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Okada (1992) provided expressions for the displacement and strain fields due to a finite rectangular source in an elastic, homogeneous and isotropic half-space. Starting with these results, we applied the correspondence principle of linear viscoelasticity to derive the quasi-static displacement, strain and stress fields in a viscoelastic, homogeneous and isotropic half-space. We assume that the medium deforms viscoelastically with respect to both the shear and the normal stresses but keeps a constant bulk modulus; in particular, the shear modulus relaxes as Maxwell fluid. We presented the viscoelastic effect on displacement, displacement gradient and stress fields, for a choice of parameter values. The viscoelastic effect due to the sudden dislocation reaches a limit value after about 10 times the Maxwell time. The expressions obtained here provide tools for the study of viscoelastic relaxation of lithosphere associated with seismic and volcanic phenomena. [ABSTRACT FROM AUTHOR]

It 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. [ABSTRACT FROM AUTHOR]

During 1997 several slow earthquakes have been recorded by a geodetic interferometer located beneath Gran Sasso, central Italy. The strain rise times of the events range from tens to thousands of seconds and strain amplitudes are of the order of 10−9. Amplitudes scale with the square root of the rise time and this suggests a diffusive behaviour of the slip propagation along the fault. In this work, we develop a model in which slip diffusion is the result of the presence of a gouge layer between fault faces, with a viscoplastic rheology. The fluid velocity field in the gouge layer diffuses in the directions of fault length and fault thickness, with different characteristic times. This model reproduces the relation between amplitude and rise time of measured strain signals. Synthetic straingrams, obtained for a horizontally layered, flat Earth and a source located a few kilometres from the instrument, are in agreement with observed signals. [ABSTRACT FROM AUTHOR]