Surface deformation during the 1928 fissure eruption of Mt. Etna (Italy): Insights from field data and FEM numerical modelling

Abstract

The 1928 CE volcanic activity on eastern Etna, Italy, produced wide surface deformation and high effusion rates along fissures, with excess volumes of about 50 million m3 of lavas. This, in conjunction with the low elevation of the main eruptive vents (1150 m a.s.l.), caused the destruction of the Mascali town. Our research focuses on a multidisciplinary study from field observations and Finite Element Method modelling through COMSOL Multiphysics ®, with the aim of reconstructing the geometry, kinematics and origin of the system of faults and fissures formed during the 1928 event. We collected quantitative measurements from 438 sites of azimuth values, opening direction and aperture amount of dry fissures, and attitude and vertical offsets of faults. From west to east, four volcanotectonic settings have been identified, related to dike propagation in the same direction: 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 as wide as 74 m and a symmetric, 39-m-wide graben without evidence of eruption, 4) alignment of lower vents along the pre-existing Ripe della Naca faults. Field data, along with historical aerial photos, became inputs to FEM numerical models. The latter allowed us to investigate the connection between diking and surface deformation during the 1928 event, subject to a range of overpressure values (1–20 MPa), host rock properties (1–30 GPa) and geometrical complexity (stratigraphic sequence, layer thickness). In addition, we studied the distribution of tensile and shear stresses above the dike tip and gained insights into dike-induced graben scenarios. Our multidisciplinary study reports that soft (e.g. tuff) layers can act as temporary stress barriers and control the surface deformation scenarios (dike-induced graben, single fracture or eruptive fissures) above a propagating dike by suppressing the distribution of shear stresses towards the surface.