Stress loading history of earthquake faults influenced by fault/shear zone geometry and Coulomb pre-stress

Whether the stress-loading of faults to failure in earthquakes appears to be random or to an extent explainable, given constraints on fault/shear-zone interaction and the build-up and release of stress over many earthquake cycles, is a key question for seismic hazard assessment. Here we investigate earthquake recurrence for a system of 25 active normal faults arranged predominantly along strike from each other, allowing us to isolate the effects of stress-loading due to regional strain versus across- and along-strike fault interaction. We calculate stress changes over 6 centuries due to interseismic loading and 25 > Mw 5.5 earthquakes. Where only one fault exists across strike, stress-loading is dominated by the regional tectonics through slip on underlying shear zones and fault planes have spatially smooth stress with predominantly time-dependent stress increase. Conversely, where faults are stress-loaded by across-strike fault interactions, fault planes have more irregular stress patterns and interaction-influenced stress loading histories. Stress-loading to failure in earthquakes is not the same for all faults and is dependent on the geometry of the fault/shear-zone system

Analysis of the spatio-temporal distribution of large earthquakes

The investigation on the spatio-temporal distribution of large earthquakes is still a controversial issue in geophysics and many works in scientific literature have been devoted to this topic. The importance of understanding the statistical distribution of large events is aimed not only to extract information on the physics of the earthquakes occurrence process, but also to make reliable earthquake forecasting. As far as theoretical aspects are concerned, a satisfactory modelling may allow, at least in principle, to test a variety of hypotheses, such as the presence of any regularity in time, and the in uence of di erent tectonic/physical factors that regulate the spatial occurrence of earthquakes. At the same time, a reliable earthquake forecasting has undoubtedly a huge social impact because it may mitigate the seismic risk

Long-range dependence in earthquake-moment release and implications for earthquake occurrence probability

Since the beginning of the 1980s, when Mandelbrot observed that earthquakes occur on 'fractal' self-similar sets, many studies have investigated the dynamical mechanisms that lead to self-similarities in the earthquake process. Interpreting seismicity as a self-similar process is undoubtedly convenient to bypass the physical complexities related to the actual process. Self-similar processes are indeed invariant under suitable scaling of space and time. In this study, we show that long-range dependence is an inherent feature of the seismic process, and is universal. Examination of series of cumulative seismic moment both in Italy and worldwide through Hurst's rescaled range analysis shows that seismicity is a memory process with a Hurst exponent H 48 0.87. We observe that H is substantially space-and time-invariant, except in cases of catalog incompleteness. This has implications for earthquake forecasting. Hence, we have developed a probability model for earthquake occurrence that allows for long-range dependence in the seismic process. Unlike the Poisson model, dependent events are allowed. This model can be easily transferred to other disciplines that deal with self-similar processe

Analysis of the spatio-temporal distribution of large earthquakes

The investigation on the spatio-temporal distribution of large earthquakes is still a controversial issue in geophysics and many works in scientific literature have been devoted to this topic. The importance of understanding the statistical distribution of large events is aimed not only to extract information on the physics of the earthquakes occurrence process, but also to make reliable earthquake forecasting. As far as theoretical aspects are concerned, a satisfactory modelling may allow, at least in principle, to test a variety of hypotheses, such as the presence of any regularity in time, and the in uence of di erent tectonic/physical factors that regulate the spatial occurrence of earthquakes. At the same time, a reliable earthquake forecasting has undoubtedly a huge social impact because it may mitigate the seismic risk

The Alto Tiberina Near Fault Observatory (northern Apennines, Italy)

The availability of multidisciplinary and high-resolution data is a fundamental requirement to understand the physics of earthquakes and faulting. We present the Alto Tiberina Near Fault Observatory (TABOO), a research infrastructure devoted to studying preparatory processes, slow and fast deformation along a fault system located in the upper Tiber Valley (northern Apennines), dominated by a 60 km long low-angle normal fault (Alto Tiberina, ATF) active since the Quaternary. TABOO consists of 50 permanent seismic stations covering an area of 120 × 120 km2. The surface seismic stations are equipped with 3-components seismometers, one third of them hosting accelerometers. We instrumented three shallow (250 m) boreholes with seismometers, creating a 3-dimensional antenna for studying micro-earthquakes sources (detection threshold is ML 0.5) and detecting transient signals. 24 of these sites are equipped with continuous geodetic GPS, forming two transects across the fault system. Geochemical and electromagnetic stations have been also deployed in the study area. In 36 months TABOO recorded 19,422 events with ML ≤ 3.8 corresponding to 23.36e-04 events per day per squared kilometres; one of the highest seismicity rate value observed in Italy. Seismicity distribution images the geometry of the ATF and its antithetic/synthetic structures located in the hanging-wall. TABOO can allow us to understand the seismogenic potential of the ATF and therefore contribute to the seismic hazard assessment of the area. The collected information on the geometry and deformation style of the fault will be used to elaborate ground shaking scenarios adopting diverse slip distributions and rupture directivity models.PublishedS03275T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

Geometry and mechanics of the active fault system in western Slovenia

Western Slovenia is part of an actively deforming region accommodating anticlockwise rotation of Adria and its continuous collision with Eurasia. The geometry of the active faulting system in this plate boundary is not well defined. In this study, detailed analysis of earthquake activity was performed with relocation of earthquakes in the period between 2006 and 2017. With inspection of the waveform data, slight temporal clustering of activity was observed. To increase the detection rate of microearthquakes we used a matched filter detection algorithm method. Templates of earthquakes were created and a database of continuous waveform data within the period 2006\u20132017 was investigated. As a result, high temporal correlation allowed us to identify swarms and earthquake sequences that affected the active fault system in the study region. Relocated seismicity allowed us to constrain the geometry of 5 nearly parallel faults, namely: Ravne, Idrija, Predjama, Selce and Ra\u161a faults. All these faults do have an expression in the geomorphology and reach a seismogenic depth of up to 20 km. Vertical and along strike extents of these active faults can favour earthquakes of moment magnitude equal to 7 or larger. The most recent large earthquake that occurred in this region is the 1511 earthquake with a magnitude 6.8. The leading fault in the system being the Idrija right-lateral strike-slip fault, experiences earthquake activity from 5 to 20 km on its northern segment, while on its southern segment no earthquake activity is detected over the decade of observations. We show that the interseismic loading on the southern segment of Idrija fault is likely unclamping the locked adjacent faults promoting the observed bursts of seismicity. Moreover, in 2009 the Predjama fault accommodated a sudden increase of the surface deformation at the extensometer accompanied by a simultaneous swarm activity at its seismogenic depth. This behaviour might correspond to velocity strengthening and weakening processes taking place at both the surface and depth terminations of a locked vertical fault. These processes can be driven by a slow-slip event on the deeper part of Idrija fault that would generate a temporary acceleration of the interseismic loading rate along with a change within the fluid circulation

The 2013 European Seismic Hazard Model: key components and results

The 2013 European Seismic Hazard Model (ESHM13) results from a community-based probabilistic seismic hazard assessment supported by the EU-FP7 project “Seismic Hazard Harmonization in Europe” (SHARE, 2009–2013). The ESHM13 is a consistent seismic hazard model for Europe and Turkey which overcomes the limitation of national borders and includes a through quantification of the uncertainties. It is the first completed regional effort contributing to the “Global Earthquake Model” initiative. It might serve as a reference model for various applications, from earthquake preparedness to earthquake risk mitigation strategies, including the update of the European seismic regulations for building design (Eurocode 8), and thus it is useful for future safety assessment and improvement of private and public buildings. Although its results constitute a reference for Europe, they do not replace the existing national design regulations that are in place for seismic design and construction of buildings. The ESHM13 represents a significant improvement compared to previous efforts as it is based on (1) the compilation of updated and harmonised versions of the databases required for probabilistic seismic hazard assessment, (2) the adoption of standard procedures and robust methods, especially for expert elicitation and consensus building among hundreds of European experts, (3) the multi-disciplinary input from all branches of earthquake science and engineering, (4) the direct involvement of the CEN/TC250/SC8 committee in defining output specifications relevant for Eurocode 8 and (5) the accounting for epistemic uncertainties of model components and hazard results. Furthermore, enormous effort was devoted to transparently document and ensure open availability of all data, results and methods through the European Facility for Earthquake Hazard and Risk (www.​efehr.​org)

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

The 2013 European Seismic Hazard Model: key components and results

The 2013 European Seismic Hazard Model (ESHM13) results from a community-based probabilistic seismic hazard assessment supported by the EU-FP7 project “Seismic Hazard Harmonization in Europe” (SHARE, 2009–2013). The ESHM13 is a consistent seismic hazard model for Europe and Turkey which overcomes the limitation of national borders and includes a through quantification of the uncertainties. It is the first completed regional effort contributing to the “Global Earthquake Model” initiative. It might serve as a reference model for various applications, from earthquake preparedness to earthquake risk mitigation strategies, including the update of the European seismic regulations for building design (Eurocode 8), and thus it is useful for future safety assessment and improvement of private and public buildings. Although its results constitute a reference for Europe, they do not replace the existing national design regulations that are in place for seismic design and construction of buildings. The ESHM13 represents a significant improvement compared to previous efforts as it is based on (1) the compilation of updated and harmonised versions of the databases required for probabilistic seismic hazard assessment, (2) the adoption of standard procedures and robust methods, especially for expert elicitation and consensus building among hundreds of European experts, (3) the multi-disciplinary input from all branches of earthquake science and engineering, (4) the direct involvement of the CEN/TC250/SC8 committee in defining output specifications relevant for Eurocode 8 and (5) the accounting for epistemic uncertainties of model components and hazard results. Furthermore, enormous effort was devoted to transparently document and ensure open availability of all data, results and methods through the European Facility for Earthquake Hazard and Risk (www.​efehr.​org)