Less than 35 mm of rain from Cyclone Harry set Niscemi's twice-failed slope moving again

Cyclone Harry is the trigger the paper names, yet the rain it left at Niscemi was modest ^[11]. The weekly total the authors describe was under 35 mm ^[17], and 21.0 mm of it fell on 20 January ^[18], so more than 60% came on one day ^[23]. The Landslide Blog, published by Eos, notes that Sicily has quite a dry climate and calls those totals remarkably low ^[19].

I think that puts most of the explanation in the ground. The blog's interpretation of the paper's cross-section is that the deep, low-angle failure was driven primarily by weak materials and high pore water pressures ^[16]. If that is right, the cyclone's rain was the last increment on a slope with little margin left ^[16][17].

That margin was hard to see from orbit. The authors used InSAR to look for ongoing or precursory deformation. They found very limited movement, on only a small part of the landslide complex, and most of the mass was stable ^[13]. The same mass then moved up to about 50 metres ^[14]. The blog post does not say whether the small moving patch is where the 2026 failure began.

The slope's history is why Niscemi was a known hazard. A large landslide struck the southern side of the mesa on 18 March 1790 ^[8]. It reactivated on 12 October 1997 over about 2 km2 ^[9], and the same slope moved again in January 2026 ^[10]. This year's footprint is roughly two-thirds of the 1997 area ^[20]. The gaps between events are 207 years ^[21], then about 28 ^[22]. Three dated failures are too few to fit a return period. A shorter second gap does not by itself show that failures are speeding up ^[8][9][10].

Press coverage went elsewhere, mostly to perceived decades of neglect by civic authorities over a known threat ^[1]. The Guardian, citing geologists and environmental experts, wrote that the landslide "is the latest sign of how the climate emergency is reshaping the Mediterranean, where there has been indifference to decades of flawed building policies and an out-of-control model of urbanisation" ^[2]. According to the blog, the paper does not really consider the neglect question ^[3]. Its rainfall figures describe a modest trigger ^[17][19].

The evidence covers one slope on one mesa. It cannot show how other clay-over-gypsum mesas behave in storms ^[7]. For Niscemi itself, I'd treat three dated failures, the last set off by under 35 mm of rain, as a case for planning around the next one ^{[8][9][10][17]}.