Politecnico di Milano

Collocation approach has been applied to get a global Moho model in spherical approximation based on a GOCE geopotential model. A simple single layer model, with known density contrast, has been considered and a linearized relationship between the spherical harmonic coefficients of the anomalous potential and those of the Moho depth has been derived. This allows the covariance propagation from gravity to Moho depth. The derived covariance functions are then used in the collocation estimate of the global Moho depth. In order to be as close as possible to the considered model, reductions for the gravity signal related to topography/bathymetry have been applied. Simulated and real data tests have been performed and the obtained global solution has been compared with Moho estimates available in literature. The obtained global Moho has been then used as a starting solution for a regional refinement assuming planar approximation. In this second step the computation has been performed in the Central Mediterranean area, based on collocation, local gravity and topography/bathymetry data.

We combine Global Positioning System (GPS) measurements with forward modelling of vis- coelastic relaxation and after-slip to study the post-seismic deformation of the 1997 Umbria- Marche (Central Apennines) moderate shallow earthquake sequence. Campaign GPS mea- surements spanning the time period 1999–2003 are depicting a clear post-seismic deformation signal. Our results favour a normal faulting rupture model where most of the slip is located in the lower part of the seismogenic upper crust, consistent with the rupture models obtained from the inversion of strong motion data. The preferred rheological model, obtained from viscoelastic relaxation modelling, consists of an elastic upper crust, underlain by a transition zone with a viscosity of 10 18 Pa s, while the rheology of deeper layers is not relevant for the observed time-span. Shallow fault creep and after-slip at the base of the seismogenic upper crust are the first order processes behind the observed post-seismic deformation. The deep after-slip, below the fault zone at about 8 km depth, acting as a basal shear through localized time-dependent deformation, identifies a rheological discontinuity decoupling the seismogenic upper crust from the low-viscosity transition zone.

Recent tomographic investigations performed down to ~300 km depth in the Calabrian Arc region gave insight in favor of the hypothesis that the Ionian subducting slab is continuous in depth beneath the central part of the Arc, while detachment of the deep portion of the subducting structure may have already taken place beneath the edges of the Arc itself. In the present study, we perform new geophysical analyses to further explore the structure of the subduction system and the structure and kinematics of the crustal units in the study area for a more comprehensive view of the local geodynamic scenario. Local earthquake tomography that we address to the exploration of the upper 40 km in the whole region of southern Italy furnishes P-wave velocity domains, suggesting southeast-ward long-term drifting of the southern Tyrrhenian unit with an advancement front matching well with the segment of Calabrian Arc where the subducting slab was found continuous and trench retreat can be presumed to have been active in the most recent times. This scenario of retreating subduction trench inducing drifting of the lithospheric unit overriding the subducting slab is further supported by the analysis of gravity anomalies, allowing us to better constrain the transitional zones between different subduction modes (continuous vs. detached slab) along the Arc. Also, the relocation of recent crustal seismicity, associated with geostructural data taken from the literature, provides evidence for NW-trending seismogenic structures in northeastern Sicily and northern Calabria that we interpret as Subduction-Transform Edge Propagator (STEP) faults guiding the southeast-ward drifting process of the southern Tyrrhenian unit. Crustal earthquake relocations show also seismolineaments in southern Calabria corresponding to the NE-trending longitudinal structures of the Arc where the great shallow earthquakes of 28 December 1908, and 5 and 7 February 1783 occurred. Seismicity and the extensional stress regime detected in these structures find also reasonable location in the proposed scenario, being interpretable in terms of shallow response of the central segment of the Arc to slab rollback and trench retreat.

The analysis of Global Positioning System (GPS) coordinates time series is a valuable tool in quantifying crustal deformations. The longer continuous GPS time series allow estimation of nonlinear signatures. As a matter of fact, besides the linear and periodic behaviors, other relevant signals are present in such time series as the so-called transient deformations. They can be related to, e.g., slow slip events, which play a crucial role in studying fault mechanisms. To give reliable estimates of these signals, an appropriate and rigorous approach for defining the deterministic and the stochastic models of the data is needed. We prove that the theory of the second order stationary random process (SOSRP) can be used to describe the stochastic behavior of the daily GPS time series. In particular, the second order stationarity condition has to be verified for the daily GPS coordinate time series to be described as a SOSRP. This method has been already used for modeling the gravity field of the earth and in predicting/filtering problems, and this work shows that it can also be useful for characterizing the colored noise in the GPS time series.

This paper presents the results of a blind experiment that is performed using two pairs of dihedral reflectors. The aim of the experiment was to demonstrate that interferometric synthetic aperture radar (InSAR) measurements can indeed allow a displacement time series estimation with submillimeter accuracy (both in horizontal and vertical directions), provided that the data are properly processed and the impact of in situ as well as atmospheric effects is minimized. One pair of dihedral reflectors was moved a few millimeters between SAR acquisitions, in the vertical and east–west (EW) directions, and the ground truth was compared with the InSAR data. The experiment was designed to allow a multiplatform and multigeometry analysis, i.e., each re- flector was carefully pointed in order to be visible in both Envisat and Radarsat acquisitions. Moreover, two pairs of reflectors were used to allow the combination of data gathered along ascending and descending orbits. The standard deviation of the error is 0.75 mm in the vertical direction and 0.58 mm in the horizontal (EW) direction. GPS data were also collected during this experiment in order to cross-check the SAR results.

We combine aftershock strain mapping, GPS measurements and leveling profiles with forward modeling of viscoelastic relaxation to study the postseismic deformation of the 1997 Umbria-Marche (Central Apennines) earthquake sequence. We explore the feasibility of GPS monitoring of postseismic transients, for the first time in Italy, generated by shallow and moderate sources. Our data allow us to distinguish a preferred coseismic faulting model as well as insight into the rheology of the Central Apennines Earth’s crust. The faulting model requires a listric geometry with most of the energy released in the lower half part of the elastic crust. The rheological model consists of an elastic thin upper crust, a transition zone of about 10 18 Pa s underlain by a low-viscosity lower crust, ranging from 10 17 to 10 18 Pa s. The postseismic deformation is, both distributed in the transition zone - lower crust and confined to the fault zone.

. In this work we analysed the time series of daily solutions of 4 Italian GPS permanent stations with the aim of investigating the presence of temporal correlations and their impact on the estimation of weekly solution and velocity field precisions. We found that precisions are remarkably lower when temporal correlations are considered; in particular, the mean horizontal precisions of weekly solutions are up to 5 times lower and the horizontal velocity precisions are about 1.5-2 times lower. This topic has 2 relevant applications: the assessment of the quality of a reference system maintenance by GPS permanent stations and the coordinate differences significance test for geodynamical applications.

A key issue in our understanding of the earthquake cycle and seismic hazard is the behaviour of an active fault during the interseismic phase. Locked and creeping faults represent two end-members of mechanical behaviours that are given two extreme rupturing hazard levels, that is, high and low, respectively. Geophysical and space geodetic analyses are carried out over the Pollino Range, an extensional environment within the Africa–Eurasia plate boundary, to disclose the behaviour of the long-lasting quiescent Castrovillari normal fault. Fault trenching evidenced at least four large earthquakes (6.5–7.0 M w ) in the past and an elapsed time of 1200 yr since the last event. Inversion of Differential Interferometric Synthetic Aperture Radar and Global Positioning System over a decade shows fast creeping at all depths of the fault plane. The velocity-strengthening creeping zone reaches maximum rates 20 mm yr −1 against an average rate of about 3–9 mm yr −1 . It limits the southern-weakening locked part of the fault. An essential condition for the generation of a large earthquake on the Castrovillari fault, as has occurred in the past, is a rupture through the velocity-strengthening zone. The Castrovillari fault yields the best evidence for being both a strong and weak fault during its earthquake cycle. Creeping at rates faster than its tectonically driven ones, it must thus consist of a mix of unstable and conditionally stable patches ready to sustain a sizeable earthquake. Quantifying and mapping the slip rate over the fault plane is important because they influence fault moment budget estimate and helps to constrain constitutive laws of fault zones. Aseismic slip also redistributes stress in the crust, thereby affecting the locations of future earthquakes.

When the results of geophysical models are compared with data, the uncertainties of the model are typically disregarded. This paper proposes a method for defining the uncertainty of a geophysical model based on a numerical procedure that estimates the empirical auto- and cross-covariances of model-estimated quantities. These empirical values are then fitted by proper covariance functions and used to compute the covariance matrix associated with the model predictions. The method is tested using a geophysical, spherical, thin-sheet finite element model of the Mediterranean region. Using a χ2 analysis, the model's estimated horizontal velocities are compared with the velocities estimated from permanent GPS stations while taking into account the model uncertainty through its covariance structure and the covariance of the GPS estimates. The results indicate that including the estimated model covariance in the testing procedure leads to lower observed χ2 values and might help a sharper identification of the best-fitting geophysical models.

This work aims at identifying and modelling statistical dependencies between empirical amplification functions of sites in central Italy and the main geological and geophysical characteristics of the region, within a geostatistical analysis framework. The empirical functions, named δS2S, are estimated by decomposing the re siduals of the median predictions of a non-ergodic ground motion model of elastic acceleration response spectra developed for the reference region. To select the model that best describes the spatial variability of the data, the performance of stationary and non-stationary spatial models is compared, the latter being able to constrain the prediction of the empirical functions to physical quantities available in the region and descriptive of the geology, topography and geographical location of the site. Finally, we obtain optimal models of δS2S, for each spectral ordinate, parameterised as a function of geographical coordinates and an input map of shear wave velocity in the upper 30 m (Vs30) constructed ad hoc by combining information gathered from two high-resolution maps available for the region. The methodology allows the development of a new practice-oriented framework for the empirical estimation of site amplification, which can be adopted for the gen eration of shaking scenarios in the context of regional hazard and seismic risk assessment.