It was early 2018 when residents of the Brazilian city of Maceió first spotted cracks appearing in buildings and roads. Heavy rainfall in mid February, followed by a small earthquake at the beginning of March, appeared to trigger the fractures. The situation in the neighbourhood of Pinheiro was so serious that 6,356 buildings were placed under demolition orders and 25,000 residents had to be moved out.
Recent research published in Scientific Reports shows that events were set in train long before the rain arrived. Using satellite measurements to assess land movement between 2004 and 2020, Mahdi Motagh and colleagues at the GFZ German Research Centre for Geosciences in Potsdam discovered that the surface subsidence began in 2004 and was sinking by up to 27cm a year by 2017.
Using geological modelling tools they show that saltmining occurring up to a kilometre underground is the most likely cause. Weak brine-filled cavities left behind when the salt has been extracted eventually collapse and, like dominoes, the rock layers above topple into the void below. Mining stopped in 2019, but the model shows that the ground still has a lot of settling out to do, and some neighbourhoods remain at high risk.
