Abstract
Scenarios of earthquakeinduced landslides are necessary for seismic microzonation (SM) studies since they must be integrated with the mapping of instability areas. The PARSIFAL (Probabilistic Approach to pRovide Scenarios of earthquake‐Induced slope FAiLures) approach provides extensive analyses, over tens to thousands of square kilometers, and is designed as a fully comprehensive methodology to output expected scenarios which depend on seismic input and saturation conditions. This allows to attribute a rating, in terms of severity level, to the landslideprone slope areas in view of future engineering studies and designs. PARSIFAL takes into account firsttime rock and earthslides as well as reactivations of existing landslides performing slope stability analyses of different failure mechanisms. The results consist of mapping earthquakeinduced landslide scenarios in terms of exceedance probability of critical threshold values of coseismic displacements (P[D ≥ Dca(t),a_{y}]). PARSIFAL was applied in the framework of level 3 SM studies over the municipality area of Accumoli (Rieti, Italy), strongly struck by the 2016 seismic sequence of Central Apennines. The use of the PARSIFAL was tested for the first time to screen the Susceptibility Zones (ZS_{FR}) from the Attention Zones (ZA_{FR}) in the category of the unstable areas, according to the guidelines by Italian Civil Protection. The results obtained were in a GISbased mapping representing the possibility for a landslide to be induced by an earthquake (with a return period of 475 years) in three different saturation scenarios (i.e. dry, average, full). Only 41% of the landslideprone areas in the Municipality of Accumoli are existing events, while the remaining 59% is characterized by firsttime earth or rockslides. In dry conditions, unstable conditions or P[D ≥ Dca(t),a_{y}] > 0 were for 54% of existing landslides, 17% of firsttime rockslides and 1% of firsttime earthslides. In full saturation conditions, the findings are much more severe since unstable conditions or P[D ≥ Dca(t),a_{y}] > 0 were found for 58% of the existing landslides and for more than 80% of firsttime rock and earthslides. Moreover, comparison of the total area of the ZA_{FR} versus ZS_{FR}, resulted in PARSIFAL screening reducing of 22% of the mapped ZA_{FR}.
Introduction
The reconstruction of landslide scenarios is a milestone for environmental planning in terms of both hazard mapping and strategic and engineering solutions to be adopted in urbanised areas. In particular, the awareness of the spatial distribution of earthquakeinduced landslides support the competent local authorities to plan intervention priorities aimed at managing the natural hazard on the basis of the available economic resources. The combined nature of the hazard related to earthquakeinduced landslides implies that such processes should be analyzed taking into account both the proneness to failure (landslide susceptibility), and the probability of occurrence of the triggering event (seismic hazard).
However, the predisposing conditions to slope failures vary according to the environmental constraints, among which the water saturation of covers and deposits or the presence of pore water pressures related to groundwater flow, more commonly originated by impulsive events such as heavy rainfall. Nevertheless, for the purposes of spatial planning and its technical products (such as SM maps), the susceptibility mapping or the analysis of the spatial distribution of hazard related to the trigger action is not sufficient. Instead, it is necessary to resort to the reconstruction of scenario maps which have the dual purpose of: (1) providing a spatial distribution of effects to a certain probability of occurrence; and (2) make it possible to take into account their variability as a function of the predisposing environmental conditions, including soil saturation.
Since the late ‘80s the prefiguration of earthquakeinduced landslides scenarios, which means analyzing over large areas the proneness of slopes to fail under seismic loading, was strongly requested by scientific and technical communities for risk mitigation purposes. This goal was initially pursued by analyzing the maximum runout distances for the expected landslide occurrence (Keefer 1984; Rodriguez et al. 1999) on the basis of datasets obtained on a global scale. Afterwards, mapping solutions were proposed based on susceptibility or hazard analyses, whose development in the last few decades—significantly favored by the constant improvement over the years of computing facilities and GIS platforms—represent a wellestablished topic in the scientific and technical communities dealing with landslide risk reduction.
A first category of methods can be classified as physicallybased approaches that are aimed at computing and mapping the safety factors and/or coseismic displacements (Newmark 1965; Rathje and Bray 2000; Rathje and Antonakos 2011) under specific seismic loading (e.g., Prestininzi and Romeo 2000; Romeo 2000; Jibson et al. 2000; Jibson 2007; Hsieh and Lee 2011; Bozzano et al. 2013; Tsai et al. 2019). Such methods have the great advantage to explicitly consider the interaction slopeseismic waves, but also require a detailed knowledge of physical–mechanical properties of the slopes, which is not easy to achieve as the stability analyses should be extensively performed over wide areas. This issue can be relevant especially for firsttime slope failures, while is less significant for landslide reactivation under seismic conditions if the geomechanical model of the slope is sufficiently constrained.
Other approaches strongly rely on indirect susceptibility or hazard assessments, such as those developed and widely used in the last decades. These are extensively described in the review papers by Reichenbach et al. (2018), encompassing a wide range of methods to assess at several scales both qualitatively and quantitatively the proneness of territories to almost each type of landslides. Most of the quantitative approaches are designed to perform susceptibility or hazard assessments on large scales; statisticallybased models are commonly used. Due to the implications for earthquaketriggered landslide applications, it is worth highlighting one of the basic principles of this kind of methods: the assumption that past and present are the key to the future (Varnes and IAEG Commission on Landslides and other MassMovements, 1984), which means that these analyses are basically back analyses. Susceptibility models, especially if based on statistic approaches, are in fact trained and validated on already occurred landslides; this implies the availability of reliable landslide inventories. To turn susceptibility analyses in proper hazard analyses, the reference inventory should be referred to a specific triggering event with its temporal probability of occurrence as an earthquake (Harp et al. 2011). As a matter of fact, several earthquaketriggered landslide susceptibility analyses were performed by means of statisticallybased methods exploiting eventbased inventories (e.g., Xu et al. 2012a; GarcíaRodríguez and Malpica 2010; Kamp et al. 2008), which is by itself a way to consider, although implicitly, the role of the trigger. In other studies triggerrelated factors (such as PGA, shake maps, Arias Intensity) referred to the triggering event have been explicitly considered as variables in the susceptibility function (e.g. Lee et al. 2008; Bai et al. 2012; Xu et al. 2012b; Lee et al. 2002; Tian et al. 2019). Such studies can be formally regarded as hazard analyses but being trained on eventbased inventories and eventspecific triggers, their results are not easily generalizable to triggering events with different magnitudes.
Based on these premises, the PARSIFAL method (Esposito et al. 2016) has been developed trying to put together the benefits of both approaches in a comprehensive, multistep methodology: a susceptibility assessment under static conditions (i.e., regardless the availability of an eventbased inventory) is a screening tool to identify areas actually prone to landslides. The analysis can be carried out with the most reliable method depending on the data availability, i.e. quantitative statisticallybased techniques or qualitative heuristic approaches, to provide the “background noise” in terms of landslide proneness of a given area, regardless the triggering factor. Subsequent physicallybased analyses can be performed in the soidentified areas to assess landslide scenarios according to “customized” seismic inputs. This kind of integrated approach can be particularly suitable for sites featured by high levels of both seismic and landslide hazards, where no eventspecific inventories are available and, thus, neither eventspecific scenarios can be assessed by means of susceptibility approaches. Furthermore, the first step (slope analysis, explained further in more detail), which is actually the integration of landslide inventorying and susceptibility assessment for soil slopes and kinematic compatibility analyses for rock slopes, provides a basic and single input for pseudostatic and pseudodynamic analyses that can be performed under different seismic scenarios.
The PARSIFAL method considers different landslide typologies (new and existing in rock or soil) and different failure mechanisms. Some test sites of the method have been: (1) a coastal sector of Calabria in Southern Italy (Bozzano et al. 2013) as part of a POR project funded by the Calabria Region, (2) some municipalities of Central Italy (Esposito et al. 2016) as part of pilot studies promoted by the Lazio Region with high seismic hazard and, (3) in Spain (Martino et al. 2018), the Alcoy area in the Community of Valencia, historically hit by strong earthquakes and today intensely urbanized. The recent seismic events that struck the Central Apennines in 2016–2017 have launched an impressive reconstruction program that involves 15 municipalities in the province of Rieti, Lazio region. Thanks to the Ordinance of the Presidency of the Council of Ministers No. 24 of 12/05/2017 (OPCM 24/2017), technical support was provided for the design phase of the reconstruction by means of quantitative studies for SM which, following the guidelines issued in 2008 by the Italian Department of Civil Protection (DPC) (ICMS 2008), have provided the inventory of already unstable and potentially unstable slopes with the purpose of defining specific areas of attention for landslides (ZA_{FR}). More specific landslides guidelines were recently published by the DPC (CTMS, 2017). According to these guidelines the areas of attention for slope failures (ZA_{FR}) must be selected among the areas of susceptibility (ZS_{FR}) that, compared to the former, constitute the subset of the areas for which: (1) the attitude to earthquakeinduced failure can be evaluated quantitatively by means of simplified analytical approaches; (2) an area of invasion by debris accumulation or an area of expansion (AE) (for enlargement, advancement or retraction) should be associated.
In this study the procedure adopted in the Municipality of Accumoli is described. The SM studies followed ODPCM 24/2017) to obtain the perimeter of the ZS_{FR} starting from the inventory of landslide areas (ZF) and potentially unstable slopes (ZPF) which, together, define the zones of attention for landslide (ZA_{FR}). This procedure, based on the construction of scenario maps referring to different types of failure mechanisms and to different conditions of saturation of deposits and covers, has been implemented starting from the previously experienced applications to SM studies and to the needs mapping standards of the DPC guidelines.
Geological and geomorphological setting
The geological and structural setting of the central Apennines is strongly influenced by the variable geodynamic context that characterized the Mediterranean area from the Triassic to the Neogene (Centamore et al. 2012; Cosentino et al. 2010).
The Accumoli area is characterised by a MesoCenozoic geological succession of limestones and marls with a thickness of about 8000 meters. The attitude of strata is strongly controlled by the folding and faulting responsible for the growth of the Apennine chain (Bigi et al. 2009) which evolved through a general northeastward migration towards the Adriatic foreland. This caused the formation of inner basin systems which hosted a thick turbiditic sedimentation during the Miocene (e.g. Laga Formation; Falcini et al. 2009, and references therein).
The present morphostructural setting of Central Apennine is strongly driven by active extensional tectonics that characterizes this area since the Upper Pliocene. This extensional phase generated NW–SEtrending normal fault systems, which are responsible for the development of intermontane basins and include the main seismogenic sources of this sector of the Apennines (Tondi and Cello 2003; Galadini and Galli 2003; Boncio et al. 2004; Pizzi and Galadini 2009).
The Municipality of Accumoli is located in the piedmont zone of the Central Apennines, between the Sibillini Mountains to the west and the Laga Mountains to the east.
Most of the area is characterised by the outcropping of deposits ascribable to the flysch of the Laga Formation while the western portion is characterized by the outcropping of MesoCenozoic limestones and marls.
In general, the lithological variability of the study area accounts for different erosional behaviour along the main slopes (Fig. 1) and represents one of the main predisposing factors for intense mass wasting of slopes due to gravitational process.
Tectonics and climate influenced the longterm landscape evolution of this sector of the Apennines and the drainage network underwent important phases of planoaltimetric reorganization, mainly consisting in stream piracy and cyclic alternation of erosional and aggradational episodes, as already testified within adjacent river basins in central Apennines (Aringoli et al. 2014; Fubelli et al. 2014).
The geomorphological setting of the western portion of the area is characterized by deeply incised, mainly Vshaped valleys and diffuse erosional landforms such as subvertical rocky cliff and ridges, which are particularly prone to landslides.
The remaining part of the area is divided by the Tronto River trunkvalley and presents a morphological asymmetry due to different characteristics of the outcropping bedrock (Fig. 1). The left valleyside, mostly carved into the arenaceous member of the Laga Formation, is featured by steep or subvertical slopes, ridges and Vshaped incised valley often interrupted, at various heights, by fluvial terraces and terracelike morphologies of different origin. The opposite valleyflank, where the mostly pelitic member of the Laga Formation crops out, shows a gentle morphology and a quasicontinuous vegetation cover.
The Municipality of Accumoli is affected by several typologies of landslides mainly including rototranslational slides involving both rock masses as well as soils and rockfalls (sensu Hungr et al. 2014).
The mostly pelitic flysch of the Laga Formation is most affected by instability, since the presence of low shear strength pelitic interlayers, is the main predisposing factor to landslides activation. The strata attitude has great influence on the hillslope erosion, which is characterized by steeper antidip slopes with respect to dip slopes. These geological conditions represent a preparatory factor for planar sliding of rock blocks (Cacciuni et al. 1995).
Materials and methods
The PARSIFAL method
The PARSIFAL method deals with the analysis of both reactivation of preexisting landslides and the occurrence of firsttime failures (Martino et al. 2018). Firsttime failures occur on slopes previously not affected by instabilities, while reactivation refer to already existing landslide that can be affected by further displacements. Both processes are triggered by either external or internal forces, such as seismic action and pore pressures increase related to intense rainfall events, respectively. Specifically, PARSIFAL analyzes reactivations of roto/translational slides and firsttime failures that fall into two main categories: rockslope failures and shallow earthslides. To properly address the first stage (slope analysis) of the methodology, field surveys are of crucial importance to inventory already existing landslides and characterize rock masses and shallow deposits to assess their proneness to firsttime failures.
The main output of PARSIFAL consists in a complete mapping of the probability of exceedance of earthquake‐induced landslide displacement in respect to an assumed threshold, for different scenarios (i.e., return times of seismic hazard and saturation conditions).
PARSIFAL is a sequential procedure based on the following three main steps (Fig. 2):
Step 1 Slope Analysis (SA)—The territory is analyzed in terms of landslide susceptibility, regardless the dynamic input. As above mentioned, the methodology aims at analyzing the most common coseismic landslides: reactivation of already existing roto/translational landslides, firsttime rockslope failures and newly generated shallow landslides of soil covers over the bedrock.
The susceptibility to reactivation is assessed by detecting and inventorying the already existing landslides which, in terms of GIS analysis, are mapped as polygons (Fig. 3). For firsttime rock failures, the procedure is more articulated. In a first instance the territory is divided by identifying, on the basis of field surveys and related geostructural and geomechanical measurements, zones that are homogeneous in terms of lithology and structural features (homogeneous geostructural zones—HGZ). Each zone is then featured by a specific geometric pattern of discontinuity sets, that are in turn characterized in terms of geomechanical properties. The HGZs are then partitioned into a grid of square cells. A test of kinematic compatibility to planar/wedge sliding and toppling is performed by GIS routines on a cellbycell basis by comparing dip and dip direction of each discontinuity sets (or trend and plunge of each line of intersection in the case of wedge mechanism) with slope and aspect of the corresponding cell, calculated starting from the available DEM (Digital Elevation Model) of the area. It is worth noting that each cell can be potentially compatible with more than one failure mechanism. With regards to firsttime shallow landslides, i.e. the deposits potentially involved in shallow landslides, the proneness to failure is assessed after dividing each polygon representing a significant outcrop of soil cover into Unique Condition Units, i.e. subareas characterized by specific classes of slope exposure and lithology. The proneness of each UCU to landslides is then assessed by applying heuristic or quantitative susceptibility methods depending on data availability.
The territorial units resulting as susceptible to failure (grid cells in HGZs and UCU) or to coseismic further displacements (polygons representing the inventoried existing landslides) are analyzed in the next step.
Step 2 Slope Stability (SS)—This step consists in the assessment of the actual potential of failure/reactivation under combined seismic and saturation loading of the areas resulted prone to failure in the previous phase. Specifically, this step aims at the computation of an exceedance probability of co‐seismic displacements based on an assumed failure threshold and evaluated by the Newmark (1965) approach, i.e. considering a critical pseudostatic acceleration, a_{y}, as unelastic displacement threshold. The degree of mobility is expressed by the exceedance probability of a critical displacement (P[D ≥ Dca(t),a_{y}]) responsible of the slope collapse, which can be assumed according to literature, as specified in the following. To calculate the degree of stability of landslides under seismic conditions, the input data considered are natural accelerometric records selected considering the seismological characteristics of the most severe seismogenic source among the ones that contribute to the seismic hazard of the site.
In case that the failure displacement threshold (a_{y}) is not exceeded, a safety factor (SF) is computed as to provide a stability level of the slope. The SF is computed according to Limit Equilibrium Methods, selected on the basis of the landslide shape H/L (where H is the depth and L is the length of the landslide mass). In particular, according to Dewitte and Demoulin (2005): infinite slope method was adopted in case of H/L < 0.01, the Janbu (1973) method was applied for slides characterized by 0.01 ≤ H/L ≤ 0.1, i.e. with a mainly translational mechanism, the Bishop (1955) method was applied for a rototranslational mechanism (where H/L > 0.1). Based on the previous reported definitions, three possible conditions can be expected by the computing: 1) for a_{y} = 0 and SF ≤ 1 then P[D ≥ Dca(t),a_{y}] = 1; 2) for a_{y} > PGA and SF > 1 then P[D ≥ Dca(t),a_{y}] = 0; 3) for a_{y} ≤ PGA and SF ≤ 1 then 0 < P[D ≥ Dca(t),a_{y}] < 1.
Step 3 Resulting scenario (RS)—This step aims at providing, for each considered scenario of combined seismic and saturation conditions, a synthetic and integrated GIS mapping of the slope stability results. The scenario map shows the different territorial units defined as elementary areas able to represent the slope stability results in terms of either failure exceedance probability or safety factor. To provide a unique value of P[D ≥ Dca(t),a_{y}] into the same cell unit, the following criteria were assumed as reported in Martino et al. (2017): (1) for the same mechanism, the P[D ≥ Dca(t),a_{y}] computed for different block volumes are solved in a unique value by performing a weight averaging; (2) combined probability is computed for each different mechanism (i.e., toppling, wedge sliding, planar sliding) using the probability values computed for each block volume; (3) in case that only SF values are computed for each block volume and landslide mechanism the lowest SF value is attributed to the cell unit, adopting a conservative solution. More in particular, in case of simultaneous presence of nunstable blocks with P(D ≥ D_{c}) > 0 for the same type of instability, the weighted exceedance probability, P(D ≥ D_{c})_{w}, is computed on the basis of the mobilized block volumes, according to the formula:
where P(D ≥ D_{c})_{i} is the probability of exceedance P[D ≥ Dca(t),a_{y}] of the ith rocky block size and V_{i} its volume.
In case more than one type of instability can occur, the overall failure probability is computed, according to the total probability theorem:
where P(D ≥ D_{c})_{j} is the exceedance probability P[D ≥ Dca(t),a_{y}] of the jth type of instability (weighted, if any).
From seismic inputs to exceedance probability
In order to define a set of natural accelerometric records representative of the local expected groundmotion to be used in the evaluation of coseismic displacements by Newmark (1965) approach, a constraint imposed by the general activity of level 3 SM in the Central Italy area struck by 201617 seismic sequence (ODPCM 24/2017) has been adopted.
In detail, Luzi (2017) defined the seismic hazard level for the municipality area of Accumoli, according to the Italian national seismic map (Montaldo et al. 2007) and the Italian building code (NTC08—CS.LL.PP. 2008), for the selection of 7 natural accelerometric timehistories to be used as seismic input for local seismic response analysis by 1D and 2D engineeringgeology numerical models.
The accelerometric records have been selected by the REXELite code (Iervolino et al. 2011) as to obtain timehistories spectrumcompatible with a target response spectrum evaluated for a Return Time of 475 years (i.e. a 10% exceedance probability within a reference time period of 50 years; obtained PGA = 0.26 g), a Eurocode 8 site ground type “A” (hard rock/rock or Vs30 ≥ 800 m/s) and ground “flat” condition (i.e., slope angle < 15°). Further seismological parameters has been set in: (1) magnitude range 5.2–6.2 M_{W} or M_{L}; (2) seismic sourcesite maximum distance 30 km; focal mechanism “normal”. The selection has been performed on the INGV ITalian ACcelerometric Archive (Luzi et al. 2008) increased with the records obtained by the temporary accelerometric network installed during the 2016–2017 Central Italy seismic sequence.
To increase the set of accelerograms to be used for the computation of the probability distributions of coseismic Newmark’s displacements, a further selection was performed (Martino et al. 2018) by querying additional accelerometric databases like ESMEngineering StrongMotion database release 1.0 (Luzi et al. 2016) or Japanese NIED strongmotion seismograph networks (website: http://www.kyoshin.bosai.go.jp/). The previously mentioned selection criteria are extended to: (1) a Eurocode 8 site ground type “A” and “B”, which are the most representative for the thickness as well as for the geotechnical and geophysical parameters of the deposits outcropping in the study area; (2) “freefield” or “ground” recordingsites; iii) Focal mechanism “normal” or “normaloblique”, i.e. in the rake interval − 20°/− 70° (normal leftlateral oblique); − 70°/− 110° (normal); − 110°/− 160° (normal rightlateral oblique); iv) hypocentral depth h ≤ 30 km. Additional constraints involved the selected timehistories in term of: PGA which must fulfill the condition 0.67 ≤ PGA ≤ 1.5 of local seismic hazard; Arias intensities (in base10 logarithm values) lying inside the 95% confidence interval of that computed from the selected records; shape similarity of response spectra compared to the target one, evaluated for a Return Time of 475 years, for different period intervals. Finally, the resulting timehistories are scaled to the local seismic hazard PGA, namely 0.26 g.
Considering both the selection criteria, a set of 25 natural accelerometric records is collected (Fig. 4a) and used for the evaluation of Newmark (1965) coseismic displacements. Assuming a critical displacement threshold of 10 cm for earth‐slides and 5 cm for rock‐slides, according to Romeo (2000), the probability of exceedance P[D ≥ Dca(t),a_{y}] for each landslide or instability area can be evaluated
In detail, for every local ground instability, the pseudostatic critical acceleration (a_{y}) evaluated at different saturation conditions allow the identification of accelerations which can induce a coseismic displacement. By a doubleintegration of timehistory accelerations overcoming the ay acceleration, a cumulative coseismic displacement is calculated (Fig. 4b). In this way, a set of 25 maximum cumulative coseismic displacement values is obtained: Fig. 4c shows the distribution of relative frequencies of obtained maximum displacements, in terms of base10 logarithm, compared as example with the xaxis position of the critical displacement threshold for rock‐slides.
It is therefore assumed that the coseismic displacements obtained by applying the Newmark’s method to the selected timehistories are distributed according to a normal distribution characterized by an average (\(\mu\)):
and a standard deviation (\(\sigma )\):
where N is the number of selected timehistories and log10(Sp_{i}) is the base10 logarithm of ith maximum cumulative coseismic displacement.
The probability of occurrence of a critical displacement calculated by means of the standard normal distribution (i.e. with mean μ = 0 and standard deviation σ = 1) is given by the equation:
where x is the base10 logarithm of critical displacement, log10(SpCr), and z is the critical displacement expressed in standard unit (zScore):
Therefore, for a given local ground instability and pseudostatic critical acceleration (a_{y}) evaluated at different saturation conditions, the exceedance probability of a critical displacement (Fig. 4d) is obtained from the equation:
Results
For both firsttime failures and reactivations the Newmark (1965) coseismic displacements were computed by using the selected time histories and by considering the previously derived a_{y} values for each typical rock block and earthslide mass at different saturation conditions. Following this approach, the landslide mass is considered like a rigid block sliding on an inclined plane (in case of planar sliding) or on a curved surface (in case of rototranslational slides) as well as on an intersection line (in case of wedge sliding). For the rock blocks, only toppling mechanism was also accounted for and driving forces are considered to produce an overturning moment. Each block has a_{y} values which represents the threshold seismic acceleration required for triggering the landslide failure. Thematic maps were obtained through GIS by synthetically reporting the slope stability conditions under dynamic action as well as the probability of exceedance of the critical displacement referred to scenarios of different saturation conditions.
Each territorial unit corresponding to a landslide mass or to a landslide prone area was classified with a color scale corresponding to the computed probability of exceedance (Fig. 5).
In case of null probability (i.e. no statistic of Newmark displacement computed) the resulting safety factor was attributed to the corresponding territorial unit. As it resulted by the PARSIFAL application to Accumoli area, up to 2% of the entire municipal territory correspond to already unstable or landslideprone areas. Within this area, 59% is represented by firsttime landslides (33% rock mass failures and 26% shallow debris covers) and 41% correspond to reactivation of already existing landslides.
In the dry condition RS (Fig. 6) for the firsttime landslides in rock mass, 17% are unstable and in no case has a probability of exceedance been obtained (0 < P[D ≥ Dca(t),a_{y}] < 1). The first time landslides in shallow debris cover are stable for the 99% while for the 1% a (0 < P[D ≥ Dca(t),a_{y}] < 1) was computed. Among the already existing landslides, the 46% have a (0 < P[D ≥ Dca(t),a_{y}] < 1) and the 8% are unstable (P[D ≥ Dca(t),a_{y}] = 1).
In the middle saturation RS (Fig. 7) for the firsttime landslides in rock mass, 83% are unstable and in no case has a probability of exceedance been obtained (0 < P[D ≥ Dca(t),a_{y}] < 1). Among the firsttime landslides, in shallows debris cover it was obtained a probability of exceedance (0 < P[D ≥ Dca(t),a_{y}] < 1) for the 41%. For already existing landslides, 29% are unstable (P[D ≥ Dca(t),a_{y}] = 1) and it was obtained a probability of exceedance (0 < P[D ≥ Dca(t),a_{y}] < 1) for the 42%.
In the full saturation scenario (Fig. 8), due to the severity of the assumed condition, for all the shallow debris cover susceptible for firsttime landslides was obtained a probability of exceedance (0 < P[D ≥ Dca(t),a_{y}] < 1) while the results for firsttime landslides in rock mass do not change respect to the middle saturation scenario. Among the already existing landslides the 58% became unstable (P[D ≥ Dca(t),a_{y}] = 1) and for the 17% probability of exceedance (0 < P[D ≥ Dca(t),a_{y}] < 1) have been computed.
Discussion
Following the DPC guidelines (ICMS 2008; CTMS, 2017) referred to landslides, the Zones of Attention (ZA_{FR}) are areas in which the available data indicate the presence of conditions predisposing the slope failures but are not sufficient to define if instability can actually occur in case of earthquake; nevertheless, through the consultation of inventories like the online CEDIT catalogue (Martino et al. 2014; Martino 2017—www.ceri.uniroma1.it/index.php/webgis/cedit), it is possible to document the occurrence of landslide events during historical earthquakes.
Susceptibility Zones (ZS_{FR}) are areas to which it is possible to attribute a nonnull earthquakeinduced hazard through simplified methods of analysis.
Respect Zones (ZR_{FR}) are areas in which it is possible to quantify the caused danger as well as the involvement area due to an earthquakeinduced landslide through quantitative approaches. The identification of the ZA_{FR} allows to perform, in a subsequent level of analysis, a “screening” operation that output from the mapped areas (including the already unstable and the potentially unstable ones) those that can be activated in case of earthquake by the use of simplified methods (pseudostatic). The landslide areas that should be unstable in case of earthquake occurrence will constitute the ZS_{FR} dataset and its related mapping. Only more specific and indepth studies will lead to verify the specific stability conditions of the landslide slopes leading to obtain the ZR_{FR} map starting from the ZS_{FR} one. The application of the PARSIFAL methodology has made it possible to define scenario maps referring to different conditions of debris and cover saturation. In particular, the methodology aimed at deducting the ZS_{FR} map starting from the scenario maps initially obtained; they were taken into account all the results obtained in the three considered scenarios reconstructed by PARSIFAL, therefore as the saturation conditions range from zero to the admissible maximum, the final susceptibility was attributed through a weighting of the probabilities of exceedance resulting from all the reconstructed scenarios.
If in two of the three considered saturation scenarios the probability of exceedance was nonzero, i.e. a statistic for the earthquakeinduced displacements was calculated, the ZA_{FR} was converted into a ZS_{FR} and deemed to be subject to further indepth studies for evaluating its admissibility as ZR_{FR}, i.e. for the purposes of planning engineering designs to be provided.
Expansion areas (AE) with a buffer of 10 meters around the landslide, following the DPC recommendations (ICMS 2008; CTMS 2017), have been defined only for already existing landslides, since in case of firsttime debris slides the distribution of activity (sensu Varnes 1978) cannot be attributed.
In case of rock slides and toppling, the AE were defined according to the shadow cone approach (as suggested by ICMS 2008), involving the maximum distances that could be reached by the detached blocks (runout distance). The shadow cones were defined through the Conefall 1.0 software (http://conefall.software.informer.com/1.0/), which allows to obtain an estimate approximate area of invasion of an unstable block from the local DEM.
Through the performed application PARSIFAL proved to be a valid methodological approach to make the screening between ZA_{FR} and ZS_{FR} (Fig. 9), as required by the DPC guidelines with the advantages:

(1)
to return the set of ZF (already existing landslides) and ZPF (first generation landslides) that can be integrated with the landslide bodies already registered by the IFFI (Inventario dei Fenomeni Franosi in Italia—http://www.isprambiente.gov.it/it/progetti/suoloeterritorio1/iffiinventariodeifenomenifranosiinitalia) and PAI (Piani di Assetto Idrogeologico  http://www.regione.lazio.it/prl_ambiente) catalogues, assigning to each identified unstable area a level of hazard, expressed as the exceedance probability with respect to a trigger threshold;

(2)
to allow a screening between ZA_{FR} and ZS_{FR} based on a comparison among different analyses of scenario that take into account the variability of environmental predisposing conditions (specifically the saturation of land and cover);

(3)
to be flexible with respect to specific needs related to the SM activities envisaged by ODPCM 24/2017, including in particular: i) the preservation of areas already bound by the PAI among the ZS_{FR} (regardless of scenario solutions); ii) the preselection of ZPF areas by means of dimensional threshold spatial filters, commensurate with the size of the building and therefore of the vulnerable elements; iii) the epistemic error due to the variability/uncertainness of mechanical parameters values that could be taken into account through parametric analyses.
More in detail, for the Municipality of Accumoli, based on the results obtained by the screening performed through PARSIFAL, the ZS_{FR} result as subdataset of the ZA_{FR} (Fig. 10). By comparing the total area of the ZA_{FR} with the total area of the ZS_{FR}, it results that the PARSIFAL screening led to a reduction of 22% of the mapped ZA_{FR}.
A limitation to the current application of PARSIFAL consists in not being able to take into account the anthropic slopes coinciding with the cuts of the road trenches both for lack of resolution of the DEM used, and for the necessity to verify the suitability of the slopetoroad system to different types of failure mechanisms: (1) shallow sliding of debris covers existing upstream of the cut; (2) sliding of the road plan; (3) sliding of the slope immediately downstream of the roadway.
Conclusions
Quantitative studies of SM provided by ODPCM 24/2017 have made it possible to test the PARSIFAL approach for the screening of landslide areas (ZA_{FR}), including the already existing landslides and potentially unstable areas, for the purpose of selecting zones susceptible to landslide (ZS_{FR}), according to the provisions of the DPC guidelines for SM studies.
To this aim, PARSIFAL has been used to compare different scenarios of earthquakeinduced landslides in the Municipality of Accumoli, thus being able to weigh the predisposition to the mobilization of already unstable or prone to failure slopes (the latter both in soil and in rock) in relation to predisposing environmental conditions, in particular the saturation from zero to the maximum admissible.
Therefore, all the ZA_{FR} were considered susceptible to landslide (ZS_{FR}) if in two of the three scenarios reconstructed by the saturation conditions it was possible to calculate a nonzero probability of exceedance with respect to the assumed threshold of coseismic displacement considered for the failure. The ZS_{FR} also included all the areas already subject to hydrogeological restrictions by the current technical legislation (PAI).
In perspective, PARSIFAL can be considered a valid screening tool between ZA_{FR} and ZS_{FR} with the advantage of being able to be applied to extended territorial scales and with the possibility to be implemented in the future for taking into account anthropic cuts, i.e. in correspondence with the road trenches where, based on the evidence gathered during the seismic crisis of 2016–2017 in the Central Apennines, the highest percentage of landslide rock falls occurred.
Change history
25 April 2019
During some overlook in typesetting process lowresolution files for Fig. 1 was used. The correct Fig. 1 is given.
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Aknowledgements
The Authors wish to thank the Municipality of Accumoli for the logistic support to the field activities; G. Di Martino, L. Falconi, C. Missori and C. Puglisi for collecting part of the field data; C. Di Marzio and D. Marchetti for providing the public technical data and reports on seismic microzonation studies that they performed at Accumoli. S. Martino coordinated the research activities; M. Della Seta and F. Troiani performed the geomorphological surveys; F. D’Alessandro performed the engineeringgeological surveys and the slope stability analyses; S. Battaglia, C. Esposito, G. Martini and F. Pallone managed the PARSIFAL procedure, computation and mapping. The study was carried out in the framework of the Ordinance of the Presidency of the Council of Ministers No. 24 of 12/05/2017 (ODPCM 24/2017) managed by the Centro per la Microzonazione Sismica e le sue Applicazioni of the Italian National Research Council (CNR). The authors recall with esteem and affection the late colleague Roberto Romeo with whom this study began.
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Martino, S., Battaglia, S., D’Alessandro, F. et al. Earthquakeinduced landslide scenarios for seismic microzonation: application to the Accumoli area (Rieti, Italy). Bull Earthquake Eng 18, 5655–5673 (2020). https://doi.org/10.1007/s10518019005891
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DOI: https://doi.org/10.1007/s10518019005891
Keywords
 Earthquakeinduced landslides
 Scenario mapping
 Seismic microzonation
 Italy