Cal Fire continues to investigate the Thomas fire, which was listed as 25 percent contained as of Wednesday morning.

to several large wildfires over the years in the state

California has a history of power lines sometimes sparking wildfires during high wind conditions, so utilities are responding in elevated fire risk situations by taking a proactive measure — turning off the electricity. Last week, Southern California Edison temporarily shut off power to thousands of people in Riverside County for some 33 hours in "extremely high-wind areas" to keep its electrical lines from sparking a wildfire. Also, San Diego Gas & Electric cut power to a mountainous area in southeastern San Diego County last week when major portions of Southern California were in red flag alerts, indicating dangerous fire weather. "De-energizing is an important tool to protect the safety of the communities we serve," SDG&E spokesperson Christy Ihrig told CNBC. The utility is part of San Diego-based Sempra Energy. SDG&E has a network of more than 170 remote weather stations with wind sensors that allow it to identify locations where there are very strong wind gusts, sometimes in excess of 80 miles per hour. The drastic step of cutting power to residents comes as California is coping with one of its worst wildfire seasons ever and as firefighters battle the monster Thomas fire in Ventura and Santa Barbara counties. "The evidence is that a lot of the big fires we've had have happened on windy days," said William Stewart, a wildfire expert and co-director of University of California-Berkeley's Center for Fire Research and Outreach. "Power lines are one source of ignition. Even if there's a car wreck on a windy day it can do the same thing."

Size of Delaware

So far this year, California wildfires statewide have burned an estimated 1.36 million acres, or more than 2,125 square miles — nearly the size of the state of Delaware, according to Cal Fire figures through Monday. It has left entire neighborhoods in ashes and also destroyed vegetation and trees in some forested areas that haven't burned in decades. "The lack of rain, the lack of moisture, the vegetation is so dry," said Cal Fire Deputy Chief Scott McLean. Significant rainfall in 2016 ended California's historic drought but also produced thick grass growth, which created ideal fuel for potential wildfires this year. Adding to the wildfire risk are an estimated 129 million dead trees in the state due to years of drought and beetle infestation, according to the U.S. Forest Service. State investigators are still looking into the cause of the 237,500-acre Thomas blaze in Southern California. The fire is believed to have started in Ventura County's Santa Paula area but not much else has been publicly disclosed. "All the major fires in Southern California are still under investigation," said Cal Fire's McLean. "It's going to be an ongoing investigation, and it does take time."

Edison's role probed

Meantime, Southern California Edison said Monday investigators may be looking at some of its infrastructure as part of the probe in the Thomas fire. There was at least one fatality linked to the fire, according to Ventura County officials. "The investigations now include locations beyond those identified last week as the apparent origin of these fires," Edison said Monday. "SCE believes the investigations now include the possible role of its facilities. SCE continues to cooperate with the investigations." Stock of SCE's parent, Edison International, plunged 6 percent on Tuesday as investors appeared to grow nervous about the Rosemead, California-based company's potential liability. Shares are expected to add to those losses Wednesday, with the stock down nearly 3 percent before the market's open. Edison also said its power grid has been damaged by the Thomas blaze, now the fifth-biggest wildfire in the state's modern history. Due to the fires, the utility said, there have been times when more than 85,000 customers in the Santa Barbara area have been without electricity.

Five major wildfires

As of Wednesday morning, the Thomas fire had destroyed 921 structures and damaged 200 others, according to Cal Fire's website. It was 25 percent contained but with some 18,000 structures still threatened. The Thomas fire is one of five major wildfires to hit Southern California this month. The Creek and Rye wildfires in Los Angeles County were at or near full containment Tuesday, while the Skirball fire further south was 85 percent contained. The Lilac fire in San Diego County was 95 percent contained, and the Liberty fire in Riverside County was fully contained. Edison equipment was damaged in the Rye, Creek and Liberty fires. Edison said its "preventative power shutoff" last week was a measure taken in the Idyllwild area in Riverside County and "driven by public safety concerns for customers in extremely high-wind areas." The power was temporarily cut just before the Liberty fire was reported last Thursday, and the outage lasted through Friday evening. Some experts say California has allowed homes to be built near wildfire-prone areas, raising the danger of more deadly disasters.

Wine country wildfire probe

In October, a series of wildfires in the wine country struck several Northern California counties, claiming 43 lives and destroying or damaging more than 14,000 homes. Cal Fire said this week the wine country fires remain under investigation. "There are still interviews to be done," said Cal Fire's McLean. "There's still investigations to be done. It's an ongoing process." Some reports have suggested downed power lines from PG&E may have played a role in causing at least one of October's wine country fires. Last month, PG&E — California's largest electrical utility company — trimmed this year's earnings forecast and said it is continuing to cooperate with agencies investigating the fires. The San Francisco-based company also said at the time during its earnings conference call that at least nine lawsuits had been filed against the company in connection with the October wildfires. Insurance claims for the wine country fires already exceed $9 billion, the state's insurance commissioner announced last week. PG&E's stock fell sharply in October on concerns its power lines may have had a role in the wine country wildfires. Its stock has yet to fully recover.

Utility liability risk