The 2018 Mount Etna Earthquake (Mw 4.9): Depicting a Natural Model of a Composite Fault System From Coseismic Surface Breaks

Abstract During the Mw 4.9 earthquake of December 26, 2018, an 8 km long arc‐shaped coseismic fracture belt formed along the southeastern flank of Etna Volcano (Sicily, Italy). We performed prompt field mapping of the 2018 coseismic fracture field, integrated with structural measurements in about 600 sites. The fracture belt comprises a northern, 3‐km‐long N130° striking segment, and a southern, 5‐km‐long N150°–170° portion. Along the former segment our data document a dextral shear zone, geometrically and kinematically coherent with the focal mechanism of the earthquake and indicating N170‐oriented compression. This shear zone shows a distinctive ground signature affecting lava flows of different ages, suggesting progressive growth of the structure through repeated coseismic cycles. The coseismic strain migrated to the NNW‐oriented segment, along which the strike‐slip deformation changed into N100–120°‐trending extension. The deformation along this segment is derived from the reactivation of a Late Quaternary tectonic boundary separating two adjacent compressional and extensional domains. The overall coseismic deformation is the result of these two different kinematics, consistent with the long‐term dynamics of both adjacent domains. The growth rate of recent cumulative morphotectonic features along the remobilized NNW‐striking fault demonstrates that the structure is still deforming at a rate comparable to the long‐period estimate. Our study points out that the 2018 seismic event, even if triggered by volcano‐tectonic processes, represents the expression of the composite regional dynamics, which are well constrained by large scale geodetic data.