The Stromboli 2002 tsunamigenic submarine slide: Characteristics and possible failure mechanisms

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007jb005172, 2008 The Stromboli 2002 tsunamigenic submarine slide: Characteristics and possible failure mechanisms Francesco L. Chiocci, 1,2 Claudia Romagnoli, 3 Paolo Tommasi, 4 and Alessandro Bosman 1,2 Received 17 May 2007; revised 31 January 2008; accepted 15 May 2008; published 4 October [1] On 30 December 2002, a major instability event, deeply involving the submarine slope, occurred on the Sciara del Fuoco, on the western flank of Stromboli volcano, in the Aeolian Islands. Tsunami waves with a maximum runup of over 10 m in Stromboli were generated, having a measurable impact as far as the Sicily coast. Just 10 months before the event, a multibeam bathymetry had been collected in the area down to 1000 m of depth. A repetition of the survey after the slide allowed the unique opportunity to verify the occurrence of a large submarine slide and to define volumes involved and morphology generated by the event, through the comparison of the preevent and postevent bathymetric grids. A morphological characterization of the slope before and after the submarine landslide is presented, showing how the preexisting features interacted with the slide event in controlling the instability. Mechanisms of the submarine failure are discussed on the basis of the geometrical characters of the landslide event, structural and stratigraphic setting of the submerged slope, and geotechnical considerations on the behavior of slope material. Citation: Chiocci, F. L., C. Romagnoli, P. Tommasi, and A. Bosman (2008), The Stromboli 2002 tsunamigenic submarine slide: Characteristics and possible failure mechanisms, J. Geophys. Res., 113,, doi: /2007jb Introduction [2] The 30 December 2002 submarine landslide at Stromboli represents a unique opportunity to characterize a tsunamigenic medium-scale submarine mass failure. In fact, the comparison of the postslide multibeam bathymetry with data from a similar survey performed before the slide allows a precise definition of the geometry of the failure event and provides robust constraints for hypothesizing the failure mechanism. [3] The reconstruction of the submarine landslide, presented here in full detail, is the result of the integration of data from different research fields including marine geology, applied geophysics, geotechnics, and volcanology. [4] Multibeam technology, which allows detailed definition of seafloor morphologies, proved to be the most valuable marine geophysical prospecting method for reconstructing instability phenomena, ranging from small slides to sector collapses and debris avalanches, and for monitoring the following morphological recovery [Chiocci et al., 2008]. [5] Moreover, in the study of volcanic islands, which are among the most favorable environments for the development of slope failures, multibeam is one of the few applicable high-resolution prospecting techniques. Reflection seismics 1 Dipartimento di Scienze della Terra, University of Roma La Sapienza, Rome, Italy. 2 Istituto di Geologia Applicata e Geoingegneria, CNR, Roma, Italy. 3 Dipartimento di Scienze della Terra e Geologico-Ambientali, University of Bologna, Bologna, Italy. 4 Istituto di Geologia Ambientale e Geoingegneria, CNR, Rome, Italy. Copyright 2008 by the American Geophysical Union /08/2007JB005172$09.00 and deep-towed side-scan sonar are, in fact, often not very effective because of the coarse and nonhomogeneous nature of the subsurface and of the uneven seafloor morphology, respectively. [6] Literature on submarine slides in coarse-grained, structurally complex deposits is extremely poor, especially that including geotechnical aspects. In this respect, the precise definition of geometry and of the succession of events represents a fundamental base datum. [7] Small- and medium-scale marine instability events are of great interest for civil protection policies. In the Mediterranean, because of the widespread occurrence of volcanic islands and fault-controlled submerged slopes, small tsunamogenic landslides are relatively frequent and potentially destructive, even if at a local scale, as witnessed by the recent Stromboli 2002 (this article), Nice 1979 [Dan et al., 2007], Gioia Tauro 1977 [Colantoni et al., 1992], and Gulf of Corinth 1963 [Galanopoulos et al., 1964, Papadopoulos et al., 2007] tsunami events. Because of their local character, such events have a rather low possibility to be recorded in historical reports so that their frequency is likely to be underestimated. [8] In the article hypotheses on failure mechanisms will be advanced by linking together: (1) the morphostructural characters of the slope; (2) the succession of events occurred in late December 2002; and (3) geotechnical considerations on the behavior of volcaniclastic debris. 2. Geological Framework [9] Stromboli Island is the subaerial portion of a large composite volcano, rising from the lower Calabrian slope, 1of11

2 [12] Basinward, the SdF scar extends into a broad canyon down to the base of the volcano (deeper than 2000 m), where it merges into the Stromboli Canyon, finally reaching the bathyal plain (Figure 1). [13] The SdF subaerial and submarine scar is partially filled by a sequence of lava flows and loose volcaniclastic debris deriving from the eruptive activity and from the redistribution of its products along the slope, with a remarkable continuity between the subaerial and the submarine slope (Figure 2). [14] During the last thousand years the Stromboli volcano has been characterized by a typical state of permanent activity consisting of mild intermittent explosions, which are periodically interrupted by paroxysmal eruptions and lava emissions [Barberi et al., 1993]. [15] Lava flows reached the sea occasionally during the main historical eruptions. However, remnants of lava deltas do not produce bulges on the SdF coastline because of wave erosion and reworking, so that, before the 2002 eruption and slide events, the SdF coastline was remarkably straight. Figure 1. Location map of the Stromboli volcano. The Sciara del Fuoco, horseshoe-shaped scar (both subaerial and submarine), is highlighted on the NW flank of the island; the fan-shaped deposit related to the Sciara Holocenic lateral collapses is in dark to light gray; the arrow represents the path of the present-day gravity flows redistributing volcaniclastic debris into the Stromboli Canyon. Profiles of Etna and Stromboli volcanoes are indicated below the map, at a similar scale, with a 2X vertical exaggeration. in the northeastern branch of the Aeolian Archipelago. The island is the tip of a large volcanic edifice, emerging from the sea only for 3% of its surface area (Figure 1). Its size is comparable with that of the Etna volcano, the main active volcano in Europe. [10] Stromboli is broadly symmetrical with respect to a NE SW axis, which also corresponds to the regional structural trend, as confirmed by the alignment of major vents and to the strike of eruptive fissures. Its volcanic and structural evolution has been characterized by the alternation of building and destructive phases: in the last 13 ka, four lateral collapses with concentric scarps have affected the NW flank of Stromboli [Tibaldi, 2001]. The most recent collapse, younger than 5000 years B.P. [Tibaldi, 2001], generated a horseshoe-shaped scar known as Sciara del Fuoco (hereafter referred to as SdF). This depression, which is partially filled by the products of the present activity of the volcano, extends below sea level down to a depth of 700 m, where Kokelaar and Romagnoli [1995] suggested that the basal collapse surface emerges. [11] On both sides the scar is bounded by two steep escarpments, which remain exceptionally high (up to 200 m) even in the offshore portion, where they slightly converge down slope (Figure 2). 3. December 2002 Submarine Slide and Tsunami [16] On 28 December, 2 days before the landslide and tsunami event, a lava eruption started at the summit crater, reaching the sea in the NE sector of SdF [Bonaccorso et al., 2003; Calvari et al., 2005]. In the following hours, a deep-seated movement disturbed the slope without evolving into a destructive slide [Tommasi et al., 2005]. At 1215 UT of 30 December a large-scale slope failure occurred, triggering tsunami waves that propagated around Stromboli and onto nearby islands, having an influence as far as the Campanian, Calabrian, and northern Sicily coast [Maramai et al., 2005]. A number of local witnesses agree on a negative first wave pulse (initial water retreat) and one of them reports that a sort of vertical cut suddenly opened in the seawater at the Sciara foot and propagated around [Tinti et al., 2005]. Tinti et al. also report that two distinct sets of tsunami waves, separated by a time lag of some minutes, reached the coast. The tsunami produced severe damage, particularly in the northern part of Stromboli, Figure 2. Digital Terrain and Marine Model (DTMM) of Stromboli volcano from the west draped with aerial photograph ( 2of11

3 Figure 3. Photograph of the Sciara del Fuoco lower slope 6 days after the landslide event (courtesy of the Civil Protection Department). The slide scar notching the coastline and the lava delta built inside it are clearly visible to left of the image. where it spread inland by 130 m and runup more than 10 m [Maramai et al., 2005; Tinti et al., 2006b]. No casualties occurred because of the paucity of inhabitants during the winter season. [17] Processing of seismograms recorded at Stromboli (the only instrumental data available) reveals that two main instability events occurred at 1215 and at 1222 UT, respectively [Pino et al., 2004]. [18] The first helicopter-borne survey, performed a few hours after the tsunami, detected a huge scar affecting the NE part of the subaerial SdF and a coastline retreat of at least m that clearly indicated that the failure affected the submarine slope. The volume involved in the subaerial slide, estimated on the basis of aerophotogrammetric surveys, was in the order of 10.3 Mm 3 [Tommasi et al., 2008]. [19] After the landslide, lava flowed inside the scar and reached the sea, building up a lava delta (Figure 3). [20] The lava flow stopped entering the sea in mid- February 2003 and the eruption ended in mid-july of the same year. 4. Data Collection and Processing [21] Ten days after the event (on 9 January 2003), a first multibeam survey was carried out in front of the Sciara del Fuoco slope. Additional surveys were then carried out on 15 and January (Table 1). [22] Surveys were performed using a small Coast Guard vessel: all the data were positioned with differential GPS, no tide correction was applied (maximum tidal range in the area is of a few decimeters). The obtained precision varies from about 1 m in shallow water to some 5 m in deep water. The resolution was some 5 times higher than the precision, being about 20 cm in shallow water and 1 m in deep water. [23] Bathymetry was collected down to a depth of 1000 m 10 months before the slide event aboard R/V Thetis of CNR in the framework of the research activities of the Italian National Group for Volcanology (GNV). [24] In order to improve the precision and resolution of the final results, rigorous procedures in data acquisition and nonstandard processing techniques were applied [Bosman, 2004]. Daily measures of sound speed profile in the water column and repeated calibration of transducers in areas close to the survey zone were performed. For each single swath, spikes due to individual false soundings were manually deleted. Local noises were therefore eliminated and regular sounding surfaces were obtained and subsequently statistically and geometrically filtered. The different swaths were mosaicked and statistic filters were applied (functions of the slope angle and sounding density). These procedures allowed a recovery of at least one third of the original soundings with a significant improvement of the signal-tonoise ratio. [25] Finally, to visualize and interpret the digital terrain model, different grids were used for different depth ranges (with grid spacing increasing with depth); tinning (triangular irregular network) and random points visualization were used when maximum resolution was required. 5. Morphostructure of the Preslide SdF Submarine Slope [26] The overall submarine SdF morphology resembles that of the subaerial slope, with roughly linear trending isobaths. Proceeding from the shoreline to the offshore, four areas with distinctive morphologies can be defined (Figure 4): (1) a narrow ephemeral beach a few meters wide; (2) a wave-formed perched terrace, up to 60 m wide, with a slope break at about 10 m, i.e., at the wave base level; (3) a regular upper slope dipping on average at 30 35, down to 300 m below sea level (bsl); and (4) a deeper slope, with an Table 1. Multibeam Surveys Carried Out in the Submarine Part of the SdF in the First Month After the Slide Date Model Depth Range (m) Frequency (khz) Feb 2002 Seabat Jan 2003 Seabat Jan 2003 Seabat Jan 2003 Seabat of11

4 [31] The above described morphologies of the submerged SdF are considered to be produced by processes acting at different temporal and spatial scales (Table 2): [32] 1. Small slides or grain flows on the subaerial slope (with weekly to monthly interval) are likely to feed the beach and the submarine terrace, with noticeable sediment redistribution along the coast. After a period of wave reworking in the littoral environment, debris is mobilized and redistributed on the submarine slope via the network of braided depressions. [33] 2. Debris fans and chutes making up the volcaniclastic apron appear to be caused by ordinary volcanic activity and failure events occurring at multiannual to decadal scale, such as the 2002 landslide or the lava flows that entered the sea in 2007, 2003, 1986, and Figure 4. Sketch of the submerged SdF slope (not to scale), with indication of main morphologic and depositional features. initial dip of 20 25, progressively decreasing with depth to less than 20. [27] The SdF submarine slope is covered by sand, gravel and blocks; sediment finer than fine sand has never been recovered. The coarser fraction is made up of lava blocks (frequently oxydized with variable, generally low, rounding) and scoriae (highly vesiculate and crushable). [28] Similarly to the subaerial slope, erosional and depositional features are present, resulting from very small slides and gravity flows, which actively redistribute the debris produced at the summit crater toward the volcano base. In detail, side-scan sonar images depict wide braided channels, depressed some tens of centimeters with respect to the surrounding seafloor, floored by coarser sediment grooving the seafloor and isolating rhomboidal morphological highs, several tens of meters in width (Figure 5). [29] In the shaded relief view of Figure 6 a series of interfingering debris fans and chutes extend with continuity from the lower subaerial slope down onto the upper submarine slope. The debris fans coalesce into a volcaniclastic apron whose base is located at an approximate depth of 300 m. Below this depth, the slope is less steep (lower than ) and smoother, except for two linear ridges running parallel to the dip direction of the slope between the depths of 340 m and 650m. The ridges are about 100 m wide and have a relief of 10 m on the seafloor (Figure 6). [30] An apparent depression (Figure 6) extends on the northwestern half of the submarine slope, from the shoreline down to a depth of some 350 m, bounded on both sides by two large debris fans. The depression is not a bare artifact produced by the two fans but represents an indentation of the average slope surface underlying the debris fans. This morphology could result from relatively old slope failures, which left a huge scar that successively was filled in part by the slide debris. Figure 5. (bottom) Along-slope side-scan sonar image of the Sciara del Fuoco seafloor, showing rhomboidal erosional and depositional features described in the text. (top) Location map of Figures 5, 10, and 11. 4of11

5 Figure 6. Close-up of the DTMM of Figure 2; the volcaniclastic apron made up of coalescing debris fans is recognizable down to 300 m water depth. Below this depth two linear ridges (black arrows) were present before the landslide. The continuity between the subaerial and submarine features is noticeable. White arrow indicates a depression on the NE part of SdF submarine slope. [34] 3. The volcaniclastic apron on the whole could result from long-term (millennial) deposition of the materials deriving from the present volcanic activity ( Strombolian activity characterized by persistent production of scoria at the summit crater) and their redistribution along the subaerial and submarine slope. The apron overlies older deposits, possibly related to a different eruptive behavior cycle and/or a different evolutionary stage of the SdF collapse scar (Figure 7 and Table 2). 6. Characters of the Slide Scar [35] A comparison between the preslide and postslide bathymetry (Figure 8) shows that the December 2002 submarine slide produced a well-defined slide scar having a maximum depth of 45 m, and a width of about 650 m. The scar depth gradually decreases proceeding downslope and, even though it exceeds some 20 m at 280 m bsl, it loses most of its relief at 320 m bsl. The northeastern boundary of the submarine slide scar extends some 260 m farther to the NE, with respect to that of the subaerial scar. In shallow water the NE boundary coincides with a rock spur, outcropping from the volcaniclastic debris (Figures 8 and 9). [36] The geometry of the submarine residuals (differences between the preslide and postslide bathymetry) is characterized by a bottleneck at 100 m bsl and by a subcircular Table 2. Morphologies and Processes on the Western Stromboli Flank Acting at Different Temporal Scales Timescale Morphology Processes Weeks months Beach and wave-formed terrace Wave reworking and grain flow Years decades Chute and debris fan Lava flow and mass failure Millennia Apron and deeper slope Change in the eruptive style Plurimillennial Island-scale scar and fan a Flank and sector collapse a Such as the island-scale fan shown in Figure 1. Figure 7. Schematic diagram of SdF setting (not to scale), showing the major morphostructural and depositional features. shape down to water depth of 300 m (red arrow in Figure 9). This area above 300 m is identified as the main scar. Below this depth and down to the survey limit, the residuals depict an elongated depression. [37] The main scar surface appeared to be relatively clean, apart from some postslide material (see also sections in Figure 8). In fact, at the moment of the first postslide multibeam survey, the upper part of the scar was already filled by debris and lava deltas emplaced just after the slide and lying on the regular scar surface, altering its original concave-up morphology. The bottom of the lava delta was thus reconstructed by extrapolating the morphology of the surrounding scar. At the time of the first survey (10 days after the event), the lava delta extended in front of the entry point of the lava flow into the sea, down to 180 m bsl with a lenticular shape and with a maximum thickness of about 12.5 m. The volume was about 1.56 Mm 3 and since then it grew by some 50,000 m 3 /d during the following 1.5 months, according to repeated multibeam surveys [Chiocci et al., 2008]. [38] The formation of a large scar with clean steep lateral scarps allowed us to observe for the first time the inner structure of the submarine SdF infilling deposit through side-scan sonar, ROV and scuba diving surveys. In fact, seismic surveys have never been able to depict the SdF subsurface structure because of its coarse grain size and heterogeneity. The first tens of meters of the SdF infilling are made up of an alternance of thin lava flows and volcaniclastic layers, which locally produce a stepped morphology of the subvertical scar wall (strata head in Figure 10). [39] A wide variability in grain size of volcaniclastic layers is to be expected. However, depositional processes form relatively fine-grained layers (gravelly sand) having remarkable extension and continuity. The marked vertical heterogeneity is in sharp contrast with the sandy-gravelly composition of the seafloor, where lavas are rarely outcropping. [40] Within the elongated depression at the foot of the main scar, i.e., from 350 to 1000 m, two along-slope elongated erosive grooves can be observed in the map of residuals (Figure 9). They are each 150 m wide and are excavated into the surrounding seafloor down to a maximum 5of11

6 Figure 8. (left) Preslide and postslide contours and boundary of subaerial slides. The white circle indicates the rock spur offshore Spiaggia dei Gabbiani discussed in the text. (right) Preslide and postslide seafloor sections parallel to the coast (location is indicated by white arrows in the contour map). The black arrow indicates the debris deposit (D in Figure 9) already present in the scar at the time of the first postslide survey. depth of 10 m. These features are interpreted as flow trails, produced by the transit of gravity-driven debris flows. The apparent lack of positive residuals inside the trails and in the surrounding areas suggests that most of the slide debris were transported farther downslope. The transit of the slide debris also completely removed the slope-parallel ridges observed in the preslide bathymetry of Figure 6. [41] According to residuals from preslide and postslide multibeam surveys, the instability phenomena removed from the seafloor over m 3 (Table 3). This is a conservative estimate because it is restricted to the first 1000 m of depth (limit of the preslide survey). In fact, postslide bathymetry indicates that the two erosional grooves inside the flow trail reach a water depth of at least 1500 m. [42] In the southwestern unfailed part of SdF, from the map of residuals two minor slide scars having maximum depth of 10 m were recognized above 100 m bsl (A and B in Figure 9). These slides, which involved 330,000 and 140,000 m 3, respectively, could have been triggered by the tsunami wave loading on the seafloor. Again no positive residuals are present downslope from these scars, suggesting that the failed mass disintegrated during sliding and ran down to greater depths or most likely was redistributed over a much wider area in the form of a veneer thinner than the multibeam precision. [43] Close to the NE limit of the SdF, an area with positive residuals (red in Figure 9), extends from the coast down to over 300 m bsl. The area with positive residuals elongates downslope for more than 560 m, with a maximum thickness of m and an estimated volume of some 450,000 m 3. Its acoustic backscatter signature (observed through side-scan sonar surveying), together with direct observations and surface sampling, indicates that it is covered by decimetric and centimetric loose subangular scorias and lava blocks. In section 7 it will be discussed whether this feature may be due to deposition from a rock avalanche entering the sea at the NE of the SdF lower slope before the 30 December event (p event in Table 3). 7. Reconstruction of the 30 December 2002 Submarine Failure [44] The sequence of subaerial instabilities that led to the tsunamigenic landslide of 30 December was reconstructed by Tommasi et al. [2005], who point out how the submarine event was the final stage of a large-scale instability process caused by the eruption that started on 28 December. Further refinements, and insights on geometry and morphological characters of the submarine instability, were presented by Chiocci et al. [2003, 2004, 2005]. [45] The bottleneck morphology of the main scar can be interpreted as the superimposition of two scars left by two distinct instability events (t 1 and t 2, Figure 9): (1) a larger scar (t 1 ) extending from the coast to 340 m bsl, with a subspherical slip surface, and (2) a shallower scar (t 2 ) which partly overlaps the former one and is opened toward the shoreline. [46] The scar t 1 refers to an entire submarine failure which displaced some m 3 (about one half of the bulk volume removed from the submarine slope, Table 3). The t 1 submarine slide occurred at the foot of a huge mass extending across the shoreline on the subaerial and the shallow marine slope (defined as a movement by Tommasi et al. [2005]) that began deforming and fragmenting h before t 1 occurred. The deforming body (a) resulted from deep-seated slope movement triggered by dike intrusion, soon after the beginning of the eruption. 6of11

7 Figure 9. Map of residuals (difference between preslide and postslide bathymetry) of the SdF submerged slope, down to about 1000 m. In the inset, t 1 and t 2 submarine slide scars are sketched; a, b, and g refer to subaerial instabilities according to Tommasi et al. [2005]; their characters are described in the text. A and B indicate minor submarine landslides in the SW SdF slope; D indicates debris deposit present within the scar at the time of the first postslide survey (see also Figure 8). Figure 10. Side-scan sonar image (without slant range correction) of the southwestern lateral boundary of the slide scar. Location is shown in Figure 5 (top). 7of11

8 Table 3. Main Mass-Wasting Phenomena That Occurred in the SdF on December 2002 Event Depth (m) Volume (Mm 3 ) Phenomena p subaerial >0.45 Early rock avalanche a t 1 50 to First major submarine collapse a t 2 0to Submarine slide due to unbuttressing of t 1 b + g b +600 to Subaerial slide due to unbuttressing of t 1 Trails 350 to > Seafloor erosion a Volumes refer to the original scars, before a lava and debris deposit was emplaced (D in Figure 9). Its estimated volume of 1.56 Mm 3 was added to the residuals actually measured. b Values after Tommasi et al. [2008]. [47] The deep scar left by the t 1 submarine failure undermined nearshore submarine deposits and destabilized the subaerial slope, which failed probably 7 min later, as witnessed by the time gap separating two groups of seismic waves reported by Pino et al. [2004]. The latter collapse produced the t 2 submarine scar and the failure of the subaerial slope (g and b scars in Figures 8 and 9). The scar t 2 likely corresponds to the lowermost part of the a deforming mass. [48] Therefore the first tsunami wave is likely to have been caused by an entirely submarine mass failure (t 1,9.5Mm 3 ), whereas the second one could be related to a mainly subaerial slide (t 2 + b + g, some 11.6 Mm 3 ). [49] This reconstruction is in agreement with eye witness accounts collected by Tinti et al. [2005], reporting a negative first wave arrival (sea retreat at first), typical in the near field for submarine landslide-generated tsunamis [Watts, 2000]. [50] The failed mass moved downslope as a gravity flow that eroded an additional 9.6 Mm 3 (at least until 1000 m) and created the two main flow trails visible in the map of residuals (Figure 9). [51] The 0.45 Mm 3 positive residual described in section 6, developed at the foot of the northeastern boundary of the SdF (close to Spiaggia dei Gabbiani, see Figure 9). This feature is sharply cut by the scar of the submarine failure t 1 (Figure 9) which, therefore, partly removed the deposit corresponding to the positive residual predating its occurrence before t 1.We relate this positive residual deposit to an early minor rock avalanche (e.g., that shown on a video taken by M. Pompilio (personal communication, 2003)), a few minutes before the tsunami generation. Such an early event occurred in the very NW part of the subaerial slope, and is defined as the p event in Table 3. [52] The p event could have produced a submarine gravity flow offshore Spiaggia dei Gabbiani that, because of its high energy, caused the failure of a rocky block some m in size (Figure 11). [53] The submarine topography (Figure 9) is likely to have confined the flow into the northeasternmost part of the submarine slope. Even if we suppose the original extension of the deposit produced by the p event was 2 or 3 times wider than the present positive residual, it likely did not extend over the main scar area. This evidence suggests that the seafloor loading caused by an earlier rock avalanche was not a cause of the submarine instability (other geotechnical considerations are discussed in section 8). 8. Geotechnical Considerations on the Failure Mechanism [54] The submarine slide (t 1 ) that followed the initial deep-seated subaerial and partially submarine movement a, was essentially controlled by the response of the volcaniclastic material under saturated conditions to different strain/load rates. [55] The initial movement of the a sliding mass was so slow to develop under drained conditions and hence not to collapse. In fact, the drained shear strength of the volcaniclastic material is higher than the maximum shear stress acting on the submarine slip surface even at large displacements [Boldini et al., 2005]. [56] The intense shear deformations experienced at depth during this stage of the slope evolution, greatly increased the fine (sand) content of the material. This behavior, due to the high crushability of the SdF volcaniclastic grains, is reported by Boldini et al. [2005], who found an increase of sand content in large-scale ring shear tests (DPRI-RS) up to 20% for an initial sandy-gravelly material. [57] In the hours preceding the t 1 submarine failure, sliding intensified dramatically and hence material comminution within the shear surfaces. We hypothesize that such a process culminated in a sudden load/strain increment that generated excess pore pressures more rapidly than they could dissipate (undrained process) causing a remarkable drop of shear strength. Such a process is similar to static liquefaction and was observed by Boldini et al. [2005] in DPRI-RS undrained tests. [58] Since the high permeability of the material entails however a rapid pore pressure dissipation, we incline to believe that fast stress/strain increment should have been produced by an abrupt local failure of a resistant part of the slope rather than a progressive rise of shear strain velocity. Figure 11. Multibeam soundings in section view, depicting the postslide morphology offshore Spiaggia dei Gabbiani (see Figure 5 for location); dashed line indicates preslide morphology. The failure of a rocky block is clearly visible as well as its precollapse position. 8of11

9 Figure 12. Geotechnical scheme of the evolution of the submerged slope under ordinary stress conditions and after occasional abrupt changes in the stress state. [59] In this respect, Boldini et al. [2005] report that a strain drop is observed also in tests performed with open drainages, where undrained conditions could have locally established with a sudden strength decrease. The high crushability of the grains would have locally caused a sudden reduction of pore volume, with a reduced capability to dissipate excess pore pressures due to the fine-grained materials produced at large displacements (Figure 12). [60] These considerations suggest that we should not regard the submarine failure as being induced by the relatively rapid loading exerted on the seafloor by the deposition of the p rock avalanche, which occurred a few minutes before the tsunami. In fact, because of the high permeability of the volcaniclastic materials, the excess pore pressure generated by the seafloor loading should have been dissipated well before the first tsunami and hence before the submarine slide. However, simple limit equilibrium analyses with circular surfaces (considering the undrained load of a 2 m thick deposit and a residual friction angle of 37,asin the work by Boldini et al. [2005]) yield to the same conclusion even if an excess pore pressure at the slip surface equal to the load at the seafloor is considered. [61] The load exerted on the seafloor by the lava flows entering the sea could also be invoked as a possible triggering factor of the submarine slide. This hypothesis seems again to be not convincing; in fact, the lava flow is slow and therefore excess pore pressure generated by the lava tongue spreading on the seafloor should have progressively dissipated as the flow advanced. Furthermore, loading was limited to a restricted nearshore area while the area involved in the submarine failure was much larger. This consideration actually applies also to the deposit of the rock avalanche p, which appears to be very narrow and relatively proximal. [62] A good a posteriori proof of this behavior is the lava delta built by the 2007 eruption. It was larger and thicker than the 2002 one, but no instability occurred on SdF submarine slope. [63] The apron deposit has the highest gradient of the submarine slope, and is formed by younger and looser volcaniclastic debris (i.e., with lower shear strength). This explains why the slide scar is virtually confined in the upper 350 m, i.e., within the apron. Also the quasi-spherical shape of the scar could be an indirect evidence of the homogeneity of the apron deposit. [64] Effects of temperature changes on the stability of the SdF slope were not quantitatively considered. This factor has been suggested by different authors as a possible source of instability, but experimental data and/or robust modeling support this hypothesis only in circumstances that are far from the case analyzed in this paper (for instance, in case of lava domes [Elsworth et al., 2007]). Temperature should essentially act toward an increase of pore fluid pressure and hence a drop of shear strength in the volcaniclastic materials [Voight and Elsworth, 1997]. In areas close to the part of the slope that revealed to be critical for failure initiation (i.e., 9of11

10 the middle-lower part), heating should be produced by shallow pencil-like intrusions. However, in the subaerial flank low saturation and high permeability of volcaniclastic material do not favor generation and maintenance of excess pore fluid pressures [Day, 1996]. In the saturated submerged slope the possible source of heath seem to be represented only by buried active lava flows, which should allow vapor formation in a very limited sector at the shoreline and in the shallower submarine slope. [65] The destabilizing effect due to temperature could essentially be the loosening of the material induced by vapor flow and possibly fluidization, however this mechanism (as well as those linked to phreatic explosions) require well-monitored physical conditions in the slope in order to be realistically modeled. 9. Conclusions [66] The availability of preslide and postslide multibeam bathymetry on the submarine Sciara del Fuoco slope gave us a unique opportunity to investigate the mechanisms of tsunamigenic underwater instability. [67] Stromboli is characterized by persistent volcanic activity, with huge production of debris at the summit crater, which is redistributed by gravity flows on the steep subaerial and submarine slope. The high production rate of volcaniclastic materials, makes Stromboli an ideal site to study instability phenomena. This study can therefore be useful to interpret submarine features or tsunami events in submarine volcanic environments. [68] The landslide that on 30 December produced tsunami waves with maximum runup of more than 10 m [Tinti et al., 2005, 2006a] was initially caused by a 9.5 Mm 3 entirely submarine slide (t 1 ). The slide affected a submarine apron made up of volcaniclastic debris arranged in coalescing fans down to a depth of 350 m. [69] On the basis of preslide and postslide morphology, succession of events and geotechnical considerations, authors hypothesize that failure was caused by a sharp drop in shear strength (static liquefaction). The collapse of the submarine slope was the final result of an intense deepseated deformation process started 2 days before, which underwent a dramatic acceleration in the hours preceding the collapse. Generation of excess pore pressures and reduced dissipation rates, which determined static liquefaction, were probably due to the very high crushability of the volcaniclastic debris grains, as a large production of fines favors pore volume reduction and reduces permeability. [70] Therefore, high crushability of the volcaniclastic debris was probably the first responsible for both generation and slower dissipation of excess pore pressure, with consequent static liquefaction. [71] The removal of confinement (unbuttressing) caused by the first submarine slide (t 1 ) then led to the failure of a mass formed by part of the subaerial slope (b + g, 10.3 Mm 3 ) and the rest of the submarine one (t 2, 1.36 Mm 3 ). The slide debris evolved into gravity flows that eroded the submarine slope to greater depths, accounting for the displacement of at least an additional 9.6 Mm 3 of material. [72] Therefore, the 30 December 2002 Stromboli case history indicates that: [73] 1. The 2002 submarine landslide was the final stage of an instability process initiated by magma intrusion during an effusive eruption. Such events are not rare at Stromboli, even though deformations do not always reach the coast, as those occurred during 2007 eruption. However, the saturated volcaniclastic debris making up the submarine slope, if involved, is susceptible to experience sudden collapses producing tsunamigenic landslides that should be regarded as a relatively fair source of risk. [74] 2. The morphostructural characters of the submarine slope, such as the presence of the debris apron, partially drove the extent and geometry of the tsunamigenic failure. [75] 3. The mobilization of more than 30 Mm 3 of debris was caused by a relatively small submarine failure which mobilized less than 1/3 of the final wasted volume. [76] 4. Tsunami waves with significant runup (maximum of 10 m) can be created even by a small submarine landslide, and therefore such phenomenon should be taken into account in managing and land use planning of coastal zones. [77] 5. Since tsunami waves were first generated by a submarine landslide that further evolved into a subaerial landslide, it is possible that some unexplained tsunami events in the past might have been caused by entirely submarine slides without inland propagation. [78] With reference to the residual hazard, one should notice that the SW unfailed portion of the submerged SdF lies in structural, lithological, and stratigraphic conditions similar to that of the failed part and has a volume that is approximately 150% greater. However, this slope seems to be quite stable in ordinary conditions, as no mass wasting occurred when it was struck by tsunami waves, except for two very minor slides. Loading induced by the 5 April 2003 paroxysm at the summit crater caused only limited displacements [Mattia et al., 2004], and the emplacement of a large lava delta in 2007 did not cause any detectable change in submarine morphology of the SW SdF. Nevertheless, if a volcanic crisis comparable to the 2002 eruption were to occur in the SW portion of SdF, a similar instability scenario should be considered. [79] Acknowledgments. Research has been funded by Civil Protection Department, which is fully acknowledged also for collaboration in organizing cruises and field activity in the island and for providing aerial photographs. A preslide survey was performed within the GNV project 15 ( ). Research was funded by INGV-DPC project V1. Ideas and reconstructions presented in the article benefit from discussion with colleagues working on the subaerial parts, namely, M. Coltelli (INGV, Catania) M. Marsella (University of Rome) and M. Pompilio (INGV, Pisa). H. Lee and J. Locat are gratefully acknowledged for their accurate review that helped us to significantly improve the paper. G. De Alteriis and R. Tonielli (CNR, Naples) collaborated in preslide data acquisition; P. De Rosa and M. Bellino are greatly acknowledged for their support during the emergency at Stromboli. Furthermore, data acquisition during the emergency took advantage of the enthusiastic participation of Italian Coast Guard and CCE s.r.l. 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