Engineers use analysis from space to understand why San Francisco skyscraper has sunk 16 inches
Engineers have tunnelled underground to try to understand why San Francisco's Millennium Tower is sinking.
Now an analysis from space shows the 58-storey skyscraper in the city's financial district is continuing to sink at a steady rate - and perhaps faster than previously known.
The luxury high-rise that opened its doors in 2009 has been dubbed the Leaning Tower of San Francisco - it has sunk about 16ins into landfill and is tilting several inches to the north west.
A dispute over the building's construction in the seismically active city has spurred numerous lawsuits involving the developer, the city and owners of its multimillion dollar apartments.
Engineers have estimated the building is sinking at a rate of about an inch a year, but the European Space Agency's Sentinel-1 twin satellites show almost double that rate based on data collected from April 2015 to September 2016.
The satellite data shows the Millennium Tower sunk 40 to 45mm - 1.6 to 1.8ins - over a recent one-year period and almost double that amount over its 17-month observation period, says Petar Marinkovic, founder and chief scientist of PPO Labs which analysed the satellite's radar imagery for the ESA, along with Norway-based research institute Norut.
"What can be concluded from our data, is that the Millennium Tower is sinking at a steady rate," Mr Marinkovic said.
The data detected a small slowdown this summer but one that needed further analysis, he said, and did not change the overall data. "There is quite a steady subsidence," he added.
The Sentinel-1 study is not focused on the Millennium Tower but is part of a larger mission by the European Space Agency tracking urban ground movement around the world, and particularly subsidence "hotspots" in Europe, said Pierre Potin, Sentinel-1 mission manager for the ESA.
The ESA decided to conduct regular observations of the San Francisco Bay Area, including the Hayward Fault, since it is prone to tectonic movement and earthquakes, said Mr Potin, who is based in Italy.
Data from the satellite, which is orbiting about 400 miles from the Earth's surface, was recorded every 24 days.
The building's developer, Millennium Partners, insists the building is safe for occupancy and could withstand an earthquake.
The developer's spokesman PJ Johnston said he had no direct comment on the satellite data but issued a statement saying that the Millennium Tower is a state-of-the-art building "designed and constructed to the extraordinarily high standards" mandated by San Francisco.
He reiterated the developers' blame for the tower's problems on the city's construction of an adjacent railway station, which they say removed ground water from beneath the Millennium Tower that caused it to sink and tilt.
The city agency, the Transbay Joint Powers Authority, blames the building's "inadequate foundation" which is not anchored to bedrock. The tower is supported by piles driven 60 to 90ft into landfill.
Engineers hired by the building and its developers have drilled deep holes around the building to test soil samples to determine if the building has stopped sinking, and if there is a way to fix it.
One of the building's tenants, Jerry Dodson, said developers had given tenants the impression that the sinking was slowing and stopping.
"To have the space agency looking at it debunks what (developers) have put out there. Now we know it's continuing to sink at an accelerated rate," said Mr Dodson, a lawyer who has helped organise homeowners' lawsuits.
"I can tell you that satellite data is way more accurate that digging in the dirt."