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 about $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.