Tephra fallout of 2001 Etna flank eruption: Analysis of the deposit and plume dispersion

Abstract

Tephra fallout represented a major source of hazard for eastern Sicily during the 2001 eruption of Mt. Etna (Italy) between 19 July and 6 August. Long-lasting explosive activity was generated from the 2570 m vent, producing a volcanic plume up to 5 km high above sea level. The eruption caused copious lapilli and ash fallout over the volcano flanks for several days. Flight operations were cancelled at the Catania and Reggio Calabria airports; health risk and economic damage put communities living close to this active volcano on the alert. The explosive activity at the 2570 m vent had three main phases characterized by phreatomagmatic, magmatic and vulcanian explosions. In this paper, we analyze the first explosive phase between 19 and 24 July that formed a tephra deposit on the volcano's south-east flanks.

Immediately after the first phase of the eruption, numerous tephra samples were collected in order to draw an isomass map, calculate physical parameters for the eruption and analyze the plume dispersion on the basis of deposit geometry. The tephra deposit shows a bilobate shape due to the change with time of both the vigour of the eruption and the wind direction and velocity that caused a higher rate of particle accumulation along two dispersal axes (SE and SSE).

The total mass of tephra erupted was calculated with two different fitting methods: exponential line segments and a power law fit on the semi-logarithmic plot of mass per unit area versus $\sqrt{\mathrm{area}}$, resulting in values of 1.02 ^⁎ 10⁹ kg and 2.31 ^⁎ 10⁹ kg, respectively. The whole deposit grain-size was calculated applying the Voronoi tessellation method, it shows a mode of 2ϕ and thus indicates a high degree of magma fragmentation during the first phase of the eruption.

Plume dispersal was investigated by an advection–diffusion model to reconstruct the tephra deposit. In the modelling, we took into account the variations of wind direction and velocity, and eruption intensity by dividing the explosive phase into sixteen sub-eruptions and considering the final deposit as the sum of the mass computed for each sub-eruption. Using best fit procedures, we find that the optimal agreement between computed values and field data is obtained by using the total mass calculated with the power law fit and a terminal settling velocity distribution with a particle aggregation model. The computed tephra dispersal was able to reproduce the bilobate shape of the real deposit. This work proves that advection–diffusion models can describe sedimentation processes of weak, i.e., bent-over, long-lasting plumes if the variations of wind direction and velocity, and eruptive intensity are included.

Introduction

Mt. Etna is the most active volcano in Europe and, as such, is a continuous source of risk for eastern Sicily. Strombolian activity and periodic Hawaiian style episodes, often associated with lava flows, frequently occur at summit craters (Fig. 1). In contrast, flank eruptions take place less frequently at intervals of some years, producing large lava flows commonly associated to variable explosive activity. Flank eruptions can be divided in two main classes following Branca and Del Carlo (2005): Class A, generally more frequent, is characterized by lava emission lasting from a few days to years and weak strombolian activity building hornitos or small scoria cones; Class B is characterized by violent strombolian explosive activity that produces long-lasting eruption plumes, ash fallout from tens to hundreds of kilometres away and large scoria cones. The July–August 2001 eruption belongs to Class B category.

From the end of the 1980's, Etna volcano has showed an increasingly explosive pattern of activity ranging from strombolian to subplinian eruptions that have injected large volumes of pyroclasts into the atmosphere (between 10⁴ and 10⁷ m³ per event) during more than 150 eruptive episodes (Branca and Del Carlo, 2005). The explosive activity reached its climax in two major flank eruptions occurred in 2001 and 2002–03, both characterised by long-lasting explosive activity. In these eruptions, volatile-rich fast-rising magma was erupted draining the deep portion of the feeding system and bypassing the central conduits, typical for Etnean eccentric eruptions (Andronico et al., 2005). During the 2001 eruption, phreatomagmatic and magmatic eruptions were observed at the 2570 m vent. A weak plume rising up to 5000 m a.s.l., was blown to the southeast and caused heavy tephra deposition on the flanks of the volcano.

Many numerical models are currently available to reproduce the dispersion and fallout processes from both weak and strong plumes; they have been successfully applied to several recent eruptions (Suzuki, 1983, Armienti et al., 1988, Bursik et al., 1992, Heffter and Stunder, 1993, Searcy et al., 1998, Hurst and Turner, 1999, Koyaguchi and Ohno, 2001, Bonadonna et al., 2002, Bonadonna and Phillips, 2003, Bonadonna et al., 2005a). Nevertheless, because volcanoes are very complex systems, single eruptions may produce a range of eruption plumes with disparate characteristics. Therefore reliable datasets from different types of eruptions are needed to calibrate numerical models more accurately and to better investigate dispersal and sedimentation processes.

In this paper, we report data from the tephra deposit produced from the 2570 m vent and deposited in the south-east sector of Etna between 21 and 24 July 2001, during the first of three phases of the explosive activity. We calculate parameters for the eruption such as the total mass and whole deposit grain-size. Finally, using the tephra dispersal model HAZMAP (Macedonio et al., 2005), we reconstruct the widespread fallout deposit of this phase and we analyze the process by comparing the computed values and the field data.