Decadal spatio-temporal reconstruction of mining-induced sinkholes activity in Gavorrano (Italy) using remote sensing and field data

The failure of mining voids and the formation of sinkholes is a major hazard at both active and dismissed mining areas that may cause important social and economic losses for the communities leaving nearby. Monitoring ground motions at mining area has been proved to successfully reduce the occurrence and the consequences of sudden sinkhole collapses. However, while most active mines are constantly monitored today, the ground instabilities around dismissed mining areas often remain disregarded. In southern Tuscany (Italy), the Gavorrano area was among the biggest pyrite (FeS₂) mines in Europe during its period of activity (1898-1981). According to mining reports, the pyrite extraction was accompanied by the failures of underground mining voids and some of them were followed by the formation of fractures at the surface. Today, the area shows significant evidence of sinkhole activity, with the major Monte Calvo sinkhole dominating the landscape of Gavorrano. However, the spatio-temporal evolution of the sinkhole phenomena, the relationship with mining, and the potential ongoing sinkhole activity in the area remained unclear. In this study we combined InSAR measurements from the European Ground Motion Service (EGMS) with historical mining reports and maps, aerial images, high-resolution Digital Surface Models (DSMs) and field surveys to reconstruct the long-term spatial and temporal evolution of ground deformation around the mining area of Gavorrano and to explore the possible relationship with the mining activity. Three sinkholes were identified: Monte Calvo, Valsecchi, and Ravi; the latter two have never been reported in the literature. The sinkholes have a spatial correlation with the mining voids and galleries. InSAR revealed that an area of ~ 700 m × 400 m around the Monte Calvo sinkhole has been subsiding with rates of ~5 mm/yr between 2016-2022. Conversely, no evidence of deformation is observed at Valsecchi, Ravi, and the nearby city of Gavorrano. The collected data suggest that the sinkhole activity has been induced by the past mining activity (until 1981) in the area. Possible scenarios to explain the observed deformation could envisage for: 1) a constant long-term subsidence; 2) an evolution characterised by multiple sudden collapses punctuated by periods of gradual subsidence; 3) a gradual stabilization of the area. Slowing down surface velocities respect to the past suggest that the Monte Calvo sinkhole is stabilizing. However, future sinking episodes cannot be ruled out if the underground stability conditions change, for example, for further mining voids failures.