Relevance of rock slope deformations in local seismic response and microzonation: Insights from the Accumoli case-study (central Apennines, Italy)

Journal Pre-proof Relevance of rock slope deformations in local seismic response and microzonation: Insights from the Accumoli case-study (central Apennines, Italy)

S. Martino, M. Cercato, M. Della Seta, C. Esposito, S. Hailemikael, R. Iannucci, G. Martini, A. Paciello, G. Scarascia Mugnozza, D. Seneca, F. Troiani PII:

S0013-7952(19)31005-1

DOI:

https://doi.org/10.1016/j.enggeo.2019.105427

Reference:

ENGEO 105427

To appear in:

Engineering Geology

Received date:

27 May 2019

Revised date:

11 November 2019

Accepted date:

19 November 2019

Please cite this article as: S. Martino, M. Cercato, M. Della Seta, et al., Relevance of rock slope deformations in local seismic response and microzonation: Insights from the Accumoli case-study (central Apennines, Italy), Engineering Geology (2019), https://doi.org/10.1016/j.enggeo.2019.105427

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© 2019 Published by Elsevier.

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Relevance of rock slope deformations in local seismic response and microzonation: insights from the Accumoli case-study (central Apennines, Italy) *

Martino S.1 , Cercato M.2, Della Seta M.1, Esposito C.1, Hailemikael S.3, Iannucci R.1, Martini G.3, Paciello A.3, Scarascia Mugnozza G. 1, Seneca D.1, Troiani F.1

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“Sapienza” University of Rome - Department of Earth Sciences and Research Center for the Geological Risks (CERI), P.le A. Moro 5 00185 Rome, Italy "Sapienza" University of Rome − DICEA, Via Eudossiana 18 00184 Rome, Italy Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) SSPT -METDISPREV, Via Enrico Fermi, 45,00044 Frascati, Rome, Italy 2

corresponding Author ([email protected])

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Abstract

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Based on a National Government Decree, detailed seismic microzonation studies were carried out after the seismic sequence of Central Italy (2016-2017) in support of post-earthquake reconstruction. During these activities mass rock creep deformations of slopes were observed in the municipality of Accumoli (Latium Region), involving two urbanized areas on the left bank of the Tronto River. The geological and geomorphological surveys as well as the geomechanical characterization of rock masses revealed a deformation process of flexural toppling which involves the sub-vertical thick sandstone layers down to 50 m of depth. Geophysical investigations, consisting of downhole tests, active surface-wave testing and seismic ambient noise measurements, were carried out in the areas involved in the deformational processes. The results highlighted that the rock mass involved in the flexural toppling exhibits peaks of the horizontal-tovertical spectral ratios (HVSR) in a frequency band ranging from 1.5 up to 2.5 Hz. In one of the two areas, H/V calculated on weak-motions recorded during the seismic sequence within the deforming rock mass, showed a clear peak in the frequency range 1.5-2.0 Hz, the same range pointed out by the ambient vibration analysis. Due to local morphological, lithological and bedding conditions, such an evidence cannot be related to a simple 1D resonance due to the seismic impedance contrasts originated from the local stratigraphy. Nevertheless, it seems more reliable that the HVSR peaks can be related to the 3D volume of intensely jointed rock mass, in agreement with the field evidence of flexural toppling, as these peaks result confined within the deforming mass. In addition, the lack of a clear polarization of the particle motion in the seismic ambient noise dataset suggests that the observed effect should not be linked to the interaction of seismic surface waves with persistent discontinuities in an anisotropic rock mass. According to the National guidelines for seismic microzonation studies, these rock masses have not been identified as unstable, i.e. involved in conventional landslide processes, but have been identified as seismic amplification prone microzones with associated amplification factors of at least 1.5 in the period range 0.1-0.5 s. The identification of such microzones within the areas involved in gravity-induced slope deformations is a significant prerequisite for planning strategies of post-earthquake reconstruction.

Keywords

Journal Pre-proof flexural toppling; geophysical investigations; seismic site effects; seismic microzonation

1. INTRODUCTION In order to define strategies for the post-earthquake reconstruction after the 2016-2017 seismic sequence occurred in the Central Apennines (mainshocks up to Mw 6.5), an Italian Government Decree issued specific investigations for quantitative seismic microzonation (SM) studies in 138 municipalities. Following the national SM guidelines, these studies aim to define seismic microzones and related ground-motion amplification factors as well as areas prone to seismically induced effects (i.e. unstable areas): ground failures, liquefaction and landslides. Landslides

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include a wide spectrum of processes that are considered in the national guidelines; an exception is represented by slope-scale deformations featured by mass rock creep (MRC) which are a peculiar type of slope instability able to produce both amplification effects and localized slope

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failures.

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The case study presented here is part of the results obtained in the SM study of the Municipality of

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Accumoli (Rieti), where MRC processes, namely flexural toppling, involve thick sandstone layers outcropping in the hamlets of Tino and Grisciano, almost completely destroyed by the aforementioned earthquakes. Integrated engineering-geology and geophysical approaches have

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been exploited to evaluate the effect of the deforming rock masses on the local seismic response.

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In the last decades, rock mass slope deformations represent a research topic of international

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interest, given their possible evolution in paroxysmal events such as slope failures and possibly rock avalanches (Evans et al., 2006). Creep processes cause an irreversible deformation of the rock mass in the medium or long term, visibly modifying geostructural setting and mechanical properties of the rock mass (Chigira, 1992). The strain rate of these processes generally requires several tens of years before they apparently modify the landforms. Among the main evidences of changes in the attitude of layers due to creep, flexural toppling (Goodman and Bray, 1976; Hungr et al., 2014) manifests itself with downslope flexure of the strata from an original high-angle counterslope attitude. Such flexure can generate a complex kinematic system, composed of stratajoints (primary discontinuities) andconjugated joints (secondary discontinuities) enveloped in a basal sliding surface. Along these surfaces, landslides with a prevalent translational mechanism can develop over time, caused by local or generalized slope collaps e (Nichol et al., 2002; El Bedoui et al., 2008). Weathering is also invoked as a possible factor for rock mass damaging in case of flexural toppling as it induces a decay in mechanical properties which contributes to anticipate the expected time to failure (Martin et al., 2011).

Journal Pre-proof The ongoing rock mass deformation revealed in slope by typical landforms consisting of escarpments, open cracks evolving in trenches, which are generally filled by debris produced from the rock mass desegregation, and terraces, which typically correspond to the debris-filled trenches (Hutchinson, 1988). Several Authors (Adhikary et al., 1997; Wyllie and Mah, 2018; Zhang et al., 2018 among the others) discussed in literature how slope stability can be quantified in case of flexural topping. In some cases, complex multiparametric monitoring systems integrated with geophysical surveys have been implemented to follow the evolution of the deformation processes until local or generalized slope failures (Delacourt et al., 2004; Jomard et al., 2007; Booth et al., 2013). Nevertheless, in the official catalogs of landslides as well as in the thematic technical maps the

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flexural toppling processes are not basically distinguished from rock slides or from the more generic deep seated gravitational slope deformations (DSGSD Auct.). Furthermore, the literature aimed at evaluating the effect of these processes on the interaction with seismic waves and on

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related amplification effects is extremely limited. Zhang et al (2016) recently discussed, by stressstrain numerical modelling in dynamic configuration, the influence of slope inclination and rock

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mass layering on the local seismic response of slopes affected by flexural toppling. Huang et al recent 2008 Wenchuan earthquake.

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(2013) have documented a flexural toppling process evolving into a landslide triggered by the The recent study by Kleinbrod et al. (2019) proposed a distinction of slopes involved in rock mass

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deformations in two main classes, based on their dynamic behavior: depth-controlled and volumecontrolled slopes. In the first class, the dynamic behavior is controlled by the depth-dependent

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seismic properties of the rock mass, which can be measured by surface waves methods, and

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where only weak, non-directional ground-motion amplifications are observed. In the second class, the dynamic behavior is controlled by the deforming volume with homogenous properties, and the seismic response is dominated by normal mode vibrations at characteristic eigenfrequencies, which result in large and frequency-dependent ground-motion amplification. In this case, the amplified frequencies are inversely proportional to the size of the deforming rock mass and directly proportional to its stiffness. A fundamental role in this response is played large-scale anisotropies related to joints, which can produce wavefield polarization and ellipticity peaks in the particle motion (Burjánek et al., 2010, 2012; Galea et al., 2014; D'Amico et al., 2018). Following the above mentioned properties, a depth-controlled dynamic behavior should imply that landslide mass has depth significantly lower than width; while in case of a volume-controlled behavior the deforming rock mass volume has comparable dimensions along the three directions and is well delimited, i.e. free to vibrate at proper eigenfrequencies .

Journal Pre-proof While lateral limits of the volume-controlled deforming rock mass can be easily determined by ambient noise measurements, the depth-controlled rock mass limits cannot be easily inferred on the basis of such measures (Kleinbrod et al., 2019).

2. GEOLOGICAL AND GEOMORPHOLOGICAL SETTING The Accumoli area is part of the Adriatic piedmont sector of Central Apennine thrust-and-fold belt,

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whose variable geodynamic context has been strongly influenced by the evolution of the

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Mediterranean area from Triassic to Neogene (Centamore et al., 2002). The bedrock of the area is

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characterized by a thick Meso-Cenozoic stratigraphic succession made up of limestones and marls overlain by prevalently terrigenous terrains. The latter are the results of the turbiditic sedimentation

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within inner basin systems formed in response to the northeastward migration of the Adriatic

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foreland during the Miocene (Marini et al., 2015). The folding and faulting responsible for the growth of the Apennine chain strongly controlled the attitude of the bedrock layers. Since the

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Upper Pliocene, extensional tectonic has affected the area and at present principally drives the

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morphodynamics of Central Apennines. In particular, the extensional tectonic phase generated along the whole Central Apennines a NW–SE-trending normal fault system. The latter is

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accountable for the formation and development of intermontane basins (Aringoli et al., 2014) and include the main seismogenic sources of the Central Apennines (Tondi and Cello, 2003; Galadini and Galli, 2003; Boncio et al., 2004; Pizzi and Galadini, 2009). The Tronto River trunk-valley traverses the Accumoli territory, part of the headwater sector of Tronto River basin (Fig. 1a), and two distinct geo-structural domains can be distinguished that account for two distinct geomorphological configurations of the opposite valley-sides. In particular, marly sandstones, ascribable to the flysch of the Laga Formation, composes the bedrock of the study area (Marini et al., 2015). The arenaceous member mostly outcrops on the left (i.e. western) valley side, while the marly one mostly outcrops on the opposite one. The lithological variability, coupled with variable attitude of the strata with respect to the slope topography, account for different erosional behaviors along the main slopes. In addition, the morphostructural setting

Journal Pre-proof strongly influenced the style and timing of the drainage network entrenchment and represents one of the main predisposing factors for the intense mass wasting process due to gravity affecting hillslopes (Martino et al., 2019). The Tronto River trunk-valley shows an evident morphological asymmetry. The left valley-side, carved on less erodible terrains, presents steep or sub-vertical slopes, ridges and, along the tributaries system, V-shaped incised valleys. On the contrary, the opposite valley-flank shows a gentle morphology. Alluvial and debris-flow dominated fan deposits overlay Holocene gravel and sand sediments that fill the Tronto River valley floor up to 15-20 m

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below the ground level.

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The presence of weak marl interlayers is the main predisposing factor to landslides activation in

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the study area. The bedding attitude has also a great influence on hillslope erosion, favoring the occurrence of several types of landslides, mainly consisting in rotational sliding (Hungr et al., 2014)

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involving both rock masses and soils, as well as rock falls.

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Two geomorphological schemes have been produced within the Tronto River valley at Grisciano and Tino hamlets, within the Accumoli territory (Fig.1b and c). Geomorphological analyses have

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been mainly concentrated on the erosional and depositional landforms, useful for detecting slope

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instability with particular emphasis on the spatial distribution of those geomorphological indicators that can suggest rock mass slope deformations. Geomorphological analyses have been based on:

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i) aerial photo interpretation using high-resolution imagery available at the scale of 1:70.000 (Volo Italia: year 1988-1989); ii) visual inspection of hillshade maps derived from a LiDAR dataset with a cell size equal to 1 m; iii) geomorphological field surveys. Within the analyzed sectors of the Tronto River valley, the main geomorphological indicators of rock mass deformations are transversal rocky scarps (Fig.2a), both transversal and longitudinal cracks upslope respect to trenches (Fig.2b), terraces, toe-slope bulging. These landforms are widespread in the study area, but mostly concentrate along the left (i.e. western) valley-side, at both Tino and Grisciano hamlets, suggesting the presence of rock mass deformations (Fig. 1b and c). In general, the involved hillslope sectors also show intense slope-wasting processes including shallow landsliding (i.e. rock and soil landslides of different typologies and state of activity with thickness generally not exceeding 5-8 m), and deeply incised stream channels, demarking the

Journal Pre-proof limits of the rock mass deformations. Longitudinal cracks favor the stream entrenchment and force the flow direction downslope. Within the study area, the type, density and spatial distribution of these features are rather dissimilar. The geomorphological indicators collected at the Grisciano hamlet cluster within a hillslope sector extending along-valley for about 700 m (Fig.1b). Several transversal cracks and a trench demark the upslope limit of the area, whereas at the central part of the slope a wide terrace occur, partially covered by very thin slope-debris deposits, mainly chaotic gravel and sand. Steep scarps bound both upslope and downslope the terrace surface. Along the

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upslope zone, a series of transversal scarps occur together with shallow landslides principally

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involving the debris cover and the shallower part of the jointed and weathered rock masses. An

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evident bulging characterizes the base of the deformed hillslope sector. Shallow landslides rock and soil rotational sliding and rock falls, diffusely occur along this hillslope portion. At the Tino

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hamlet the geomorphological indicators of rock mass deformations are to some extent most

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frequent and well-developed then at Grisciano area. In particular, within an area about 700 m wide along the valley and about 500 m extended upslope, fresh scarps and fractures (both transversal

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and longitudinal) are widespread evident.. Several terraces can be identified at different heights,

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partially covered by a very thin colluvial cover.. Incised stream channel flow at the margins of the deformed hillslope area. Several scarps, generally clustered at the downslope sector, mark the

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emergence of the less erodible, sub-vertical arenaceous bedrock layers (Fig.3a). Based on the collected geomorphological evidences, a more advanced evolutionary stage of the deformation can be attributed to the Grisciano slope, as proved by: i) the highest and well defined scarp at the hilltop; ii) a 100 m extended terrace, which delimits uphill a released zone where open trenches; iii) the involvement of the Tronto River alluvial plain in a bulging, responsible for the formation of a wide meander that marks the hillslope toe and, in proximity of the Grisciano hamlet, causes the watercourse to abruptly turn toward west. Nonetheless, upstream the Tronto River valley from the Grisciano hamlet area, missing geomorphological and sedimentological evidences of stream damming or deviation, such as lacustrine deposits, cut-and-fill stream terraces, dam debris, epigenetic gorges as well as the absence of landslide debris in the alluvial plain, allow to exclude that a generalized collapse involved the Grisciano slope. Based on these considerations it is more

Journal Pre-proof reliable to attribute to the Grisciano slope an advanced stage of MRC deformations that not yet

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reached an ultimate stage which should imply a generalized collapse phase.

Figure 1 – (a) Location of the study area and hillshade map of the headwaters sector of the Tronto River basin at the Accumoli territory. Geomorphological schemes of the Grisciano (b) and Tino (c) valley-slopes with emphasis on the indicators of rock mass deformations. Legend refers to the geomorphological schemes.

Journal Pre-proof On the contrary, the lack of similar evidences at Tino allows to attribute an less evolved stage of the slope deformation, only responsible for originating several scarps and terraces without major elements which delimits released zones uphill and without evidences of bulging in the alluvial

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plain.

Figure 2 - Evidences from field surveys: a) subvertical scarp within the deforming rock mass at Grisciano; b) trench within the deforming zone at Grisciano; c) regularly jointed rock mass outside the deforming zone at Tino; d) intensely jointed rock mass inside the deforming zone at Tino.

3. GEOMECHANICAL FEATURES OF THE DEFORMING SLOPES Detailed field surveys have been performed in the areas of Tino and Grisciano to collect geostructural and geomechanical data and provide the related models of the slopes. Geostructural and geomechanical evidence strongly support and refine the hypotheses about the deformational processes inferred by means of geomorphological observations. Indeed, a rock mass jointing

Journal Pre-proof degree (i.e. joint density) heavier than the surrounding zones is observed within a deforming slope

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portion (Figs.2c, 2d).

Figure 3 - Geomechanical maps of the deforming slopes at Tino (a) and Grisciano (b). For Tino rock mass jointing: low (Jv 1-10); middle (Jv 10-20); high (Jv 20-30); for Grisciano rock mass jointing: low (Jv 1-10); middle (Jv 10-15); high (Jv 15-20)

Figure 3 shows the location of the survey sites, where the rock masses have been characterized according to the ISRM (1978) standards, such as volumetric joint count (Jv), Rock Mass Rating (RMR) and Geological Strength Index (GSI) which account for jointing conditions and allow to attribute equivalent mechanical properties to the rock mass (Table 1).

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Grisciano

class

(joint/m3)

GSI

RQD

low

1-10

A (50) - B (45)

95-82

middle

10-20

B (45) - D (35)

82-49

high

20-30

D (35)

49-32

low

1-10

A (50) - B (45)

95-85

middle

10-15

B (45) - C (40)

78-65

high

15-20

C (40)

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Tino

Jv

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Locality

jointing

65-59

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Table 1. Geomechanical parameters of joints measured on the outcropping rock masses. Jointing

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classes are referred to Fig.3

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Starting from the geostructural data, a first relevant evidence in both Tino and Grisciano slopes is the sub-vertical bedding, which is the most persisting joint set. This feature is itself a predisposing factor for topple-like deformation processes. Furthermore, while a dip-slope attitude of bedding is

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prevalent, clusters of reverse-slope attitude can be found in the slope areas where the

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geomorphological evidences indicate a deforming process (Fig. 3).

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Figure 4 - Geomechanical cross sections, along traces AB and CD of Fig.3, for the deforming slopes of Tino (a) and Grisciano (b). For Tino rock mass jointing: low (Jv 1-10); middle (Jv 10-20); high (Jv 20-30); for Grisciano rock mass jointing: low (Jv 1-10); middle (Jv 10-15); high (Jv 15-20) The analysis of spatial distribution and variation of rock mass jointing can provide useful clues as they can be the result of gravity-driven, slope-scale deformation processes: deformed slopes are likely to show higher jointing degree and/or modifications of the joint attitude. For the here presented cases, even if outcrops are quite rare and thus the spatial distribution of field measurements sites is neither dense nor homogeneous, a zonation of the slopes in term of Jv has been performed, according to the two following criteria.

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The Jv values were grouped in three classes based on their frequency distribution to

identify and map zones of the slopes with different rock mass jointing. -

The Jv zones have been outlined according to a heuristic approach, also due to the

relatively small amount of data (i.e., reliable survey sites) that limit the use and efficiency of conventional interpolation techniques. Specifically, after having clustered survey sites with similar Jv class values, the limits between different zones have been drawn accounting for the joint set (i.e., a new one or the one whose decrease in spacing causes a Jv change) that differentiate a cluster from the adjacent one. Based on the resulting zonation (Fig. 3), it is possible to point out a predominance of high Jv values (i.e., higher jointing degree) in the slope sections within and across the geomorphic

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lineaments hypothesized as the boundaries of the deforming zones. It is worth stressing that the Jv classes for Tino and Grisciano do not include to the same values as they have been obtained based on the site-specific value distributions. This implies that the relative jointing of the deforming

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zone respect to the undeformed one is comparable in the two sites even if the absolute Jv values are different.

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By merging geostructural and geomechanical data and in light of geomorphic evidence the 1)

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following listed considerations can be drawn for both Tino and Grisciano deforming slopes. The spatial distribution of rock mass classes fits well with the effect of a local deformation

controlling fault line. 2)

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rather than the inheritance of tectonic jointing, which is expected to have a linear pattern along the There is a good correspondence between the boundaries of zones with higher Jv and the By plotting the geostructural and geomechanical data over representative cross sections of

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3)

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“geomorphic limits” of the deforming zones (Figs. 3a, 3b). both slopes (Fig. 5), a kinking of bedding planes in the upper part of the sections is clearly visible. This evidence strongly recalls the effect of topple-like deformations, that also account for grabenlike depressions in the middle and upper parts of the slopes. The lack of high-persistence, low angle discontinuities suggests a flexural toppling mechanism affecting all or part of the slope. Furthermore, by enveloping the hinges of the kink folds it is possible to estimate a maximum depth of the deformational process of about 50 m for both Tino and Grisciano slopes. 4)

The bedding joint set (j0 in Fig.5) is associated to a conjugated joint set (j1 in Fig.5), having

a similar strike but with a reverse slope dipping. Such a set can be linked to the rock mas deformations associated to the flexural toppling. 5) Inside the deforming zones the j0 and j1 joint sets show a higher back-tilt respect to outside the deforming zones. 5) At Grisciano the observed geomorphic evidences allow to recognize a more intensely deformed rock mass volume respect to Tino, where the ongoing gravity-driven slope deformations are still at an initial stage.

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Figure 5 - Synthetic stereoplots (equal-areal Schmidt projection from the lower hemisphere) obtained for Tino (a) and Grisciano (b) slopes inside and outside the deforming zones.

Journal Pre-proof 4. GEOPHYSICAL DATA AND METHODS In the last decades, surface geophysical investigations have been widely applied to study slope instability processes by improving conventional methods as well as developing new and innovative techniques (Bogoslovsky and Ogilvy, 1977; McCann and Forster, 1990; Hack, 2000; Jongmans and Garambois, 2007; Maurer et al., 2010). In particular, passive and active seismic surveys have been used to evaluate fundamental features of rock masses involved in landslide processes, e.g. geometry, elastic properties including their lateral and vertical variations, content of water, rate of movement, dynamic behavior (Lévy et al., 2010; Hibert et al., 2012; Panzera et al., 2012; Bottelin et al., 2013; Galea et al., 2014; Colombero et al., 2018; Iannucci et al., 2018). In this study, single-station seismic ambient noise measurements and Multichannel Analysis of

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Surface Waves (MASW) were carried out in the hamlets of Tino and Grisciano to study the dynamic behavior of the two areas involved in the MRC process. Ambient noise measurements were analyzed by the horizontal-to-vertical spectral ratio method (HVSR) and by the Time-

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Frequency Polarization Analysis (TFPA). HVSR method (Nogoshi and Igarashi, 1970, 1971; Nakamura, 1989) is well known for providing a fast and reliable dynamic characterization of the

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subsurface through the estimation of the fundamental resonance frequency (f0) of the investigated

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site. In fact, under 1D site conditions of a single soft deposit overlying a stiff seismic bedrock, the HVSR curve exhibits a sharp peak at the site fundamental resonance frequency f0 (BonnefoyClaudet et al., 2006). The resonance frequency is related to the 1D shear-wave velocity (Vs)

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structure down to the main seismic impedance contrast between the bedrock and the overlying deposits. This technique has been successfully applied for the dynamic characterization of

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unstable slopes (Havenith et al., 2002).

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TFPA method (Burjanek et al., 2010) is an improvement of the polarization analysis proposed by Vidale (1986) which allows the identification of frequency-dependent directional resonances in unstable rock mass.

MASW survey method allows for the estimation of the Vs profile by analyzing the dispersive properties of propagating surface waves artificially generated by a source and recorded through a linear array of seismic receivers (Park et al., 1999; among many others). The obtained Vs profiles were compared with previous results obtained by downhole tests in closeby sites and with HVSR results for co-located measurements to evaluate the consistency of the site characterization. In addition, weak-motion records acquired at Tino were analyzed by using a non-reference site response method (Lermo and Chavez-Garcia, 1993; Field and Jacob, 1995) to evaluate the occurrence of frequency dependent ground-motion amplification within the deforming rock mass. 4.1 Single-station seismic ambient noise measurements Single-station seismic ambient noise measurements were performed between September 2016 and November 2018 in the areas of Tino and Grisciano hamlets where the flexural toppling

Journal Pre-proof processes have been observed. Overall 90 single-station measurements (Fig.6) were carried out, of which 37 at Grisciano and 53 at Tino hamlets, by different velocimetric instruments: i) SL06 and SR04 24 bit digitizers with built-in SS20 three-component velocimetric sensor (2 Hz eigenfrequency) manufactured by SARA Electronic Instruments; ii) three-component LE-3D/5s Lennartz three-component seismometers (0.2 Hz eigenfrequency) coupled to Reftek 130-01 digitizer and iii) digital tromometer TROMINO (Micromed). The sampling frequency was 200 Hz for the SL06 and SL04 instruments, 250 HZ for the LE-3D/5s Lennartz sensor and 128 Hz for TROMINO. The measurements had generally a duration of 60 minutes and minimum recording of

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at least 30 minutes.

Figure 6 - Location of the HVSR measurements on the hill shade maps of the deforming slopes at Tino (a) and Grisciano (b) with related values of the main HVSR peaks (f0).

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The measurement stations were spread all over the areas under study and in case of uncertain results were repeated some tens of meters afar. The signals were processed by Geopsy software (www.geopsy.org); each record was de-trended, cut into 40 s time-windows, 5% cosine tapered, converted to the frequency domain and smoothed by a Konno-Ohmachi function (Konno and Ohmachi, 1998) with parameter b set to 40. For each measurement, the Horizontal-to-Vertical Spectral Ratio (HVSR) was calculated as the quadratic mean of the NS/V and EW/V spectral ratios and the final HVSR function as the geometric average from all the 40 s time windows. Unfortunately, several HVSR curves are difficult to interpret, as they show more than one peak of low level (between 2 and 3) or peaks that are not significant

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according to SESAME (2004) criteria. These curves were interpreted taking into account the

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results obtained from nearby HVSR measures or were not considered at all.

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Figure 7 - HVSR obtained at Tino (a) and Grisciano (b) grouped according to the obtained f0 values. The HVSR curves were grouped according to the frequency of the main HVSR peak (f0), when observed (Fig.7); curves with HVSR peaks at frequencies larger than 9 Hz were grouped together with the curves that do not show any significant HVSR peak (NR), as these higher frequencies can be ascribed to few meters thick colluvial debris filling the shallower part of trenches. All over the Tino area the HVSR curves show a “bump-like” shape around 1.5 Hz that cannot be considered a significant HVSR peak; in about 15 stations however, these frequencies correspond to significant HVSR peaks (green dots in Fig.6). In Grisciano area about one-third of the HVSR curves can be classified as NR or with HVSR peaks at frequencies larger than 9 Hz, one-third shows a HVSR peak at about 2.5 Hz, while the remaining show HVSR peaks in the range 4-6 Hz.

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Single-station seismic ambient noise measurements were also analyzed in terms of polarization and ellipticity of the particle motion (Vidale, 1986) by the WAVEPOL package (Burjánek et al., 2012), that allows to obtain a 3D ellipse representing the particle motion at each time-frequency pair by adopting the Continuous Wavelet Transform (CWT). The ellipticity of the particle motion is defined as the ratio between the semi-minor axis and the semi-major axis of the ellipse (i.e. 1 for circular motion and 0 for linear motion) for each frequency; the polarization of the particle motion is represented by a polar strike plot, indicating the azimuth of the semi-major axis projected to the horizontal plane from North. This analysis is able to show a polarization effect of the particle motion, pointed out by a high degree of linearity on the ellipticity diagram and an azimuthal

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direction on the polar strike plot for the same frequencies.

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In general, the polarization analysis of the recorded data does not show marked features of polarization or linearity of the particle motion at both sites, neither in the stable areas nor in the

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zones involved by the instability process (Fig.8). In particular, the HVSR peak at around 1.5 Hz at Tino does not show low values of ellipticity and any directionality in the polarization plot; instead,

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significant linearity of particle motion.

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the HVSR peak at about 2.5 Hz at Grisciano shows a weak EW polarization but not related to any

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Figure 8. Examples of HVSR(f) function (top panels, the dashed black lines show the standard deviation of the curve), ellipticity diagram (middle panels) and polar strike plot (bottom panels) for the unstable areas of Tino (left) and Grisciano (right).

4.2 Weak-motion records Soon after the 24th August 2016, Mw 6.0, Amatrice earthquake which started the long lasting and damaging 2016-2017 Central Italy seismic sequence, several research institutions installed a dense temporary seismic network of 50 strong- and weak-motion recording stations in the epicentral area of the Amatrice mainshock (Cara et al., 2019). Among the aims of this network deployment, the estimation of site response was a prevalent goal. One weak-motion station of 3A network, station code MZ104, was installed in the Tino hamlet (Fig. 6), in the uphill central portion of the slope involved in rock mass deformation. This recording station was equipped with a 24 bit VELBOX digitizer and built-in 3-component SS20 velocimetric sensor (2 Hz eigenfrequency)

Journal Pre-proof manufactured by SARA electronic instruments. MZ104 was installed at the basement of a small one-storey building to ensure power supply and connected to external GPS antenna for time synchronization. The station recorded signals in continuous mode with sampling rate of 200 Hz and gain set to minimum in order to avoid signal saturation. MZ104 operated in the time span September-October 2016 and recorded hundreds of earthquakes. From this huge dataset, a list of more than 100 events, with magnitude in the range 2.0-3.0, epicentral distance mainly within 30 km and back-azimuth mainly in the range N300°-N350°, was selected for estimating the amplification function at MZ104 site by means of spectral analysis. In general, experimental methods for site response estimation rely on the comparison of the ground-motion recorded at the site of interest with that recorded at a nearby reference site, usually

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located on outcropping rock (Borchertd, 1970; Andrews, 1986; Field and Jacob, 1995). In particular the distance between stations should be at least a fifth of the epicentral distance of the earthquake waveforms used for the analysis (Le Brun et al, 1999). Considering the proximity of the selected

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earthquakes, a reference recording site at a sufficiently small distance from MZ104 was not identified since the closest reference station (belonging to the IV network managed by INGV,

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T1299) was installed in Amatrice at about 10 km distance. Therefore, also in this case the

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Horizontal-to-Vertical spectral ratio (H/V) method has been applied (Lermo and Chavez-Garcia, 1993; Field and Jacob, 1995). To this aim, the earthquake signals were processed by the SAC code (Goldstein and Snoke, 2005). Earthquake records were corrected for the instrument response

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and windowed around the S phase arrival using a 5% cosine-taper window which started about 1 s prior the S-phase onset and ended 15 s later. The resulting signals were pass -band filtered in the

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frequency range 0.2–20 Hz and converted to the frequency domain. For each selected event, the

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spectra were smoothed with the Konno-Omachi window (Konno-Ohmachi, 1998), then smoothed spectra of the NS, EW and V components were used to calculate the NS/V and EW/V spectral ratios. Finally, the single event H/V curve was calculated as the geometric average of the NS/V and WE/V ratios.

The H/V amplitude dependence on the azimuth was evaluated by calculating the spectral ratio on rotated horizontal components (Spudich et al., 1996). For this purpose, the S-wave windowed horizontal components were rotated clockwise on the horizontal plane between 0 and 180° in angular steps of 10°, before conversion to the frequency domain. The rotated and smoothed horizontal spectra were divided by the vertical one and the H/V curve for each event and azimuth was obtained as the geometric average of the two rotated ratios. This estimation of the spectral ratio does not provide the total horizontal motion (normalized by the vertical one) at the selected azimuth but the rotated spectral ratio amplitude can be directly compared with the non-rotated H/V. The single-event directional spectral ratios were then averaged over the available earthquake data set, assuming a lognormal distribution.

Journal Pre-proof The results obtained analyzing over 100 local earthquakes show that a significant site amplification occurs at the MZ104 site in the 1-2 Hz frequency band (Fig.9). This is the same frequency band of the H/V peaks observed at Tino by ambient noise measurements. The average amplitude of the function in the specified frequency range is about 4 (Fig.9, top), which can be considered as a lower bound for the amplification level at the site (Parolai et al., 2004; Pilz et al., 2009). Furthermore, the computed function shows a weak directional dependence on the considered horizontal direction, with larger amplitude values observed in the N40°-N140° azimuth range (Fig.9, bottom). These results allow to assume that for both the Tino and Grisciano slopes also the observed H/V

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peaks from ambient noise can be related to an amplification effect due to the deforming rock mass.

Figure 9. Average H/V function from weak-motions recorded at Tino (top) and directional H/V function (bottom)

4.3 MASW and downhole tests The seismic velocity profiles as well as the reference stratigraphy for the downhole seismic surveys performed at Tino (TIN1-DH) and Grisciano (GR1-DH), as indicated in Fig.6, are reported in Fig.10a and Fig. 10b, respectively.

Journal Pre-proof According to the official reports of SM studies, the geological units involved in the slope deformations are ascribable to the Laga Formation and to Holocene deposits. In terms of lithotechnical units the Laga Formation can be split in

undeformed rock mass (ALS), and

deforming rock mass (SFALS) while the Holocene deposits are distinguished in fine grained (GMin) and coarsely grained (GHMin). The stratigraphic log of TIN1-DH consists of 5m thick Holocene deposits (GMin) and of more than 30 m thick SFALS, i.e. the ALS was not reached in the borehole. On the other hand, the seismic downhole GR1-DH is located on the alluvial plain and the stratigraphic log consists of 1 m of GMin and 21 m of GHmin, representing Holocene alluvial

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deposits and 10 m of SFALS, i.e. the ALS was reached at almost 32 m below the ground level.

Fig. 10 Results of downhole investigations at (a) Tino (TIN1-DH) and (b) Grisciano (GR1-DH). For each downhole survey, the reference stratigraphies and the shear-wave velocity profiles are also reported. The geological units are labeled as follows: ALS - marly-arenaceous bedrock; SFALS jointed bedrock; GMin - fine grained deposits, GHMin - coarsely grained deposits.

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Fig.11 Results of MASW inversion at Tino (TN1) and Grisciano (GR1). (a) TN1: example f-c panel

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and observed dispersion curve. (b) TN1: shear-wave velocity profile of the minimum misfit inverted model. (c) Geological interpretation of (b). (d) TN1 a posteriori reflectivity modelling and

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comparison with observed and predicted modal dispersion. (e) GR1: example f-c panel and observed dispersion curve. (f) GR1: shear-wave velocity profile of the minimum misfit inverted

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model (g) Geological interpretation of (f). (h) GR1 a posteriori reflectivity modelling and comparis on

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with observed and predicted modal dispersion.

The MASW (Multichannel Analysis of Surface Waves) results are reported in Fig.11 for both sites. At Tino site, the seismic line is located on the unstable area, whereas at Grisciano site the seismic line was carried out near the GR1-DH location, due to logistic restrictions and steep topography, with the purpose to extend the point results of the downhole survey. The linear arrays employed for the two MASW experiments, consist of 48 vertical receivers (4.5 Hz geophones) spaced 2 m apart. The overall array length is 94 m and the maximum employed offset is 14 m. The seismic data, after pre-processing (trace balancing, amplitude spreading correction, surgical muting and lowpass filtering) are transformed into the f-c (frequency-phase velocity) domain via the phase-shift method (Park et al., 1999). An example f-c panel for a shot gather at Tino is reported in Fig.11a. The observed dispersion curve on top of the panel (open black circles) is the average curve resulting from the whole picking process. Similarly, in Fig.11e is reported a f-c panel corresponding to a single shot gather at Grisciano. The average dispersion curve observed at this site (open black circles) is superimposed on the panel. In both cases, the observed dispersion curves are inverted

Journal Pre-proof by the global simulated-annealing algorithm described by Cercato (2011). The results of the inversion in terms of the shear-wave velocity of the minimum misfit model are reported in Fig.12b and Fig 11f for Tino and Grisciano, respectively. An a posteriori reflectivity modeling (Cercato, 2018) is also performed to reproduce the dispersion pattern of the Rayleigh waves associated to the inverted model. The results are shown in Fig.11d and Fig.11h respectively at Tino and Grisciano. The results of TN1-MASW inversion identify a sharp velocity contrast at about 45 m of depth, where the velocity is slightly below 900 m/s. This cannot be consistent with the TN1-DH downhole investigation because of the different location (see Fig.6). On the other hand, the GR1MASW inversion is consistent with the downhole profile and the alluvial deposits are about 20 m

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thick in the area.

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Single-station HVSR measurements can be employed to estimate the behavior of the Rayleigh wave ellipticity function versus frequency (Fäh et al. 2001; Hobiger et al. 2009). Since the

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proportion between Rayleigh and Love waves is site and source dependent and cannot be assumed a priori (Cercato 2018), advanced signal analysis interpretation such as the RayDec

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(Rayleigh wave ellipticity by using the random decrement technique) software (Hobiger et al. 2009) are needed in principle to accurately estimate the amplitude of the observed ellipticity.

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Nevertheless, when the comparison between the observed and theoretical ellipticity corresponding to a certain soil model is confined to the analysis of the main resonant peak, more conventional HVSR processing techniques can be employed with confidence. The HVSR curve shown in

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Fig.12a was collected at the TN1 site and the seismic station was located at the center of the ellipticity computed

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MASW array as indicated in Fig.6a. It is compared with the fundamental mode Rayleigh wave for the TN1-MASW inverted model in Fig.11b. The theoretical ellipticity

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associated to the inverted model does not fit the low-frequency peak (1.5 Hz) of the HVSR, although the curve resembles a secondary peak located at around 2.5-3 Hz. The low frequency peak (about 1.5Hz) probably indicating that a deeper velocity contrast, since the inverted shear-wave velocity profile does not show values larger than 1000 m/s a Tino, whereas the massive bedrock generally exhibits larger values as in the Grisciano site. The HVSR data collected at the GR1 site (located in Fig. 6b) are shown in Fig. 13b where they are compared with the fundamental mode Rayleigh wave ellipticity computed for the GR1-DH downhole and the GR1-MASW inverted model (Fig. 10b and 11f, respectively). The main peak of the predicted ellipticity curves is coherent with the well-defined peak of the HVSR curve.

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Fig.13 Comparison of recorded HVSR from single-station seismic noise measurements (black

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lines) with the predicted ellipticity of the Rayleigh-wave fundamental mode from MASW inversions (red lines). (a) TN1 site. The MASW minimum misfit model is reported in Fig.11b. (b) GR1 site. The

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MASW minimum misfit model is reported in Fig.11f. The predicted ellipticity of Rayleigh waves

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5. DISCUSSION

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associated to the downhole model GR1-DH reported in Fig.10b is also displayed (grey line)..

The geological, geomorphological and geomechanical data as well as the results of the

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geophysical investigations pointed out that flexural toppling deformations involve a large portion of the ESE facing slopes in the areas of Tino and Grisciano hamlets. In these areas, the geomorphological delimitation of the deforming rock mass allows to estimate a volume in the order of 106 - 107 m 3 that corresponds to the rock mass characterized by intensely jointing conditions with respect to the surrounding bedrock. The increased jointing due to the ongoing deformations lead to a mostly isotropic behavior of the rock mass in terms of mechanical and dynamic properties. Deformations significantly modified the structural setting of the slope and intensified its jointing so that the specific dynamic behavior of the deforming rock mass is clearly revealed through ambient noise measurements. In both cases, the results obtained from single-station seismic noise measurements projected on the traces of sections of Figure 14 show that in the portion of the slope where deformations are ongoing a clear HVSR peak within a very narrow range (1.5-2.0 Hz in Tino and about 2.5 Hz in Grisciano) is observed. In the case of Tino, the analysis of weak-motions showed that ground-motion amplification occurs within the deforming rock mass in the same frequency range pointed out by ambient noise measurements. HVSR peaks are not observed in

Journal Pre-proof the portion of the slope closest to the scarps which bound the deforming rock mass uphill as well as immediately upslope. The time-frequency polarization analysis of the ambient noise measurements does not reveal any strong wavefield polarization. Similar results can be inferred from the directional analysis of the weak motions recorded at Tino inside the deforming rock mass, where the H/V amplitude variation with the azimuth is limited within 20% in the amplified frequency band (1-2 Hz). According to the finding by Burjánek et al. (2010), these results allow to exclude the presence of joint systems with strong anisotropy and high persistence in the deforming rock mass. In addition, it cannot be excluded that local portions of the deforming mass (i.e. dislodged by the trenches which identify different volume of the deforming mass) could be involved in separated vibrational modes which

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could explain the diffusely observed HVSR peaks in a higher frequency range (up to 3 Hz). Vs values for the rock mass involved in the deformational process were estimated by active seismic methods while no correlation was possible with the RQD as it was not determined on

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borehole cores. In the Grisciano hamlet, both MASW and DH tests were performed in the alluvial plain at the bottom of the slope involved in the rock mass deformation (Fig. 6). The obtained Vs

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profiles showed that the geological bedrock, overlain by alluvial gravel deposits, is found at 22 m

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depth, where the Vs increases from about 400 m/s to 900 m/s (Fig. 11, right panel). Moreover, the DH results showed a sharp transition at about 32 m depth between the jointed geologic al bedrock (SFALS) and the intact geological bedrock (ALS) with Vs of about 1500 m. This sharp Vs transition

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represents the buried limit between the toe of the rock mass deformation volume (SFALS) above the undeformed rock mass. This transition is observed at a different depth (28 m) in the MASW Vs

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profile, where Vs changes from 1200 to 1500 m/s (Fig. 12 bottom panel), likely due to the lower

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sensitivity of surface-wave method with respect to the bedrock Vs (Arai and Tokimatsu, 2005). In Tino hamlet, the Vs profiles were obtained in the uphill portion of the deformation zone located at about 950 m a.s.l. and on a terrace located at about 900 m a.s.l. (Fig. 6). The DH test provided Vs estimates for the jointed rock mass (SFALS) in the range 800-1000 m/s (Fig. 10 left panel), in agreement with the estimates obtained at Grisciano. The MASW showed Vs below 500 m/s for 45 m below ground level and an abrupt increase at larger depths (Fig. 12 b panel). This depth is the limit of the deforming rock mass volume and is consistent with the geomechanical interpretation which identifies the shear zone of the deforming rock mass at almost 50 m below ground level. In addition, the lower Vs values obtained for the SFALS in the shallower portion of the Tino slope prove that the increase of jointing of the rock mass may result in a strong reduction of the shearwave velocities of the geological bedrock. In particular, these values are lower than the threshold value of 800 m/s that is used to identify the seismic bedrock, according to the national seismic building code. The significant difference of Vs values for SFALS from DH and MASW measurement at Tino can be ascribed to a lateral change in jointing conditions as reported in Fig. 13, which upgrade the reconstruction of Fig. 4 only based on field surveying.

Journal Pre-proof Given this engineering-geological setting, it is worth noting that the HVSR peaks at lower frequencies, 1-2 Hz at Tino and 2-3 Hz at Grisciano, are mainly clustered within the deforming rock mass characterized by the higher jointing, where the Vs values may be strongly reduced with respect to the undeformed rock mass. This observation suggests that the SFALS volumes (corresponding to the deforming rock mass) could theoretically be responsible for the observed HVSR peaks and may cause, at least for the Tino case, a significant ground-motion amplification. At the same time, it cannot be excluded that deeper impedance contrasts (i.e. between differently jointed ALS) can justify the observed peaks at lower frequency (i.e. <3Hz). However, to provide a more general interpretation to the resulting site amplification , a 1D resonance phenomenon does not explain the observed fundamental frequencies, which are lower than those that can be

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estimated by the simplified equation under the 1D approximation:

where H is the thickness of a softsoil above a seismic bedrock and Vs is the soft soil shear-wave

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velocity. A more complex 2D-3D seismic response should be assumed, influenced by the geometry of the deforming rock mass as well as by the heterogeneity of its mechanical properties, although it

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is probable that at the Tino site a deeper velocity contrast in the bedrock can be responsible of the

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low-frequency resonant peak . A sound investigation of the observed resonance frequencies requires complex 3D numerical modelling of the site response in these areas, which is beyond the

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research.

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scope of the present study. Nevertheless, such analyses can be planned in a next stage of

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Fig.13 - Values of measured f0 from the ambient noise measurement performed at Tino (a) and Grisciano (b) projected preserving their altitude on the geomechanical cross sections obtained along traces AB and CD of Figure 3. For Tino rock mass jointing: low (Jv 1-10); middle (Jv 10-20); high (Jv 20-30); for Grisciano rock mass jointing: low (Jv 1-10); middle (Jv 10-15); high (Jv 15-20)

According to Kleinbrod et al. (2019), the obtained results allow to assume that the observed dynamic behavior of the deforming rock mass should be classified as mainly depth-controlled. Nevertheless, in the presented cases, the depth of the deforming rock mass volume is quite large, which poses them in an intermediate condition with respect to the above considered systematic classification.

Journal Pre-proof The data discussed above showed the complex conditions and peculiar dynamic behavior of slopescale MRC deformations which may affect several municipalities in the Central Apennines. These conditions suggest the need of a specific approach for resilience strategies. In practice, beyond the identification of first generation and reactivated conventional landslides, SM should also address the zonation of slope-scale MRC, distinguishing between the areas prone to groundmotion amplification and areas where the rock mass is also potentially unstable, as in the here presented case studies.

6. CONCLUSIONS

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The study of slopes involved in MRC is a complex issue as these areas, which are often urbanized

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in the Italian Apennine territory,

may be affected by local ground-motion amplification and local or generalized slope failure by

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earthquake triggering. This is the case of the studied areas, where the marly sandstones are affected by slope-scale flexural toppling and related rock-mass damaging which causes a drastic

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reduction of the shear-wave velocity with respect to the undeformed rock mass. A more advanced evolutionary stage of the flexural toppling can be observed at Grisciano, where the deforming rock

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mass already involved the valley floor causing a bulging and influencing the watercourse of the

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Tronto River.

Frequency dependent ground-motion amplification was observed in the highly damaged Tino

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hamlet by weak-motion analysis; the average lower bound of the amplification level is 4 in the 1-2 Hz frequency range. This ground-motion amplification level and the frequency band of maximum

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amplification could be attributed to a complex effect involving the 3D geometry of the deforming rock mass. More in particular, such an amplification is likely controlled by: i) the dimension of the deforming volume, ii) the distribution of mechanical properties inside the volume with respect to the surrounding undeformed material and iii) the morphology of the slope itself. The dynamic behavior of the deforming rock mass may be studied by HVSR analysis of seismic noise records. In fact, clear HVSR peaks were observed within the ongoing deformation zone in both the investigated slopes and the peak frequency range agrees with that of the observed maximum ground-motion amplification in the case of Tino. Therefore, the integration of rapid and low-cost HVSR analysis of seismic noise records with geomorphological and geomechanical surveys represents a very effective approach to define boundaries of deforming rock masses , responsible for ground-motion amplification. In the framework of SM, an exhaustive zonation approach of slopes affected by MRC should distinguish areas with different combinations of seismic response in terms of both ground-motion amplification and coseismic failure. Such an approach could integrate the National SM guidelines

Journal Pre-proof presently adopted for conventional landslides, that are analyzed in terms of expected coseismic displacement under dynamic conditions.

7. Acknowledgments The Authors wish to thank the Municipality of Accumoli for authorizing field surveys and investigations; J. Burjànek for the constructive discussion on the collected data; A. Screpanti for his technical support to field measurements. DH and MASW results were provided by the Center for Seismic Microzonation and its applications (CNR-IGAG). In the present study: S. Martino was the scientific responsible and coordinator of the res earch group; D. to

engineering-geological

and

geomorphological

surveys

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contributed

and

to

geophysical

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investigations in the framework of his master thesis; M. Cercato, S. Hailemikael, R. Iannucci, G. Martini, A. Paciello performed geophysical investigations and analyses; C. Esposito, S. Martino, G. Scarascia

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Mugnozza performed engineering-geological surveys and modelling; M. Della Seta, F. Troiani performed

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geomorphological surveys and mapping.

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8. REFERENCES

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Adhikary, D.P., Dyskin, A.V., Jewell, R.J., Stewart, D.P., 1997. A Study of the Mechanism of Flexural Toppling Failure of Rock Slopes. Rock Mech. Rock Eng., 30(2), 75-93 Andrews, D. J., 1986. Objective determination of source parameters and similarity of earthquakes of different size. In S. Das, J., Boatwright, and C. H. Scholtz, editors, Earthquake Source Mechanics, volume 6, pages 259 –267. American Geophysical Union, Washington D. C., 1986 Arai, H., and Tokimatsu, K. (2005), S ‐ wave velocity profiling by joint inversion of microtremor dispersion curve and Horizontal-to-Vertical (H/V) spectrum spectrum, Bull. Seismol. Soc. Am., 95, 1766– 1778 Aringoli, D., Cavitolo, P., Farabollini, P., Galindo-Zaldivar, J., Gentili, B., Giano, S.I., Lopez-Garrido, A.C., Materazzi, M., Nibbi, L., Pedrera, A., Pambianchi, G., Ruano, P., Ruiz-Constan, A., Sanz de Galdeano, C., Savelli, D., Tondi, E., Troiani, F., 2014. Morphotectonic characterization of the quaternary intermontane basins of the Umbria -Marche Apennines (Italy). Rendiconti Lincei 25/2, 111-128 Bogoslovsky, V.A., Ogilvy, A.A., 1977. Geophysical methods for the investigation of landslides. Geophysics 42(3), 562 – 571 Boncio, P., Lavecchia, G., Pace, B. 2004. Defining a model of 3D seismogenic sources for seismic hazard assessment applications: the case of central Apennines (Italy). J Seismol 8, 407–425 Bonnefoy-Claudet, S, Cornou, C, Bard, P.Y, 2006. H/V ratio: A tool for site effects evaluation. Results from 1 -D noise simulations. Geophysical Journal International 167, 827 -837 Booth, A.M., Lamb, P.M., Avouac, J.P., Delacourt, C., 2013. Landslide velocity, thickness, and rheology from remote sensing: La Clapière landslide, France. Geophysical Research Letters 40, 4299 –4304 Borcherdt R.D., 1970. Effects of local geology on ground-motion near San Francisco Bay, Bull. seism. Soc. Am. , 60(1), 29–61 Bottelin, P., Jongmans, D., Baillet, L., Lebourg, T., Hantz, D., Lévy, C., Le Roux, O., Cadet, H., Lorier, L., Rouiller, J. -D., Turpin, J., Darras, L., 2013. Spectral analysis of prone-to-fall rock compartments using ambient vibrations. J. Environ. Eng. Geophys. 18(4), 205-217 Burjánek, J., Gassner-Stamm, G., Poggi, V., Moore, J.R., Fäh, D., 2010. Ambient vibration analysis of an unstable mountain slope. Geophys. J. Int. 180(2), 820-828 Burjánek, J., Moore, J.R., Molina, F.X.Y., Fäh, D., 2012. Instrumental evidence of normal mode rock slope vibration. Geophys. J. Int. 188, 559–569 Cara, F., Cultrera, G., Riccio, G., Amoroso, S., Bordoni, P., Bucci, A., D’Alema, E., D’Amico, M., Cantore, L., Carannante, S., Cogliano, R., Di Giulio, G., Di Naccio, D., Famiani, D., Felicetta, C., Fodarella, A., Franceschina, G., Lanzano, G., Lovati, S., Luzi, L., Mascandola, C., Massa, M., Mercuri, A., Milana, G., Pacor, F., Piccarreda, D., Pischiutta, M., Pucillo, S., Puglia, R., Vassallo, M., Boniolo, G., Caielli, G., Corsi, A., de Franco, R., Tento, A., Bongiovanni, G., Hailemikael, S., Martini, G., Paciello, A., Peloso, A., Verrubbi, V., Gallipoli, M.R., Tony Stabile, T.A., Mancini, M., 2019. Temporary dense seismic network during the 2016 Central Italy seismic emergency for microzonation studies. Nat Sci Data, 6-1, pp. 182. Centamore, E., Fumanti, F., Nisio, S., 2002. The Central-Northern Apennines geological evolution from Triassic to Neogene time. Boll Soc Geol It 1, 181–197

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Cercato, M. 2011. Global surface wave inversion with model constraints. Geophysical Prospecting 59, 210 -226. Cercato, M., 2018. Sensitivity of Rayleigh wave ellipticity and implications for surface wave inversion. Geophysical Journal International, 213, 489-510 Chigira, M., 1992. Long term gravitational deformations of rock mass by mass rock creep. Engineering Geology 32, 157 184 Colombero, C., Baillet, L., Comina, C., Jongmans, D., Larose, E., Valentin, J., Vinciguerra, S., 2018. Integration of ambient seismic noise monitoring, displacement and meteorological measurements to infer the temperature controlled long-term evolution of a complex prone-to-fall cliff. Geophys. J. Int., 213(3), 1876-1897 D’Amico, S., Panzera, F., Martino, S., Iannucci, R., Paciello, A., Lombardo, G., Galea, P., Farrugia, D., 2018. Chapter 12: Ambient noise techniques to study near-surface in particular geological conditions: a brief review. In.: Innovation in near-surface geophysics, Persico R., Piro S. and Linford N. Eds., 419-460 Del Gaudio, V., Muscillo, S., Wasowski, J., 2014. What we can learn about slope response to earthquakes from ambient noise analysis: An overview. Engineering Geology 182, 182 -200 Del Gaudio, V., Luo, Y., Wang, Y., Wasowsky, J., 2018. Using ambient noise to characterise seismic slope response: The case of Qiaozhuang peri-urban hillslopes (Sichuan, China). Engineering Geology 246, 374 –390 Delacourt, C., Allemand, P., Casson, B., Vadon, H., 2004. Velocity field of the ‘‘La Cl apiére’’ landslide measured by the correlation of aerial and QuickBird satellite images. Geophysical Research Letters 31, L15619 Delgado, J., Garrido, J., Lenti, L., Lopez-Casado, C., Martino, S., Sierra, F.J., 2015. Unconventional pseudostatic stability analysis of the Diezma landslide (Granada, Spain) based on a high -resolution engineering-geological model. Engineering Geology 184, 81–95 El Bedoui, S., Guglielmi, Y., Lebourg, T., Perez, J.L., 2008. Deep-seated failure propagation in a fractured rock slope over 10,000 years: The La Clapière slope, the south-eastern French Alps. Geomorphology 105, 232–238 Esposito, C., Martino, S., Pallone, F., Martini, G., Romeo, R., 2016. A methodology for a comprehensive assessment of earthquake-induced landslide hazard, with an application to pilot sites in Central Italy. In: Landslides and engineered slopes. Experience, theory and practice, Taylor and Francis Inc. 2: 869 –877 Evans, S.G., Scarascia Mugnozza, G., Strom, A., Hermanns, R.L., 2006. Landslides from Massive Rock Slope Failure. Nato Science Series IV. pp. 662 Field, E.H., Jacob, K.H., 1995. A comparison and test of various site -response estimation techniques, including three that are not reference-site dependent. Bulletin of the Seismological Society of America, 85, 1127 –1143 Galadini, F., Galli, P. 2003. Paleoseismology of silent faults in the central Apennines (Italy): the Mt. Vettore and Laga Mts. faults. Annals of Geophysic 46, 815–836 Galea, P., D’Amico, S., Farrugia, D., 2014. Dynamic characteristics of an active coastal spreading area using ambient noise measurements -Anchor Bay, Malta. Geophys. J. Int. 199, 1166-1175 Goodman, R.E., Bray, J., 1976. Toppling of rock slopes. ASCE, Proc. Specialty Conf. on Rock Eng. for Foundations and Slopes, Boulder, CO, 2, 201–34 Goldstein, P., Snoke, A., 2005. SAC Availability for the IRIS Community. Incorporated Research Institutions for Seismology Newsletter, UCRL-JRNL-211140, pp 6

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Journal Pre-proof Highlights

Results of local seismic response in a rock mass deforming area are presented . In hole and surface measurements were used and compared. A conceptual interpretative model was based on a detailed engineering -geological reconstruction of the deforming slopes.

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Local seismic amplification resulted in the deforming slopes usable for seismic microzonation studies .

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