The leak was discovered at a storage facility in the Santa Susana Mountains in late October. Southern California Gas Company has not been able to stop it yet.
"Today's proclamation builds on months of regulatory and oversight actions from seven state agencies mobilized to protect public health, oversee Southern California Gas Company's actions to stop the leak, track methane emissions, ensure worker safety, safeguard energy reliability and address any other problems stemming from the leak," read a statement from Brown's office.
The governor toured the facility and met with residents earlier this week. Many are upset.
The leak has forced people from their homes and students from area schools.
Yitz Dekel spoke to CNN last month outside his his temporary home, a residential hotel in Calabasas, California.
"In a way, without fire and brimstone, this is a natural disaster," he said. "A technological failure and natural disaster."
According to the Los Angeles County Department of Public Health, the leak is having health effects in some people.
"Exposures to these chemicals are generally not expected to lead to permanent or long-term health problems. However, short-term, recurrent symptoms may occur with these exposures as long as the odors persist, and some individuals may be more sensitive than others," it said in a fact sheet.
When the Health Department held a community meeting in Porter Ranch on exposure to methane and mercaptans (the foul-smelling odorants added to natural gas), officials said nausea, dizziness, vomiting, shortness of breath and headaches are among the health complaints "consistent with inhalation exposure to mercaptans."
Dekel complained of "inexplicable fatigue." He said his wife experienced some dizziness.
"And my neighbors, some in ill health, had major coughing and irritation in the eyes and throat. Some had nose bleeding," he said.
SoCalGas says it's working as quickly and safely as it can to stop the leak.
In the meantime, the company said that as of late December more than 2,290 households have accepted relocation and that it was working with 3,660 of them in various stages of finalizing agreements.
It estimated it should be able to stop the leak between late February and late March.
"Our focus remains on quickly and safely stopping the leak and minimizing the impact to our neighbors in Porter Ranch," said President and CEO Dennis Arriola.
"SoCalGas reaffirms our prior commitment to mitigate the environmental impact of the actual amount of natural gas released from the leak. We look forward to working with state officials to develop a framework that will achieve this goal."