In the Mediterranean area, cliff slopes represent widespread high-risk landforms as they are highly frequented touristic places often interested by landslide processes. Malta represents a significant case study as several cliffs located all around the island are involved in instability processes, as evidenced by wide block-size talus distributed all along the coast line. These diffused instabilities are related to the predisponding geological setting of Malta Island, i.e. the over-position of grained limestone on plastic clay deposits, that induces lateral spreading phenomena associated to falls and topples of different-size rock blocks and is responsible for a typical landscape with stable plateau of stiff rocks bordered by unstable cliff slopes.

The ruins of Għajn Ħadid Tower, the first of the thirteen watchtowers built in 1658 by the Gran Master Martin de Redin, stand out in the Selmun area. Currently the safety of this important heritage site, already damaged by an earthquake on October 12th 1856, is threaten by a progressive moving of the landslide process towards the stable plateau area. During autumn 2015, a field-campaign was realized to characterize the jointed rock mass. A detailed engineering-geological survey was carried out to reconstruct the geological setting and to define the mechanical properties of the rock mass. Based on the surveyed joint spatial distribution, 58 single-station noise measurements were deployed to cover both the unstable zone and the stable area. The obtained 1-hour records were analyzed in the frequency domain for associating vibrational evidences to different instability levels, i.e. deriving the presence of already isolated blocks by the local seismic response.

The here presented results can be a useful contribute to begin to asses defense strategies for the Selmun Promontory, in the frame of managing the landslide risk in the study area and preserving the local historical heritage.

1. Introduction

Cliff slopes are high-risk landforms in the Mediterranean area due to the diffused landslide processes that affect sites of touristic relevance as well as buildings which are part of the cultural heritage. Malta Island represents a significant case study as its geological setting, i.e. the over-position of grained limestone on plastic clay deposits, predisposes large lateral spreading processes associated to falls, slides and/or topples of different-size rock blocks. These instabilities interest countryside areas, e.g. the rock slabs where the city of Mdina and Citadel are built [1], as well as sea cliffs all along the coast line, especially in the north-west part of Malta and in Gozo island.

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