Data from Sentinel-1 satellites acquired between Feb. 22, 2015, and Sept. 20, 2016, show that San Francisco's Millennium Tower is sinking by about 40 millimeters a year in the satellites' line of sight. This translates into a vertical subsidence of almost 50 millimeters a year, assuming no tilting. Colored dots represent targets observed by the radar. (Credit: Contains modified Copernicus Sentinel data (2015-16)/ESA SEOM INSARAP study/PPO.labs/Norut/NGU)
European Space Agency (ESA) Sentinel-1 satellites have shown that San Francisco's 58-storey Millennium Tower is sinking by a few centimeters a year. A complete study of the city is helping scientists improve the monitoring of urban ground movements.
Completed in 2009, the Millennium Tower has recently been showing signs of sinking and tilting. Although the cause has not been pinpointed, it's believed that the movements are connected to the supporting piles not firmly resting on bedrock.
To probe these subtle shifts, scientists combined multiple radar scans from the Copernicus Sentinel-1 twin satellites of the same area to detect subtle surface changes”down to millimeters. The technique works well with buildings because they better reflect the radar beam.
Working with ESA, a team from Norut, PPO.labs and Geological Survey of Norway also mapped other areas in the wider San Francisco Bay Area, including buildings along the earthquake-prone Hayward Fault as well as subsidence of the newly reclaimed land in the San Rafael Bay.
An uplift of land was detected around the city of Pleasanton, possibly from the replenishment of groundwater following a four-year drought that ended in 2015.
