Ground deformation and source geometry of the 30 October 2016 Mw 6.5 Norcia earthquake (Central Italy) investigated through seismological data, DInSAR measurements, and numerical modelling

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

Integrating seismological data, DInSAR measurements and numerical modelling to analyse seismic events: the Mw 6.5 Norcia earthquake case-study

Questa tesi di dottorato è incentrata sull’analisi dettagliata di sequenze sismiche e sull’applicazione di un approccio multidisciplinare basato sull’integrazione di diverse tecniche geofisiche e geodetiche. Nel contesto di uno studio più generale delle sequenze sismiche, abbiamo concentrato il lavoro sull’analisi ed il confronto di dieci sequenze sismiche (cinque estensionali e cinque compressive), al fine di comprendere le differenze tra questi due ambienti tettonici in termini di durata degli aftershocks. Infatti, il numero di aftershocks decade nel tempo in funzione di vari parametri che risultano essere peculiari di ogni area sismogenetica; tra questi si possono annoverare la magnitudo del mainshock, la reologia crostale e le variazioni dello stress lungo la faglia. Tuttavia, il ruolo esatto svolto da questi parametri nel controllo della durata delle sequenze di aftershocks non è ancora noto. Utilizzando due diverse metodologie, abbiamo evidenziato che l’ambiente tettonico gioca un ruolo primario nell’influenzare la durata degli aftershocks. In media e per una data magnitudo del mainshock, (i) le sequenze di aftershocks sono più lunghe e (ii) il numero di terremoti è maggiore negli ambienti tettonici estensionali rispetto a quelli compressivi. Una possibile spiegazione consiste nel fatto che questa differenza possa essere correlata al diverso tipo di energia dissipata durante i terremoti; in dettaglio, (i) un effetto congiunto di forza gravitazionale e di energia elastica governerebbe i terremoti estensionali, mentre (ii) il rilascio di pura energia elastica controllerebbe i terremoti compressivi. Infatti, le faglie normali operano a favore della gravità, preservando così l'inerzia per un periodo più lungo, e la sismicità dura fino a quando l'equilibrio gravitazionale non viene nuovamente raggiunto dal sistema. Viceversa, i thrusts agiscono contro la gravità, esauriscono la loro inerzia più velocemente e la dissipazione di energia elastica viene controbilanciata dalla forza gravitazionale. Quindi, per sequenze sismiche con magnitudo e parametri reologici paragonabili, gli aftershocks durano più a lungo negli ambienti estensionali poiché la gravità favorisce il collasso dei volumi di hangingwall. Il verificarsi della sequenza sismica del Centro Italia nel 2016 ha fornito un banco di prova per un'analisi dettagliata di un altro terremoto estensionale. Per questo motivo, abbiamo analizzato il terremoto di Norcia (Mw 6.5; Italia Centrale) per aggiungere un’altra sequenza sismica estensionale ai casi di studio precedentemente esaminati. I risultati di questa analisi mostrano che anche la sequenza sismica di Norcia presenta lo stesso comportamento delle altre sequenze estensionali in termini di evoluzione temporale e spaziale degli aftershocks. Inoltre, abbiamo deciso di prendere in considerazione il terremoto di Norcia come caso di studio per l’applicazione di un approccio multidisciplinare, al fine di cercare di comprendere la possibile cinematica e il ruolo della gravità durante i processi di enucleazione degli eventi estensionali. In particolare, abbiamo investigato il terremoto di Norcia, ricorrendo all’utilizzo di dati sismologici, di misure DInSAR e della modellazione numerica. In particolare, abbiamo prima di tutto preso in considerazione gli ipocentri rilocalizzati con 0.1≤Mw≤ 6.5, verificatisi tra il 24 agosto e il 29 novembre 2016 e registrati dalla rete sismometrica INGV; la proiezione su sezioni e la successiva analisi degli ipocentri considerati hanno consentito di comprendere quali strutture geologiche siano state coinvolte durante il processo di enucleazione del terremoto. In seguito, abbiamo analizzato la componente verticale (sollevamento e subsidenza) dei displacements che hanno interessato i blocchi di hangingwall e di footwall della faglia sismogenetica, precedentemente identificata in profondità mediante l’analisi della distribuzione ipocentrale; per fare ciò, abbiamo utilizzato le misure DInSAR ottenute dalla combinazione delle coppie di dati SAR cosismici acquisite dal sensore ALOS-2 lungo orbite ascendenti e discendenti. La mappa di deformazione verticale ottenuta mostra tre aree di deformazione principali: (i) una maggiore subsidenza che raggiunge il valore massimo di circa 98 cm in prossimità delle zone epicentrali vicine alla città di Norcia; (ii) due piccoli lobi di sollevamento che interessano sia il blocco di hangingwall (dove raggiunge valori massimi di circa 14 cm) sia quello di footwall (dove raggiunge valori massimi di circa 10 cm). Partendo da queste evidenze, abbiamo calcolato i volumi di roccia interessati dai fenomeni di sollevamento e subsidenza, evidenziando che quelli coinvolti dal fenomeno di subsidenza sono caratterizzati da valori di deformazione significativamente più alti di quelli affetti da sollevamento (circa 14 volte). Al fine di fornire una possibile interpretazione di questa asimmetria volumetrica, abbiamo esteso l'analisi elaborando un modello numerico 2D basato sul metodo degli elementi finiti, implementandolo in un quadro strutturale-meccanico e sfruttando i dati geologici e sismologici disponibili. I risultati della modellazione sono stati poi confrontati con le misure della deformazione del suolo ottenute dall'analisi DInSAR. Nel corso della realizzazione del modello numerico, abbiamo collaudato gli effetti di geometrie diverse, considerando in particolare due scenari: il primo si basa su una singola faglia immergente a sud-ovest, il secondo su una faglia principale immergente a sud-ovest e una fascia antitetica. In questo contesto, il modello caratterizzato dalla presenza della fascia antitetica fornisce il miglior fit quando confrontato con il pattern cosismico di deformazione superficiale. Questo risultato consente di interpretare i fenomeni di subsidenza e sollevamento causati dal terremoto di Norcia come il risultato di un collasso gravitazionale del blocco di hangigwall lungo la faglia principale e della forza frizionale che agisce in direzione opposta, consistentemente con il meccanismo di doppia coppia lungo il piano di faglia.This Ph.D. thesis is focused on the detailed analysis of seismic sequences and on the application of a multidisciplinary approach based on the integration of several geophysical and geodetic techniques. In the context of a more general study of seismic sequences, we focus this work on the analysis and comparison of five extensional and five compressional seismic sequences to understand the differences between these two tectonic settings in terms of aftershocks duration. In fact, aftershocks number decay through time, depending on several parameters peculiar to each seismogenic regions, including mainshock magnitude, crustal rheology, and stress changes along the fault. However, the exact role of these parameters in controlling the duration of the aftershock sequence is still unknown. Here, by using two methodologies, we show that the tectonic setting primarily controls the duration of aftershocks. On average and for a given mainshock magnitude, (i) aftershock sequences are longer and (ii) the number of earthquakes is greater in extensional than in compressional tectonic settings. We suggest as possible explanation that this difference can be related to the different type of energy dissipated during earthquakes; in detail, (i) a joint effect of gravitational forces and pure elastic stress release governs extensional earthquakes, whereas (ii) pure elastic stress release controls compressional earthquakes. Accordingly, normal faults operate in favour of gravity, preserving inertia for a longer period and seismicity lasts until gravitational equilibrium is reached. Vice versa, thrusts act against gravity, exhaust their inertia faster and the elastic energy dissipation is buffered by the gravitational force. Hence, for seismic sequences of comparable magnitude and rheological parameters, aftershocks last longer in extensional settings because gravity favours the collapse of the hangingwall volumes. The occurrence of the 2016 Central Italy seismic sequence furnishes a test-bed for a detailed analysis of a normal fault earthquake. Therefore, we analyse also the Mw 6.5 Norcia (Central Italy) earthquake to add another extensional seismic sequence to the previously examined case-studies. The results of this analysis show that, with respect to the other considered extensional seismic sequences, also the Mw 6.5 Norcia seismic sequence present the same behaviour about the aftershocks temporal and spatial evolution. Moreover, we decide to take into account the Mw 6.5 Norcia mainshock as case-study for the application of a multidisciplinary approach, in order to understand the kinematics and the role of gravity during nucleation processes of extensional events. In particular, we investigate the Mw 6.5 Norcia earthquake by exploiting seismological data, DInSAR measurements and a numerical modelling approach. In particular, we first take into consideration the relocated hypocentres with 0.1≤Mw≤ 6.5 that occurred between August 24th and November 29th, 2016, recorded by the INGV seismometric network; the projection onto sections and the subsequent analysis of the considered hypocentres allow us to identify the geological structures that were involved during earthquake nucleation process. Then, we 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 hypocentres 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). Also GPS measurements were used to compare the displacements recorded next to the epicentral area. 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 the analysis by running a 2D numerical model based on the finite element method, implemented in a structural-mechanic framework and exploiting the available geological and seismological data. Modelling results are compared with the ground deformation measurements retrieved from the multi-orbit ALOS-2 DInSAR analysis. In the modelling approach, we test the effects of different geometries, by considering two different scenarios: the first is based on including only a single SW-dipping fault, the second includes 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 of the frictional force acting in the opposite direction, consistently with the double couple fault plane mechanism

Volume unbalance on the 2016 Amatrice - Norcia (Central Italy) seismic sequence and insights on normal fault earthquake mechanism

We analyse the M w 6.5, 2016 Amatrice-Norcia (Central Italy) seismic sequence by means of InSAR, GPS, seismological and geologic data. The >1000 km 2 area affected by deformation is involving a volume of about 6000 km 3 and the relocated seismicity is widely distributed in the hangingwall of the master fault system and the conjugate antithetic faults. Noteworthy, the coseismically subsided hangingwall volume is about 0.12 km 3 , whereas the uplifted adjacent volumes uplifted only 0.016 km 3 . Therefore, the subsided volume was about 7.5 times larger than the uplifted one. The coseismic motion requires equivalent volume at depth absorbing the hangingwall downward movement. This unbalance regularly occurs in normal fault-related earthquakes and can be inferred as a significant contribution to coseismic strain accomodated by a stress-drop driven collapse of precursory dilatancy. The vertical coseismic displacement is in fact larger than the horizontal component, consistent with the vertical orientation of the maximum lithostatic stress tensor

Testing Different Tectonic Models for the Source of the M w 6.5, 30 October 2016, Norcia Earthquake (Central Italy): A Youthful Normal Fault, or Negative Inversion of an Old Thrust?

We adopted a multidisciplinary approach to investigate the seismotectonic scenario of the 30 October 2016, Mw 6.5, Norcia earthquake, the largest shock of the 2016\u20132017 central Italy earthquake sequence. First, we used seismological and geodetic data to infer the dip of the main slip patch of the seismogenic fault that turned out to be rather low\u2010angle (~37\ub0). To evaluate whether this is an acceptable dip for the main seismogenic source, we modeled earthquake deformation using single\u2010 and multiple\u2010fault models deduced from aftershock pattern analyses. These models show that the coseismic deformation generated by the Norcia earthquake is coherent with slip along a rather shallow\u2010dipping plane. To understand the geological significance of this solution, we reconstructed the subsurface architecture of the epicentral area. As the available data are not robust enough to converge on a single fault model, we built three different models encompassing all major geological evidence and the associated uncertainties, including the tectonic style and the location of major d\ue9collement levels. In all models the structures derived from the contractional phase play a significant role: from controlling segmentation to partially reusing inherited faults, to fully reactivating in extension a regional thrust, geometrically compatible with the source of the Norcia earthquake. Based on our conclusions, some additional seismogenic sources falling in the eastern, external portions of the Apennines may coincide with inherited structures. This may be a common occurrence in this region of the chain, where the inception of extension is as recent as Middle\u2010Upper Pleistocene

Are normal fault earthquakes due to elastic rebound or gravitational collapse?

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Numerical analysis of interseismic, coseismic and postseismic phases for normal and reverse faulting earthquakes in Italy

The preparation, initiation, and occurrence dynamics of earthquakes in Italy are governed by several frequently unknown physical mechanisms and parameters. Understanding these mechanisms is crucial for developing new techniques and approaches for earthquake monitoring and hazard assessments. Here, we develop a first-order numerical model simulating quasi-static crustal interseismic loading, coseismic brittle episodic dislocations, and postseismic relaxation for extensional and compressional earthquakes in Italy based on a common framework of lithostatic and tectonic forces. Our model includes an upper crust, where the fault is locked, and a deep crust, where the fault experiences steady shear. The results indicate that during the interseismic phase, the contrasting behavior between the upper locked fault segment and lower creeping fault segment generates a stretched volume at depth in the hanging wall via extensional tectonics while a contracted volume forms via compressional tectonics. The interseismic stress and strain gradients invert at the coseismic stage, with the interseismic dilated volume contracting during the coseismic stage, and vice versa. Moreover, interseismic stress gradients promote coseismic gravitational subsidence of the hanging wall for normal fault earthquakes and elastic uplift for reverse fault earthquakes. Finally, the postseismic relaxation is characterized by further ground subsidence and uplift for normal and reverse faulting earthquakes, respectively, which is consistent with the faulting style. The fault is the passive feature, with slipping generating the seismic waves, whereas the energy activating the movement is stored mostly in the hanging wall volume. The main source of energy for normal faulting and thrust is provided by the lithostatic load and elastic load, respectively

Study of the 2016 central Italy post seismic displacement through an independent component analysis

Tra l’Agosto e l’Ottobre 2016 una serie di terremoti moderati ha colpito l’Italia centrale. Gli epicentri degli eventi principali sono enucleati vicino i paesi di Amatrice, Visso e Norcia, perciò questa sequenza sismica è anche conosciuta come sequenza di Amatrice-Visso-Norcia. Lo scopo di questa tesi è quello di tracciare una mappa afterslip attraverso lo studio della deformazione post-sismica misurata dal GPS. Il primo passo necessario per raggiungere questo obiettivo è la detrendizzazione delle serie temporali geodetiche. Successivamente è stata eseguita una "Independent Component Analysis" (ICA) della distribuzione spazio temporale del campo di spostamento. L’ICA è una tecnica di analisi statistica multivariata che permette di ricostruire e separare le sorgenti fisiche che producono lo spostamento misurato. Nello specifico, tramite una ICA, è possibile decomporre il segnale geodetico in un numero fissato di Componenti Indipendenti (IC). Il vincolo di indipendenza statistica permette di distinguere meglio gli effetti (segnali) delle diverse sorgenti fisiche che hanno prodotto il dataset, rispetto ad altre tecniche di statistca multivariata. Lo step finale richiede l’inversione della distribuzione spaziale del segnale post sismico, che nel nostro caso è stato mappato nella prima componente indipendente (IC1) e interpretato come dovuto ad afterslip su faglia. Per fare ciò è necessario fissare la geometria della sorgente sismica che è stata interessata da afterslip, ovverosia quali faglie sono state attivate durante la fase postsismica della sequenza. Il risultato finale consiste in una mappa della distribuzione di afterslip sulle faglie considerate, con aree attivate nella fase post sismica compatibili con le zone coinvolte nella fase cosismica e con la distribuzione di sismicità. A quanto risulta finora, questo è il primo tentativo di vincolare il segnale post sismico della sequenza dell’Italia centrale del 2016

Multiple Lines of Evidence for a Potentially Seismogenic Fault Along the Central-Apennine (Italy) Active Extensional Belt–An Unexpected Outcome of the MW6.5 Norcia 2016 Earthquake

The Apenninic chain, in central Italy, has been recently struck by the Norcia 2016 seismic sequence. Three mainshocks, in 2016, occurred on August 24 (MW6.0), October 26 (MW 5.9) and October 30 (MW6.5) along well-known late Quaternary active WSW-dipping normal faults. Coseismic fractures and hypocentral seismicity distribution are mostly associated with failure along the Mt Vettore-Mt Bove (VBF) fault. Nevertheless, following the October 26 shock, the aftershock spatial distribution suggests the activation of a source not previously mapped beyond the northern tip of the VBF system. In this area, a remarkable seismicity rate was observed also during 2017 and 2018, the most energetic event being the April 10, 2018 (MW4.6) normal fault earthquake. In this paper, we advance the hypothesis that the Norcia seismic sequence activated a previously unknown seismogenic source. We constrain its geometry and seismogenic behavior by exploiting: 1) morphometric analysis of high-resolution topographic data; 2) field geologic- and morphotectonic evidence within the context of long-term deformation constraints; 3) 3D seismological validation of fault activity, and 4) Coulomb stress transfer modeling. Our results support the existence of distributed and subtle deformation along normal fault segments related to an immature structure, the Pievebovigliana fault (PBF). The fault strikes in NNW-SSE direction, dips to SW and is in right-lateral en echelon setting with the VBF system. Its activation has been highlighted by most of the seismicity observed in the sector. The geometry and location are compatible with volumes of enhanced stress identified by Coulomb stress-transfer computations. Its reconstructed length (at least 13 km) is compatible with the occurrence of MW≥6.0 earthquakes in a sector heretofore characterized by low seismic activity. The evidence for PBF is a new observation associated with the Norcia 2016 seismic sequence and is consistent with the overall tectonic setting of the area. Its existence implies a northward extent of the intra-Apennine extensional domain and should be considered to address seismic hazard assessments in central Italy

The influence of subsurface geology on the distribution of earthquakes during the 2016‐-2017 Central Italy seismic sequence

Abstract In 2016–2017, a destructive sequence of earthquakes affected a wide portion of Central Italy, activating a complex, 80-km long system of SW-dipping normal faults and causing impressive surface faulting and widespread damage. Former studies providing reconstructions of the fault systems activated during this sequence, are mostly based on high-resolution seismological and geodetic data. In this paper, we integrate surface and subsurface geological data with the ones obtained by an irregular network of seismic reflection profiles, aimed at providing a comprehensive reconstruction of the subsurface lithologies and structures in this area. We have constructed a set of five geological cross-sections, passing through the mainshock epicentral areas (Mw > 5.5) of the seismic sequence. The cross-sections are extrapolated down to a depth of ca. 12 km, along which we have plotted relocated seismicity. Combined geological and seismological data support a new 3D seismotectonic model, illustrating the propagation through time and space of the seismic ruptures during the sequence. Our results show that the litho-mechanical stratigraphy exerted a primary control on the distribution of seismicity, as it is mostly hosted in the more competent lithologies (i.e. the Late Triassic-Paleogene succession, consisting of carbonates and evaporites). In addition, we illustrate the crucial role played by the inherited compressional structures in determining the lateral and vertical variations of the rheological properties of the upper crust and, eventually, the overall geometry and segmentation of the seismogenic extensional system. The workflow proposed here can be applied to other seismogenic zones throughout the world, since reliable seismotectonic models require an accurate reconstruction of the subsurface geological setting, based on a close integration of geological, geophysical and seismological data

Variations in slip-rate and earthquake occurrence across 3D structural complexities on active normal faults

This PhD thesis provides a series of studies on the relationship between the non-planar geometry and the seismic behaviour of active normal faults. Herein, several examples show that throw and fault dip increase within along-strike fault bends in order to preserve the horizontal strain-rate within the bend and along the fault. This has been demonstrated for a variety of normal faults (a) located in different geodynamic domains and (b) for measurements of throw taken over different time periods. Furthermore, throw enhancement within fault bends has been observed on (1) immature faults, where fault bends are still propagating up to the surface and are not yet fully established, (2) well-established single fault segments, where fault bends affect one continuous fault segment, and (3) densely-spaced fault systems arranged across strike (with fault spacing < 5 km), where a change in strike across several fault segments creates an overall bend in the system. The results presented in this thesis suggest that the relationship between the non-planar geometry and the distribution of throw is scale-independent, and can act across systems of faults if they are closely spaced across strike. Moreover, 36Cl-cosmogenic dating of tectonically-exposed fault planes on faults spaced > 5 km across-strike shows that these faults are clustered, with a non-systematic alternance of periods of rapid slip accumulation (i.e. earthquake clustering) and periods of quiescence (i.e. earthquake anti-clustering); this suggests that parallel faults interact in terms of sharing the regional strain-rate, with switching activity that affects the slip-rate on a single fault. The results have implications for numerous and diverse aspects of the earthquake geology, such as interpretation of palaeoseismology studies including at trench sites and cosmogenic dating of fault planes, empirical scaling relationships, PSHA, and off-fault deformation