1,721,116 research outputs found
Landslide activity maps for landslide hazard evaluation: Three case studies from Southern Italy
No full textThis paper focuses on the application of landslide activity maps for evaluating the mass movement hazard in selected areas of the Southern Apennines of Italy: Bisaccia, Calitri, and Buoninventre. The availability of multi-year aerial photo coverage helped to assess the morphological changes which occurred in the last 40 years. This information, integrated with historical data on slope instability and field checks, were used to produce landslide activity maps. These maps represent a short-cut in the assessment of mass movement hazard, because they focus on the effects of slope instability rather than on the causative conditions and processes; however, if kept simple and prepared at large scale, they may help the local administrators and land-use planners to reduce the socio-economic costs of landslides. Furthermore, the comparative study of landslide activity represents a relatively inexpensive and quick method for evaluating the performance of the engineering control efforts. The quantification of landslide activity in terms of areal frequency can represent an additional step, useful to determine the relative landslide hazard (zonation in more or less hazardous areas). For example, the estimates of areal frequency of active landsliding for the last 40 years demonstrated the great influence of the 1980 Irpinia earthquake (M(s) = 6.9) on the stability of slopes situated close to its epicenter (within a radius of about 20 km)
Advances and problems in understanding the seismic response of potentially unstable slopes
No full textThe influence of site effects on landslide triggering during earthquakes has been inferred in several studies,
but its evaluation is made difficult by the complexity of factors controlling the dynamic response of potentially
unstable slopes and also by the lack of local ground motion instrumental observations. This work explores this
problem and reports new findings based on an ongoing long term accelerometric monitoring conducted on a
landslide-prone test area in the Apennine Mountains, Italy, where the presence of site effects enhancing
seismic susceptibility of local slopes has been invoked on the basis of historic accounts of landsliding triggered
at large epicentral distance. The recordings relative to low-to-moderate magnitude earthquakes showed
significant amplifications affecting hillslope portions covered by thick (N5 m) colluvia and pronounced
amplification maxima oriented along the local maximum slope direction on a recent deep-seated landslide.
While the amplifications seem most likely linked to high impedance contrast between surface materials and
underlying substratum, the causes of directivity are less clear. The case of the monitored test site together
with evidence of site response directivity identified on other hillslopes, suggest that the directivity
phenomena can result from a combination of topographic, lithological and structural factors that act together
to re-distribute shaking energy, focusing it on site-specific directions. Thus, it is difficult to single out the
critical factors controlling such phenomena and no general criterion for the identification of sites affected by
directivity is proposed here. Nevertheless, the presence and orientation of site response directivity can be
revealed through reconnaissance techniques by using recordings of seismic weak motion and/or ambient
microtremors, and calculating azimuthal variation of shaking energy and horizontal-to-vertical ground
motion spectral ratios. A comparison with the recordings obtained during the recent MW=6.3 earthquake
that hit the Abruzzo region in April 2009 demonstrated that analysis relying on data from low energy events
can furnish valid indications for slope behaviour also under stronger shaking, provided the data are well
differentiated in terms of distance, azimuth and source characteristics. Furthermore, the comparative analysis
of the Abruzzo earthquake recordings at a landslide and reference sites showed that directivity properties of
strong shaking had been correctly anticipated using earlier weak motion observations. However, under the
complex slope conditions the identification of resonance frequencies from horizontal-to-vertical spectral
ratios estimated from weak-motion accelerometric recordings does not seem reliable, and better results have been obtained by velocimetric microtremor recordings
Time probabilistic evaluation of seismically-induced landslide hazard in Irpinia (Southern Italy)
No full textA recently proposed method, which incorporates the Newmark model to evaluate the earthquake-induced landslide hazard at regional scale, was applied to Irpinia, one of the most seismically active regions of Italy. The method adopts a probabilistic approach to calculate values of critical acceleration ac representing the minimum strength required for a slope not to fail at a fixed probability level in a given time interval. Regional probabilistic hazard maps were generated for the two failure types most common in Irpinia (slump-earthflows and rock falls). The results suggest that quite moderate critical acceleration (0.05-0.08 g) could suffice to keep the slope failure probability low. However, the available data indicate that potential slide surfaces with ac below these values could be common in Irpinia, where, perhaps in relation to particular geo-environmental conditions, a relative large number of marginally stable slopes might survive other destabilising actions and fail even on occasion of not particularly strong earthquake shaking
Preface: Special issue from the symposia on remote sensing and ground-based geophysical techniques for recognition, characterisation and monitoring of unstable slopes
No full textRisposta di sito in un’area franosa: osservazioni accelerometriche a Caramanico Terme (PE)
No full textEvaluating seismically-induced mass movement hazard in Caramanico Terme (Italy)
No full textMass movements and earthquakes represent two major geological hazards in the municipal territory of Caramanico Terme (south-central Apennines). Available records revealed the contemporaneous occurrence of earthquakes and slope failures on four occasions in the last four centuries (1627, 1706, 1933, and 1984). These events, with local intensities ranging from VI to IX, generated mass movements varying from a rotational slope failure to rock/block falls. All occurred in the southern periphery of the town and involved a thick carbonate megabreccia caprock and coarse colluvia which overlie a clayey substratum. Field investigation and review of historical records helped to delimit the areas susceptible to seismically triggered rockfalls. The mapping of historic and pre-historic rockfall deposits revealed their dispersal patterns and provided the basis for a determination of potential hazard zones. We approximate the temporal hazard assessment by relating the rockfall occurrence to the probability of earthquake triggering. Considering the VI degree triggering threshold indicated by local historical data, the statistical analysis of the regional seismic activity shows that events capable of inducing rockfalls have an approximately decennial recurrence in Caramanico. The approach presented could be readily applied to other potential risk areas of Italy by exploiting the rich long-term record of historical seismicity. In general, temporal hazard estimates at relatively low intensity levels will be possible even where the seismic history of the site is only well documented for a relatively limited time interval, provided that this interval is much longer than the recurrence time of the events exceeding the threshold considered
Slope dynamic response to earthquakes: Insight from the 2009 L’Aquila earthquake
No full textLong term accelerometric monitoring of a tectonically and geomorphologically active site in Central Italy (Caramanico Terme) is used to study the dynamic response of slopes to seismic shaking. By combining the 2009 Mw 6.3 L’Aquila earthquake data with results based on lower magnitude events recorded earlier, it is shown that a valuable assessment of site response properties can be obtained by averaging weak motion data, provided the latter are sufficiently dif-ferentiated in terms of azimuth location, distance, energy and source characteristics. The case of directional amplification affecting a slope prone to deep-seated landsliding is described to offer some practical indications on how to estimate the presence and significance of site effects on slopes susceptible to seismic failures. For the monitored slope we estimate that amplifications of shaking energy, expressed through Arias intensity, vary on average from 2 to 16, if calculated relatively to sites on soft soil or rock, respectively
Can landslide-prone slope response to strong shaking be inferred from weak motion data? First answer from 2009 L’Aquila earthquake
No full textThe complexity of factors controlling the dynamic response of marginally stable slopes and the scarcity of direct ground motion recordings acquired on landslide-prone areas make it difficult to evaluate the role of site response in seismic landslide triggering. A long term accelerometric monitoring, conducted at a tectonically and geomorphologically active site of Abruzzo in Central Italy (Caramanico Terme) has provided interesting evidence of amplification with a pronounced directional character parallel to the local slope direction on a landslide consisting of colluvial deposits overlying mudstone substratum. However, until 2008, these observations were based only on recordings of events of low to moderate magnitude. The 6.3 Mw earthquake that on 6 April 2009 hit L’Aquila, 60 km from Caramanico, allowed to test whether these previous findings hold true also for the landslide site response at higher shaking levels. The comparison of the 2009 mainshock recordings of the accelerometric station located on the landslide (CAR2) to those from two nearby stations, one sited on soft soils similar to landslide substratum (CAR1) and the other on colluvial material (CAR5), showed relative amplifications (in terms of peak horizontal acceleration PHA) very close to the average values estimated from smaller events (about 1.5 and 1.0 relatively to CAR1 and CAR5, respectively). The similarity of PHA observed on colluvium, regardless of its involvement in landsliding, confirmed that, in terms of total shaking energy, the main factor controlling amplification is the impedance contrast between colluvium and mudstone substratum. The comparison between the station on landslide and a reference station on rock (CAR4) showed a relative amplification increasing with magnitude, probably because of the stronger response of the rock site to the higher frequencies prevailing in wavetrains coming from nearby small sources. This suggests that in such cases amplification assessments based on weak motion data can lead to considerable underestimates. On the other hand, the landslide site response directivity showed similar properties for the entire range of the observed magnitudes (1.4 – 6.3). It is possible that in the near field source effects modify the ratio between directional maximum and minimum of shaking energy, without, however, significantly altering the orientation of shaking maxima. In terms of spectral properties, directivity of major peaks in horizontal to spectral ratios (HVSR) was the same at any magnitude, even though at higher magnitudes spectral ratio amplitudes tend to decrease at higher frequencies and increase at lower ones. However the comparison of HVSR with standard spectral ratios (SSR) between the station on landslide and the reference site on rock indicated that the inferences on resonance frequencies derived from single station seismic weak motion measurements (like HVSR) could be unreliable under the complex conditions of a landslide-prone slope. It appears that more reliable indications can be derived from ambient noise measurement acquired with velocimetric instruments. Thus, overall, weak motion recordings proved to provide useful information on landslide site response characteristics, especially having a dataset sufficiently differentiated in terms of azimuth location, distance, energy and source characteristics. However, the extrapolation of inferences based on recordings of small magnitude events to stronger earthquake scenarios requires some caution
Risposta sismica di un area in frana a Caramanico Terme (PE): nuovi dati dal terremoto aquilano del 6.4.2009
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