Investigation of cliff instability at Għajn Ħadid Tower (Selmun Promontory, Malta) by integrated passive seismic techniques

Għajn Ħadid Tower represents an important cultural heritage of Malta (Central Mediterranean Sea). This tower, built in 1658 on the Selmun Promontory on the North-East coast of the island, was severely damaged by the 1856 Crete earthquake (MW 7.7). The area where the tower ruins stand is involved in a significant landslide process of lateral spreading. During 2015 and 2016, engineering geological surveys and passive seismic measurements were carried out to evaluate the main resonance frequencies of the Promontory and monitor the gravity-induced instability process. Seismic ambient noise measurements in the area of the tower show significant Horizontal-to-Vertical Spectral Ratio (HVSR) resonance peaks at 3.3-3.5 Hz characterised by linearity and polarization of the particle motion. These features are not present in the measurements carried out on the stable zone and can be related to the vibrational behaviour of the unstable rock block. Additionally, a shear wave velocity profile for the area was obtained by using a seismic array. At the same time, two different approaches were tested for monitoring the landslide process: i) a Seismic Navigating System (SNS) array detected 20 natural microseismic events caused by the ongoing landslide process; ii) few-days continuous seismic noise measurements on the unstable rock block were used to carry out a preliminary study of the variation of specific parameters over time.