Dragoni, Michele

Time-dependent piezomagnetic fields due to inclined rectangular faults embedded in a viscoelastic, homogeneous half-space were investigated. A viscoelastic rheology of the surrounding medium was assumed to relate piezomagnetic changes at the surface to the stress field at depth. The viscosity of the medium strongly influences time-dependent stress changes. Especially in volcanic areas, rocks near magmatic sources are considerably heated. The presence of higher temperatures produces a lower effective viscosity in the crust, making it necessary to consider its inelastic properties. Rocks no longer behave in a purely elastic manner but permanently deform because the viscosity is significantly lowered. To determine the time-dependent piezomagnetic fields in a viscoelastic medium, we applied the Correspondence Principle to the analytical elastic solutions for dislocation sources. Among all the possible rheological models, we investigated three cases in which the bulk modulus is purely elastic and the shear modulus relaxes as for (i) a Maxwell solid, (ii) a standard linear solid (SLS) and (iii) a Kelvin solid. The piezomagnetic field completely vanishes after the relaxation process for a Maxwell rheology, whereas it is found to decrease over time and reach some finite offset values for SLS and Kelvin rheologies. A real case study concerning the magnetic anomalies observed during the 2002–2003 Mt Etna eruption is also investigated. Post-eruptive magnetic variations were in general agreement with a viscoelastic relaxation process of a SLS rheology undergoing in the volcano edifice.

We estimate the long-term vertical deformation of Basiluzzo island, located in the volcanic arc of the Aeolian Islands, Italy, by inferring the subsidence of a Roman age submerged wharf dated 2000 ± 50 years ago.We model the crustal deformation that produced this displacement as a cooling magma chamber emplaced during the formation of the island. This is the first attempt to model crustal deformation using the subsidence of a submerged archaeological structure. Nowadays the top of the wharf, which can be considered an unconventional leveling benchmark, is located near Punta Levante, at an average depth of 3.20 ± 0.10mbelow actual sea level, and it is still in good conservation. Its present location is due to combined effect of sea level rise and volcanic and tectonic activity that occurred since it was built. Taking into account the architectural features of the wharf and that the mean sea level rise of the Mediterranean sea has been estimated as 0.45 m during the last 2000 years [Flemming andWebb, 1986],we estimate a total subsidence of 3.75 ± 0.10m, at a rate of about 1.87 mm yr 1 [Anzidei et al., 2002]. We propose a possible mechanism for the long-term subsidence, which occurred on a timescale of thousands of years, by considering the Earth’s crust as a Maxwell body. We assume that the magmatic source is located directly beneath Basiluzzo dome and underwent progressive solidification and subsequent volume reduction since its emplacement 50,000 years ago.

We show the results of application of Differential SAR Interferometry to the MW 7.4, August 17, 1999, Izmit earthquake, Western Turkey. The differential interferogram is obtained using an interferometric ERS2 ascending pair with a time interval of 35 days (August 13th - September 17th). The fringe pattern clearly defines the coseismic displacement field extended in an area of about 100 km N-S and 120 km E-W. The analysis of the interferogram shows the right-lateral strike-slip movement on the activated section of the North Anatolian fault system. The maximum SAR-detected displacement ranges between 117.6 cm and 134.4 cm in the proximity of Gölcük. We invert SAR data for uniform dislocation on a single fault plane using a Montecarlo procedure, with the aim of testing a large set of a priori possible asperity distributions on the fault. We then use a forward modeling approach to evaluate the slip variability for the dislocation using additional constraints as surface offsets and seismicity distribution: in this case we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. Misfits between SAR data and modeled slant range displacements are generally low for all our models (~ 12 cm). Our results indicate that slip is concentrated in the central-western part of the fault, in the upper 10-15 km, tapering to the fault tips. For the Izmit case, we note that a well constrained fault model can be obtained only integrating DInSAR data with additional observations. This is mainly due to an undersampling of the displacement field by DInSAR, caused by decorrelation and lack of image data.

By means of a 2D finite-element procedure, we tested how heterogeneities at the scale of seismogenic fault affect the displacement. We defined one or more slip distributions for two typical normal-faulting earthquakes in the Central Apennines, computed the displacement occurred within different structures including lateral heterogeneities, and compared the different displacement profiles to isolate the effect of the crustal structure. To understand at what magnitude the heterogeneities affect the observation significantly, we compared the predicted coseismic displacement with GPS and DInSAR data for the Colfiorito 1997 earthquake. We find that heterogeneities significantly affect the observable horizontal coseismic displacement for the larger magnitudes, whereas for smaller quakes, they affect horizontal displacement close to the fault trace only.

Lava flows are not only a fascinating scientific problem, involving many branches of continuum mechanics and thermodynamics, but are natural events having a strong social impact. A reliable evaluation of volcanic hazard connected with lava flows depends on the availability of physical models allowing us to predict the evolution of these phenomena. In this regard, the rheological properties of lavas are of major importance in controlling the dynamics of lava flows. Lava is a multi-phase and chemically heterogeneous system. This entails a characteristic, non-Newtonian behaviour of lava flows, which is emphasized by the fact that the rheological parameters are strongly temperature dependent and are therefore affected by the progressive cooling of lava after effusion. Physical models of lava flows show us the complex relationships between the many quantities governing this process and in the near future they may allow us to predict the dynamics of lava flows and to take effective measures for the reduction of volcanic risk.

A power-law, viscoelastic constitutive equation for lithospheric rocks, is considered. The equation is a nonlinear generalization of the Maxwell constitutive equation, in which the viscous deformation depends on the n-th power of deviatoric stress, and describes a medium which is elastic with respect to normal stress, but relaxes deviatoric stress. Power-law exponents equal to 2 and 3, which are most often found in laboratory experiments, are considered. The equation is solved by a perturbative method for a viscoelastic layer subjected to a constant, extensional or compressional, strain rate and yields stress as a function of time, temperature and rock composition. The solution is applied to an ideal extensional boundary zone and shows that the base of the crustal seismogenic layer may be deeper than predicted by a linear rheology.

An instability of frictional sliding driven by tectonic stress is assumed to be the source of earthquakes. Empirical slip laws indicate that, under constant ambient conditions, friction depends on time, slip rate and slip history. Regular stick slip behaviour is induced by velocity weakening, a decrease of friction with slip rate. Velocity weakening is introduced into a model for a propagating Somigliana dislocation under slowly increasing shear stress in an elastic space. Two distributions of static friction are considered, characterized by asperities with sharp borders and smooth borders respectively. The instability occurs when the rate at which friction decreases becomes greater than the rate at which the applied stress must increase to produce an advance of fault slip. The possibility that this condition is fulfilled depends on the velocity dependence and on the spatial distribution of friction on the fault. In the case of sharp asperity borders, instability can take place only when some amount of slip has occurred on the fault, while this condition is not required in the case of smooth borders.

The rationale of lava flow deviation is to prevent major damage, and, among the possible techniques, the opening of the flow leve¤es has often been demonstrated to be suitable and reliable. The best way to open the leve¤es in the right point, in order to obtain the required effect, is to produce an explosion in situ, and it is then necessary to map with the highest precision the temperature field inside the leve¤es, in order to design a safe and successful intervention. The leve¤es are formed by lava flows due to their non-Newtonian rheology, where the shear stress is lower than the yield stress. The leve¤es then cool and solidify due to heat loss into the atmosphere. In this work we present analytical solutions of the steady-state heat conduction problem in a leve¤e using the method of conformal mapping for simple geometrical shapes of the levee cross-section (triangular or square). Numerical solutions are obtained with a finite element code for more complex, realistic geometries.

We present here the results from dynamical and thermal models that describe a channeled lava flow as it cools by radiation. In particular, the effects of power-law rheology and of the presence of bends in the flow are considered, as well as the formation of surface crust and lava tubes. On the basis of the thermal models, we analyze the assumptions implicit in the currently used formulae for evaluation of lava flow rates from satellite thermal imagery. Assuming a steady flow down an inclined rectangular channel, we solve numerically the equation of motion by the finite-volume method and a classical iterative solution. Our results show that the use of power-law rheology results in relevant differences in the average velocity and volume flow rate with respect to Newtonian rheology. Crust formation is strongly influenced by power-law rheology; in particular, the growth rate and the velocity profile inside the channel are strongly modified. In addition, channel curvature affects the flow dynamics and surface morphology. The size and shape of surface solid plates are controlled by competition between the shear stress and the crust yield strength: the degree of crust cover of the channel is studied as a function of the curvature. Simple formulae are currently used to relate the lava flow rate to the energy radiated by the lava flow as inferred from satellite thermal imagery. Such formulae are based on a specific model, and consequently, their validity is subject to the model assumptions. An analysis of these assumptions reveals that the current use of such formulae is not consistent with the model.