Shares of PG&E Corp. (PCG - Get Report) fell sharply on Wednesday after the company said in an SEC filing that it doesn't have enough insurance to cover potential liabilities tied to a deadly wildfire in Northern California.

The stock fell 21.6% to $25.66 on Wednesday, its worst percentage decline since Aug. 1, 2002. The stock has lost nearly half its value since Nov. 7, the day before the the so-called Camp Fire began. The fire began last Thursday in Butte County, about 200 miles northeast of San Francisco.

It has killed at least 48 people, destroyed more than 8,000 homes and businesses in and around Paradise, Calif., and burned nearly 200 square miles, filling the air over much of Northern California with acrid smoke prompting health warnings and school closures.

In an SEC filing late Tuesday, PG&E said that "on November 8, 2018 at approximately 0615 hours, PG&E experienced an outage on the Caribou-Palermo 115 kV Transmission line in Butte County. In the afternoon of November 8, PG&E observed by aerial patrol damage to a transmission tower on the Caribou-Palermo 115 kV Transmission line, approximately one mile north-east of the town of Pulga, in the area of the Camp Fire."

PG&E said it has insurance coverage for wildfire events totaling $1.4 billion. The company reported cash and cash equivalents of $3.45 billion.

"While the cause of the Camp Fire is still under investigation, if the Utility's equipment is determined to be the cause, the Utility could be subject to significant liability in excess of insurance coverage that would be expected to have a material impact on PG&E Corporation's and the Utility's financial condition, results of operations, liquidity, and cash flows," PG&E said in the SEC filing.