Belardinelli, Maria Elina

The aseismic sliding on shallow strike-slip faults, under the assumption of a non linear constitutive equation (velocity strengthening), is here treated as a two-dimensional quasi-static crack problem whose equations are solved numerically (boundary elements method). Results are compared with the corresponding one-dimensional («depth averaged») model by a suitable choice of the effective stiffness of the fault. In the one-dimensional case also the inertial term was taken into account in the evolutive equation. The current results are in agreement with an earlier one-dimensional model for afterslip as long as the state variable evolution is neglected a priori and friction depends only on velocity. In general, if the state variable is allowed to evolve, the previous approximation is valid for velocity strengthening slipping section of faults extending down to several kilometers in depth. For smaller sections of fault the evolution of the state variable affects the coseismic and early postseismic phase and accordingly it cannot be neglected. Moreover, in the presence of rheological heterogeneities, for fault sections shallower than 1 km depth, the comparison between the two-dimensional and one-dimensional models suggests the need to employ the two-dimensional model, possibly taking into account inertial effects.

After the April, 6 2009 Mw=6.3 L’Aquila earthquake, the coseimic deformation has measured by > 70 Global Positioning System (GPS) stations. We use a rectangular uniform-slip dislocation and a constrained, non-linear optimization algorithm, obtaining a rupture occurred on a N129°E striking and 50° SW-ward dipping normal fault. Our distributed slip model exhibits the highest slip of the order of ∼1 m. We do also bootstrap and resolution analysis to quantify goodness of our model. We analyze the Coulomb stress change affected by the major aftershocks, and compare the results obtained from the uniform slip and the heterogeneous slip models.

We analyze the coseismic stress redistribution during the seismic sequence of June 17 2000 in South Iceland in which a mainshock (MS 6.6) was followed by three quite large events within few tens of seconds (8, 26 and 30 s respectively) at a distance up to about 90 km. We use this observational case to investigate the possibility of fault interaction by purely transient coseismic stress changes and in particular nearly instantaneous triggering. We compute the stress changes as functions of time in a stratified elastic half space by means of the discrete wavenumber and reflectivity method (Cotton and Coutant, 1997). We evaluate the dynamic stress caused by the mainshock at the three hypocenters of the subsequent events. Our results show that the onset of the last two events is slightly delayed with respect to the arrival time of the second positive peak of Coulomb Failure Function variation, while the first event stroked after the first positive peak. We also analysed the response of a rate- and state-dependent springslider model of fault perturbed by the shear stress and the normal stress variations that we computed as generated by the June 17 2000 mainshock at the three hypocenters. Assuming an initial sliding velocity comparable with tectonic velocity of the region, for the last two events, we obtained failure times close to the observed origin times, provided that the value of the initial effective normal stress is low enough, whereas the 8 s event requires closer to failure initial conditions to be reproduced. The 8 s event might already be close to failure at the time of the mainshock, due to its vicinity to the main event and the subsequent June 21 (MS 6.6) mainshock. Therefore the first aftershock does not provide us a clear evidence of dynamic triggering.

We present a model of seismogenesis on an extended 3-D fault, subject to the external perturbations of coseismic stress changes due to an earthquake occurring on another fault (the causative fault). As an application, we consider the spatio-temporal stress redistribution produced on the Hvalhn´ukur fault by the MS 6.6 2000 June 17 mainshock in the South Iceland Seismic Zone (SISZ). The latter is located nearly 64 kmfrom the causative fault and failed 26 s after the main shock with an estimated magnitudeMw = 5.25 ± 0.25, providing an example of instantaneous dynamic triggering. The stress perturbations are computed by means of a discrete wavenumber and reflectivity code. The response of the perturbed fault is then analysed solving the truly 3-D, fully dynamic (or spontaneous) problem accounting for crustal stratification. In a previous study, the response of the Hvalhn´ukur fault was analysed by using a spring–slider fault model (SS fault model), comparing the estimated perturbed failure time with the observed origin time. In addition to the perturbed failure time, this model can provide numerical estimates of many other dynamic features of the triggered event, which can be compared with available observations—the rupture history of the whole fault plane, its final extent and the seismic moment of the induced event.We show the key differences existing between a mass–spring model and this extended fault model; in particular, we show the essential role of the load exerted by the neighbouring slipping points of the fault. By considering both rate- and state-dependent laws and non-linear slip-dependent law, we show how the dynamics of the 26 s fault strongly depend on the assumed constitutive law and initial stress conditions. In the case of rate- and state-dependent friction laws, assuming an initial effective normal stress distribution that is suitable for the SISZ and consistent with previously stated conditions of instantaneous dynamic triggering of the Hvalhn´ukur fault, we obtain results in general agreement with observations.

The Emilia-Romagna seismic sequence in May 2012 was characterized by two mainshocks which were close in time and space. Several authors already modeled the geodetic data in terms of the mechanical interaction of the events in the seismic sequence. Liquefaction has been extensively observed, suggesting an important role of fluids in the sequence. In this work, we focus on the poroelastic effects induced by the two mainshocks. In particular, the target of this work is to model the influence of fluids and pore-pressure changes on surface displacements and on the Coulomb failure function (CFF). The fluid flow and poroelastic modeling was performed in a 3D half-space whose elastic and hydraulic parameters are depth dependent, in accordance with the geology of the Emilia-Romagna subsoil. The model provides both the poroelastic displacements and the pore-pressure changes induced coseismically by the two mainshocks at subsequent periods and their evolution over time. Modeling results are then compared with postseismic InSAR and GPS displacement time series: the InSAR data consist of two SBAS series presented in previous works, while the GPS signal was detected adopting a variational Bayesian independent component analysis (vbICA) method. Thanks to the vbICA, we are able to separate the contribution of afterslip and poroelasticity on the horizontal surface displacements recorded by the GPS stations. The poroelastic GPS component is then compared to the modeled displacements and shown to be mainly due to drainage of the shallowest layers. Our results offer an estimation of the poroelastic effect magnitude that is small but not negligible and mostly confined in the near field of the two mainshocks. We also show that accounting for a 3D fault representation with a nonuniform slip distribution and the elastic-hydraulic layering of the half-space has an important role in the simulation results.

The Umbria-Marche Apennines are characterized mainly by SW-NE oriented extensional deformation and most of major historical and instrumental earthquakes occurred mainly on the western side of chain, bounded by west-dipping buried high-angle normal faults. Recent studies about the northernmost part of Umbria-Marche region show seismic and tectonic activity on correspondence of the east-dipping Alto Tiberina (AT) low-angle normal fault (LANF), which is widely documented by geological data and deep seismic reflection profiles. In this area which of the known fault systems play a major role in accommodating the extension, and which are the modes (seismic VS aseismic deformation) this extension is taken up, is still a debated topic. During last years on Umbria-Marche Apennines close to Gubbio fault (GuF) a dense network of continuous GPS stations, belonging to the RING-INGV network, has been installed, improving significantly the spatial resolution of the detectable geodetic gradients. We used a self-consistent kinematic block modeling to study this sector of the Umbria-Marche Apennines, in order to understand which fault system is accommodating the tectonic extension. We found that both fault systems, i.e. the Alto Tiberina LANF and the antithetic high-angle normal faults, are needed to better reproduce the nearfield GPS velocities, obtaining kinematic agreement with geological slip-rates. Moreover we parameterized the ATF fault as a, more realistic, curved surface to infer the distribution of interseismic coupling (IC), which is validated by numerous resolution tests. The obtained IC distribution shows a correlation between relocated microseismicity and uncoupled patches attributed to aseismic creeping behavior, which could be explained by the presence of fluid overpressure. Otherwise this correlation has been verified with a very small quantity of events (almost 400) and it might be of interest to evaluate this correlation with future available data.

The three largest events of the 1997 Umbria-Marche (Italy) sequence occurred on September 26, 1997 at 00:33 GMT (Event 1, MW=5.7) and 09:40 GMT (Event 2, MW=6.0) in the Colfiorito area and on the October 14, 1997 at 15:23 (Event 3, MW=5.6) in the Sellano area. The availability of different sets of geodetic and seismological data allowed several studies to characterize the extended sources of events 1-3. In this work, I review some of the studies that obtain the properties of the seismic sources by inversion of available data. Generally these studies assume the seismic sources as dislocations or distributions of equivalent point sources in elastic half-spaces. Following their chronological order, they model increasing complexities of the sources by using an increasing number of data. Some of the differences between results obtained, such as the top edge depth estimates, are shown to be due to the different approaches used. Commonly a 1-D crustal model is used in inverting strongmotion data. Instead homogeneous elastic half-spaces are mainly assumed in inverting geodetic data to obtain the three main sources of the 1997 Umbria-Marche sequence. Assuming the same crustal structure is important to make comparable results obtained analyzing seismological data or geodetic data separately, as it has been done till now for this sequence.

Our ability to estimate surface deformation rates in the Central Mediterranean has considerably enhanced in the last decade thanks to the growth of continuous Global Navigation Satellite System (GNSS) networks. Focusing on the Apennine/Alpine seismogenic belt, this area offers the opportunity to test the use of geodetic strain rates for constraining active tectonic processes and for seismic hazard assessments. Given the importance of geodetic strain rate models in modern hazard estimation approaches, however, one has to consider that different approaches can provide significantly different strain rate maps. Despite the increasing availability of GNSS velocity data, in fact, strain rate models can significantly differ, because of the spatial heterogeneity of GNSS station locations and inherent strategies in computing strain rates. Using a dense GNSS velocity dataset, this study examines three methods for estimating horizontal strain rates, described in the recent literature, and selected to represent approaches of increasing mathematical complexity. The advantages, drawbacks, and optimal settings of each method are discussed. The main result is an ensemble of strain rate models that enable the evaluation of epistemic uncertainties in seismicity rate models constrained by geodetic velocities.

Analyzing the displacement time series from continuous GPS (cGPS) with an Independent Component Analysis, we detect a transient deformation signal that correlates both in space and time with a seismic swarm activity (maximum Mw =3.69±0.09) occurred in the hanging wall of the Altotiberina normal fault (Northern Apennines, Italy) in 2013–2014. The geodetic transient lasted ∼6 months and produced a NW-SE trending extension of ∼5.3 mm, consistent with the regional tectonic regime. The seismicity and the geodetic signal are consistent with slip on two splay faults in the Altotiberina fault (ATF) hanging wall. Comparing the seismic moment associated with the geodetic transient and the seismic events, we observe that seismicity accounts for only a fraction of the measured geodetic deformation. The combined seismic and aseismic slip decreased the Coulomb stress on the locked shallow portion of the ATF, while the transition region to the creeping section has been loaded.