Proposal of a method to define areas of landslide hazard and application to an area of the Dolomites, Italy

A numerical–cartographical method has been developed to create landslide hazard maps. This method allows the assigning of a rating to the various parameters which contribute to landslides. The parameters considered are: (1) erodibility and degradability of the rocks and Quaternary deposits; (2) permeability of the ground to identify areas prone to hydraulic overpressure; (3) the geometric ratio between discontinuities and slope, and thickness of Quaternary deposits; (4) angle of the slopes; and (5) land use. A thematic map is constructed for each factor considered which defines different areas through ratings, after which all the thematic maps are overlaid and the ratings added up (or multiplied). The map which is thus obtained is reclassified in order to create the final map of landslide hazard. This method, which has already been tested in various areas, has produced excellent results in this case too, allowing a map to be constructed which corresponds to the actual instability problems.     

Neural networks and landslide susceptibility: a case study of the urban area of Potenza

For those working in the field of landslide prevention, the estimation of hazard levels and the consequent production of thematic maps are principal objectives. They are achieved through careful analytical studies of the characteristics of landslide prone areas, thus, providing useful information regarding possible future phenomena. Such maps represent a fundamental step in the drawing up of adequate measures of landslide hazard mitigation. However, for a complete estimation of landslide hazard, meant as the degree of probability that a landslide occurs in a given area, within a given space of time, detailed and uniformly distributed data regarding their incidence and causes are required. This information, while obtainable through laborious historical research, is usually partial, incomplete and uneven, and hence, unsatisfactory for zoning on a regional scale. In order to carry this out effectively, the utilization of spatial estimation of the relative levels of landslide hazard in the various areas was considered opportune. These areas were classified according to their levels of proneness to landslide activity without taking recurrence periods into account. Various techniques were developed in order to obtain upheaval numerical estimates. The method used in this study, which was applied in the area of Potenza, is based on techniques derived from artificial intelligence (Artificial Neural Network—ANN). This method requires the definition of appropriate thematic layers, which parameterize the area under study. These are recognized by means of specific analyses in a functional relationship to the event itself. The parameters adopted are: slope gradient, slope aspect, topographical index, topographical shape, elevation, land use and lithology.     

Geomorphology and landslide susceptibility assessment using GIS and bivariate statistics: a case study in southern Italy

In this article, the results of a study aimed to assess the landslide susceptibility in the Calaggio Torrent basin (Campanian Apennines, southern Italy) are presented. The landslide susceptibility has been assessed using two bivariate-statistics-based methods in a GIS environment. In the first method, widely used in the existing literature, weighting values (Wi) have been calculated for each class of the selected causal factors (lithology, land-use, slope angle and aspect) taking into account the landslide density (detachment zones + landslide body) within each class. In the second method, which is a modification of the first method, only the landslide detachment zone (LDZ) density has been taken into account to calculate the weighting values. This latter method is probably characterized by a major geomorphological coherence. In fact, differently from the landslide bodies, LDZ must necessarily occur in geoenvironmental classes prone to failure. Thus, the calculated Wi seem to be more reliable in estimating the propensity of a given class to generate failure. The thematic maps have been reclassified on the basis of the calculated Wi and then overlaid, with the purpose to produce landslide susceptibility maps. The used methods converge both in indicating that most part of the study area is characterized by a high–very high landslide susceptibility and in the location and extent of the low-susceptible areas. However, an increase of both the high–very high and moderate–high susceptible areas occurs in using the second method. Both the produced susceptibility maps have been compared with the geomorphological map, highlighting an excellent coherence which is higher using method-2. In both methods, the percentage of each susceptibility class affected by landslides increases with the degree of susceptibility, as expected. However, the percentage at issue in the lowest susceptibility class obtained using method-2, even if low, is higher than that obtained using method-1. This suggests that method-2, notwithstanding its major geomorphological coherence, probably still needs further refinements.     

Large reactivated landslides in weak rock masses: a case study from the Northern Apennines (Italy)

This case study paper is about a large rotational rock and earth slide—earth flow located in the Secchia River Valley, in the Northern Apennines of Italy, that has displayed multiple reactivation phases between 2002 and 2004. The main geological constraints of the mass movement are related to the overlap of flysch rock masses over clayey complexes that allows rock slides to take place in the source area. The disarrangement and weathering of rock masses following slope movements causes large amount of fine-grained debris to be accumulated on the slope and mobilised by earth sliding and flowing. Analysis of rainfall data at the onset of reactivation events has proved that they occurred after periods with cumulated values higher than the averages of the last 30 years. The quantification of the morphological modifications induced by these reactivations has been made possible by comparing pre- and post-event digital elevation models. Depletion and accumulation has been in the range of 30 m in different parts of the slope. In particular, an advancement of the landslide toe of more than 400 m, which caused a 30-m thick landslide tip to deposit, has been clearly seen. Monitoring data regarding subsurface movements and surface tension crack widening (tension cracks so large as to be properly described at trenches) has shown that sliding surfaces as deep as 43 m exist in the upper part of the landslide, while the accumulation lobe has moved by sliding and flowing over surfaces as deep as some 10 m. Velocities of cm/day have been recorded in the deep surfaces and in widening trenches of the source area, while the advancement of the accumulation lobe has been estimated as having velocities of up to 10 m/day. Groundwater in the landslide body has been observed at depths of 5–15 m in the upper areas, while it is estimated as being at the ground level in the toe. On this basis, it is concluded that the landslide still has a high potential for further development, both in the upper landslide zone and in the toe area.     

Late Tertiary–Quaternary tectonics of the Southern Apennines (Italy): New evidences from the Tyrrhenian slope

This paper summarises the results of combined structural and geomorphological investigations we carried out in two key areas, in order to obtain new data on the structure and evolution of the Tyrrhenian slope of the southern Apennines. Analysis by a stress inversion method [Angelier, J., 1994. Fault slip analysis and paleostress reconstruction. In: Continental Deformation. P.L. Hancock Ed., Pergamon Press, Oxford, 53–100] of fault slip data from Mesozoic to Quaternary formations allowed the reconstruction of states of stress at different time intervals. By integrating these data with those deriving from the stratigraphic and morphotectonic records, chronology and timing of the sequence of the deformation events was obtained.The tectonic history of the region can be related to four deformation events. Structures related to the first event, that was dominated by a strike-slip regime with a NW–SE oriented σ1 and was active since Mid–Late Miocene, do not significantly affect the present day landscape, as they were strongly displaced and overprinted by subsequent deformation events and/or deleted by erosion. The second and third events, that may be considered as the main responsible for the morphostructural signature of the region, are comparable with the stretching phases recognised offshore and considered to be responsible for the opening and widening of the Tyrrhenian basin. In particular, the second event (with an E–W oriented σ3), took place in the Late Miocene/earliest Pliocene and was first dominated by a strike-slip regime, that was also responsible for thrusting and folding. Since Late Pliocene, it was dominated by an extensional regime that created large vertical offsets along N–S to NW–SE trending faults. The third event, that was dominated by extension with a NW–SE oriented σ3, started in the Early Pleistocene and was responsible for formation of the horst-and-graben structure with NE–SW trend that characterises the Tyrrhenian margin of the southern Apennines. The fourth deformation event, which is characterised by an extensional regime with a NE–SW trending σ3, started in the late Middle Pleistocene and is currently active.     

Evaluating seismically induced mass movement hazard in Caramanico Terme (Italy)

Mass 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.     

UNESCO World Heritage sites in Italy affected by geological problems, specifically landslide and flood hazard

A National Research Council, Research Institute for Hydrogeological Protection (IRPI) study on Italian monuments included in the UNESCO World Heritage List has revealed that many are affected by geological, geomorphological, and engineering geological problems. These monuments are static entities set in an environment that often manifests highly dynamic processes. As part of the efforts to protect sites of cultural and natural heritage, this study has applied a preliminary and empirical Geographical Information System-based method developed to characterize the environmental hazards at the sites where the monuments are located. Because the study of hydrogeological degradation falls within the province of IRPI, this hazard zoning focuses on river and mountain slope dynamics specifically concerning landslides and floods.     

An objective method to rank the importance of the factors predisposing to landslides with the GIS methodology: application to an area of the Apennines (Valnerina; Perugia, Italy)

The present work aims at contributing to further our knowledge on the predisposing factors to landslides as well as proposing a method that allows us to weigh up, as objectively as possible, the influence that the various factors have on landslides in order to construct a realistic map of potential landslide hazards. In order to assess the differential incidence of very predisposing factor to landslides, maps of the various factors considered were drawn up with the aid of IDRISI software (1997). The factors were as follows: the distance from faults, parallelism between the fractures and the landslide scarps, land use, lithology, distance from the streams, orientation and steepness of slopes, orientation of layers compared to the slope. The global analysis of the different incidence of the factors analyzed on the landslides present in the study area was carried out by comparing the respective maps with that of the scarps; in this way the number of pixels forming the scarps which fell into the various classes of the maps was calculated. The result thus obtained can form the basis of an objective assignation of the different ratings to be attributed to the various factors under consideration. In fact, the analysis carried out in this study has shown that the factors act differently and, for every factor, only some of the classes considered have marked importance.     

The radiolarian assemblages of the Bonarelli Horizon in the Umbria-Marche Apennines and Southern Alps, Italy

The Bonarelli Horizon is an important regional marker level in the Umbria-Marche Apennines. It is characterized by consisting of black bituminous shales that yield a distinctive radiolarian fauna. Black shales referable to the Bonarelli Horizon are also present in the Southern Alps. The horizon is of uniform lithology and contains radiolarian assemblages of late Cenomanian-early Turonian age throughout both the Umbria-Marche Apennines and Southern Alps. Several new radiolarian taxa have been recognised and described from the Bonarelli Horizon of both areas. Two new species are defined in this paper, namelySethocapsa bossoensisandXitus picenus.     

Appraise the structural mitigation of landslide risk via numerical modelling: a case study from the northern Apennines (Italy)

The Ca’ Lita landslide is a large and deep-seated mass movement located in the northern Apennines, about 70 km west of Bologna (Northern Italy). It consists of a composite landslide that affects Cretaceous to Eocene flysch rock masses and chaotic complexes. Many of the sectors making up the landslide have resumed activity between 2002 and 2006, threatening some villages and an important road connecting several key industrial facilities located in the upper watershed. This paper presents the management of the emergency, dealing with the investigation campaigns (geological, geomorphological and LiDAR surveys, borehole drillings, seismic surveys), with the monitoring (in situ instrumentation) and with the design and construction of mitigation measures. The whole process, from landslide reactivation to date, has been modelled on a numerical basis with the finite difference code FLAC 2D, to assess the efficiency of the mitigation system and to propose further countermeasure works in different scenarios.     

Integrated geophysical and geomorphological approach to investigate the snowmelt-triggered landslide of Bosco Piccolo village (Basilicata, southern Italy)

Most of Basilicata region in the southern Italian Apennines is characterized by landslides often developing in clayey–marly formations. Many events have been triggered by extreme rainfall or snowmelt. The most important happened (on February–March 2005) at Bosco Piccolo 5 km far from Potenza. This landslide developed subsequently to rapid snowmelt occurred during alternating short periods of high temperatures and intense and continuous snowfalls. This complex landslide affected 4 ha of surface and reached a maximum depth of 20 m inducing damage and collapse of about 80% of the buildings in the village. An integrated multidisciplinary approach has been adopted to study the landslide. A multitemporal aerial photo interpretation and detailed geomorphological surveys have been carried out. Combined Electrical Resistivity Tomography (ERT) and Self-Potential (SP) measurements have been performed and calibrated with boreholes stratigraphy. Such an integrated approach allowed us to partially reconstruct the geometry of the investigated body and to evaluate the effectiveness of drainage system planned for the area.     

Risk assessment strategies for the reactivation of earth flows in the Northern Apennines (Italy)

The aim of this paper is to analyze the reactivation mechanism of ancient earth flows, with a view to gleaning information that can subsequently be utilized to formulate a risk-reduction strategy. All considerations made herein are the result of direct experience and observation of actual events which have occurred over the past few decades in the Northern Apennines. Particular attention has been paid to the analysis of the evolution of landslides during actual reactivation, acknowledging a typical, recurring succession of events that precede the failure of the slope. The hazard assessment of these large landslide bodies, which are of slope scale, constitutes a thorny problem, especially in view of the inapplicability of traditional deterministic models such as limit equilibrium stability analysis. Nevertheless, a site-specific assessment of probability of reactivation of these large and ancient earth flows is fundamental to effective land-use planning.     

Rainfall patterns and related landslide incidence in the Porretta-Vergato region,Italy

An analysis of landslide occurrence in the low permeability terrain of Porretta-Vergato, Italy, related to prolonged rainfall patterns is presented. Data sets collected over nearly a century are statistically analysed. The pattern of the landslide hazard is considered and related to precipitation at the basin scale in order to enhance the understanding between the two parameters and assess their temporal changes, as well as interrelationships. Landslide incidence generally follows the periodic pattern of precipitation with a lag of approximately six months, which is believed to relate to the time necessary for the ground water to reach a critical level to initiate slope failure. There also appears to be a two-stage pattern of precipitation which induces most landslides: a preparatory period, where the landslide is destabilized and conditioned for slope failure, followed by a more intense period of rainfall that triggers or provokes the event. These initial findings point to the need for further studies to verify such unstable situations.     

Spatio-temporal estimation of shallow landslide hazard triggered by rainfall using a three-dimensional model

A shallow landslide triggered by rainfall can be forecast in real-time by modeling the relationship between rainfall infiltration and decrease of slope stability. This paper describes a promising approach that combines an improved three-dimensional slope stability model with an approximate method based on the Green and Ampt model, to estimate the time–space distribution of shallow landslide hazards. Once a forecast of rainfall intensity and slope stability-related data, e.g., terrain and geology data, are acquired, this approach is shown to have the ability to estimate the variation of slope stability of a wide natural area during rainfall and to identify the location of potential failure surfaces. The effectiveness of the estimation procedures described has been tested by comparison with a one-dimensional method and by application to a landslide-prone area in Japan.     

Geological features and landslide mechanisms of an unstable coastal slope (Petacciato, Italy)

On the basis of geological and geomorphological surveys, landslide phenomena are analysed on a slope along a stretch of the Adriatic coast, near Petacciato (Molise, Italy).

Locally, a blue clay sequence of Pleistocene outcrops, stratified with silty-sandy layers; bedding dips 3–8°NE and the slope has a similar attitude. This sequence evolves upwards to sands and conglomerates, with thickness of up to 40 m, on which the built-up area is located.

Several episodes of landslide reactivation occurred in the past century, involving the zone between the built-up area and the sea, along a coastal slope of over 2000 m long and 200 m high. Important roads and railway lines have been heavily damaged as well as the town itself.

The typology of the movement is a rotational–translational slide; the displacement reaches tens of centimetres at each reactivation episode along the entire coastal slope, extending well beyond the shore line.

Detailed studies already exists, but different mechanism failures have been proposed to explain the landslide phenomena. In this study, a new failure mechanism is proposed, (sudden spreading of Terzaghi, K., Peck, R.B., 1948. Soil Mechanics in Engineering Practice. Wiley and Sons New York) and analytical approaches have been adopted to evaluate the slope stability, based on the geotechnical and monitoring data and the geometrical and geological features of the slope.     

Different approaches to the seismic hazard of Sannio-Matese (Southern Italy)

The seismic hazard of Sannio-Matese is calculated using a new seismogenetic zoning of southern Italy of seven areas and different, azimuth-dependent, attenuation laws. Various approaches (Gumbel's first and third asymptotic distribution, Cornell) lead to similar results for the different exposure times considered (100 and 200 years) and probability levels (37 and 68%). The present seismic regulation proposed in 1980 by the Italian National Council for Researches, and based on a different approach, is, in general, confirmed by the results.     

Critical parameter investigation for earthquake hazard assessment in the Sannio-Matese area of Southern Italy

Probabilistic methods have been applied for the assessment of seismic hazard in a selected region of Southern Italy (Sannio-Matese). This method is mainly suitable for engineering and planning purposes and was first introduced in 1968 by Cornell and efficiently codified into a FORTRAN computer program by McGuire. Special attention is paid in this paper to the specific input parameters, i.e. completeness of data catalogues, time variability of seismic activity, different forms of frequency distributions, and regional attenuation characteristics.     

Parametrical landslide modeling for the hydrogeological susceptibility assessment: from the Crati Valley to the Cavallerizzo landslide (Southern Italy)

The article deals with a tool for landslides susceptibility assessment as a function of the hydrogeological setting at different scales. The study has been applied to a test area located in Southern Italy. First, a 3D groundwater flow model was implemented for a large-scale area. The simulation of several groundwater conditions compared with the landslide activity map allows drawing a hydrogeological susceptibility map. Then, a slope scale analysis was carried out for the Cavallerizzo landslide. For this purpose, a 2D groundwater parametrical modeling was coupled with a slope stability analysis; the simulation was carried out by changing the values of the main hydrogeological parameters (recharge, groundwater supply level, etc.). The results enabled to connect the slope instability to some hydrogeological characteristics that are easy to survey and to monitor (e.g., rainfall, piezometrical level, and spring discharge), pointing out the hazard thresholds with regards to different triggering phenomena.     

The geotechnical effects of long human habitation (2000< years): Earthquake induced landslide hazard in the city of Zefat, northern Israel

This work studies the effects of long human habitation on site geotechnical conditions. It is focused on the city of Zefat that is located on the borders of the Dead Sea Transform in northern Israel. The city of Zefat, suffered severe damage and loss of life in historical earthquakes, as a consequence of earthquake induced landslides (EILS). In this work we evaluate the current EILS hazard for the city of Zefat using a GIS-based regional Newmark analysis, with calibration of the calculated Newmark displacement (representing EILS hazard) using maps of field evidence and historical documents testifying to slope instability that occurred in historical earthquakes.

We found that the core city of Zefat is built on a layered anthropogenic material, few meters deep which, was deposited as a result of more than 2000 years of human habitation. The anthropogenic material is mechanically weak, susceptible to slope failure and to amplification of seismic-shaking. It is responsible for the city's devastation in historical earthquakes and it is the source for the current high seismic hazard as well.

Our model shows that earthquakes of magnitudes (M_w) 5, 6 and 7 at distances of up to 10 km, 50 km and more than 100 km, respectively, are likely to induce landslides in the core city of Zefat. The current engineering status of the city is poor, and as a consequence severe damage and loss of life are expected in future earthquakes due to EILS, unless major engineering efforts are made. Cities in the Eastern Mediterranean with comparable long habitation histories (e.g., Jerusalem, Tiberias, Nablus, Amman) are expected to have similar geotechnical problems in their old sections and are advised to take appropriate engineering steps to reduce damage and loss of life in future earthquakes.

Evaluation of historical earthquake magnitudes based on reported local-damage may, however, lead to overestimated magnitudes where the damaged sites are built on anthropogenic talus (a common setting in the vicinity of the Dead Sea Transform).