What causes wildfires? A look at 10 of the deadliest and most destructive wildfires in California history shows that power lines, fireworks and other elements of human civilization are often to blame.

What causes wildfires? A look at 10 of the deadliest and most destructive wildfires in California history shows that power lines, fireworks and other elements of human civilization are often to blame.

People cause the vast majority of California wildfires, whether it’s an arsonist lighting up a hillside or a driver’s tractor releasing a spark that turns into an inferno. Networks of power lines built into wildland are also behind some of the worst blazes.

Yet, in many cases, the cause simply can’t be determined.

As fire season ramps up, state officials are warning that heavier-than-usual rain and snow in recent months have knocked down trees and led to an overgrowth of grass and shrubs that will soon dry out and become ready-made fuel for 2019 wildfires.

California Fire Map: Wildfire Tracker for Northern, Central and Southern California

A fire’s cause typically doesn’t affect how it grows or moves, said Battalion Chief Amy Head, a spokeswoman for the California Department of Forestry and Fire Protection. Wind, humidity and vegetation are the primary factors that drive fire behavior.

Still, finding out what happened at the outset is critical.

“It’s important for us to know the causes to hopefully prevent more of those fires in the future, and also to determine who’s possibly responsible,” Head said.

To discover the origin and cause of a fire, investigators use the scientific method: collect evidence, test and refute hypotheses, and analyze data. Evidence could range from someone seeing a suspect vehicle’s license plate to a dead squirrel under a power line, said Dan White, chief of the fire prevention and law enforcement division for Cal Fire.

“We interview witnesses, look at burn patterns and burn indicators to find a general origin area, then a specific origin area,” he said. “Every fire investigation starts as a criminal investigation until determined otherwise.”

By the end of a probe, Cal Fire assigns blame to one of a dozen causes: arson, debris burning, electrical, equipment, campfire, lightning, playing with fire, railroad, smoking, vehicle, miscellaneous or undetermined.

According to Cal Fire data from 2013 to 2017, the most recent five-year period available, the cause of about a quarter of all fires was found to be “undetermined.”

Usually when a cause is undetermined, either firefighting activities have disrupted the origin area or investigators have winnowed down the possibilities but remain unsure between a few of them, White said.

The second-most common cause of California wildfires falls under “miscellaneous,” which includes fireworks, explosives, glass refraction, shootings and “those oddball things that don’t completely fit in another” category, White said.

On one occasion, the cause of a fire he investigated turned out to be a horseshoe hitting a rock.

Cal Fire also tracks when every fire starts. In any given year, fires tend to follow a time-of-day bell curve, reaching an apex around 3 p.m. Blazes late at night or early in the morning are less common than those in the afternoon.

Here are the causes behind 10 of the largest, deadliest and most destructive wildfires in state history. A couple of the worst ones — the Mendocino Complex and the Woolsey Fire in Southern California — remain under investigation.

Camp Fire, November 2018

The Camp Fire burning in 2018. State investigators found that PG& equipment sparked the deadly fire. State legislators are now voting on an overhaul of the way California deals with wildfires sparked by utilities. less The Camp Fire burning in 2018. State investigators found that PG& equipment sparked the deadly fire. State legislators are now voting on an overhaul of the way California deals with wildfires sparked by ... more Photo: Gabrielle Lurie / The Chronicle 2018 Photo: Gabrielle Lurie / The Chronicle 2018 Image 1 of / 2 Caption Close How California’s biggest wildfires ignited: power lines, cars, arsonists, fireworks 1 / 2 Back to Gallery

County: Butte

Acres burned: 153,336

Structures destroyed: 18,804

Deaths: 86

Cause: Electrical transmission lines

Power equipment owned and operated by Pacific Gas & Electric Co. caused the deadliest and most destructive wildfire in state history near the town of Pulga, according to Cal Fire’s investigation.

The probe identified a second ignition site in the town of Concow, where vegetation fell onto PG&E distribution lines. The fire that started in Pulga consumed the second one.

Cal Fire forwarded its findings to Butte County District Attorney Mike Ramsey, who could file criminal charges.

Carr Fire, July 2018

Counties: Shasta and Trinity

Acres burned: 229,651

Structures destroyed: 1,614

Deaths: 8

Cause: Human

A tire on a trailer belonging to an older couple went flat and dragged on the pavement, sending sparks into parched grass near Whiskeytown Lake, investigators said.

Thomas Fire, December 2017

Counties: Ventura and Santa Barbara

Acres burned: 281,893

Structures destroyed: 1,063

Deaths: 2

Cause: Power lines

Energized power lines in Santa Paula (Ventura County) — operated by Southern California Edison — came into contact with each other, authorities said. The connection released molten aluminum particles onto surrounding dry vegetation and sparked the fire, arson investigator Christine Saqui, of the Ventura County Fire Department, wrote in her report.

Tubbs Fire, October 2017

Counties: Napa and Sonoma

Acres burned: 36,807

Structures destroyed: 5,636

Deaths: 22

Cause: Electrical

Lead investigator John Martinez, a battalion chief with Cal Fire, and his team of experts found that a privately owned conductor or electrical equipment at a Calistoga residence caused the fire. They reached the conclusion through an arduous process of elimination.

Valley Fire, September 2015

Counties: Lake, Napa and Sonoma

Acres burned: 76,067

Structures destroyed: 1,955

Deaths: 4

Cause: Electrical

Investigators found that homeowner John Pinch installed a circuit to power a hot tub outside his Cobb (Lake County) residence. The circuit connection didn’t follow electrical code standards and a breaker tripped.

“The temperature at the electrical connection was at least 1,981 degrees Fahrenheit as the copper wire was melted,” James Engel, a Cal Fire deputy chief, wrote in the investigative report. “The wire connection was in contact with dry grass and lead litter, which ignited.”

Prosecutors declined to file charges.

Rim Fire, August 2013

County: Tuolumne

Acres burned: 257,314

Structures destroyed: 112

Deaths: 0

Cause: Human

An illegal campfire in the Stanislaus National Forest burned out of control, investigators said. Bow hunter Keith Matthew Emerald was indicted, but federal prosecutors dropped the charges, citing two key witnesses’ deaths.

Witch Fire, October 2007

County: San Diego

Acres burned: 197,990

Structures destroyed: 1,650

Deaths: 2

Cause: Power lines

Power lines operated by San Diego Gas and Electric came into contact with each other in the wind and arced, releasing hot particles onto vegetation. The company agreed to pay the state more than $14 million to settle accusations that it was responsible for the Witch Fire and other 2007 blazes in the area.

Zaca Fire, July 2007

County: Santa Barbara

Acres burned: 240,207

Structures destroyed: 1

Deaths: 0

Cause: Human

Workers at La Laguna Ranch were using grinding equipment to repair a metal pipe, which emitted sparks. The corporate owners of the ranch, without admitting guilt, agreed to a $17 million settlement.

Cedar Fire, October 2003

County: San Diego

Acres burned: 273,246

Structures destroyed: 2,820

Deaths: 15

Cause: Human

Sergio Martinez, a West Covina man on a deer-hunting trip, started the fire when he became separated from his companion. Dehydrated and worried he might die, he lit the fire to signal for help. He was convicted of arson and sentenced to six months in a work-furlough program and 960 hours of community service.

Tunnel Fire, October 1991

County: Alameda

Acres burned: 1,600

Structures destroyed: 2,900

Deaths: 25

Cause: Rekindle

A grass fire that started the day before rekindled to cause the major Oakland Hills firestorm. Several arson fires had occurred in the hills before and after the Tunnel Fire, including at least one on the same day, but no link was made to any of those incidents.

Kimberly Veklerov is a San Francisco Chronicle staff writer. Email: kveklerov@sfchronicle.com Twitter: @kveklerov