Italy 2016 Earthquake sequence

On 24 August 2016, at 01:36 UTC¸ the intra-Apennine extensional fault system of Central Italy released a destructive earthquake (Amatrice 2016, M_W 6.0 TDMT, M_W 6.2, QRCMT). It produced widespread damage and fatalities, killing about 300 people and severely destroying the town of Amatrice and other small localities. After few hours, the Amatrice earthquake was followed by a significant aftershock (M_W 5.5, QRCMT), which nucleated ~15 km NW-ward. In the following days, five events having M_W between 4.5 and 5.0 were released and the sequence mainly grew northward.

The epicentral area extends in the NNW-SSE direction, for a length of about 25-30 km. It is located at the hanging-wall of the WSW-dipping Vettore-Gorzano active extensional fault system. Relevant co-seismic deformations were highlighted soon after the main shock. This was located at a depth of about 8 km; its epicenters are located within the relay zone between the two en-echelon fault segments. The epicentral area well coincides with the pattern revealed by DInsAR measurements, which is characterized by a double-eyed co-seismic shape. In particular, we generated several interferograms by using ALOS and Sentinel 1-A and B constellation data acquired on both ascending and descending orbits to show that most displacement is characterized by two main subsiding lobes of about 20 cm on the fault hanging-wall; this is consistent with the calculated focal mechanism. By inverting the generated interferograms, following a classical Okada analytical approach, the modelling results account for two sources related to main shock and more energetic aftershock. The time interval between the ascending and descending (31 August 2016) does not discriminate the effects derived from the main 24 August event and by its aftershock, but the low magnitude of the second event can only very marginally contribute to the overall deformation pattern.

The reconstructed 3D fault model consists in two major interconnected fault segments, Vettore and Gorzano, which are individual at depth shallower than about 7-8 km and converge into a unified surface at higher depths. The Vettore-Gorzano unified surface has a length of 65 km, dips WSW-ward with an angle of about 45-50° and reaches a depth of 11 km, where, according to the proposed reconstruction, detaches on an east-dipping basal detachment.

Through Finite Element numerical modelling that jointly exploits DINSAR deformation measurements and structural-geological data, we reconstruct the 3D source of the Amatrice 2016 normal fault earthquake which well fit the main shock. The inversion shows that the co-seismic displacement area was partitioned on two distinct en echelon fault planes, which at the main event hypocentral depth (8 km) merge in one single WSW-dipping surface. Slip peaks were higher along the southern half of the Vettore fault, lower along the northern half of Gorzano fault and null in the relay zone between the two faults; field evidence of co-seismic surface rupture are coherent with the reconstructed scenario. 

The Central Apennines in Italy were struck by multiple moderate-size but damaging shallow earthquakes in 2016. In this study, we optimize the fault geometry and invert for fault slip based on coseismic GPS and Interferometric Synthetic Aperture Radar (InSAR) analysis of Copernicus Sentinel-1A and -1B, JAXA ALOS-2 data, and ASI COSMO-SkyMed for the 2016 M_w6.2 Amatrice, M_w6.1 Visso, and the M_w6.4 Norcia earthquakes in Italy. For the Amatrice event, there was less than 4 cm static surface displacement at the town Amatrice where the most devastating damage occurred. Landslides triggered by earthquake ground shaking are not uncommon, but triggered landslides with sub-meter movement are challenging to be observed in the field. We find evidence of coseismically triggered deep-seated landslides northwest and northeast of the epicenter where coseismic peak ground acceleration was estimated > 0.5 g. By combining ascending and descending InSAR data, we are able to estimate the landslide thickness as at least 100 and 80 m near Mt. Vettore and west of Castelluccio, respectively. The landslide near Mt. Vettore terminates on the pre-existing fault Mt. Vettore Fault (MVEF) scarp. Our results imply that the long-term fault slip rate of MVEF estimated based on paleoseismic studies could potentially have errors due to triggered landslides from nearby earthquake events. Two months after the Amatrice earthquake, the M_w6.1 Visso and M_w6.4 Norcia earthquakes stroke Central Apennines in late October. Both events occurred ~30 km north of the Amatrice earthquake. We combine ascending/descending InSAR and GPS measurements to constrain the fault geometry as well as the slip distribution. The geodetic data infer that the majority of slip is on a west-dipping moderately dipping normal fault. However, the InSAR result suggests antithetic normal faults above a shallow detachment with normal sense of motion also slipped. The antithetic faults and the detachment all slipped during or right after the Norcia earthquake. Although the complicated slip on multiple faults cannot be well constrained by strong motion seismic data, the aftershocks recorded three months following the earthquake illuminate the antithetic fault as well as the detachment. Our results demonstrate how earthquakes can illuminate geological structures and the significant advantage of using space geodesy to obtain detail of surface deformation during earthquakes.

In this study we analyse in detail the multiple-mainshock sequence occurred in Central Italy since August 24th till October 30th 2016. We use all the available InSAR data, from different satellites, to reconstruct the complex mechanism of the seismic sequence, characterized by 5 mainshocks from magnitude 5.4 (August 24th and October 26th) to magnitude 6.5 (October 30th). In addition to a detailed reconstruction of  the mainshocks faulting mechanisms, we also computed the time evolution of the stress field, starting from a background stress consisting of the regional tectonic static field plus the time-dependent Coulomb stress changes generated by the previous earthquakes in the region since 1979 (Norcia earthquake). FInally, we compute the actual stress field in the region, resulting from the whole set of mainshocks (with magnitude over 5), from 1979 till now, and its time evolution in the next decades. In order to compute time-dependent stress changes due to earthquake occurrence, we use a theoretical description involving a flat, layered, self-gravitating, compressible Earth model. The viscosity of the lower crust, which is the most crucial parameter affecting the time evolution of the static stress changes, has been inferred by previous studies based on the analysis of post-seismic ground deformation data around the largest seismic event occurred in the Apennines in the last century, namely the 1980 Irpinia earthquake (M=6.9).

Results of our work, besides giving a very accurate and reliable recontruction of the multiple-fault mechanism of this important Central Italy sequence, hilights, for the first time, the close relationship among the earthquake occurrence in the Apennines and the time evolution of the stress changes. Such a close relationships holds both at the individual sequence scale (i.e. among the 5 mainshocks of the 2016 sequence) and, more interestingly, among adjacent faults and tectonic domains. Differently from previous studies about stress interaction in the Apennines, this is the first time that a time dependent stress model is applied to reconstruct the Coulomb stress evolution in an important part of the Apennine chain, allowing to identify the future evolution of the areas more prone to seismic activation.

4.01.b   Italy 2016 Earthquake sequence