This week’s explosion of fire activity in Southern California may have many wondering: How do wildfires start?

In short, by humans, says John Keeley, who has been researching fire records from the past 100 years to determine the historical causes of fires in the state.

During Santa Ana winds one of the most common causes is power lines being blown down. Another is arson.

“Humans are the only sources of fires during Santa Ana winds,” said Keeley, a research ecologist with United States Geological Survey. “You don’t get lightning during Santa Ana wind conditions. Humans are responsible for all the fires, either directly or through the power lines.”

Though official causes for the current wildfires have not been released, Keeley said history has shown that sparks from lines are a huge factor in igniting wildfires.

According to Cal Fire statistics, seven of the top 20 most destructive California wildfires were caused by power lines or arson. Another seven on that list are still under investigation or undetermined.

“A lot of arson-ignited fires occurs under Santa Ana wind conditions because I suppose the arsonist sees that as an opportunity to create a really big fire,” he said by phone Thursday afternoon.

As for other causes reported by CalFire and the U.S. Forest Service, Keeley said his research has found:

• Debris burning

• Smoking

• Campers leaving fire unattended

• Kids playing with matches

• Sparks from trains

• Outdoor equipment

• Weed whackers

• Vehicles

With vehicles, Keeley said, “catalytic converters get real hot and you pull your car off to the grass alongside the road and a lot of fires get started that way.”

RELATED: How to keep your car from starting a wildfire

In coastal California, lightning accounts for about 1 percent of all fires; in the Sierra Nevada, about 45 percent; and in northeastern California, 60 percent, he said.

What’s not a factor?

Climate change, Keeley said.

He said climate change would only figure in California’s more heavily forested landscapes in the state.

Looking at 100 years of climate data and fire data for the state, “in Southern California we could not find any relationship between climate and fire,” he said. “We believe the reason is, every single year it’s hot enough and dry enough for a big fire.”

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“That’s usually people igniting fires under bad conditions,” he said.