3d GeoPhysicAl modellinG of The vAJonT lAndslider. francese, m. Giorgi, G. böhm OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy

Introduction and Motivations. The 1963 Vajont landslide, mostly because of its size and of the catastrophic effects, has been studied for several tens of years by many different authors (Selli and Trevisan, 1964; Rossi and Semenza, 1965; Martinis, 1978; Hendron and Patton, 1985; Semenza and Ghirotti, 2000). The majority of the studies focused on the triggering mechanisms (Kilburn and Petley, 2003) and on the post-failure geology. A comprehensive review is given by Semenza and Ghirotti (2005). Although the collapse has been largely studied some of the factors controlling the dynamic of the movement are still not completely clear. Among the major issues there is the high velocity of the sliding mass itself and the movement almost as a unitary rock block that caused such an unexpected large wave.

A geophysical parameterization of the landslide body could result in a better insight in both in the understanding of the geometry settings and the topology of the collapsed units and in a better estimate of the changes of the elastic properties caused by the collapse. This last information could be used as a vital constrain in the recently developed collapse model. Very few geophysical data are presently available for the landslide body. Some measurements of the P- and S- wave velocity were undertaken on the northern scarp of the Monte Toc before the collapse but with conflicting results and no clear indications about the rock quality (Caloi and Spadea, 1960, 1961). After the collapse, on behalf of the Court of Belluno, was carried out a seismic campaign with P-wave velocity and borehole sonic measurements on the landslide body (Morelli and Carabelli, 1965).

A new and comprehensive geophysical investigation, based on 2D and 3D seismic and resistivity imaging was then undertaken since the year 2011 and it’s still in progress. This geophysical experiment was designed and conducted integrating the re-interpreted geological (Bistacchi et al., 2013) and structural data (Massironi et al., 2013). A novel series of borehole stratigraphies made recently available by ENEL were also incorporated in the geophysical modelling. Finally some accurate surface models obtained processing aerial and terrestrial laser data were extremely useful to constrain the inversion of resistivity and seismic data and to properly account for the deformation of the electrical field caused by the rough topography.

A geophysical profile collected along the rock wall below Casso on the other side of the Vajont valley was used a reference for the geophysical response of the geological units involved in the landslide. The geophysical images of the landslide body, especially in the near surface, showed a very good correlation with the post-failure geology (Rossi and Semenza, 1965).

An initial correlation between lithology and physical parameters has been proposed for the various geological units embedded in the landslide mass. This 3D physical model of the landslide introduces a series of new constrains for an accurate numerical simulation of the landslide kinematics.

Geological setting. Geology in the Vajont valley is comprised of a Jurassic-Cretaceous carbonate sequence (Carloni and Mazzanti, 1974; Semenza, 1965; Martinis, 1978). The thickness of the various formations, at the landslide scale, could be considered roughly constant. The base of the Jurassic sequence is marked by a massive (Vajont Formation) limestone (350-400 m) overtopped by a layered cherty (10-40 m) limestone (Fonzaso Formation) and followed by the nodular limestone of the Ammonitico Rosso Formation (15 m). The Cretaceous sequence is comprised of the Soccher Limestone (200-250 m) and of the layered marly limestones and marls of the Scaglia Rossa Formation (about 300 m). The upper part of the Fonzaso Formation and the Soccher Limestone Formation were involved in the landslide. Rossi and Semenza (1965), analyzing the post-failure accumulation, mapped six different lithological members indicated with letters from a to f (from the older to the youngest).

190

GNGTS 2013 SeSSione 3.3

131218 - OGS.Atti.32_vol.3.27.indd 190 04/11/13 10.39

Some very thin clayey interbeds were identified in the upper part of the Fonzaso Formation (unit a’) and indicated by Hendron and Patton (1985) as the stratigraphic level where the initial sliding happened. There are just few outcrops of this unit as during the failure the unit was dragged and crunched along the sliding surface. The bottom level of unit a’’ is represented by the Ammonitico Rosso Formation (Rossi and Semenza, 1965) while the body of unit a’’ is comprised of an alternation of layered cherty limestone and marly limestones. Unit b is represented by a thin conglomerate layer and it is very important because it represents an isochron surface and it’s a marker level within the landslide. Units c, d and e are comprised of massive limestones grading to layered marly and cherty limestones. Finally unit f represents the top of the Soccher Formation and it is comprised of layered marly cherty limestones.

A review of available structural data, associated with some new field observations, has been recently completed (Massironi et al., 2013). According to these data the E-W trending Erto syncline (Giudici and Semenza, 1960) is further folded by a N-S trending syncline with it’s axis elongated along the pre-landslide Massalezza valley. The interference of these two sets is exposed on the sliding surface and in some cases the undulations generated steps that interrupted the continuity of the surface itself.

The ensemble of these structural features appears to be a major constrain in the kinematics of the landslide. In particular the association of the stratigraphy and of the N-trending bedding planes with the curved shape of the sliding surface converging on the Massalezza valley behave as a track for the 1963 failure.

The physical database. The success of the geophysical experiment, in reconstructing the landslide stratigraphy and structure, strongly depends on the possibility of measuring some key properties of the in-situ formations. A reliable and accurate physical reference model of the upper part of the Fonzaso Formation and of the Soccher Limestone Formation was then constructed to reduce the uncertainty in correlating physical properties (e.g. velocity and resistivity) to specific lithological units. This robust tie could be also a constrain while attempting to estimate the degree of fracturing of the landslide mass itself. The first indicator of physical properties is the rock coherency. Hard rocks (HR) will be probably characterized both by high velocity and high resistivity while soft rocks (SR) are generally low velocity and low resistivity. In this simplified approach units a’, a’’ and f could be classified as moderately soft rocks while units b, c, d and e are mostly referable to moderately hard rocks.

Some of the boreholes drilled on the landslide accumulations where also used as major constrains in assisting geophysical data analysis, processing and interpretation. The majority of these boreholes reached the depth of the in-situ bedrock.

The few geophysical data available for the left side of the Vajont valley, before the failure, are represented by four seismic profiles collected immediately after the discovery of the paleolandslide (Caloi and Spadea, 1960, 1961). Results from a first seismic survey indicated how the P-wave velocity in the uppermost layers was higher than 5000 m/s. Some authors (Semenza, 1965; 2001; Selli and Trevisan, 1964) pointed out that these values appear to be too high also for compact limestones. A second seismic survey (Semenza, 2001) carried out after the discovery of the perimetrical crack, showed more realistic P-wave velocities around 2500-3000 m/s. The Court of Belluno to collect new evidences for the trail during the preparation of the case required a new seismic survey. The major purpose of this new survey was to collect data inside and outside the landslide area. Seismic velocities were measured at few locations (Morelli and Carabelli, 2005) inside and outside the landslide area. Unfortunately the measurements on the landslide body were carried out in boreholes mostly below the sliding surface making this new data not directly comparable with the previous surveys. Other measurements carried out on the Cretaceous sequence outside the landslide show P-wave velocities ranging from 2100 m/s to 3000 m/s. These numbers are more and less similar to the values measured during the second seismic survey before the failure.

191

GNGTS 2013 SeSSione 3.3

131218 - OGS.Atti.32_vol.3.27.indd 191 04/11/13 10.39

Under a theoretical point of view (Telford et al., 1990; Dobrin and Savit, 1988) the seismic and electrical properties of a medium mostly depend upon density (also related to lithology and age), porosity and fluid content. Water in the Vajont landslide body is almost absent due to the very high permeability of the collapsed mass. The water table in the accumulation is directly related to the water level in the residual lake located eastern than the landslide body. Several attempts to measure the water level in the boreholes were made but a water table has been never detected. The unknowns are then two: lithology and porosity (or fracturing) and in the equation there is just one parameter (velocity and resistivity). To reduce the uncertainty resistivity and seismic velocities should be measured on the same formations but outside the landslide. For this purpose a reference geophysical profile was then collected along the rock wall below the village of Casso. On this rock wall there is almost a full exposure of the geological sequence involved in the landslide. Unit a’ is the only missing as it’s covered by the talus deposits.

Geophysical data acquisition and processing. Geophysical data acquisition was not straightforward due to the complex morphology of the accumulation and of the associated complications in coupling electrodes and geophones.

The geophysical reference profile, along the rock wall, was comprised of a 24-channel seismic line and of a 48-electrodes electrical line. The spacing was set to 10 m and to 5 m in the seismic line and in the electrical line respectively. Three component geophones were firmly tightened with the rock using special screws while the electrodes were hammered into holes filled with conductive medical gel. Data resulted good quality and the tomographic inversion of both the two datasets was carried out with a misfit lower than 5%.

The landslide body is comprised of three major lobes: the Massalezza lobe and two separate masses (defined as the “eastern lobe” and the “western lobe”) probably collapsed just after the washout of the Massalezza lobe (Semenza, 1965, 2001).

Fig. 1 – Key features of the geophysical campaign on the Vajont landslide.

192

GNGTS 2013 SeSSione 3.3

131218 - OGS.Atti.32_vol.3.27.indd 192 04/11/13 10.39

Seismic data were collected on the Massalezza lobe only (lobe A in Fig. 1) using a DMT Summit modular system with more than 200 double channel A/D conversion units. Each receiving station was equipped with a three component 10-Hz geophone to detect the incoming signal. The three component sensors were laid out along four lines (L100, L200, L300, L400). The average station spacing was 10.0 m for a total of 276*3 live channels. Elastic waves were generated and propagated into the ground using a medium size vibrator (both in P-wave and S-wave modes). The source stations were located along the roads bounding and crossing the Massalezza lobe.

Recorded seismic data were generally of good quality; first breaks, in P-wave mode, were sharp and easy to pick even at offsets larger than 500 m while in S-wave mode the signal was slightly lower amplitude. A total of about 80000 first arrivals were picked and pre-processed and lately inverted using a 3D traveltime tomography. Inversion was carried out with the CAT3D proprietary software. Data resolution was improved using staggered grids (Vesnaver and Bohm, 2000).

Resistivity data were collected on the two landslide lobes (A and B in Fig. 1) using a 48-electrode Syscal R1 system, a 96-electrode Syscal Pro system and an experimental 36-electrode wireless resistivity system manufactured by MultiPhase Technologies LLC. A total of six ERT profiles, in Wenner and in Wenner-Schlumberger configuration, and two ERT volumes, in pole-dipole configuration, were collected on the Massalezza lobe. Three additional ERT profiles and a ERT volume were collected on the eastern lobe. Data were collected in separate sessions during early spring and middle autumn after several days of heavy rain to improve the coupling. The high permeability of the landslide mass allowed for assuming the subsurface layers as homogeneously wet.

Recorded resistivity data also resulted of good quality and just few points needed to be removed from the dataset prior to run the inversions. The inversion was carried out using the package ERTLAB+ that is based on a sophisticated reweight (Morelli and LaBreque, 1996) of the inversion parameters at each iteration.

Results and discussion. Unit a’’ in the reference section appears to be low both in resistivity (0.5-1.0 KΩ·m) and P-wave velocity (2200-2700 m/s). Unit b, is very thin and, due to the large sensor spacing, is outside the resolution capability of both the two techniques. Units c, d and e in the middle part of the section are moderately compact limestones and appear show a similar geophysical response. In these units the resistivity is fairly high (2.5-4.5 KΩ·m) as well as the P-wave velocity (3400-3800 m/s). At the top of the rock wall, where unit f outcrops, there is a sudden lowering of both the resistivity (0.5-1.0 KΩ·m) and the velocity (2300-2800 m/s).

The terrain resistivity in the landslide accumulation exhibit a large degree of fluctuation. The majority of the values range from 0.15-0.20 KΩ·m to 3.50-4.00 KΩ·m.

The resistivity distribution along the profile ERT1 (Fig. 2) appears to be quite complex and the Average values are slightly lower as compared to the reference profile. This is somewhat related to the fracturing and to the severe changes occurred in the geological layers involved in the landslide. Unfortunately along profile ERT1 there are no borehole data available to constrain the interpretation. The deep conductive unit a’’ was utilized to define the initial geological layout along the profile. The prominent structure is a narrow syncline fold located in the middle portion of the profile. The axis is probably elongated along the old Massalezza valley. The bottom layers are probably belonging to the a’’ unit while in the top layers there are the typical lithologies of the c unit. Some resistive bodies could be associated with a partly undifferentiated c-d-e lithological sequence while the conductive unit, visible in the distance interval 50-200 m, according to Rossi and Semenza (1965) still belongs to units c, d and e. Possible explanation of this anomaly are the high degree of fracturing in the landslide or the occurrence of lateral changes of lithology moving from the right to the left side of the Vajont valley. These folding with an apparent N-S trend of the lithological units is also visible on the sliding surface (Massironi et al., 2013). The preservation of the structural settings, before and

193

GNGTS 2013 SeSSione 3.3

131218 - OGS.Atti.32_vol.3.27.indd 193 04/11/13 10.39

after the 1963 failure, confirm the hypothesis of a rigid roto-translation of the collapsed mass. The large anomalous thickness of the c-d-e sequence is probably caused by some detachment phenomena occurred along existing or newly developed high-angle discontinuities (Rossi and Semenza, 1965; Martinis, 1978) during the failure. The profile also show some low angle detachment planes (see for example at the x-coordinate interval 200-300 m) that could explain the anomalous thickness of the c, d and e units in depth.

The resistivity distribution along profile ERT5 (Fig. 2), crossing lobe B, is more homogeneous and it’s more and less comparable with the post-failure geological map of Rossi and Semenza (1965). The outcrops are represented by an undifferentiated unit a covering almost the entire lobe B (Fig. 1) and by some minor exposures of the unit b. The general structure appears to be folded up into an anticline with some major displacements. In this case also the geometry of the bottom layers of lithological unit a’’ was utilized as the prominent geophysical marker to define the settings. In the reference section unit a’ is not exposed and hence there are no indications about its possible geophysical response. Carloni and Mazzanti (1964) suggest a total thickness of about 80-90 for the undifferentiated unit a with a maximum thickness of unit a’’ around 50 m. According to several other authors (Rossi and Semenza, 1965; Martinis, 1978; Hendron and Patton, 1985) unit a’ is similar to unit a’’ with the sole difference of the presence of the interbedded clays in the base layers. The two units should then exhibit a comparable geophysical signature. The increase of the resistivity in the deeper portion of the profile (where unit a’ was expected) is rather difficult to explain. There are possible explanations: the upper part of unit a’ is more resistive than expected; more realistically unit a’’ overthrusts the resistive terrains belonging to the c-d units in the deeper portion of profile. Several high and low angle discontinuities are also visible in the profile. These planes disrupt the former continuity of the geological layers generating an ensemble of pop-up like structures. Unfortunately on this lobe the sliding surface is not constrained by post-landslide data and also during the failure occurred a partial overlap of lobe B on the Massalezza lobe. The deeper portion of the geophysical image could be then really complex.

Fig. 2 – 2-D Resistivity inversion images along tomography ERT 1 (top) and ERT 5 (bottom) (see Fig. 1). The investigated landslide mass could be then considered dry.

194

GNGTS 2013 SeSSione 3.3

131218 - OGS.Atti.32_vol.3.27.indd 194 04/11/13 10.39

The P-wave velocity field in the landslide accumulation range from 700-1000 m/s to 3500-4000 m/s. Seismic data resulted comparable to electrical data with a reasonable degree of confidence.

The correspondence between resistivity and P-wave velocity has been analysed in details along the section of profile ERT1 (Fig. 3). A profile was extracted from the 3D velocity model and compared to the 2D ERT. The syncline structure below the Massalezza ditch is visible also in the seismic data but it is not completely resolved as in the resistivity image probably because the geophone line is located 60 m northern of the electrode line. Right in the middle of the P-wave section there is a high-velocity bottom layer that is not visible in the resistivity section. The high-velocity layer is located at a depth where the signal to noise ratio of the resistivity data is very low. It is known that a deep resistive layer below a conductive layer requires very large AB spacing to be sampled by an electrical field that is forced into the conductor. The seismic data are more reliable because there is a geophone line exactly along the Massalezza ditch (Fig. 1) and the sources are located on the nearby road. This high velocity layer (or high resistivity) below unit a’’ is quite difficult to explain without assuming the presence of a detachment plane that duplicate the sequence.

Conclusion. The study of a large landslide based on the 2D and 3D geophysical parameterization of the involved geological units is particularly difficult due to the expected complexity of a chaotic accumulation. The Vajont landslide, given its large volume, was even more complicated. The different units involved in the landslide, due to their lithological changes were expected to have a distinct geophysical signature. The reference section collected along the rock wall below the village of Casso confirmed this hypothesis. The pre-slide geological sequence from the bottom to the top is a sort of sandwich of “conductive/low velocity” – “resistive/high velocity” – “conductive/low velocity” layers.

The initial correlation of the geophysical images from the landslide body with the post-failure geology confirmed the observations from the reference section. The conductive unit a’’ is an excellent geophysical marker to guide the interpretation. The pre-landslide stratigraphy appears to be quite well preserved in the shallow layers while in depth the geophysical response is rather complex. Both the resistivity and the seismic images along profile ERT1 highlight a syncline that is fully exposed on the sliding surface. The geophysical images along profile ERT1 also show a series of small-scale folds with north-south axes that are probably pre-landslide as the stress occurred during the failure folded the strata generating east-west axes. This mode of folding is clearly visible in resistivity profile ERT5, collected on lobe B.

These results are satisfactory but further investigations are anyhow required to achieve a reliable reconstruction of the landslide accumulation settings. Unfortunately borehole data collected, before and after the landslide, are of limited use because of the high lateral variability of the physical properties occurring in the collapsed mass. This is somewhat confirmed by the borehole stratigraphy reconstructed by mean of micropalaeontological analysis of drilled

Fig. 3 – Comparison between 3D seismic tomography (top) and 2D electrical resistivity tomography (bottom) response. The profiles are oriented from W to E. The geophysical data are presented as 10m by 10m cell scalars without interpolation. The two sections are computed along the trace of profile ERT1.

195

GNGTS 2013 SeSSione 3.3

131218 - OGS.Atti.32_vol.3.27.indd 195 04/11/13 10.39

samples. Results of this last analysis suggests the existance of several duplication of the original in-situ sequence.

Further work that will be undertaken in a very near future includes: 1) geophysical parameterization of unit a’, 2) inversion of the electrical data into a 3D resistivity volume of the entire landslide, 3) correlation of the seismic and of the resistivity responses to improve interpretation and finally 4) processing of S-wave data in order to estimate the elastic parameters of the landslide body.Acknowledgements. We acknowledge the Friuli Venezia-Giulia Region for providing the funding for the project (project 35935/2010). A special thank to Alessia Rosolen for her personal support and to Ketty Segatti. A final thank to Giovanni Rigatto and to Nuccio Bucceri for their assistance during topographic measurements.referencesBistacchi, A., Massironi, M., Superchi, L., Zorzi, L., Francese, R., Giorgi, M., Genevois, R., and Chistolini, F., 2013. A 3D

Geological Model of the 1963 Vajont Landslide. Vajont 2013 Conference.Caloi, P., and Spadea, M., C., 1960. Serie di esperienze geosismiche seguite in sponda sinistra a monte della diga del Vajont

(dicembre 1959). SADE Technical Report. Caloi, P., and Spadea, M., C., 1961. Indagine geosismica condotta nel mese di dicembre 1960 a monte della diga del Vajont

in sponda sinistra. SADE Technical Report. Carloni, G., C., and Mazzanti, R., 1964. Rilevamento geologico della frana del Vajont, In Selli R. et al., “La frana del

Vajont”, Giornale di Geologia, 2, 32/1.Castellanza, R., Agliardi, F., Bistacchi, A., Massironi, M., Crosta, G.B., and Genevois, R., 2013. 3D finite-element

modelling of the Vajont landslide initiation stage. Vajont 2013 Conference.Dobrin, M.B., and Savit, C.H., 1988. Introduction to Geophysical Prospecting – IV Edition. McGraw-Hill, 867 pp.Genevois R., and Ghirotti, M., 2005. The 1963 Vaiont Landslide, Giornale di Geologia Applicata 1, 41-52.Giudici, F., and Semenza, E., 1960. Studio geologico del serbatoio del Vajont. SADE Technical Report.Hendron A.J., and Patton, F.D., 1985. The Vaiont slide, a geotechnical analysis based on new geological observations of

the failure surface. Tech. Rep. GL-85–5, Department of the Army, US Army Cops of Engineers, Washinghton D.C., 2 vols.

Kilburn, C.J., and Petley, D.N., 2003. Forecasting giant, catastrophic slope collapse: lessons from Vajont, Northern Italy. Geomorphology, 54, 1-2, 21-32.

Martinis B., 1978. Contributo alla stratigrafia dei dintorni di Erto-Casso (Pordenone) ed alla conoscenza delle caratteristiche strutturali e meccaniche della frana del Vajont. Memorie di Scienze Geologiche, Università di Padova, 32, 1-33.

Massironi, M., Superchi, L., Zampieri, D., Bistacchi, A., Ravagnan, R., Bergamo, A., Ghirotti, M., and Genevois, R., 2013. Geological Structures of the Vajont landslide. Vajont 2013 Conference.

Morelli, C., and Carabelli, E., 1965. Misure di velocità delle onde elastiche longitudinali nella roccia del bacino del Vajont. Court of Belluno. Lerici Foundation. Research 410.

Morelli, G., and LaBrecque, D. J., 1996. Robust scheme for ERT inverse modelling. European Journal of Environmental and Engineering Geophysics, 2, 1-14.

Müller, L., 1959. Talsperre Vaiont – 6 Geotechnischer Bericht. SADE Technical Report.Müller, L., 1964, The rock slide in the Vaiont valley. Felsmechanik und Ingenieur-geologie, 2, 148-212.Petley, D.N., and Petley, D.J., 2006. On the initiation of large rockslides: perspectives from a new analysis of the Vaiont

movement record. In Massive Rock Slope Failure. Mugnozza, G., Strom, A. & Hermanns, R. L. Rotterdam. NATO Science Series, Earth and Environmental Sciences, 49, 77-84.

Rossi, D., and Semenza, E., 1965. Carte Geologiche del versante settentrionale del M. Toc e zone limitrofe prima e dopo il fenomeno di scivolamento del 9 Ottobre 1963. Scala 1:5000. Ist. Geol. Univ. Ferrara, Italy.

Semenza, E., 1960. Nuovi studi tettonici nella valle del Vajont e zone limitrofe. Acc. Naz. Lincei, Rend. Cl. Sc. MMFFNN, 8, 28/2.

Semenza, E., 1965. Sintesi degli studi geologici sulla frana del Vaiont dal 1959 al 1964. Memorie del Museo Tridentino di Scienze Naturali, 16, 1-52.

Semenza, E., 2001. La storia del Vaiont raccontata dal geologo che ha scoperto la frana. Tecomproject Ed., Ferrara, Italy. 279 pp.

Semenza, E., and Ghirotti M., 2000. History of the 1963 Vaiont Slide. The importance of the geological factors to recognise the ancient landslide. Bulletin of Engineering Geology and the Environment, 59, 87-97.

Telford, W., M., Geldart. L., P., and Sheriff, R., E., 1990. Applied Geophysics – II edition. Cambridge University Press, 770 pp.

Vesnaver, A., and Bohm G., 2000. Staggered or adapted grids for seismic tomography?, The Leading Edge, 19, 9, 944-950.

196

GNGTS 2013 SeSSione 3.3

131218 - OGS.Atti.32_vol.3.27.indd 196 04/11/13 10.39