DOI: 10.5061/dryad.3r2280ggn
title: The 30 December 2002 tsunamigenic landslides of Stromboli volcano: A reappraisal

description: Volcanic mass flows constitute an important trigger of tsunamis, resulting
 in more local, although sometimes larger, impacts comparable to
 earthquake-induced tsunamis. Bearing in mind the destructive potential of
 these phenomena, the tsunamigenic landslide that occurred on 30 December
 2002 on the unstable NW slope of Stromboli volcano in Italy has been
 re-examined here, by means of unpublished helicopter-borne (visible and
 thermal) images, and slope stability analysis. The main result of this
 study is that the sequence of landslides triggering the 2002 Stromboli
 tsunami can be defined as the final stage of a lateral magma intrusion
 that exerted a high thrust at high altitude, destabilizing the entire
 slope. This study, allowing a fuller understanding of the event that took
 place in Stromboli on 30 December 2002, also allows us to generalize the
 results, making a contribution to comprehending the volcanic edifice
 failures that can trigger tsunamis.
description: The slope’s
 stability at Stromboli is evaluated with Slope Stability Analysis Program
 (SSAP; Borselli et al., 2020), a free software that investigates the
 Factor of Safety (FS) of sliding surfaces through bi-dimensional LEM
 analysis by assigning the geo-mechanical parameters (according to
 Mohr-Coulomb, generalized Hoek and Brown or Barton-Bandis non-linear
 failure criterion), setting the water table position, the loading
 conditions and the presence of retaining structures, already used to
 estimate the stability of volcanic edifices in different geological
 contexts (Borselli et al., 2011; Dondin et al., 2017). In this study, the
 Factor of Safety is estimated through the Morgenstern–Price method
 (Morgenstern and Price, 1965), a general method of slices developed on the
 basis of limit equilibrium. It requires satisfying the/an equilibrium of
 forces and moments acting on individual blocks. The research of critical
 surfaces is carried out through the “Random Search” engine, that generates
 possible sliding surfaces through the Monte Carlo methods. SSAP generally
 adopts a feature called “Dynamic Surface Search Attractor” that
 progressively reduces the initial research area set by the user or
 provided automatically by the software, according to the surfaces with
 lower FS gradually identified. This function is extremely useful because
 it allows the user to find ever more critical surfaces within a specific
 area. At the same time, the software tends to discard potentially unstable
 areas if not identified in the early stage of the research. SSAP gives as
 output a graphical representation of the surfaces with lowest FS, internal
 distribution of forces and pressure, distribution of local FS, local
 distribution of the numerical reliability of general FS numerical solution
 (namely RHO index) and a 2D color map with distribution of average local
 FS obtained by local stress distribution, defined as quasi-Finite Element
 Analysis (qFEM; Schofield and Wroth, 1968; Griffiths and Lane,1999) and
 over stress ratio (OSR) maps (Farias and Naylor, 1998). These maps, based
 on a combination of LEM and FEM procedures, can highlight the presence of
 local areas within the slope that may display a critical state of stress.
 Although the actual state of deformation is not calculated by the software
 (SSAP does not allow the user to include the strain’s parameters in the
 slope characterization), this feature provides a graphical representation
 of those areas in which progressive failure can originate. Based on
 Schofield and Wroth (1968) and Griffiths and Lane (1999), the qFEM map
 describes the local distribution of the FS, including a representation of
 the potential direction of plasticization for those areas where
 FS&lt;1. A local OSR map, based on Farias and Naylor (1998), exhibits
 in terms of local mean stress (principal stress and path stress) the areas
 where the maximum local shear stress is greater than the local shear
 strength. The areas with OSR &gt;1.0 are the most likely to develop
 progressive failure. The slope’s
 topography is obtained by digitalizing the contour lines included in
 Tommasi et al. (2008) and Baldi et al. (2008). Then a slope profile line
 that passes through the area mostly affected by failures and deformation
 during the 30 December 2002 landslide sequence was selected for the LEM
 analysis (maximum number of 100 nodes imposed by the SSAP). In each
 simulation performed in static and seismic conditions, the water table is
 assumed to be at the sea level, and above it the rock properties refer to
 dry condition. This assumption is justified by the absence of springs a
 few meters above the sea level, the general dry state of investigated
 outcrops, and the low average annual precipitation on the island (Revil et
 al., 2011). The SdF behavior in
 response to both static and loading conditions has been reproduced
 considering a mechanical stratigraphy as follows: - a shallow layer in
 the subaerial part of the slope made up of rockfill material (Tommasi et
 al., 2004). The failure’s envelope of the rockfill material is described
 by the Barton-Kjaernsli criterion (1981) implemented in SSAP as described
 by Barton & Bandis (1991), Barton (2013) and Prassetyo et al.
 (2017); - Lava-Breccia unit
 (Apuani et al., 2005a), considered as a lower limit for the slope’s
 geotechnical characterization. Non-linear failure envelopes as the
 Generalized Hoek and Brown criterion GHB (Hoek et al. 2002), implemented
 in SSAP following Carranza-Torres (2004) and Lei et al. (2016),
 accurately/efficiently describe the behavior of the volcanic edifice both
 at shallow and higher depths; - a stronger
 substratum mainly composed of the Lava Unit (Apuani et al., 2005a). The
 Mohr- Coulomb failure criterion is considered for this zone.
description: <b>Geomechanical/geotechnical parameters following the GHB (Hoek et al., 2002) or the Mohr-Coulomb Criterion</b> Layer UCS (MPa) GSI (-) m<sub>i (-)</sub> D (-) γ<sub>dry </sub>(kN/m<sup>3</sup>) γ<sub>sat </sub>(kN/m<sup>3</sup>)   Rock-fill (GHB- Criterion) 40 30 19 0 19 22   Lava Breccia (GHB- Criterion) 40 30 19 0 19 22     φ’ (°) c’ (kPa) γ<sub>dry </sub>(kN/m<sup>3</sup>) γ<sub>sat </sub>(kN/m<sup>3</sup>)       Substratum (Mohr -Coulomb Criterion 45 500 22 24       <b>Barton-Kjaernsli Criterion (1981),</b> <b>following parametrization of Barton & Bandis (1991) and Lunardi et al. (1994)</b> Layer JRC (°) JCS (Mpa) φ<sub>r</sub> (°) L<sub>0 </sub>(m) L (m) β (°) Dβ(°) Rock-fill 20 10 32.00 1.00 150.00 40.00 20.00 Lava Breccia 20 20 32.00 1.00 150.00 40.00 20.00   File Content   All files can be read using the SSAP software, which can be downloaded for free from the website https://www.ssap.eu/downloads.html   The software developer is Prof. Lorenzo Borselli, co-author of the article, who can be contacted at lborselli@gmail.com.   Below is the list of files and their meaning.   falda.fld -&gt; water table file geomeccanici.geo -&gt; gemechanical parameters Impostazioni.par -&gt; setting file modello_6_mod.mod -&gt; input file for SSAP (making the software read this file, the software automatically reads all the others as well) profilo_2016_SdF_D_Factor.dat strato_sup_modificato.jrc Magma Thrust\10kn.dxf -&gt; failure surface Magma Thrust\s_new.dxf -&gt; failure surface Magma Thrust\s3_20kn.dxf -&gt; failure surface Magma Thrust\s3_100kn.dxf -&gt; failure surface Magma Thrust\singola_3.sin -&gt; input file for SSAP for failure surface stability analysis (making the software read this file, the software automatically reads all the others as well)   For questions Federico Di Traglia (fditraglia@inogs.it; federico.ditraglia@unifi.it) Lorenzo Borselli (lborselli@gmail.com)