Seismometer arrays have been widely applied to record collapse by controlled explosion in mines and caves. However, most underground failures are natural events, and because they can occur abruptly, underground failures represent a serious geological hazard. An accelerometric array installed on 4 September 2008 has been used to manage the geological risk of the Peschiera Springs drainage plant of Rome's aqueduct, which is located in the Central Apennines approximately 80 km from Rome, Italy. The plant occupies a karstified carbonatic slope that is extensively involved in gravitational deformations, which are responsible for underground failures such as cracks and collapses. To distinguish among different types of recorded events, an automated procedure was implemented based on the duration, peak of ground acceleration (PGA) and PGA variation in the recordings of the plant's accelerometric stations. The frequencies of earthquakes and micro-earthquakes due to underground failures are, in general, well correlated. Nevertheless, many underground failure sequences can be directly associated with the continuous deformations that affect the slope. The cumulative Arias intensity trend derived for the underground failures combined with the failure and earthquake frequencies enabled the definition of a control index (CI) that identifies alarming or emergency conditions. The CI can be used as a tool for managing the geological risk associated with the deformational processes that affect the drainage plant.
An integrated monitoring system is operative in the Peschiera Springs slope (Central Apennines, Italy) to manage the landslide risk related to the plant of Rome aqueducts. Since 2008, an accelerometric network has been operating in order to integrate the stress-strain monitoring system. Nowadays about 1300 microseismic signals due to instabilities have been recorded; these events can be distinguished in failures and collapses. Whereas the failures are related to the rock mass deformation, the collapses are mainly associated with the aquifer discharge changes (about 16–21 m3/s). A Control Index (CI), based on the frequency of occurrence and the cumulative energy of the recorded local instabilities was tested for providing three levels of alert. In 2014, a nanoseismic array (Seismic Navigation System) was installed inside the drainage plant that is contributing to identify sequences of microseismic pre-failure events, allowing to assess the related landslide hazard.
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 adid 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.
With the aim of evaluating the seismic hazard and vulnerability of villages with valuable historical monuments in Central Italy, since the late 80's ENEA operated an accelerometric network at the Cerreto di Spoleto village in the area of Central Apennines.The area is characterized by high level instrumental seismicity and seismic hazard.The accelerometric network, consisting of one station located at a rock site close to the hill base and of two accelerometers located inside a building at the relief top, recorded the Umbria-Marche aftershocks of 1997 as well as more recent earthquake sequences.In 2001 a temporary seismometric network was deployed at Cerreto di Spoleto village in order to better characterize the ground motion spatial variability along the ridge.The accelerometric network, consisting of one station located at a rock site close to the hill base and of two accelerometers located inside a building at the relief top, recorded the Umbria-Marche aftershocks of 1997 as well as more recent earthquake sequences.Analysis of the accelerometric records pointed out broadband seismic amplification above 1 Hz at the relief top.In 2001 a temporary seismometric network was deployed at Cerreto di Spoleto village in order to better characterize the ground motion spatial variability along the ridge.The seismic station were deployed at the ridge base and along the crest.To this aim we analyzed waveform in the frequency domain through the Horizontal-tovertical spectral ratio method.Except for the trapped waves effect, the results showed no clear evidence of ground motion amplification for the stations located on the crest.This evidence suggests that the seismic amplification observed on the accelerometric records is not related to topographic amplification.
The cliff slope of the Selmun Promontory, located in the Northern part of the island of Malta (Central Mediterranean Sea) close to the coastline, is involved in a landslide process as exhibited by the large block-size talus at its bottom. The landslide process is related to the geological succession outcropping in the Selmun area, characterized by the overposition of a grained limestone on a plastic clay, that induces a lateral spreading phenomenon associated with detachment and collapse of different-size rock blocks. The landslide process shapes a typical landscape with a stable plateau of stiff limestone bordered by an unstable cliff slope. The ruins of Għajn Ħadid Tower, the first of the 13 watchtowers built in 1658 by the Grand Master Martin de Redin, stand out on the Selmun Promontory. The conservation of this important heritage site, already damaged by an earthquake which struck the Maltese Archipelago on 1856 October 12, is currently threatened by a progressive retreat of the landslide process towards the inland plateau area. During 2015 and 2016, field surveys were carried out to derive an engineering geological model of the Selmun Promontory. After a high-resolution geomechanical survey, the spatial distribution of the joints affecting the limestone was obtained. At the same time, 116 single-station noise measurements were carried out to cover inland and edge of the limestone plateau as well as the slope where the clays outcrop. The obtained 1-hour time histories were analysed through the horizontal to vertical spectral ratio technique, as well as polarization and ellipticity analysis of particle motion to define the local seismic response in zones having different stability conditions, that is, related to the presence of unstable rock blocks characterized by different vibrational modes. The results obtained demonstrate the suitability of passive seismic geophysical techniques for zoning landslide hazard in case of rock slopes and prove the relevance of anisotropies in conditioning the polarization of vibrational modes for dislodged rock masses.
