The destructive 1928 fissure eruption of Mt Etna (Italy): surficial deformation revealed by field data and FEM numerical modelling

The present research is aimed at evaluating the wide surficial deformation associated with the destructive 1928 fissure eruption on Mt. Etna, Italy: with its high effusion rates and the low elevation of the main eruptive vents, this eruption caused the destruction of the Mascali town. The main aim of our work is to reconstruct the geometry, kinematics and origin of the system of faults and fissures formed during the 1928 event. Our study has been performed through a multidisciplinary approach consisting of field observations, aerial photo interpretation and Finite Element Method (FEM) modeling through COMSOL Multiphysics® (v5.6). Field data consist of 438 quantitative measurements: azimuth values, opening direction and aperture of dry/eruptive fissures, as well as attitude and offsets of faults. Our detailed structural analysis allowed us to detect four different tectonic settings related to dike propagation scenarios, which, from west to east, are: 1) a sequence of 8 eruptive vents surrounded by a 385-m wide graben, 2) a 2.5-km long single eruptive fissure, 3) a half-graben up to 74-m-wide and a symmetric 39-m-wide graben without evidence of eruption, 4) alignment of lower vents along the pre-existing Ripe della Naca faults. 

As a next step, several numerical models have been developed to investigate the relationship between diking and surficial deformation. We performed sensitivity analyses, by modifying crucial parameters, such as a range of dike overpressure values (1-20 MPa), host rock properties (Young modulus ranging from 1 to 30 GPa), stratigraphic sequence, and layer thickness. Furthermore, the distribution of tensile and shear stresses above the dike tip has been evaluated. Results revealed the presence of temporary stress barriers, which consist of soft (e.g. tuff) layers, that control the surficial deformation above a dike propagating to the surface by suppressing the distribution of shear stresses.