27th February 2014 San Leo landslide

on 27th February 2014 a large landslide affected the historical town of San Leo, in the Montefeltro area of northern Italy, which I reported at the time.  In a paper in the journal Landslides, Lisa Borgatti and colleagues (Borgatti et al. 2015) provide an overview of the San Leo landslide, including a historical perspective on large failures of the massif.  There is no doubt that San Leo is both a spectacular location and somewhat prone to slope failures:

Wikipedia Commons

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The town dates from Roman times; the churches were built in the 12th Century, as does the castle.  Unfortunately, the spectacular setting means that landslides are inevitable – the limestone / sandstone block that forms the citadel is underlain by varicoloured clays, which provide a poor foundation, allowing movements to slowly develop in the main block.

The collapse on 27th February 2014 occurred on the flank of the limestone block, and fortunately did not destroy any buildings or strike any people. The best view of the landslide is from drone footage captured in a Youtube video:

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Seismic data suggest that it occurred in two distinct events about 90 seconds apart.  Borgatti et al. (2014) postulate a progressive failure followed by a topple event and rapid disintegration of the block:

Tentative reconstruction of the time evolution of the landslide event (Borgatti et al. 2015)

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But perhaps the most interesting aspect of the paper is this image, which compares a watercolour painted in 1626 by Francesco Mingucci with a modern day photograph from the same perspective:

Comparative pictures showing the evolution of the north-eastern cliff face of San Leo village (Borgatti et al. 2015)

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Even allowing for artistic license the degree to which landslide has removed sections of the massif is very clear.  In 1737 it is reported that a landslide from the block killed up to 100 people .  These landslides will have profound implications for the preservation of this important historical site.

Reference

Borgatti, L., Guerra, C. Nesci, O. Romeo, R.W., Veneri, F., Landuzzi, A., Benedetti, G., Marchi, G., Lucente, C.C. 2015. The 27 February 2014 San Leo landslide (northern Italy). Landslides. Doi: http://dx.doi.org/10.1007/s10346-015-0559-4