slurry starting December 22 had created a 25-feet-deep crater around the wellhead, the danger of a blowout increased.

"[30] Mitchell Englander, the Los Angeles City Councilman representing Porter Ranch, criticized SoCal Gas "operating a facility of this magnitude, [...] feeding 20 million addresses" for not having a backup plan, the delay in bringing necessary equipment on site "from the Gulf states like they did in this particular situation" and the delay in catching the brine, oil and chemical mist landing on people's homes and blackening their cars.

[53] The Center for Biological Diversity criticized Governor Brown's slow response "because state regulators' hands-off approach to underground injection helped set the stage for this catastrophe."

On December 7, 2015, Los Angeles City Attorney Mike Feuer sued SoCal Gas over its handling of the well failure.

[1] Feuer's Office went to court again on December 23, 2015, and secured a judicially-enforceable agreement with SoCal Gas for the company to speed up relocation efforts of residents affected by the massive leak.

He issued stepped-up inspections and safety measures for all natural gas storage facilities in California; further injections at Aliso Canyon had already been prohibited.

[4] In January 2016, two California senators asked the heads of the US Department of Justice and US Department of Transportation and the Environmental Protection Agency for a "legal analysis of any federal authorities that could apply to this incident and storage fields in general" and a "technical analysis of whether Southern California Gas Company could more quickly reduce the gas stored in the facility.

"[67] Los Angeles Times wrote that no politician is "pursuing the issue as hard as Caforio," who on January 6, 2016, spoke to a group of disgruntled Porter Ranch residents who attended a meeting co-hosted by environmental activist Erin Brockovich.

[68][69] While Knight stated he did not want to politicize the disaster, fellow Republican Michael Antonovich, a Los Angeles County supervisor who has voted consistently against regulation efforts, was outspoken about his determination to hold SoCal Gas responsible.

[70] Elected officials, including Congressman Brad Sherman and Senator Henry Stern, took the position that no decision about whether to reactivate the facility should be made until the health study and an ongoing search for the leak's cause were complete.

Since the multiple investigations into the cause of the leak have not been completed, Congressman Brad Sherman insisted that the process of reopening the storage facility must proceed with extreme caution.

Aliso Canyon is not deemed ready to resume normal operations by regulators or chief politicians that are involved with the oversight of the securing and cleanup of the leak.

Congressman Brad Sherman urged that the gas company be restricted to minimal operations required to prevent power interruptions.

[76] On February 2, 2016, Los Angeles County filed criminal charges against SoCal Gas for its failure to report the leak immediately.

Under the agreement, SoCal Gas pleaded no contest to one misdemeanor count of failing to immediately report the October 2015 leak and was ordered to pay a $4 million fine.