A. Bonaccorso

The initial phases of the 2008–2009 Mount Etna eruption: A multidisciplinary approach for hazard assessment

Accepted for publication in Journal of Geophysical Research. Copyright (2010) American Geophysical Union

Dynamic inversion of ground deformation data for modelling volcanic sources (Etna 1991–93)

The 1991–93 Etna eruption, that represented the most important recent eruptive event both in terms of duration (472 days) and total volume of erupted lava (ca. 250 · 106 m³), caused marked ground deformation measured by using different geodetic techniques such as EDM, GPS, levelling, and tiltmetry. An inversion, termed “dynamic” because it took into account the sequence of events considering both the separate and cumulative effects of two sources, (a shallow tensile crack and a deeper ellipsoidal deflating source), was performed. It was based on 163 data collected by the different techniques plus 2 geometrical conditions for constraining the geometry of the crack on the surface. The final solution furnished a good fit for all measurements, defining a shallow tensile crack located inside the volcanic edifice, which represents the effect of the intrusion, and a depressuring body with centre at ca. 3000 m b.s.l., which could represent the centre of a generalized and more complex deflation of the volcanic edifice during the course of the eruption.

Models of ground deformation from vertical volcanic conduits with application to eruptions of Mount St. Helens and Mount Etna

We present simple analytical models of ground deformation from inflation of vertical volcanic conduits or pipes. We compare three cases corresponding to (1) a pressurized pipe with closed end at the top that resists the internal pressure, (2) a pressurized pipe with top open for which the internal deformation is mainly dislocation of the cylindrical walls, and (3) a pipe-shaped region that dilates. For the closed pipe we use Eshelbys inclusion theory to model deformation from a thin, pressurized, elongated, prolate ellipsoid in a full-space, which we express in terms of double forces. To satisfy surface boundary conditions, we superimpose image solutions by using double forces derived from Mindlins half-space solution for the point force. For the open pipe we apply the Volterra integral to dislocation across a cylindrical surface and generalize it to the half-space using Mindlins point force solution. These two solutions show marked differences from the line of dilatation solution. Ratios of maximum horizontal deformation to maximum vertical deformation for the pipe models are significantly greater than values for the center or line of dilatation models. This, and the more gradual fall off of the deformation with distance may be used as diagnostics for discriminating between pipe-like sources and dilatational sources on volcanoes, where both components of the deformation field are available. As examples, we compare the closed pipe model with deformation associated with dome building on Mount St. Helens volcano, and tilt predicted by the open pipe model with tilt measured during a period of explosive eruptive activity on Mount Etna, Sicily, in 1995. Comparison between model values and the measurements suggests that the effective elastic moduli of the volcanic cones are very low.

Fast deformation processes and eruptive activity at Mount Etna (Italy)

The seismic and deformation patterns observed on Mount Etna before and during the 1991–1993 eruption, the third largest since the seventeenth century in terms of lava volume, are consistent with the regional tectonic framework of eastern Sicily. The pattern of the stress field acting on the intermediate and lower crust was defined at the local scale by focal mechanisms of microearthquakes occurring at depths between 10 and 25 km beneath the volcano. They provide evidence for a strike-slip compressional stress regime with the maximum compressive component acting approximately N-S. The fault plane solutions and the spatial and temporal distribution of seismicity indicate that usually sinistral shear ruptures occur along approximately NE-SW trending fault zones, while dip-slip ruptures affect approximately NNW-SSE trending fault zones. The latter include the avenues along which magma ascended during the 1991–1993 eruption. Seismic observations indicate a local inversion of the stress field acting on the upper crust (depth < ∼10 km) underlying Mount Etna, which was initiated less than 2 months before the eruptive event and disappeared with its end. This is consistent with a local tensile regime that favored the magma ascent through the shallow crust. The events preceding the eruption and accompanying its onset (tilt anomalies and seismic swarms) and those occurring shortly after its beginning (a mainshock-aftershock seismic sequence and associated coseismic tilts) provide important evidence for understanding the dynamics of the two main volcano-tectonic structures (NE-SW and NNW-SSE trending fault zones) and associated intrusive mechanisms on Etna. The shape and location of the eruption-feeding dike have been modeled from ground deformation data. This approximately NNW-SSE modeled dike, the seismicity, and the position of the fractures are consistent with the regional stress field characterized by σ1 oriented approximately N-S. The geophysical data presented and analyzed in the present paper strongly suggest an overall regional tectonic control as well as an active role for the intruding magma in the dynamics of the volcano.

Shear response to an intrusive episode at Mt. Etna volcano (January 1998) inferred through seismic and tilt data

Abstract The space–time evolution of a seismic swarm (MDmax=3.7) occurring on 9–14 January 1998 at Mt. Etna volcano is analysed. The hypocentral pattern and most reliable focal mechanisms indicate that seismic activity during 10–14 January mainly occurred within a fault-zone located in the volcanos western flank. The fault showed an approximately NE–SW overall trend, and shallow rupture mode of right-lateral strike-slip type. It is noteworthy that earthquakes occurring during the initial phase (9 January) of the swarm showed prevalent dip–slip rupture mechanisms, which are consistent with tensile opening of an approximately NNW–SSE-trending fault-zone. This NNW–SSE structural lineament belongs to a regional lithospheric structure, which has also been identified as a main volcano-genic structural trend of Mt. Etna. The observed continuous tilt data are not explainable by the seismic slip associated with strain released along the NE–SW structure. However, an intrusive source would be compatible with the rupture mechanisms observed during the initial phase of the swarm and could explain the observed tilt. Therefore, we evidence that an intrusive episode occurred along the NNW–SSE structure starting 9 January and during the following days it forced the NE–SW fault system to work in a right-lateral mode. These mechanisms, leading to the February–November 1999 summit eruption, are analogous to the ones observed before the 1991–93 eruption.

Evidence of a dyke‐sheet intrusion at Stromboli Volcano Inferred through continuous tilt

A tiltmeter, installed in the northern sector of the island of Stromboli (Aeolian archipelago, Tyrrhenian Sea), recorded a co-seismic offset at the time of the main shock (ML 3.7) of the 7 November, 1994 swarm located 5 km north of the island. During the three months following this event a transient tilt anomaly, which was interpreted as due to a lateral intrusion and modelled through a dyke-sheet dislocation, was recorded. The tilt data were used in an analytical inversion based on a propagating tensile source with length and width increasing in time. The final result is consistent with the tectonic and structural framework of the volcano area.

Multidisciplinary investigation on a lava fountain preceding a flank eruption: The 10 May 2008 Etna case

A multidisciplinary approach integrating a wide data set ranging from bulk rock compositions of the erupted products to volcanic tremor, long-period events, and tilt and gravity signals is used to investigate the source depth and magma dynamics of the 10 May 2008 lava fountain at Southeast crater (SEC) of Mount Etna. The investigation was undertaken in the framework of the previous 2007 explosive activity as well as the subsequent effusive eruption beginning 13 May 2008 and lasting up to July 2009. All the data concur in indicating that the 10 May lava fountain was generated by the fragmentation of a foam layer trapped at the top of a shallow reservoir, about 1500–1700 m below the summit of SEC. The shift from the episodic strombolian/lava fountain activity occurring in 2007 at SEC to the more powerful 10 May 2008 lava fountain is explained by the intrusion of a new more primitive magma into the shallow reservoir. Data also indicate that an attempted magma intrusion east of the summit area occurred during the 10 May fire fountain. This event caused the fracturing and weakening of the surrounding rocks and created a preferential pathway for the penetration of the magma that, only 3 days later, started to feed the 2008–2009 effusive eruption.

Modeling of ground deformation associated with recent lateral eruptions: Mechanics of magma ascent and intermediate storage at Mt. Etna

The last twenty years have been a very active time in the eruptive history of Mt. Etna. Both summit and flank or lateral eruptions have occurred repeatedly, but the main magma discharge came from flank eruptions. In this work, results are reported from fitting analytic models of the deformation sources by inversion of ground deformation data recorded by different geodetic techniques (electro-optical distance measurements or EDM, leveling, tilt, GPS). The various models obtained, either single or double sources, indicate that the eruptions are related to different feeding mechanisms such as deep intrusions, with emplacement lasting from only a few days (2001) up to several months (1981) to prolonged inflation of the intermediate storage chamber of the volcano followed by either rapid shallow intrusion and eruption (1989) or extended eruption (1991-93). The results and the principal conclusions suggested by the modeling are discussed. A revealing feature, common to all the modeled eruptions, is that their final intrusions, both near-surface and at depth, take place within the volcanic edifice in an approximately NNW-SSE to N-S direction, which coincides with a main tectonic trend crossing the volcano. Another important conclusion, supported by deformation associated with the 1991-93 eruption, is that a shallow intermediate storage or magma chamber is present beneath Mt. Etna located approximately 3 km below sea level (b.s.1.). Finally, we discuss the volcano inflation in 1993-2001, which included several summit explosive eruptions culminating in the 2001 lateral eruption.

Dislocation modelling of the 1989 dike intrusion into the Flank of Mount Etna, Sicily

We have applied a dislocation model, consisting of a dipping tabular body with constant dislocation perpendicular to the plane of the body, to surface displacements related to an intrusion in the south flank of Etna volcano, Sicily. Ground deformation was measured by a trilateration array and automatic tilt measurements during the 1989 eruption of the volcano. Significant line length changes and tilt were measured associated with an episodic 0.8 km propagation of the intrusion into the measurement array. We incorporated the orientation and location of measured surface cracking into a Bayesian nonlinear inversion of the data to determine the geometry of the intrusion. We suggest the intrusion caused a 1-m-wide opening of a vertical crack that extended from a depth of 1 km to within a few hundred meters of the surface. We relate the distribution of surface cracking and seismicity associated with the fracture to stresses generated in the slip-weakening zone associated with the crack edge.

An investigation into shallow borehole tiltmeters

Shallow borehole tiltmeters are frequently used for ground deformation monitoring in volcanic areas, where they are usually installed within a few meters of the surface. A major problem concerns the noise affecting the installation at shallow depth where large tilt and strain are caused by temperature effects. The long term stability of these instruments and the best shallow depth of installation, in terms of best cost/benefit, are the crucial points. We conducted an experiment in order to compare the signals of three shallow borehole electronic tiltmeters installed in the same place at different depths. We verified the signal reliability in revealing the long term slow deformation and looked into the reduction of the temperature effects with depth. We present three years of data and discuss the limits and the advantages of the different installation depths.