Seismically-induced landslide susceptibility evaluation: Application of a new procedure to the island of Ischia, Campania Region, Southern Italy

Engineering Geology 114 (2010) 10–25

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Seismically-induced landslide susceptibility evaluation: Application of a new procedure to the island of Ischia, Campania Region, Southern Italy A. Rapolla, V. Paoletti ⁎, M. Secomandi Department of Earth Sciences, University of Naples Federico II, Largo S. Marcellino 10, I-80138 Naples, Italy C.U.G.RI. – Inter-University Centre for Prevision and Prevention of Severe Risks, Viale Kennedy, 5, I-80125 Naples, Italy

a r t i c l e

i n f o

Article history: Received 17 July 2009 Received in revised form 16 March 2010 Accepted 28 March 2010 Available online 1 April 2010 Keywords: Seismic hazard Landslide susceptibility Grade-2 zonation Seismic landslides Ischia island

a b s t r a c t In this paper we present an approach for evaluating landslide susceptibility in seismic areas. It is known that earthquake-induced landslide susceptibility is related to several, often interplaying, factors. Nevertheless, an effective grade-2 zonation should be characterized by a good balance between simplicity, quickness and reliability. The GIS-based procedure we present employs only three factors that we believe are the most significant in this susceptibility assessment: the type of outcropping rocks/soils, the slope angle and the MCS intensity. The local annual precipitation, certainly an essential factor, is considered here as a parameter whose seasonal pattern is constant in time and space. Each of the three parameters is expressed as a Significance percentage and the resulting Seismic Landslide Susceptibility level of an area is given by the average of the significances of the first two factors multiplied by the significance of the third factor. The procedure was set and tested on the volcanic island of Ischia (southern Italy), which was affected by several historical earthquakeinduced landslides. The results of this susceptibility zonation test at Ischia show a very good match between the distribution of the sources of historical landslides and the areas we identified as the most susceptible ones. © 2010 Elsevier B.V. All rights reserved.

1. Introduction It is well known that landslides represent one of the most important and damaging hazards connected to earthquakes. Evaluation of landslide phenomena aimed at land use planning represents a major problem for engineering geology (e.g., Fell et al., 2008). In seismic areas such a problem has to be faced by considering the seismic effects too. In some cases the effect of seismically-induced landslides on human lives and facilities may exceed the damage directly connected to the shaking (Jibson et al., 2000). Therefore, seismic hazard evaluation should not only concern the computation of the seismic transfer function — which provides estimates of local ground motion amplification and seismic signal spectrum modifications — to give a correct input signal to builders, but also the Seismic Landslide Susceptibility (Rapolla, 2004, 2008). Several methods have been developed for the evaluation of the hazard connected to earthquake-triggered landslides. The Technical Committee for Earthquake Geotechnical Engineering of the International Society for Soil Mechanics and Geotechnical Engineering, quoted here as TC4 (1999), has pointed out three different grades for approaching this evaluation: grade-1 refers to regional or nationwide territory (scale 1:1 000 000–1:50 000); grade-2 refers to smaller territory (scale 1:100 000–1:10 000), grade-3, finally, refers to specific sites (scale

⁎ Corresponding author. Tel.: +39 081 679293. E-mail address: [email protected] (V. Paoletti). 0013-7952/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2010.03.006

1:25 000–1:5000). The same threefold-phase approach was suggested for local seismic hazard evaluation in the Campania Region, southern Italy (Cascini et al., 2006). Basic principles for grade-1 were first stated by Keefer (1984) and then developed by other authors (e.g., Keefer and Wilson, 1989; Rodriguez et al., 1999). Keefer (1984) suggested a simple and effective method based on the seismic event Magnitude (or on the epicentral Intensity) and on the distance from the epicenter to evaluate a given region's susceptibility to seismic-induced landslides. Evaluation of grade-3 has also been discussed by many other authors (e.g., the methods proposed by Wilson et al. (1979), Siyahi and Ansal (1993) and Ausilio et al. (2008)) who suggested different mathematical approaches. Because of the complexity of the methods and the number of parameters that need to be accounted for, in some cases this led to the production of computer programs whose correct use requires accurate and complete laboratory and in-situ data acquisition. As regards grade-2, TC4 (1999) describes three methods based on the peak ground acceleration or MCS (Mercalli–Cancani–Sieberg Scale) intensities and on several other parameters. In our paper we will test two of them (see Section 4). Other authors proposed grade-2 methods that use instead Arias Intensities and/or displacements. Luzi and Pergalani (1996) evaluated the vulnerability of landslides in dynamic conditions by using the Newmark method and produced slope stability maps in terms of final displacements in an area of the Marche Region, Italy. Miles and Ho (1999) and Jibson et al. (2000) performed a seismic landslide hazard zonation also employing the Newmark analysis and applied it to different areas of California. Del Gaudio et al. (2003) made

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use of three parameters, i.e. Arias intensity, critical acceleration and Newmark displacement to represent, respectively, the level of seismic shaking, the slope resistance to failure and the conditions for seismic triggering of landslides. Del Gaudio and Wasowski (2004) applied this method to evaluate seismically-induced landslide hazards in Irpinia, southern Italy. Silvestri et al. (2005) carried out a similar approach applying it to an area near the city of Benevento in southern Italy. Miles and Keefer (2007) proposed an elaborate system for a comprehensive areal model of earthquake-triggered landslides. More recently, Saygili and Rathje (2008) presented an empirical model that reduces the uncertainty in the sliding displacement prediction by combining multiple ground motion parameters, i.e., peak ground acceleration, peak ground velocity and Arias Intensity. All these grade-2 methods are not always straightforward and quick to apply as they require the knowledge of several parameters. We believe instead that a grade-2 base evaluation of the landslide

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susceptibility in seismic areas should be simple, quick and — most importantly — inexpensive. In fact, land use planning in these areas needs information about such susceptibility at a sustainable cost; more complete, detailed and expensive studies should be focused only on areas where a detailed zoning is necessary (Fell et al., 2008). In this paper we shall describe the application to the island of Ischia (Fig. 1) in the Campania Region, southern Italy, of two procedures suggested by TC4 (1999) for grade-2. We shall also describe and apply a qualitative zoning on potential seismic landslides that we believe to be simple, cheap and effective. Following the guidelines for land use planning (Fell et al., 2008), the island of Ischia may represent an interesting case of earthquake-triggered landslide susceptibility zoning and a good test field for the method we propose. In fact, the island was affected by several earthquakes and historical landslides on natural slopes including: rock falls from steep cliffs and slopes, debris flows and earth slides from previously failed slopes and

Fig. 1. A) Campanian Plain Volcanic Region. The square shows the area considered in this study; B) Topographic map of the island (data resolution: 40 m), with tectonic lineaments derived from De Vita et al. (2006).

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deep-seated slides. Furthermore, the area is characterized by weathered volcanic soils and rocks and by areas with different slope angles, some very high. All these features make landsliding a potential issue in land use planning at Ischia. 2. Geological, volcanological and geophysical sketch of the island of Ischia The island of Ischia encompasses the westernmost active volcanic field of the Campanian Plain (southern Italy), a K-alkaline volcanic district that includes the Phlegrean Fields, the Procida Island and the Mt. Somma–Vesuvius complex (Fig. 1A). The volcanic activity in this area, which started in the Upper Pleistocene accompanying extensional processes, is controlled by regional strain fields along NE–SWand subordinately NW–SE-trending fractures (Vezzoli, 1988). The island of Ischia is the emerged top of a large volcanic complex located along one of the NE–SW trends. It extends over an area of about 42 km2 and is morphologically dominated by Mt. Epomeo (787 m a.s. l.), an active resurgent block uplifted about 1000 m in the last ∼30 ky (Gillot et al., 1982) (Fig. 1B). The island is composed of volcanic rocks (trachytes, alkali-trachytes, trachybasalts, latite, and phonolites) (e.g., Orsi et al., 1991), marine sediments and landslide deposits. De Vita et al. (2006) showed that the caldera resurgence has to be considered as a main factor for inducing intense slope instability, as it is accompanied by activation of faults and renewal of volcanism that produces over-steepening of slopes and generates seismicity that can trigger landslides. As regards the Mt. Epomeo resurgent block, there are different hypotheses regarding its uplift (see Orsi et al., 1991, Luongo et al., 1995; Tibaldi and Vezzoli, 1998; Acocella and Funiciello, 1999; Molin et al., 2003). Several authors (e.g., Carlino et al., 2006; Nunziata and Rapolla, 1987) proposed that the uplift, the volcanic and seismic activity and the ground deformation of the past 2000 years are connected to the existence of a laccolith. More specifically, this body, with a hypothesized diameter of about 10 km and a depth of up to 1 km in the centre of the island, is thought to have triggered the caldera resurgence after the Mount Epomeo Green Tuff eruption (55 ky ago) and to have controlled the dynamic of the island afterwards. A recent study of the structural setting of the island of Ischia using high-resolution aeromagnetic and self-potential data (Paoletti et al., 2009) confirmed the presence of an igneous structure located at about 1.2–1.7 km b.s.l. This intrusion, or group of intrusions, is characterized by de-magnetization processes connected to the high geothermal gradient measured in the central-western portion of the island. The oldest known products (130–150 ky) crop out along the coast. At about 55 ky a major sub-aerial eruption occurred, emplacing the Green Tuff (e.g., Gillot et al., 1982; Vezzoli, 1988), a welded tuff forming the structure of Mt. Epomeo and often characterized by superficial degradation due to exfoliation, weathering and thermochemical alteration processes. The Green Tuff eruption formed a caldera depression and led to the emplacement of the marine terrigenous formations in the central part of the island (Barra et al., 1992). Between 44 and 33 ky the Citara Tuff was deposited in the western coast of the island. Within the last 33 ky moderate volcanism occurred mainly in the eastern sector of the island (e.g., Orsi et al., 1992; Bortoluzzi et al., 1995). The most recent activity concentrated in the last 2.9 ky and was accompanied by subsidence. The last eruption (Arso lava flow) occurred in 1301 AD and was preceded and followed by the emplacement of landslides and mudflows in the northwestern and western sector of the island. The most important phenomena recorded presently on the island are low intensity shallow seismicity, bradyseism activity and intense hydrothermalism. All of these phenomena described in the island, which are connected to the interplay of tectonism, volcanism, erosion and

sedimentation in the past and recent history of Ischia, led to a rough morphology and intense slope instability. 3. Seismic activity and historical seismically-induced landslides at Ischia Almost all the strong historical earthquakes have occurred in the northern and northwestern sector of the island and for most of them the epicentral area was located in the village of Casamicciola (Alessio et al., 1996; Boschi et al., 1997). Fig. 2 shows the location of the epicenters of all known historical earthquakes of the island. Table 1 reports date, location and intensity of only the main and/or most recent earthquakes of the island. In Fig. 2B–C we show the MCS intensities of the 1828 and 1881 earthquakes. The pattern of the macroseismic fields for the two earthquakes is similar but with higher intensities for the 1881 earthquake. The damaged areas of these two earthquakes and of the 1796 one are shown in Fig. 2D. The latest and strongest earthquake occurred in 1883 (I0 = X MCS) in the same area and caused major damage triggering several landslides in the northwestern sector of the island. As we can see, the pattern of the macroseismic field of the 1883 earthquake (Fig. 3A) resembles those of the 1828 and 1881 earthquakes, with MCS intensities ranging from VI to X. The 1883 earthquake has been related to the resurgence activity and, more specifically, to the E–W striking faults that affect the northernmost area of Ischia (Cubellis, 1985; Alessio et al., 1996). The restricted area of the maximum damage suggests a shallow source for the earthquakes recorded on Ischia (Chiodini et al., 2004). Indeed, Luongo et al. (1987) calculated the hypocentre of the 1883 earthquake to be at a depth of about 3 km b.s.l. At present, low intensity seismic activity characterizes the western slope of Mt. Epomeo (http://www.ov.ingv.it/ufmonitoraggio/italiano/index.htm). As far as the seismic hazard of the island is concerned, the most recent Italian seismic laws (“Nuove Norme Tecniche per le Costruzioni”, Ministry Decree of 14/01/2008, www.cslp.it) have incorporated national scale estimates (http://zonesismiche.mi.ingv.it/; Gruppo di Lavoro “Mappa della Pericolosità Sismica”, 2004) that attribute to Ischia values of the peak ground acceleration on bedrock ranging from 0.13 g to 0.16 g for a return period T of 475 years (Fig. 3B). These values should then be modulated on the basis of the site outcropping Ground Types, as reported in Ordinanza del Presidente del Consiglio dei Ministri (2003) — quoted here as OPCM (2003) — and Eurocode 8 (British Standards Institution, 2004). All the acceleration values shown in Fig. 3B correspond to MCS intensities around VIII, as it can be inferred from the relation between intensity and peak ground acceleration (PGA) by Medvedev and Sponheuer (1969). They show a pattern of regular decrease from ENE to WSW, as the distance from the Apennine seismogenic zone increases, suggesting that Ischia seismicity was not adequately taken into account by the Italian regulations (Cascini et al., 2005). This appears to be in contrast with the well known, above mentioned, information reporting several strong historical earthquakes having epicenter in the Casamicciola area. Moreover, no landslides caused by Apennine earthquakes have been reported at Ischia. Therefore, for our purposes we cannot have confidence in the local seismic pattern given at Ischia by the above quoted Ministry Decree of 14/01/2008. However, to estimate the Seismic Landslide Susceptibility at Ischia we need to know the expected actual local seismic activity. In doing so, we did not take into account the Italian seismic regulations but instead derived the PGA values directly from the MCS intensities of the major Casamicciola 1883 earthquake (Fig. 3A). Historical data (Luongo et al., 1987; Alessio et al., 1996; Luongo et al., 2006) show in fact that all significant seismicity was from that same seismogenic area. Therefore the 1883 earthquake may be considered the representative earthquake for the island seismic scenario as far as the MCS field distribution and the maximum intensity are concerned. Moreover this earthquake is characterized by the most complete and

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Fig. 2. A) Map of the epicenters of historical earthquakes in the island of Ischia (modified from Alessio et al., 1996); B) Macroseismic field map of the 1828 Casamicciola earthquake (I0 = VIII–IX MCS) (modified from Alessio et al., 1996); C) Macroseismic field map of the 1881 Casamicciola earthquake (I0 = IX MCS) (modified from Alessio et al., 1996); D) Damaged areas connected to the 1796, 1828 and 1881 Casamicciola earthquakes (modified from Luongo et al., 1987).

reliable information on the surface effects. To derive the acceleration values from the intensities we used the well known empirical relation reported by Medvedev and Sponheuer (1969) (see Table 2 and Fig. 3A). We found this relation to give more reliable estimates with respect to other relations such as those quoted in Murphy and O'Brien (1977). More specifically, it gives estimates of PGA/MCS intensities in

Table 1 Main historical earthquakes at Ischia (modified from De Vita et al., 2006). Year

Epicenter location

Epicentral MCS intensity

1883 1881 1867 1863 1841 1828 1796 1767 1762 1557 1302 1228 II–III Cent. A.D. IV Cent. B.C.

Casamicciola Casamicciola Casamicciola Casamicciola Casamicciola Casamicciola Casamicciola Northern Sector Casamicciola Campagnano Arso Casamicciola Western Sector Western Sector

X IX VI VII VII VIII–IX VII VII–VIII VII VII–VIII VIII IX–X – –

a better correspondence with PGA and MCS intensity having the same return time, derived by hazard estimates carried out in Italy. With regard to earthquake-induced landslides, evidence all over Ischia shows that the island's morphological evolution was and still is controlled by several slope instability phenomena of different types. Morphological surveys, historical reports and archeological evidences demonstrate that most of the landslide phenomena occurring on the island are related to major historical earthquakes (Mele and Del Prete, 1998). Table 3 reports the inventory of the earthquake-induced landslides whose location was detected with good confidence (see Fig. 4). Occurrence of earthquake-induced landslide has been inferred by Guadagno and Mele (1995) and Mele and Del Prete (1998) on the basis of geo-morphological and archeological studies and from historical chronicles. When such information was not considered reliable, the authors omitted the year of occurrence. Mele and Del Prete (1998) state that contrary to other types of landslides, rock falls were triggered basically by all the earthquakes of the island. The Mt. Epomeo Green Tuff formation, outcropping in the highest cliffs of the mountain, is indeed fractured by many discontinuities influencing the morphologic evolution of the slopes (Figs. 1B and 4). More specifically, the tuff cliffs are characterized by an intense shallow degradation related to processes of exfoliation, case-hardening, aeolian weathering and thermo-chemical alteration due to the fumarolic activity along fractures. Furthermore, the slopes are affected by a high degree of fracturing oriented along the main tectonic

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Fig. 3. A) MCS Intensities of the 1883 Casamicciola earthquake (from Luongo et al., 2006). The numbers referred to each isoseismical curve represent the PGA values computed using the Medvedev and Sponheuer (1969) relation; B) Peak horizontal acceleration on bedrock (with Vs30 N 800 m/s) at Ischia given by the Italian seismic regulations for Tr = 475 years (“Nuove Norme Tecniche per le Costruzioni”, Ministry Decree of 14/01/2008, www.cslp.it).

lineaments of the island. This results in large discontinuities — with separations varying from centimeters to meters — that divide the Green Tuff into blocks whose volume can range from tens of centimeters to several cubic meters. These blocks tend to fall through mechanisms of sliding and toppling (Mele and Del Prete, 1998). Other seismically-induced landslides of the island involved the debris deposits covering the formation of the Green Tuff in the

Table 2 Relation between MCS intensities and peak horizontal acceleration values (Medvedev and Sponheuer (1969). Macroseismic intensity (MCS)

Peak horizontal acceleration (g)

V VI VII VIII IX X XI XII

0.0125 0.025 0.05 0.1 0.2 0.4 0.8 1.6

piedmont areas: some are debris slides, whereas others are deepseated slumps evolving in debris flows (Guadagno and Mele, 1995; Mele and Del Prete, 1998) (Table 3 and Fig. 4). Table 3 Inventory of the Ischia earthquake-induced landslides (Guadagno and Mele, 1995; Mele and Del Prete, 1998). The location is referred to the source scarps (see Fig. 4). Landslide number

Landslide type

Year

1 2 3–4–5–6 7–8–9–10 11 12 13 14 15 16 17 18 19 20 21

Debris flow – Debris flow – Rock falls 1828, 1863, 1881, 1883 Rock falls 1828, 1863, 1881, 1883 Debris slide 1883 Debris slide – Debris slide – Debris slide – Debris slide – Debris slide 1883 Debris slide 1883 Debris slide II–III Cent. A.D. Debris slide II–III Cent. A.D. Deep-seated slump IV Cent. B.C. Deep-seated slump IV Cent. B.C.

Average Epicentral slope distance (km) 30° 30° 42° 43° 20° 30° 30° 22° 24° 28° 29° 30° 30° 30° 25°

1.68 1.47 1.73 1.33 1.98 1.64 1.26 1.57 1.91 1.14 0.95 2.72 3.05 1.65 1.72

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Fig. 4. Geo-lithologic sketch map of the Ischia Island (Vezzoli, 1988; ABNOC, 2002) with overlapped the source scarps and deposition areas of the historical earthquake-triggered landslides (Guadagno and Mele, 1995; Mele and Del Prete, 1998). Landslide numbers refer to Table 3.

4. The methods suggested by TC4 for grade-2 susceptibility evaluation The Technical Committee for “Earthquake Geotechnical Engineering — TC4” published a manual for zonation of seismic geotechnical hazards (TC4, 1999). The manual describes accepted approaches for assessing three kinds of geotechnical phenomena: local ground response, slope instability and liquefaction. As regards grade-2 methods for the evaluation of slope instability, TC4 (1999) discusses three methods proposed by the Kanagawa Prefecture, Mora and Vahrson and the Japan Road Association which differ in the context and for the factors which are considered significant in predicting susceptibility to slope failure. As the third method proposed by the manual applies only to cut slopes along roads, in this paper we dealt only with the first two methods and applied them to the earthquakeinduced landslide susceptibility evaluation at Ischia. We slightly modified the two methods to exploit the detail of our data (40 m by 40 m mesh). However, in some cases we had to use the larger mesh employed in the original procedures (500 m by 500 m mesh for the first method and 1 km by 1 km mesh for the second one) as some of the weights given by the procedures seem to be strictly related to the geometrical features of the mesh. 4.1. The application of a modified Kanagawa Prefecture procedure The method proposed by the Kanagawa Prefecture (TC4, 1999) is based on the studies on slope failures carried out during three large earthquakes that occurred in Japan in the 1970s and '80s. Slope failure susceptibility zones are plotted on a 500 m by 500 m mesh area on maps with a 1:50 000 or 1:25 000 scale. The method identifies seven main factors as governing slope instability and assigns a given

weighting to each of them. The factors considered significant within a mesh square are: 1) the maximum ground acceleration; 2) the length of a contour line at the average elevation; 3) the maximum difference in elevation; 4) the hardness of rock in a typical slope; 5) the total length of faults; 6) the total length of artificial cuts or filled slopes; 7) the topography of typical slopes in a mesh as classified into four morphological categories. The “susceptibility to slope failure in each mesh” is obtained — in terms of number of slope failures that are likely to occur — by the summation of the seven weighted factors. The application of the Kanagawa procedure to the island of Ischia was unsatisfactory for us because of the large dimension of the cells with respect to the relatively small investigated area (Fig. 5A). Given this and the complex morphological, geo-lithological and structural setting of the island, we did not use for all the above mentioned factors a 500 m by 500 m mesh. Instead, in an attempt to improve the result by exploiting the detail of our topographic and geo-lithological data, we employed when possible a 40 m by 40 m mesh. More specifically, factors # 1 and 4 were evaluated on a 40 m by 40 m mesh. Factors # 2, 3, 5 and 7 had instead to be evaluated on a 500 m by 500 m mesh, being the weights used by the Kanagawa Procedure strictly linked to the dimensions of the cell adopted. Thus, in the summation of the different weights we assigned for factors # 2, 3, 5 and 7 the same weight to all the 40 m by 40 m cells included within a 500 m by 500 m cell. As regards factor # 6, it was here considered zero as the island is characterized by a negligible number of artificial cuts or filled slopes. It should be noted that the peak ground acceleration (factor # 1) was determined by using the empirical relation reported in Table 2. The earthquake-triggered landslide susceptibility map of Ischia obtained by employing this modified Kanagawa Procedure (Fig. 5B) led to a more significant result with respect to the map obtainable

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Fig. 5. Earthquake-triggered landslide susceptibility map of Ischia obtained applying: A) the Kanagawa Prefecture Procedure (TC4, 1999); B) a modified version of the procedure. The landslide source scarps are overlapped.

with the strict Kanagawa Procedure. All the susceptible areas are placed very close to the epicenter of the 1883 earthquake and resemble closely the shape of the IX and X isoseismal curves (see Fig. 3A), while for the areas where MCS intensities are lower than IX (most of the island) the susceptibility is zero. This is clearly due to the high weighting of factor # 1 with respect to the other factors. It is indeed extremely high for ground accelerations higher than 0.2 g (corresponding to MCS Intensities higher than IX) and, on the contrary, it equals zero for ground accelerations lower than 0.2 g (MCS corresponding to Intensities lower than IX). This result appears in contrast with what we observe for Ischia, i.e. some of the landslides occurred in areas of intensity VIII, corresponding to 0.1 g. From the analysis of the susceptibility maps obtained by the strict Kanagawa Procedure (Fig. 5A) or its modified version (Fig. 5B), we see that the most susceptible areas include only some of the historical seismic-induced landslides, while some failures occur in areas of low or zero susceptibility. We can conclude that probably due to the attribution of zero weighting to accelerations lower than 0.2 g, the Kanagawa Procedure does not allow a good zonation at Ischia. It is worthwhile to note that this weighting attribution is also not in agreement with the studies on historical landslides by Keefer (1984) and Rodriguez et al. (1999) who found that in most cases the

minimum MCS intensity that can trigger landslides has a value of about V–VII, depending on the type of slide. Finally, a further fundamental inconsistency in this procedure appears to us. It may give values, expressed as number of seismicsusceptible slopes per unit area, different from zero even in the case of non-seismic areas. For example this can occur in an area characterized by medium or high values of factors # 2–7 but having no horizontal ground accelerations, i.e., having factor # 1 equal zero. In this case the area will be characterized by medium-high values of the seismic susceptibility to slope failure (up to a number of failure susceptible slopes greater than 9 for each 500 m by 500 m cell), even though it is an aseismic area.

4.2. The application of a modified Mora and Vahrson procedure The method proposed by Mora and Vahrson (TC4, 1999) is based on studies of slope failures in historical earthquakes and of those induced by heavy rainfall in Central America. The method considers three factors — relative relief, lithologic conditions and soil moisture — as factors influencing the susceptibility. In addition, two factors — seismicity (in terms of MCS intensities) and rainfall intensity — are incorporated as triggering factors. The susceptibility, defined as “a degree of slope failure

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hazard” (Classes from I to VI), is obtained by combining these five factors. Similarly to the Kanagawa Procedure, we employed a 40 m by 40 m mesh, as the 1 km by 1 km mesh quoted by the method would have resulted in a too coarse estimation of the hazard within the morphologically and geologically complex volcanic island of Ischia (Fig. 6A). We evaluated again all factors on a 40 m by 40 m mesh except for factor # 1 (Relative Relief) that, being strictly related to the dimensions of the cell adopted by the method, was calculated on a 1 km by 1 km mesh. Furthermore, we used the same criteria of combining meshes of different sizes, as done for the above described modified Kanagawa Procedure. The earthquake-triggered landslide susceptibility map of Ischia so obtained (Fig. 6B) allows some improvement with respect to the map obtained by the strict Mora and Vahrson Procedure as it outlines with better detail the pattern of the most susceptible area. However as we can see from both maps (Fig. 6A and B), the island susceptibility turns out to be almost entirely moderate (Class III), with only an extremely small area of medium susceptibility (Class IV). This is probably caused by the fact that the method was developed by Mora and Vahrson (TC4, 1999) for regions of heavy rainfall, while Ischia's rainfall is near the average for temperate areas (about 1000 mm/y). According to TC4 (1999), the

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authors discuss this issue stating that in dry regions (no or very low rainfall), the resulting susceptibility may reach a maximum of only Class III or IV, which conforms to the result for Ischia. Moreover, even considering Class IV as the highest susceptibility class, the pattern of the slope instability susceptible areas at Ischia doesn't match the actual spatial distribution of seismically-induced landslides. Therefore, we conclude that the method doesn't allow an acceptable zonation in areas such as Ischia. 5. Description and application of a new procedure A crucial factor for developing a simple and significant procedure to evaluate the Seismic Landslide Susceptibility of an area is to single out few and proper parameters. We believe that there are basically two non time-dependant parameters that need to be considered for a proper result, i.e., the type of outcropping rocks and/or soils and the slope. These are Predisposing Factors, influencing the potential of a slope to fail. The obvious time-dependant parameter is the triggering force, i.e., the seismic action. Moreover, there is another main parameter, the quantity and the type of rain, which at Ischia is seasonally-dependant. A relatively small area, approximately a hundred km2, is generally considered for a grade-

Fig. 6. Earthquake-triggered landslide susceptibility map of Ischia obtained applying: A) the Mora and Vahrson Procedure (TC4, 1999); B) a modified version of the procedure. The landslide source scarps are overlapped.

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2 study. Thus the annual climatic conditions may be considered in general the same within all the studied territory and therefore this parameter is in many cases a constant in space. Furthermore, assuming the constancy of the average annual climatic conditions of an area, this parameter may also be considered constant in time. As a matter of fact, possible strong monthly variations in the quantity and type of precipitations make the influence of rain an important triggering factor as well. Nevertheless, we here take into account only the triggering effect of earthquakes considering, as said, the precipitation a constant in the susceptibility evaluation of a relatively small area. Obviously, in a given climate such susceptibility may be seasonally dependent. For example, in the test area of Ischia century-old average data show that the quantity of rain reaches the highest values in the cold season (rain peaks are in December and March–April). Thus we assume that the only necessary parameters to be considered for a grade-2 qualitative evaluation of the landslide susceptibility in seismic areas are: A) the Type of Outcropping Rocks/ Soils, B) the Slope Angle and C) the seismic action in terms of MCS intensities. We quantify these three factors in terms of relative weight expressed as Significance percentage. We assume that the first two parameters contribute to failure susceptibly in the same manner and therefore we consider their average as Predisposing Factor. The seismic action is instead regarded as the parameter that modulates the Predisposing Factor. The seismicallyinduced landslide susceptibility level of an area will be given by: Slð%Þ =

ðSA + SB Þ ⋅SC 2

where Sl is the Susceptibility Level (in %), SA and SB are the Significances of the Predisposing Factors (Parameters A and B, respectively) and SC is the Significance of the seismic action (Parameter C). Consequently, for areas that are basically aseismic (MCS lower than V, see below), the seismically-induced landslide susceptibility level Sl equals zero. 5.1. Parameter A: Geo-lithological, geophysical and geotechnical characteristics As far as the technical characteristics and the response to a mechanical action are concerned, rocks and soils can be classified in different ways, according to various qualitative/quantitative criteria. Following the same rock and soil classifications reported by OPCM

(2003) and Eurocode 8, based on three different criteria (geolithological, geotechnical and geophysical) it is possible to classify the outcropping soils and rocks on the basis of what is reported in Table 4. The parameter we believe to be the most suitable for evaluating the response of materials to the seismic action is their shear modulus and hence their transversal wave velocity (Vs). If these data are not available, a classification of the materials is still feasible on the basis of their geo-lithological and/or geotechnical characteristics. We assume an inverse proportionality between the shear wave velocities (Vs) and the Lithology Significance (percentage) for the susceptibility assessment. This proportionality is shown in Fig. 7A: the significance — in terms of landslide susceptibility — of the parameter related to Geo-Lithological, Geophysical and Geotechnical Characteristics is assumed to be zero for rocks having Vs higher than 1.5 km/s, i.e., hard, non-fractured rocks. On the other side, the significance reaches a value of 100% for deposits having Vs lower than 0.18 km/s, i.e., cohesionless soils or pseudo-coherent (clayey) materials with high natural humidity. In Fig. 4 we show the geo-lithological sketch map of the island of Ischia derived from the geological map of Vezzoli (1988) and the geolithological map of Autorità di Bacino Nord Occidentale della Campania quoted here as ABNOC (2002), both maps having a scale 1:10000. The soils and rocks at Ischia were classified on the basis of their geotechnical, geological and geoseismic features following the criteria of Table 4. The geotechnical and seismic studies were performed by the Municipalities of Ischia and in several cases provided us with direct information about the average shear wave velocities of the lithological units of the island. This allowed us to set the correlation reported in the last column of Table 4 and compute the map of the Lithology Significance of the island (Fig. 7B). The highest Significance values for this parameter are mainly located in the eastern side of the island, which is mostly characterized by loose and in some cases re-worked pyroclastics deposited on gentle slopes (Figs. 1B and 4). As mentioned above, the main lithological units involved in the seismic-induced instability phenomena of the island are: a) the Green Tuff formation in correspondence with the Mt. Epomeo cliffs and b) the debris deposits that cover the tuff in the piedmont areas. As regards the tuff formation, laboratory analyses (Guadagno and Mele, 1995) classify it as “soft rock”; it is affected by exfoliation, weathering and thermo-chemical alteration processes. The slopes of Mt. Epomeo are covered by debris deposits and soils deriving from the degradation of the Green Tuff. Granulometry

Table 4 Outcropping rock and soil classification based on geo-lithological, geotechnical and geophysical features. In the last column we report the attribution of the geo-lithological units of Ischia to the different ground types. Geo-lithological characteristics: – Ground type (OPCM, 2003; British Standards Institution, 2004) – Internal disruption level – Natural humidity

Geotechnical parameters

Geophysical parameter

NSPT

Vs (km/s) (average value)

Cu (kPa)

Ground type A 1) Coherent, non-fractured materials 2) Coherent, slightly fractured materials

N1.5 1.5–0.8 (1.15)

Ground type B Coherent, strongly fractured materials; deposits of stiff soil

N 50

N 250

0.8–0.36 (0.58)

Ground type C Deposits of dense or medium-dense soil

15–50

250–70

0.36–0.18 (0.27)

Ground type D Deposits of cohesionless soil

b 15

b 70

b0.18

N 50 b 50

N0.18 b0.18

Pseudo-coherent (clayey) materials 1) Low natural humidity 2) High natural humidity

Geo-lithological units at Ischia (see Fig. 4)

Lavas Siltstones Pumice breccias and welded scoriae Tuff (welded facies, disrupted at the surface) Debris deposits Tuff (unwelded facies) Pyroclastic deposits Re-worked pyroclastic deposits Sand and filling materials

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Fig. 7. A) Correlation between Vs (km/s) and its Significance (%) in the susceptibility evaluation on earthquake-induced landslides. B) Map of the Lithology Significance at Ischia. For reference see Table 4 and Fig. 4.

analyses show that these deposits are mainly sandy, while the clay fraction is negligible or absent. Drained shear tests show a friction angle of 32° (Guadagno and Mele, 1995). 5.2. Parameter B: Slope angle The minimum slope inclinations reported in literature for seismicinduced landslides are as gentle as about 2° for rapid soil flows and increase for other types of slides (Keefer, 1984; Rodriguez et al., 1999). In areas characterized by medium-to-low shear wave velocity values — i.e., Ground Types C and D and clayey materials (Table 4) — seismic-induced landslides occur typically on 5°–20° slopes. Following Wasowski et al. (2002), a 20°–25° slope range seems to provide a separation between soil and rock failures the latter generally occurring on slopes steeper than 35°. As regards failures on rocky slopes, Keefer (1984) reports a threshold of 15° for coherent slides (e.g. rock block slides) and a limit of 35° for disrupted slides (e.g. rock slides). We can conclude that, depending if we are dealing with loose/clayey materials or coherent/ rocky materials, the slope significance in terms of instability is characterized by two different laws, with different angle intervals of failure occurring. Thus, in our procedure we set the correlation between slope angle (degrees) and its significance (percentage) to have different lower and upper limits for loose soils and rocks (Fig. 8A). That is, the significance for soil slopes with loose/clayey materials is directly proportional to the incline from 0° to 25° and saturates over 25°, while the significance for rock slopes is set to be zero for angles lower than 15° and is proportional to the incline angle up to 40°, over which this significance is kept constant to the maximum. This last limit was chosen on the basis of above mentioned studies but also following the Mora and Vahrson Procedure, which assigns a maximum weight to slope angles exceeding about 43°. In Fig. 8B we show the map of the Slope Significance at Ischia obtained by the application of these two distinct relations for slopes characterized by loose/clayey soils and coherent/rocky materials.

5.3. Parameter C: MCS intensity With regard to the seismic action, we note that when used as single shaking parameter PGA does not appear optimal to estimate destabilizing effects on slopes (Saygili and Rathje, 2008). Therefore, we here employ a seismic factor based on MCS Intensities and we assume linearity between MCS values and Intensity Significance (see Fig. 9A). Studies carried out on historical seismic-induced landslides by Keefer (1984) and Rodriguez et al. (1999) show that in almost all cases the lower boundary for activation of landslides is MCS V (corresponding to a peak ground acceleration of 0.0125 g) (see Table 2). We therefore set this value as the lower limit of the linear correlation between intensity and this parameter significance. As far as the upper limit of this correlation is concerned, studies on the global scenarios of Keefer (1984) and Rodriguez et al. (1999) show that for most earthquakes this boundary is MCS VIII and in a few cases it is as big as IX (corresponding to a peak ground acceleration of 0.2 g). This implies that the critical seismic actions for slope failures are unlikely to be larger than 0.2 g. On the other side, there may be slopes with a critical seismic action larger than 0.2 g, and for them the occurrence of an earthquake of intensity higher than IX has not the same effect as intensities that equal IX (Anonymous Reviewer's personnel communication). In addition to this remark we note that the highest intensity observed in the study area is as big as X. Therefore we set an intensity of X as upper boundary of the proportionality between seismic action and parameter significance, assuming a constant significance for higher intensities (Fig. 9A). In Fig. 9B we show the map of the MCS Intensity Significance at Ischia obtained by using the isoseismal curves of the 1883 Casamicciola earthquake (Fig. 3A). It should be noted that we used intensity values gradually varying between each isoseismal boundary. As mentioned in Section 3, we consider the seismic action derived from the 1883 earthquake more significant for the local seismic hazard evaluation than that derived from the Italian law. Nevertheless, we

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Fig. 8. A) Correlations, for soil and rock slopes, between slope angle (°) and its significance (%) in the susceptibility evaluation on earthquake-induced landslides. B) Map of the Slope Significance at Ischia derived from the application of these two distinct relations. Dashed areas are those characterized by coherent/rocky materials.

Fig. 9. A) Correlation between MCS intensities and their significance (%) in the susceptibility evaluation on earthquake-induced landslides. B) Map of the MCS intensity Significance at Ischia derived from the isoseismal curves of Fig. 3A. Dashed areas are those characterized by coherent/rocky materials.

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Fig. 10. Map of the Significance related to the Predisposing Factors (lithology and slope). The source scarps and deposition areas of the historical earthquake-triggered landslides are also shown.

computed the Intensity Significance by using the seismic action derived from the Italian law as well. This was done in order to compare the susceptibility evaluations obtained from the two seismic inputs (see Section 6). 5.4. Seismic-induced landslide susceptibility evaluation As mentioned, the seismic-induced landslide susceptibility of an area will be given by the average of the significances of the two Predisposing Factors (Parameters A and B) multiplied by the significance of the triggering factor (Parameter C). As regards the predisposing factors, we show in Fig. 10 the map obtained by accounting only for the lithology and slope angle. We note here that the Predisposing Factor influence on the susceptibility is strongly dependent on the climatic context of an area. We stress that the map of Fig. 10 has not a quantitative meaning but a qualitative one as it provides a degree of susceptibility in relative terms (Fell et al., 2008). A quantitative significance related to the Predisposing Factors, expressed for example as number of possible landslides per unit area in a given season, should in fact be related to the seasonal climatic variations. The presence of rainwater may in fact change the soil mechanical properties and hydrogeological conditions, decreasing the slope strength. At Ischia, as practically everywhere, the Vs characteristics of a soil depend on the above mentioned variation of the rainfall and thus the quantitative significance of the map will assume different values if referred to rainy or dry periods within the year. As the Vs data have been collected by the Municipalities of the island in different periods of the year an average annual significance can attributed to the Predisposing Factor map of Fig. 10. The potential triggering effect of earthquakes was thus accounted for by multiplying the significance percentage related to the

Predisposing Factors of Fig. 10 by the Acceleration Significance. The Seismic Landslide Susceptibility map of Ischia so obtained is shown in Fig. 11. This map has the same qualitative character of the map of Fig. 10. As we can see, the pattern of the most dangerous areas matches rather well the distribution of the sources of historical landslides. Furthermore, in agreement with the JTC-1 Joint Technical Committee on Landslides and Engineered Slopes (Fell et al., 2008), we note that all the landslides occur in areas characterized by the higher susceptibility classes while the spatial area for these classes is kept to a minimum (see Section 6). This derives from the fact that all seismically triggered landslides take place in areas with high lithology and slope Significances and only where the acceleration significance reaches its highest values. A comparison between the maps of Figs. 10 and 11 shows a somewhat different pattern. The Predisposing Factor map (Fig. 10) is characterized by high values in several areas: along the steep southeastern cliffs near P.ta San Pancrazio and in correspondence with Mt. Rotaro, Mt. Trippodi and Mt. Epomeo. The seismic-induced landslide susceptibility map (Fig. 11), instead, is characterized by a main area with the highest susceptibility values located along the northwestern slopes of Mt. Epomeo. The presence of this area is of course related to the expected seismicity of the island which is basically concentrated in the northwestern area (see Figs. 2 and 3A). 6. Discussion and conclusions Earthquake-induced landslide susceptibility is related to several, often interplaying, factors. They involve, of course, the seismic ground shaking but also the geo-lithologic, geomorphologic, geotechnical, tectonic, hydrogeological and hydrological features of a study area. Nevertheless, an effective grade-2 zonation should be characterized by

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Fig. 11. A) Map of the Seismic Landslide Susceptibility level at Ischia obtained by using the MCS intensity values of the 1883 Casamicciola earthquake. B) Histogram of the susceptibility data.

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a good balance between simplicity, rapidity and reliability. To this aim we developed an approach for a qualitative evaluation of the landslide susceptibility in seismic areas. This procedure provides a degree of susceptibility in relative terms making use of only three factors that we believe are the most significant in this susceptibility assessment: the type of outcropping rocks/soils, the slope inclination and the seismic action. These inputs are expressed in quantitative terms as shear wave velocities, slope angles and MCS intensities, respectively. The relative Significances of the three factors for the Susceptibility Level assessment are here considered independent from each other. It should be noted that different failure types can be characterized by different critical Significance values of each of the three factors. For example, the lithological characteristics influence the slope angle for which different types of landslides may occur. Our method accounts for this aspect (see Section 5, Parameter B). The level of the seismic action necessary to trigger landslides also depends on the lithological characteristics and may control the type of landslide that is activated. Literature data show that different landslides may be activated by seismic shaking of slightly different intensities. For example, Keefer (1984) reports that for a few earthquakes, disrupted landslides may occur at MCS intensity levels as low as IV, while for coherent landslides this level is equal to V. Rodriguez et al. (1999) finds instead that for both types of failures the minimum intensity level is V. In this study we assume MCS intensity V as the minimum triggering level for all landslide types. The procedure was developed and tested on the volcanic island of Ischia (southern Italy), which was affected by several historical earthquake-induced landslides. The method was applied by using as input the MCS intensity values of the Casamicciola 1883 earthquake (Fig. 3A) — considered the representative earthquake of the island as far as the MCS field distribution and the maximum intensity of the expected

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shaking are concerned — but also the intensities derived from the PGA values of the Italian national scale hazard estimates (http://zonesismiche.mi.ingv.it/) (Fig. 3B). In the first case the results (Fig. 11A) show a very good match between the distribution of the sources of historical landslides and the areas we identified as the most dangerous ones. That is, all the landslides occur in areas characterized by the higher susceptibility classes while the spatial area for these classes is kept to a minimum. In the case of the island of Ischia the areas with mediumhigh susceptibility (greater than 60%) are about the 8% of the total surface of the island. The areas with susceptibility 30%–60% are about the 52% of the surface of the island, while the areas with susceptibility less than 30% are about the 40% of the surface of the island (see Fig. 11B). In the second case (Fig. 12), the map is characterized by the presence of high susceptibilities also in the southeastern areas. This is due to the practical constancy of the acceleration input given by the national scale hazard estimates, which does not take into account the local important seismicity. This constancy makes the map of Fig. 12 very similar to the Predisposing Factor map of Fig. 10. As a matter of fact, the southeastern areas of the island are affected by significant landslide phenomena (Del Prete and Mele, 2006), but none of them have been reported to be earthquake-triggered. We therefore believe the susceptibility map derived from local earthquake data (Fig. 11) to be more reliable and effective. It's worthwhile to note that, obviously, when carrying out a complete hazard evaluation not only the landslide source zones but also the transit and deposition areas should be considered (Fell et al., 2008). At Ischia earthquake-induced landslides affected either an altered rock deposit (Green Tuff) or debris flow deposits deriving from the degradation of the tuff. Landslides affecting the tuff unit occurred on slopes exceeding 40°, while the debris flow deposits were affected by landslides in a slope range of 20°–30°.

Fig. 12. Map of the Seismic Landslide Susceptibility level at Ischia obtained by using the MCS Intensities derived from the PGA values of the Italian national scale hazard estimates (http://zonesismiche.mi.ingv.it/).

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As the island of Ischia is characterized by deposits with a negligible or absent clay fraction, we are also currently testing our procedure in areas characterized by the presence of loose and/or clayey materials, such as Irpinia, southern Italy. This region of high seismicity is characterized by a complex morphology and by clay, gravel and sand units. Two of the methods described by TC4 (1999) were also applied to Ischia. Neither of them produced an acceptable zonation at the island (Figs. 5 and 6) as those methods seem to work properly only for specific areas and/or contexts. In fact, the Kanagawa Prefecture Procedure is adjusted on peak ground acceleration values that appear too high and therefore may apply well only to very high seismicity areas. The Mora and Vahrson Method that accounts also for rainfall intensity as triggering factor, is instead set for regions of heavy rainfall and seems to fail in temperate areas with moderate rainfall, such as Ischia. To this regard we add that according to Keefer (1984) and Rodriguez et al. (1999), it's arguable that rainfall-induced and earthquake-induced landslide hazards should be assessed simultaneously. As a matter of fact, given the extent of areas generally considered for a grade-2 study and assuming the constancy of average annual climatic conditions in areas of this size, the quantity of rain can be in many cases assumed constant in space and time on a yearly base. Possible strong changes in the quantity and type of precipitation related to seasonal variations and/or extreme climatic events certainly make rainfall an important influencing factor, as it can significantly modify the slope strength. However, this should not change the overall picture of the earthquake-triggered susceptibility map of a small area like Ischia. Acknowledgments The authors acknowledge support to A.R. from Italian National Interest Research Project (PRIN) 2007, “Evaluation of geophysical and geological aspects of landslide susceptibility to severe natural events and relative land zoning” (project 2007LE8ZC5_003). The authors are grateful to two anonymous reviewers whose constructive suggestions were a great help in improving our paper. In particular, one of them reviewed the paper thoroughly. He got to the heart of our work finding a number of weak and unclear points and suggesting how to clarify and/or overcome them. The authors are also very grateful to Dr. D.K. Keefer for his encouragement and his helpful comments and suggestions. They acknowledge the Municipalities of the Island of Ischia for providing with reports about the geological setting of the study area and B. Garofalo for his help in collecting this material. The authors are also thankful to Bill McGann for language revision. References ABNOC — Autorità di Bacino Nord Occidentale della Campania, 2002. Piano Stralcio per l'Assetto Idrogeologico (in Italian). http://www.autoritabacinonordoccidentale. campania.it/pianificazione_di_bacino/assetto_idrogeologico.asp. Acocella, V., Funiciello, R., 1999. The interaction between regional and local tectonics during resurgent doming: the case of the island of Ischia, Italy. J. Volcanol. Geotherm. Res. 88, 109–123. Alessio, G., Esposito, E., Ferranti, L., Mastrolorenzo, G., Porfido, S., 1996. Correlazione tra sismicità ed elementi strutturali nell'isola di Ischia. II. Quaternario 9 (1), 303–308 in Italian. Ausilio, E., Costanzo, A., Silvestri, F., Tropeano, G., 2008. Prediction of seismic slope displacements by dynamic stick-slip analyses. AIP Conf. Proc., July 2008, Vol. 1020, pp. 475–484. Barra, D., Cinque, A., Italiano, A., Scorziello, R., 1992. Il Pleistocene superiore marino di Ischia: paleoecologia e rapporti con l'evoluzione tettonica recente. Studi Geol. Camerti (1), 231–243 (in Italian). Bortoluzzi, G., Donadio, C., Mele, R., Romano, P., Russo, F., Santangelo, N., Sgambati, D., 1995. Campi Flegrei-Isola d'Ischia. Excursion Guide. Gruppo Nazionale Geografia Fisica e Geomorfologia. Fall Meeting, September 28, 1995 (in Italian). Boschi, E., Guidoboni, E., Ferrari, G., Valensise, G., Gasperini, P., 1997. Catalogo dei forti terremoti in Italia dal 461 a.C. al 1990. Istituto Nazionale di Geofisica and Storia Geofisica Ambiente, 644 pp. (in Italian). British Standards Institution, 2004. Eurocode 8: Design of Structures for Earthquake Resistance: General Rules, Seismic Actions and Rules for Buildings, BS EN 1998-1:2004, London. 232 pp.

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