Abstract
We investigate the Mw 6.5 Norcia (Central Italy) earthquake by exploiting seismological data, DInSAR measurements, and a numerical modelling approach. In particular, we first retrieve the vertical component (uplift and subsidence) of the displacements affecting the hangingwall and the footwall blocks of the seismogenic faults identified, at depth, through the hypocenters distribution analysis. To do this, we combine the DInSAR measurements obtained from coseismic SAR data pairs collected by the ALOS-2 sensor from ascending and descending orbits. The achieved vertical deformation map displays three main deformation patterns: (i) a major subsidence that reaches the maximum value of about 98 cm near the epicentral zones nearby the town of Norcia; (ii) two smaller uplift lobes that affect both the hangingwall (reaching maximum values of about 14 cm) and the footwall blocks (reaching maximum values of about 10 cm). Starting from this evidence, we compute the rock volumes affected by uplift and subsidence phenomena, highlighting that those involved by the retrieved subsidence are characterized by significantly higher deformation values than those affected by uplift (about 14 times). In order to provide a possible interpretation of this volumetric asymmetry, we extend our analysis by applying a 2D numerical modelling approach based on the finite element method, implemented in a structural-mechanic framework, and exploiting the available geological and seismological data, and the ground deformation measurements retrieved from the multi-orbit ALOS-2 DInSAR analysis. In this case, we consider two different scenarios: the first one based on a single SW-dipping fault, the latter on a main SW-dipping fault and an antithetic zone. In this context, the model characterized by the occurrence of an antithetic zone presents the retrieved best fit coseismic surface deformation pattern. This result allows us to interpret the subsidence and uplift phenomena caused by the Mw 6.5 Norcia earthquake as the result of the gravitational sliding of the hangingwall along the main fault plane and the frictional force acting in the opposite direction, consistently with the double couple fault plane mechanism.
Highlights
The crustal seismicity along the axial part of the Apennines belt (Central Italy) is mainly dominated by extensional faulting and is confined in the upper crust, apart from deeper events associated with a westerly directed subducting slab; the lower crust is rather seismically silent [1]
The DInSAR measurements show that during the Norcia earthquake (NEQ) the hangingwall block was affected by broad subsidence with a maximum vertical amplitude of about 98 cm and by a significantly less extended region characterized by uplift; the footwall block was affected by uplift
The starting point of our study is the analysis of a set of ALOS-2 coseismic DInSAR maps, which reveals three distinct surface deformation patterns: (i) a major subsidence that reaches a maximum value of about 98 cm near the epicentral zones, nearby the town of Norcia; (ii) two smaller uplift lobes that affect both the hangingwall and the footwall blocks
Summary
The crustal seismicity along the axial part of the Apennines belt (Central Italy) is mainly dominated by extensional faulting and is confined in the upper crust, apart from deeper events associated with a westerly directed subducting slab; the lower crust is rather seismically silent [1]. The 2016 Norcia earthquake (NEQ) fits the tectonic setting of the Central Apennines fold-thrust belt and reflects extensional tectonics affecting this area since, at least, Pliocene time [6] This extensional tectonics, associated with the opening of the Tyrrhenian back-arc basin in the west [7], followed and replaced earlier contractional tectonics that formed the accretionary prism, presently shifted to the east (western Adriatic Sea) [8]. Avallone et al [10] show lengthening at a rate of ~4 mm/year between Rieti and Ascoli Piceno, resulting in a strain-rate of ~60 nanostrain/year, consistent with available geological and seismotectonic information, and with the preliminary coseismic deformation observed through DInSAR measurements for the Amatrice earthquake [3] This region has more than 600 years record of historical earthquakes and detailed constraints on the locations and slip rates of its active normal faults [11]. On 30 October the largest event of the sequence (Mw 6.5) occurred near the town of Norcia along the MVFS and hit the area included between the previous events (Figure 1b)
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