Mt. Etna: Volcano Laboratory

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We assume that this layer deforms accordingly to a Drucker-Prager rheology. They are very similar to those in Fig. The predicted displacement Fig. A local minimum develops above the reservoir, while increases with respect to model CLAY. Normalized horizontal solid red lines and vertical solid black lines displacements predicted by the models described in Fig.

The deformation is induced by a spherical, isotropic source common to all models. In the following we introduce a new deformation source represented by a vertical dike placed below the summit craters. In the first case the dike is considered as a passive pre-existing discontinuity able to release some of the deviatoric stress provided by the isotropic expanding source located below. Keep in mind that the purpose here is not to find best fitting parameter for this data set but just to catch the main features of the observed deformation.

The comparison with the data set from Fig. Crosses and diamonds are the projections of the normalized displacements from Aloisi b for the two episodes June— May and May—December, respectively. Red and black colours are for horizontal and vertical components, respectively. The GPS sites are those confined within the black dashed lines in Fig.

The normalization factor is either the maximum vertical displacement [ a , b ] or maximum horizontal displacement [ c , d ], according to the two types of active sources. A well-developed U z local minimum is now predicted above the magmatic sources. The displacement curves are continuous suggesting that no shear movement occurs along the dike, and neither opening reaches the surface.

However, the deformation patterns are different from what are predicted in model CLAY suggesting that the dikes should play a role even though not that relevant. Indeed, the differences between surface displacements for models SD-CLAY and DD-CLAY are really small indicating that the isotropic source has an overall similar behaviour in both models resulting in the opening at depth between 2. Since 1. The slightly larger displacement computed for model DD-CLAY is due to the small vertical propagation m length of the deepest part of the dike just above the magma chamber.

Comparing the normalized ground deformation to normalized data collected during time interval June— May shows that the trends are different, the greatest discrepancy being observed for U z : while geodetic displacement is characterized by diffuse subsidence starting less than 5 km from the summit craters, the model prediction signal is positive or weakly negative 15 km away of the deformation centre. Focusing on the May—December time interval a partial agreement between data and predictions may be speculated either for horizontal and vertical components.

Despite that the predicted eastward displacements are large compared to those obtained by uniform models, the retrieved trend is always characterized by a maximum located approximately 8—9 km from the craters and a gentle slow decay toward the Ionian Sea. The geodetic data set, on the other hand, places the maximum amplitude at almost 15—20 km from the craters. None of the proposed models is able to reproduce this feature suggesting that the frictional deformation in the Clays layer is not efficient enough to allow the overriding volcanic cover to move eastward as a rigid block.

We have also tested lower values of cohesion for the Clays layer but, in these cases the volcanic cover is continuously drifting eastward due to the effect of gravity, and the isostatic equilibrium cannot be attained see Section 3. The predicted displacement indeed amounts to hundreds if not thousands of meters, an unrealistic value for Mount Etna. Figs 8 c and d show the normalized horizontal and vertical displacements predicted by the active dike opening.

The first thing we note is that is now larger than. Both U x and U z are discontinuous at the surface, with opening prevailing over shear movement. U z , also characterized by two relative maxima in proximity to the summit, reverses its sign between 5 and 10 km, and remains close to zero at greater distances. Both horizontal and vertical displacements are almost symmetric with respect to the deformation centre meaning that, even the active dike source is not able to cause any significant frictional deformation in the compliant Clays layer, despite being very close model SD-CLAY or even in direct contact with it model DD-CLAY.

This suggests that the surface of discontinuity located between the Volcanites and the Flysch transfers deformation more efficiently with respect to the Clays layer. Here, the dike opens 10 cm maximum opening and slides at depth between 2. Moreover, the first m of the subhorizontal plane closer to the dike slides eastward favouring the asymmetric deformation pattern. What happens if the dike is not confined to the volcanic cover but extends deeper to the magmatic source as for DD-PLAY model?

Now the differences in Fig. Dike opening is confined between 2. The subhorizontal discontinuity does not slide. The resulting ground horizontal deformation is characterized by two local maxima, the smallest above the dike and the largest further east of the summit but not far enough to fit the observed data. U x changes its sign almost 4 km west of the summit. In this model is negative just above the dike in agreement with observations.

Left and right of the minimum the model predicts two very asymmetric relative maxima. Then U z approaches to zero never assuming the large negative values recorded on the east flank. The same as in Fig. In the case of the deeper dike Fig. Predicted vertical and horizontal movements are almost completely confined to the east flank.

The comparison with the normalized data relative to June— May and May—December Aloisi et al. Moreover, we are not able to fit, at the same time, both horizontal and vertical deformations and the west and east sectors of the volcano as well. In the east flank predicted vertical displacements are characterized by a strong uplift not evidenced by data, while the maximum of horizontal displacements is still located at about 5 km from the summit.

To give a contribution to the open debate about the role of a shallow or deep sliding plane accommodating eastward sliding of the east flank of Mount Etna we have taken into account a model with a sliding surface located at the base of the Clays layer of Fig. For this model the results slightly differ less than 5 per cent from those plotted in Figs 9 b and d.

We recall that sliding occurs along the predefined discontinuity if the ratio between shear and normal stresses overcomes the effective static friction coefficient. The deeper plane is closer to the source and certainly it is subject to an higher shear stress but also to an higher lithostatic load, while the shallower plane, being more distant from the source, undergoes a lower shear stress from the source, but an equally decreased lithostatic load. As a consequence, in both cases eastward sliding takes place only along a segment of about 5—8 km from the source so that the surface deformation is quite similar.

The usual approach in volcano deformation modelling is to account for a forward or inverse model aimed to the interpretation of a specific data set collected in a well-defined time-interval. In nature, however, every deformation episode is just an event in the volcano evolution. If this approximation is justified in the case of a purely elastic rheology where the effects of every source are additive, this is not the case when dealing with frictional plastic rheology where the final deformation depends on the sequence in which loads are applied.

Our purpose here is to evaluate how model predictions may change if we neglect the fact that every single deformation episode is just one part of a complex deformation history. One simple way is to setup a model where an inflation and deflation cycle is repeated numerous times. The recent activity of Mount Etna may suggest a cycle consisting of 3 yr inflation followed by a rapid 3 months deflation.

Afterwards, displacements follow the time evolution of the source suggesting that the system is turned to a conservative one and basically only the elastic properties of the materials are significant. The U x and U z displacements in Fig. In the following cycles an essentially elastic behaviour with full recover of the deformation after the load removal is observed.

After two cycles the horizontal and vertical displacements stop showing the sawtooth behaviour of the first cycle and sensibly reduce in amplitude. In the first inflation the contact surface between Volcanites and Flysch layers slides with the typical stick-slip behaviour to release the shear stress accumulated due to the magmatic source.

From the third cycle on, again an elastic, conservative response is observed. This probably can be explained by considering that in the deflation phase the recovery of the elastic part of the deformation is also inhibited by the gravity load, preventing any back sliding on the subhorizontal discontinuity. This certainly may be a very strong and conservative approximation. Of course, in case of strong increase of overpressure at the source, the system may again will be forced to release the stress reactivating the frictional deformation or sliding.

For the specific case of Mount Etna, these results suggest that it is unlikely that the flank instability is maintained active by the action of a magmatic source because this would require a continuous rising of the deviatoric stress. Displacement history of a node located on the surface, 1. The flank instability is a very complex and multifactor peculiarity of the east side of Mount Etna which results from the complex interplay between forces due to magma intrusions, gravity and tectonics acting in a heterogeneous environment such as a volcanic edifice.

In the last few years each of these mechanisms has been reclaimed and proposed as the most important factor according to the data or model analysed. For example, both Solaro et al.

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Other papers instead focus on the leading role of dike type intrusions in the presence of elastic heterogeneities Aloisi et al. From this simple and incomplete review of the latest published papers, we can note how, despite the increasing amount of high quality data produced by the geophysics and geodetic monitoring networks deployed, the possibility to discriminate between causal effects of the various processes involved seems still very far from being addressed. In this paper we have been looking for a combination of structural features, rheological properties and active sources being able to focus a significant amount of horizontal deformation away from the volcano summit, which is the most important manifestation of flank instability.

Among these factors the most important are: frictional rheology, elastic heterogeneities associated to the presence of the asymmetric DC volume, the vertical dike discontinuity below the summit craters and the subhorizontal sliding plane. This study suggests that a careful inference of the source parameters based on deformation data should be aware of the effects of these elements.

At lower elevations and larger distances from the volcano summit, approaching the coast, the flank instability cannot be directly related to magma accumulation. Indeed, none of the models we investigated has been able to transfer a significant amount of horizontal deformation up the coast line where large deformations have been recorded, also associated with a negative vertical displacement, never predicted by our computations.

Our findings support the hypothesis that the sliding at low elevations could be related to local, shallow-depth, structures. This view is as well supported both by InSAR deformation in the last 15 years and seismic activity. InSAR data show clear evidence of a deformation field partitioned into blocks with coherent internal behaviour Solaro et al.

On the other hand, seismic studies such as Alparone et al. This missing link supports the hypothesis that the flank dynamics at low elevations can be conditioned by the mechanical properties of the uppermost layers and does not involve deep structures.

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The goal of this paper has been to study the surface deformation caused by simple sources embedded in a layered domain with frictional rheology resembling a 2-D section of Mount Etna across the east flank. Taking full advantage of the a priori information about the internal structure we have studied the predicted deformation checking for the effects of compliant layers or sliding planes. We have shown that anisotropic sources in a homogeneous, isotropic and poissonian half-space provide a lesser degree of asymmetry when compared to isotropic sources embedded in a layered and laterally heterogeneous domain.

For the specific case of Mount Etna, the DC volume underneath summit craters significantly contributes to increase the deformation in the east sector. The frictional rheology is endorsed because it provides a more realistic stress distribution, while purely elastic models can easily predict deviatoric stress levels non-sustainable by crustal rocks. On the other hand, it requires a consistent initialization of the stress field. We have shown that, according to different assumptions about the initial stress distribution, different results can be obtained due to a total or partial relaxation of the differential stress due to the gravity load.

This result suggests that initial stress assumptions can severely influence the assessment of the physical mechanisms that may lead to flank instability. For the two episodes of Mount Etna deformation considered we find that our modelling is effective to enhance deformation in the east flank up to 10 km from the source. However, the high rates of horizontal displacements observed in the sites at larger distances and lower altitudes require a further mechanism in addition to the isotropic source, dyke opening and sliding planes included in the modelling.

In other words the hysteresis loop is substantially reduced after the first cycle and the system quickly tends towards an elastic behaviour where the deformation is almost totally recovered. This model, predicting variations in the mechanical response of the volcanic system as a function of the loading history, suggests another important element that may impact the interpretation of volcanic deformation. Wassermann, P. Lundgren and an anonymous referee for their constructive comments. Aloisi and M.

We thank F. Mazzarini for fruitful discussions and enthusiastic encouragement. Acocella and G. Puglisi are acknowledged for support. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation.

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Journal of Geophysical Research, , B, doi Bonaccorso, A. Bousquet, N. Bruno, G. Budetta, T. Caltabiano, S. Calvari, D. Carbone, M. Coltelli, R. Corsaro, P. Del Carlo, C. Del Negro, S. Falsaperla, F. Greco, G.

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Lanzafame, D. Pompilio, E. Privitera, G. Puglisi, R. Romano, S. Rapporto Istituto Naz.

Geophysical Research Letters, 40, , doi Bulletin of Volcanology, 75, Doi: Geophysical Research Letters, 30, 18, Earth and Planetary Science Letters, —, — , doi: Burton M. Geophysical Research Letters, vol.

Sonia Calvari | Istituto Nazionale di Geofisica e Vulcanologia -

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