Growth of a sinkhole in a seismic zone of the Northern Apennines (Italy)

Sinkhole collapse is a major hazard causing substantial social and economic losses. However, the surface deformations and sinkhole evolution are rarely recorded, as these sites are known mainly after a collapse, making the assessment of sinkholes-related hazard challenging. Furthermore, 40 % of the sinkholes of Italy are in seismically hazardous zones; it remains unclear whether seismicity may trigger sinkhole collapse. Here we use a multidisciplinary dataset of InSAR, surface mapping and historical records of sinkhole activity to show that the Prà di Lama lake is a long-lived sinkhole that was formed over a century ago and grew through several events of unrest characterized by episodic subsidence and lake-level changes. Moreover, InSAR shows that continuous aseismic subsidence at rates of up to 7.1 mm yr−1 occurred during 2001–2008, between events of unrest. Earthquakes on the major faults near the sinkhole are not a trigger to sinkhole activity but small-magnitude earthquakes at 4–12 km depth occurred during sinkhole unrest in 1996 and 2016. We interpret our observations as evidence of seismic creep in an active fault zone at depth causing fracturing and ultimately leading to the formation and growth of the Prà di Lama sinkhole.