The fires that have been raging in Los Angeles County this week may be the “most destructive” in modern US history.

In just three days, the blazes have covered tens of thousands of acres of land and could potentially have an economic impact of up to $150bn (£123bn), according to private forecaster Accuweather.

Sky News has used a combination of open-source techniques, data analysis, satellite imagery and social media footage to analyse how and why the fires started, and work out the estimated economic and environmental cost.

More than 1,000 structures have been damaged so far, local officials have estimated. The real figure is likely to be much higher.

“In fact, it’s likely that perhaps 15,000 or even more structures have been destroyed,” said Jonathan Porter, chief meteorologist at Accuweather.

These include some of the country’s most expensive real estate, as well as critical infrastructure.

Accuweather has estimated the fires could have a total damage and economic loss of between $135bn and $150bn.

“It’s clear this is going to be the most destructive wildfire in California history, and likely the most destructive wildfire in modern US history,” said Mr Porter.

“That is our estimate based upon what has occurred thus far, plus some considerations for the near-term impacts of the fires,” he added.

The calculations were made using a wide variety of data inputs, from property damage and evacuation efforts, to the longer-term negative impacts from job and wage losses as well as a decline in tourism to the area.

The Palisades fire, which has burned at least 20,000 acres of land, has been the biggest so far.

Satellite imagery and social media videos indicate the fire was first visible in the area around Skull Rock, part of a 4.5 mile hiking trail, northeast of the upscale Pacific Palisades neighbourhood.

These videos were taken by hikers on the route at around 10.30am on Tuesday 7 January, when the fire began spreading.

At about the same time, this footage of a plane landing at Los Angeles International Airport was captured. A growing cloud of smoke is visible in the hills in the background – the same area where the hikers filmed their videos.

The area’s high winds and dry weather accelerated the speed that the fire has spread. By Tuesday night, Eaton fire sparked in a forested area north of downtown LA, and Hurst fire broke out in Sylmar, a suburban neighbourhood north of San Fernando, after a brush fire.

These images from NASA’s Black Marble tool that detects light sources on the ground show how much the Palisades and Eaton fires grew in less than 24 hours.

On Tuesday, the Palisades fire had covered 772 acres. At the time of publication of Friday, the fire had grown to cover nearly 20,500 acres, some 26.5 times its initial size.

The Palisades fire was the first to spark, but others erupted over the following days.

At around 1pm on Wednesday afternoon, the Lidia fire was first reported in Acton, next to the Angeles National Forest north of LA. Smaller than the others, firefighters managed to contain the blaze by 75% on Friday.

On Thursday, the Kenneth fire was reported at 2.40pm local time, according to Ventura County Fire Department, near a place called Victory Trailhead at the border of Ventura and Los Angeles counties.

This footage from a fire-monitoring camera in Simi Valley shows plumes of smoke billowing from the Kenneth fire.

Sky News analysed infrared satellite imagery to show how these fires grew all across LA.

The largest fires are still far from being contained, and have prompted thousands of residents to flee their homes as officials continued to keep large areas under evacuation orders. It’s unclear when they’ll be able to return.

“This is a tremendous loss that is going to result in many people and businesses needing a lot of help, as they begin the very slow process of putting their lives back together and rebuilding,” said Mr Porter.

“This is going to be an event that is going to likely take some people and businesses, perhaps a decade to recover from this fully.”

The Data and Forensics team is a multi-skilled unit dedicated to providing transparent journalism from Sky News. We gather, analyse and visualise data to tell data-driven stories. We combine traditional reporting skills with advanced analysis of satellite images, social media and other open source information. Through multimedia storytelling we aim to better explain the world while also showing how our journalism is done.

They are the displaced and there are tens of thousands of them, 600 in an evacuation centre we visited.

From elderly people who fled without their medication, to pregnant mothers desperate to escape the smoke, they had nowhere else to go.

Jim Mayfield, who has lived in the northern suburb of Altadena for 50 years, wept as he told me his dogs, Monkey and Coca, were all he had left.

He said: “The fire was coming down, a ball of fire, it hadn’t made it to my house, but then I woke up and I seen it so I had to start evacuating.

“I had to grab my dogs, I didn’t have enough water and my house is burned down to the ground.”

Sheila Kraetzel, another elderly resident, relived the sense of terror as homes were engulfed by the flames.

She said: “I smelt smoke, I was sleeping, and my dog alerted me that there was trouble.

“When I looked outside, there were embers floating across my yard.

“My whole neighbourhood is gone.”

“It was a beautiful, unique place,” she added, smiling.

Asked how she could smile, she fought back tears and replied: “Well, there’s tomorrow you know.”

How anyone could find hope amid the destruction we have witnessed here is beyond me.

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In pictures: Before and after the blazes
What caused the fires?

There are people handing out food and water, medical staff doing what they can. Volunteers have rallied from far and near.

One of them, Stephanie Porter, told me it felt “heavy” inside the centre.

“You walk through and see the despair on people’s faces, not knowing what their next step is, not knowing if their house is still standing,” she said.

“I had to take a few moments… and kind of cry, and then you go back to serve.

“It just breaks your heart.”

Three miles up the road, Altadena resembles a war zone, but residents have not been allowed to return.

When they finally do, they’ll discover there’s nothing left of the material lives they left behind.

Authorities in California have vowed to arrest anybody caught looting in burnt-out neighbourhoods, with one official warning: “We are not screwing around with this.”

Five separate wildfires continue to burn across Los Angeles County, including the Pacific Palisades blaze – which has torn through more than 20,000 acres of land and destroyed an estimated 5,000 structures.

Los Angeles sheriff Robert Luna said a curfew enforced overnight on Thursday would start again at 6pm local time on Friday (2am on Saturday, UK time).

LA fires updates:
Wildfires burn for fourth day killing at least 10

The curfew – which forbids anyone from entering mandatory evacuation areas between 6pm and 6am – was brought in after officers arrested several people for looting in the burned areas. It will be “strictly enforced”, Sheriff Luna added.

“We’re not screwing around with this, we don’t want people taking advantage of our residents that have already been victimised,” he said at a press conference.

The punishment for looting is a $1,000 fine and even potential jail time.

The National Guard has been deployed to help secure areas affected by the fires. They are helping to manage restriction zone checkpoints and prevent looting.

Pacific Palisades, which has borne the brunt of the destruction, is an exclusive neighbourhood loved by celebrities – many of whom have seen their homes completely burnt out.

Paris Hilton’s house in the nearby Malibu has also been destroyed, along with a number of other beachfront properties.

Authorities are cracking down on illegal drone usage in fire traffic areas after a fire-fighting ‘super scooper’ plane was grounded due to being damaged by a drone.

They are continuing to investigate what caused the fires. A suspected arsonist was arrested near the Kenneth fire on Thursday afternoon local time.

The officials’ warning to looters came as 153,000 people remain under evacuation orders. While the fires are still burning, some evacuated residents have been able to make brief trips to their neighbourhoods – where many have discovered their homes reduced to ashes.

Authorities have also confirmed at least 10 people have been killed.

Among them was Rodney Nickerson, an 82-year-old who decided to stay in his home in Altadena, a suburb north of Los Angeles.

His daughter Kimiko Nickerson told Sky News: “He just didn’t want to evacuate. He’s been living here since 1968, and he’s been in Altadena my whole life.

“Like all of us on this block, in four blocks, he didn’t think it was going to be this devastating.”

Read more:
In pictures: Before and after the blazes
What caused the fires?
Furious woman challenges LA governor

The Palisades blaze – the biggest of the five – is just 8% contained, while the Kenneth fire, which threatens another celebrity-loved neighbourhood, Calabasas, has burnt through 1,000 acres and is 35% contained.

At least 10,000 structures, including thousands of homes, have been destroyed across the region.