Development of Fault-induced Sinkhole in a Seismic Zone of the Italian Apennines

Sinkholes have been reported along seismically active faults indicating a causal link between sinkholes and tectonics. The processes responsible for their formation may be related to fracturing in the fault damage zone, promoting fluid circulation and weathering of soluble rocks at depth. However, the surface deformations and sinkhole evolution are rarely recorded, as these sites are known mainly after a collapse, making the assessment of sinkhole-related hazard challenging. In Italy, a total of 750 sinkholes have been identified and the 40% of them are along active faults. It remains unclear whether seismicity may trigger sinkhole collapses. The sinkhole of Prà di Lama, near the village of Pieve Fosciana (Italy), is a circular depression filled by a lake, located in the seismically active Apennines range, at the intersection of two active faults. Here we integrate InSAR analysis, surface mapping and historical records of sinkhole activity to show that the Prà di Lama lake is a long-lived sinkhole that was formed in the 10th century in an active fault zone and grew through several events of unrest characterised by sudden subsidence, episodic fracturing and lake-level changes, including the two most recent events of unrest in 1996 and 2016. Hot springs are also present at Pieve Fosciana, suggesting that fluids migrate along the existing fault planes. InSAR time-series analysis shows that continuous subsidence at rates of up to 7 mmyr-1 occurred during 2003–2008, between events of unrest. Earthquakes on the major faults near the sinkhole do not trigger sinkhole activity but low-magnitude earthquakes at 4–12 km depth occurred during sinkhole unrest in 1996 and 2016, indicating that a link between low-magnitude seismicity and sinkhole activity exists. We conclude that the Prà di Lama sinkhole is a fault-induced geomorphologic feature and we interpret our different observations as evidence of seismic creep at depth causing fracturing and ultimately leading to the development of the Prà di Lama sinkhole.