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Analytical expressions for the static displacement field produced by a centre of dilation and by a pressure source in a viscoelastic half-space are derived. The associated stress fields are also computed. The rheology of a standard linear solid (SLS) is adopted for the shear modulus, while the incompressibility is kept elastic. An instantaneous dilation or variation of pressure is considered as responsible for the deformation. In the centre of dilation model, if the two rigidities of the SLS are of the same order of magnitude, the viscoelastic contribution to the deformation is negligible; if the short-term rigidity is at least two orders of magnitude higher than the other one, the results are indistinguishable from those obtained with a Maxwell solid rheology. In this case, it is found that the initial elastic displacement is amplified by 20 per cent. In the pressure source model, if the rigidities of the SLS are of the same order of magnitude, the initial elastic displacement is amplified by a factor of about 2, but unrealistically high pressure values are required. On the other hand, for a Maxwell solid rheology the displacement grows indefinitely in time, following a sudden application of a finite pressure. The uplift rate is evaluated and it is shown that, for obtaining values of the order of 1 m over one characteristic relaxation time, more reasonable values of pressure are allowed. Applications to ground deformation in volcanic areas are discussed, taking as an example the Campi Flegrei zone, near Naples, Italy.

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We introduce a 3D model for near-vent channelized lava flows. We assume the lava to be an isothermal Newtonian liquid flowing in a rectangular channel down a constant slope. The flow velocity is calculated with an analytical steady-state solution of the Navier-Stokes equation. The surface velocity and the flow rate are calculated as functions of the flow thickness for different flow widths, and the results are compared with those of a 2D model. For typical Etna lava flow parameters, the influence of levees on the flow dynamics is significant when the flow width is less than 25 m. The model predicts the volume flow rate corresponding to the surface velocity, taking into account that both depend on flow thickness. The effusion rate is a critical parameter to evaluate lava flow hazard. We propose a model to calculate the effusion rate given the lava flow width, the topograhic slope, the lava density, the surface flow velocity, and either the lava viscosity or the flow thickness

S U M M A R YVolcanic rocks forming sills, dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement. We model a sill as a steadystate flow of a Bingham fluid, driven by a pressure gradient in a horizontal conduit. The magma velocity as a function of depth is calculated from the motion and constitutive equations. Vorticity and strain rate are determined for a reference system moving with the fluid. The angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain. Magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations. It is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction, and that the magnetic foliation plane changes periodically from horizontal to vertical. The results are compared with the magnetic fabric of Ferrar dolerite sills (Victoria Land, East Antarctica) derived from low-field susceptibility measurements.

The formation of lava tubes is a common phenomenon on some basaltic volcanoes, such as Etna. A model for tube formation by roofing of a channel is proposed and involves first describing lava as a Bingham liquid flowing down a slope. It is further assumed that lava flows in a channel with rectangular cross section: as a result of heat loss into the atmosphere, a crust is gradually formed on the upper surface of the flow and this crust eventually welds to the channel levees. We assume that a lava tube is formed when such a crust is sufficiently thick to resist the drag of the underlying flow and to sustain itself under its own weight. The minimum thickness of the crust satisfying such conditions depends on the tensile strength and shear strength of the crust itself. Assuming that the growth of the crust produces a downflow linear increase of the shear stress at the interface between flowing lava and the crust, the distance is evaluated between the eruption vent and the point where the tube is formed. The model predicts that if the flow rate is constant, the thickness of the flow increases as the crust fragments grow and weld to each other, and the velocity of the crust decreases to zero. Once the lava tube is formed, the initial flow rate can be achieved by a Row thickness smaller than the vertical size of the tube, with the same viscous dissipation: this may explain why under steady state conditions, the lava level inside a tube is frequently lower than the roof of the tube itself

S U M M A R YIt is generally agreed that the occurrence of seismic sequences implies a kind of interaction between different fault segments. The coseismic stress transfer produced by each dislocation is the most obvious component of such an interaction. However, the time intervals elapsing between subsequent events in a sequence indicate that the coseismic stress is not sufficient to trigger other seismic events by itself. We investigate the possibility that the coseismic stress field may induce flow of pore fluids, altering the pore pressure distribution in the region. Since the crust is a fluid-saturated medium at many locations, we consider the crust as a poroelastic solid. Because poroelastic materials exhibit time-dependent stress fields, we examine if this behaviour can explain the triggering of aftershocks. We consider some available analytical solutions for a semi-infinite plane fault. Permeable and impermeable dislocation planes are considered. We compare the solutions for a poroelastic medium with those for a porous medium, and evaluate the effect of the coupling between deformation and fluid diffusion. We find that the Coulomb stress changes due to the main shock may be initially negative at some locations, but become positive as pore fluids are redistributed. These changes are significantly large. If the crust were to behave as an isotropic, fluid-filled, poro-elastic medium, as we assume here, Coulomb stress triggering by means of pore fluid diffusion is likely an important mechanism for aftershock generation over distances and widths of about 2.5 and 0.5 fault lengths, respectively. These distance ranges are smaller than those predicted by previous models which disregarded the mechanical interactions between elastic deformation and pore fluid diffusion. For typical porosities, the stress changes due to fluid flow are diminished greatly after about 1 yr after the main shock.