Tectonic geomorphology and active strain of the Northern Apennines mountain front

The Northern Apennines (NA) chain is the expression of the active plate margin between Europe and Adria. Given the low convergence rates and the moderate seismic activity, ambiguities still occur in defining a seismotectonic framework and many different scenarios have been proposed for the mountain front evolution. Differently from older models that indicate the mountain front as an active thrust at the surface, a recently proposed scenario describes the latter as the frontal limb of a long-wavelength fold (> 150 km) formed by a thrust fault tipped around 17 km at depth, and considered as the active subduction boundary. East of Bologna, this frontal limb is remarkably very straight and its surface is riddled with small, but pervasive high- angle normal faults. However, west of Bologna, some recesses are visible along strike of the mountain front: these perturbations seem due to the presence of shorter wavelength (15 to 25 km along strike) structures showing both NE and NW-vergence. The Pleistocene activity of these structures was already suggested, but not quantitative reconstructions are available in literature. This research investigates the tectonic geomorphology of the NA mountain front with the specific aim to quantify active deformations and infer possible deep causes of both short- and long-wavelength structures. This study documents the presence of a network of active extensional faults, in the foothills south and east of Bologna. For these structures, the strain rate has been measured to find a constant throw-to-length relationship and the slip rates have been compared with measured rates of erosion. Fluvial geomorphology and quantitative analysis of the topography document in detail the active tectonics of two growing domal structures (Castelvetro - Vignola foothills and the Ghiardo plateau) embedded in the mountain front west of Bologna. Here, tilting and river incision rates (interpreted as that long-term uplift rates) have been measured respectively at the mountain front and in the Enza and Panaro valleys, using a well defined stratigraphy of Pleistocene to Holocene river terraces and alluvial fan deposits as growth strata, and seismic reflection profiles relationships. The geometry and uplift rates of the anticlines constrain a simple trishear fault propagation folding model that inverts for blind thrust ramp depth, dip, and slip. Topographic swath profiles and the steepness index of river longitudinal profiles that traverse the anti- clines are consistent with stratigraphy, structures, aquifer geometry, and seismic reflection profiles. Available focal mechanisms of earthquakes with magnitude between Mw 4.1 to 5.4, obtained from a dataset of the instrumental seismicity for the last 30 years, evidence a clear vertical separation at around 15 km between shallow extensional and deeper compressional hypocenters along the mountain front and adjacent foothills. In summary, the studied anticlines appear to grow at rates slower than the growing rate of the longer- wavelength structure that defines the mountain front of the NA. The domal structures show evidences of NW-verging deformation and reactivations of older (late Neogene) thrusts. The reconstructed river incision rates together with rates coming from several other rivers along a 250 km wide stretch of the NA mountain front and recently available in the literature, all indicate a general increase from Middle to Late Pleistocene. This suggests focusing of deformation along a deep structure, as confirmed by the deep compressional seismicity. The maximum rate is however not constant along the mountain front, but varies from 0.2 mm/yr in the west to more than 2.2 mm/yr in the eastern sector, suggesting a similar (eastward-increasing) trend of the apenninic subduction.

Seismic tomographic full-waveform inversion for the Vrancea sinking lithosphere structure using the adjoint method.

The Vrancea region, at the south-eastern bend of the Carpathian Mountains in Romania, represents one of the most puzzling seismically active zones of Europe. Beside some shallow seismicity spread across the whole Romanian territory, Vrancea is the place of an intense seismicity with the presence of a cluster of intermediate-depth foci placed in a narrow nearly vertical volume. Although large-scale mantle seismic tomographic studies have revealed the presence of a narrow, almost vertical, high-velocity body in the upper mantle, the nature and the geodynamic of this deep intra-continental seismicity is still questioned. High-resolution seismic tomography could help to reveal more details in the subcrustal structure of Vrancea. Recent developments in computational seismology as well as the availability of parallel computing now allow to potentially retrieve more information out of seismic waveforms and to reach such high-resolution models. This study was aimed to evaluate the application of a full waveform inversion tomography at regional scale for the Vrancea lithosphere using data from the 1999 six months temporary local network CALIXTO. Starting from a detailed 3D Vp, Vs and density model, built on classical travel-time tomography together with gravity data, I evaluated the improvements obtained with the full waveform inversion approach. The latter proved to be highly problem dependent and highly computational expensive. The model retrieved after the first two iterations does not show large variations with respect to the initial model but remains in agreement with previous tomographic models. It presents a well-defined downgoing slab shape high velocity anomaly, composed of a N-S horizontal anomaly in the depths between 40 and 70km linked to a nearly vertical NE-SW anomaly from 70 to 180km.

Evaluation of slope stability under water and seismic load through the Minimum Lithostatic Deviation method

The main objective of this thesis is to obtain a better understanding of the methods to assess the stability of a slope. We have illustrated the principal variants of the Limit Equilibrium (LE) method found in literature, focalizing our attention on the Minimum Lithostatic Deviation (MLD) method, developed by Prof. Tinti and his collaborators (e.g. Tinti and Manucci, 2006, 2008). We had two main goals: the first was to test the MLD method on some real cases. We have selected the case of the Vajont landslide with the objective to reconstruct the conditions that caused the destabilization of Mount Toc, and two sites in the Norwegian margin, where failures has not occurred recently, with the aim to evaluate the present stability state and to assess under which conditions they might be mobilized. The second goal was to study the stability charts by Taylor and by Michalowski, and to use the MLD method to investigate the correctness and adequacy of this engineering tool.

A comprehensive study of the 26th December 2004 Sumatra earthquake: possible implications on Earth rotation and investigations on the coseismic and postseismic stress diffusion associated with the seismic rupture

In this work a multidisciplinary study of the December 26th, 2004 Sumatra earthquake has been carried out. We have investigated both the effect of the earthquake on the Earth rotation and the stress field variations associated with the seismic event. In the first part of the work we have quantified the effects of a water mass redistribution associated with the propagation of a tsunami wave on the Earth’s pole path and on the length-of-day (LOD) and applied our modeling results to the tsunami following the 2004 giant Sumatra earthquake. We compared the result of our simulations on the instantaneous rotational axis variations with some preliminary instrumental evidences on the pole path perturbation (which has not been confirmed yet) registered just after the occurrence of the earthquake, which showed a step-like discontinuity that cannot be attributed to the effect of a seismic dislocation. Our results show that the perturbation induced by the tsunami on the instantaneous rotational pole is characterized by a step-like discontinuity, which is compatible with the observations but its magnitude turns out to be almost one hundred times smaller than the detected one. The LOD variation induced by the water mass redistribution turns out to be not significant because the total effect is smaller than current measurements uncertainties. In the second part of this work of thesis we modeled the coseismic and postseismic stress evolution following the Sumatra earthquake. By means of a semi-analytical, viscoelastic, spherical model of global postseismic deformation and a numerical finite-element approach, we performed an analysis of the stress diffusion following the earthquake in the near and far field of the mainshock source. We evaluated the stress changes due to the Sumatra earthquake by projecting the Coulomb stress over the sequence of aftershocks taken from various catalogues in a time window spanning about two years and finally analyzed the spatio-temporal pattern. The analysis performed with the semi-analytical and the finite-element modeling gives a complex picture of the stress diffusion, in the area under study, after the Sumatra earthquake. We believe that the results obtained with the analytical method suffer heavily for the restrictions imposed, on the hypocentral depths of the aftershocks, in order to obtain the convergence of the harmonic series of the stress components. On the contrary we imposed no constraints on the numerical method so we expect that the results obtained give a more realistic description of the stress variations pattern.

Trasmission and Reflection (ATR)Far-Infrared Spectroscopy Applied in the Analysis of Cultural Heritage Materials

FIR spectroscopy is an alternative way of collecting spectra of many inorganic pigments and corrosion products found on art objects, which is not normally observed in the MIR region. Most FIR spectra are traditionally collected in transmission mode but as a real novelty it is now also possible to record FIR spectra in ATR (Attenuated Total Reflectance) mode. In FIR transmission we employ polyethylene (PE) for preparation of pellets by embedding the sample in PE. Unfortunately, the preparation requires heating of the PE in order to produces at transparent pellet. This will affect compounds with low melting points, especially those with structurally incorporated water. Another option in FIR transmission is the use of thin films. We test the use of polyethylene thin film (PETF), both commercial and laboratory-made PETF. ATR collection of samples is possible in both the MIR and FIR region on solid, powdery or liquid samples. Changing from the MIR to the FIR region is easy as it simply requires the change of detector and beamsplitter (which can be performed within a few minutes). No preparation of the sample is necessary, which is a huge advantage over the PE transmission method. The most obvious difference, when comparing transmission with ATR, is the distortion of band shape (which appears asymmetrical in the lower wavenumber region) and intensity differences. However, the biggest difference can be the shift of strong absorbing bands moving to lower wavenumbers in ATR mode. The sometimes huge band shift necessitates the collection of standard library spectra in both FIR transmission and ATR modes, provided these two methods of collecting are to be employed for analyses of unknown samples. Standard samples of 150 pigment and corrosion compounds are thus collected in both FIR transmission and ATR mode in order to build up a digital library of spectra for comparison with unknown samples. XRD, XRF and Raman spectroscopy assists us in confirming the purity or impurity of our standard samples. 24 didactic test tables, with known pigment and binder painted on the surface of a limestone tablet, are used for testing the established library and different ways of collecting in ATR and transmission mode. In ATR, micro samples are scratched from the surface and examined in both the MIR and FIR region. Additionally, direct surface contact of the didactic tablets with the ATR crystal are tested together with water enhanced surface contact. In FIR transmission we compare the powder from our test tablet on the laboratory PETF and embedded in PE. We also compare the PE pellets collected using a 4x beam condenser, focusing the IR beam area from 8 mm to 2 mm. A few samples collected from a mural painting in a Nepalese temple, corrosion products collected from archaeological Chinese bronze objects and samples from a mural paintings in an Italian abbey, are examined by ATR or transmission spectroscopy.

Imaging of crustal scatterers using multiple seismic arrays

Array seismology is an useful tool to perform a detailed investigation of the Earth’s interior. Seismic arrays by using the coherence properties of the wavefield are able to extract directivity information and to increase the ratio of the coherent signal amplitude relative to the amplitude of incoherent noise. The Double Beam Method (DBM), developed by Krüger et al. (1993, 1996), is one of the possible applications to perform a refined seismic investigation of the crust and mantle by using seismic arrays. The DBM is based on a combination of source and receiver arrays leading to a further improvement of the signal-to-noise ratio by reducing the error in the location of coherent phases. Previous DBM works have been performed for mantle and core/mantle resolution (Krüger et al., 1993; Scherbaum et al., 1997; Krüger et al., 2001). An implementation of the DBM has been presented at 2D large-scale (Italian data-set for Mw=9.3, Sumatra earthquake) and at 3D crustal-scale as proposed by Rietbrock & Scherbaum (1999), by applying the revised version of Source Scanning Algorithm (SSA; Kao & Shan, 2004). In the 2D application, the rupture front propagation in time has been computed. In 3D application, the study area (20x20x33 km3), the data-set and the source-receiver configurations are related to the KTB-1994 seismic experiment (Jost et al., 1998). We used 60 short-period seismic stations (200-Hz sampling rate, 1-Hz sensors) arranged in 9 small arrays deployed in 2 concentric rings about 1 km (A-arrays) and 5 km (B-array) radius. The coherence values of the scattering points have been computed in the crustal volume, for a finite time-window along all array stations given the hypothesized origin time and source location. The resulting images can be seen as a (relative) joint log-likelihood of any point in the subsurface that have contributed to the full set of observed seismograms.

Earthquake forecasting and seismic hazard analysis: some insights on the testing phase and the modeling

This thesis is divided in three chapters. In the first chapter we analyse the results of the world forecasting experiment run by the Collaboratory for the Study of Earthquake Predictability (CSEP). We take the opportunity of this experiment to contribute to the definition of a more robust and reliable statistical procedure to evaluate earthquake forecasting models. We first present the models and the target earthquakes to be forecast. Then we explain the consistency and comparison tests that are used in CSEP experiments to evaluate the performance of the models. Introducing a methodology to create ensemble forecasting models, we show that models, when properly combined, are almost always better performing that any single model. In the second chapter we discuss in depth one of the basic features of PSHA: the declustering of the seismicity rates. We first introduce the Cornell-McGuire method for PSHA and we present the different motivations that stand behind the need of declustering seismic catalogs. Using a theorem of the modern probability (Le Cam's theorem) we show that the declustering is not necessary to obtain a Poissonian behaviour of the exceedances that is usually considered fundamental to transform exceedance rates in exceedance probabilities in the PSHA framework. We present a method to correct PSHA for declustering, building a more realistic PSHA. In the last chapter we explore the methods that are commonly used to take into account the epistemic uncertainty in PSHA. The most widely used method is the logic tree that stands at the basis of the most advanced seismic hazard maps. We illustrate the probabilistic structure of the logic tree, and then we show that this structure is not adequate to describe the epistemic uncertainty. We then propose a new probabilistic framework based on the ensemble modelling that properly accounts for epistemic uncertainties in PSHA.

Seismic fragility assessment of unreiforced masonry aggregate buildings

Aggregate masonry buildings have been generated over the years, allowing the interaction of different aggregated structural units under seismic action. The first part of this work is focused on the seismic vulnerability and fragility assessment of clay brick masonry buildings, sited in Bologna (Italy), with reference, at first, to single isolated structural units, by means of the Response Surface statistical method, taking into account some variabilities and uncertainties involved in the problem. The seismic action was defined by means of a group of selected registered accelerograms, in order to analyse the effect of the variability of the earthquakes. Identical and different structural units chosen by the Response Surface generated simulations are then aggregated in row, in order to compare the collapse PGA referred to the isolated structural unit and the one referred to the aggregate structure. The second part is focused on the seismic vulnerability and fragility assessment of stone masonry structures, sited in Seixal (Portugal), applying a methodology similar to that used for the buildings sited in Bologna. Since the availability of several information, the analyses involved the assessment of the most prevalent structural typologies in the area, considering the variability of a set of structural and geometrical parameters. The results highlighted the importance of the statistic procedures as method able to consider the variabilities and the uncertainties involved in the problem of the fragility of unreinforced masonry structures, in absence of accurate investigations on the structural typologies, as in the Seixal case study. Furthermore, it was showed that the structural units along the unreinforced clay brick or stone masonry aggregates cannot be analysed as isolated, as they are affected by the effect of the aggregation with adjacent structural units, according to the different directions of the seismic action considered and to their different position along the row aggregate.

Evaluation of seismic losses of RC buildings: the case of the industrial areas after the 2012 Emilia Earthquake

The topic of seismic loss assessment not only incorporates many aspects of the earthquake engineering, but also entails social factors, public policies and business interests. Because of its multidisciplinary character, this process may be complex to challenge, and sound discouraging to neophytes. In this context, there is an increasing need of deriving simplified methodologies to streamline the process and provide tools for decision-makers and practitioners. This dissertation investigates different possible applications both in the area of modelling of seismic losses, both in the analysis of observational seismic data. Regarding the first topic, the PRESSAFE-disp method is proposed for the fast evaluation of the fragility curves of precast reinforced-concrete (RC) structures. Hence, a direct application of the method to the productive area of San Felice is studied to assess the number of collapses under a specific seismic scenario. In particular, with reference to the 2012 events, two large-scale stochastic models are outlined. The outcomes of the framework are promising, in good agreement with the observed damage scenario. Furthermore, a simplified displacement-based methodology is outlined to estimate different loss performance metrics for the decision-making phase of the seismic retrofit of a single RC building. The aim is to evaluate the seismic performance of different retrofit options, for a comparative analysis of their effectiveness and the convenience. Finally, a contribution to the analysis of the observational data is presented in the last part of the dissertation. A specific database of losses of precast RC buildings damaged by the 2012 Earthquake is created. A statistical analysis is performed, allowing deriving several consequence functions. The outcomes presented may be implemented in probabilistic seismic risk assessments to forecast the losses at the large scale. Furthermore, these may be adopted to establish retrofit policies to prevent and reduce the consequences of future earthquakes in industrial areas.

The Moho reflectivity from ambient seismic noise autocorrelations beneath the Western Alps

The lower crustal structure beneath the Western Alps -- including the Moho -- bears the signature of past and present geodynamic processes. It has been the subject of many studies until now. However, its current knowledge still leaves significant open questions. In order to derive new information, independent from previous determinations, here I wish to address this topic using a different method --- ambient seismic noise autocorrelation --- that is for the first time applied to reveal Moho depth in the Western Alps. Moho reflections are identified by picking reflectivity changes in ambient seismic noise autocorrelations. The seismic data is retrieved from more than 200 broadband seismic stations, from the China--Italy--France Alps (CIFALPS) linear seismic network, and from a subset of the AlpArray Seismic Network (AASN). The automatically-picked reflectivity changes along the CIFALPS transect in the southwestern Alps show the best results in the 0.5--1 Hz frequency band. The autocorrelation reflectivity profile of the CIFALPS transect shows a steeper subduction profile,~55 to ~70 km, of the European Plate underneath the Adriatic Plate. The dense spacing of the CIFALPS network facilitates the detection of lateral continuity of crustal structure, and of the Ivrea mantle wedge reaching shallow crustal depths in the southwestern Alps. The data of the AASN stations are filtered in the 0.4--1 and 0.5--1 Hz frequency bands. Although the majority of the stations give the same Moho depth for the different frequency bands, the few stations with different Moho depths shows the care that has to be taken when choosing the frequency band for filtering the autocorrelation stacks. The new Moho depth maps by using the AASN stations are a compilation of the first and second picked reflectivity changes. The results show the complex crust-mantle structure with clear differences between the northwestern and southwestern Alps.

Seismic vulnerability of buildings and non-structural elements: insights from the 2011 Lorca earthquake

This PhD dissertation presents a profound study of the vulnerability of buildings and non-structural elements stemming from the investigation of the Mw 5.2 Lorca 2011 earthquake; which constitutes one of the most significant earthquakes in Spain. It left nine fatalities due to falling debris from reinforced concrete buildings, 394 injured and material damage valued at 800 million euros. Within this framework, the most relevant initiatives concerning the vulnerability of buildings and the exposure of Lorca are studied. This work revealed two lines of research: the elaboration of a rational method to determine the adequacy of a specific fragility curve for the particular seismic risk study of a region; and the relevance of researching the seismic performance of non-structural elements. As a consequence, firstly, a method to assess and select fragility curves for seismic risk studies from the catalogue of those available in the literature is elaborated and calibrated by means of a case study. The said methodology is based on a multidimensional index and provides a ranking that classifies the curves in terms of adequacy. Its results for the case of Lorca led to the elaboration of new fragility curves for unreinforced masonry buildings. Moreover, a simplified method to account for the unpredictable directionality of the seism in the creation of fragility curves is contributed. Secondly, the characterisation of the seismic capacity and demand of the non-structural elements that caused most of the human losses is studied. Concerning the capacity, an analytical approach derived from theoretical considerations to characterise the complete out-of-plane seismic response curve of unreinforced masonry cantilever walls is provided; as well as a simplified and more practical trilinear version of it. Concerning the demand, several methods for characterising the Floor Response Spectra of reinforced concrete buildings are tested through case studies.