Wednesday's firestorm in a wealthy area of Los Angeles could be the final straw that breaks California's insurance market. The state's insurance market has been teetering on the edge of insolvency for years thanks to catastrophic wildfires that have driven many insurers to stop writing new policies and drop existing ones.

The National Park Service says more than 85 percent of wildfires are caused by human activity. Flames can spark from unattended campfires, burning debris, equipment use and malfunctions, discarded cigarettes and arson, among other reasons. Fires also can start from natural causes, such as lightning strikes.