1,721,177 research outputs found
On the preparatory processes of the M6.6 earthquake of June 17th, 2000, in Iceland
No full textA model is proposed to explain the spatial distribution of foreshocks of the June 17th 2000, Ms 6.6 earthquake in the South Iceland Seismic Zone (SISZ) and the high stress drop of the mainshock. Fluids of magmatic origin, ascending at near-lithostatic pressure through a low permeability layer perturb the regional stress field, inhibiting fluid flow laterally, where a high strength asperity is left. The asperity is modeled as elastic, embedded within a medium with low effective rigidity. Regional stresses due to tectonic motions are perturbed by the presence of the asperity, enhancing the production of hydrofractures and foreshocks in the NW and SE quadrants and increasing considerably the shear stress within the asperity, leading to the NS striking mainshock.
Citation: Bonafede, M., C. Ferrari, F. Maccaferri, and R. Stefansson (2007), On the preparatory processes of the M6.6 earthquake of June 17th, 2000, in Iceland, Geophys. Res. Lett., 34, LXXXXX, doi:10.1029/2007GL031391
On strike‐slip dislocations in an elastic half space in the presence of localized distributions of strain nuclei
No full textThe Somigliana dislocation theory is applied to the study of strike‐slip faulting in an isotropic, homogeneous, elastic half space in the presence of localized distributions of strain nuclei. Vanishing shear tractions on the dislocation surface are assumed and the slip amplitude as a function of depth is derived, together with the stress drop pattern and the energy release. The results show indications that the condition of continuous stable sliding prevailing at depth along the locked sections of faults may strongly affect the slip, the stress drop and the energy release of a seismic dislocation event. Moreover, fault widths derived from dislocation models driven from underneath are greater than those evaluated from models with a uniform initial stress field and thus tend to explain in simple terms the common finding in some regions that earthquake foci are deeper than evaluated widths of dislocation surfaces
A crack model of creep processes on deep sections of faults
No full textA crack model in antiplane shear configuration is shown representing creep processes interpreted in terms of ‘viscous’ deformation of a narrow plastic layer, characterized by inhomogeneous rheological properties, embedded within a homogeneous elastic medium. The evolution in time of slip and stress over the crack plane is studied through a truncated expansion in Chebyshev polynomials, and convergence is proved to be fast in the simple examples considered. Finite‐stress solutions are found which are compatible with constitutive relations of elasto‐plastic materials and furthermore these allow us to simulate creep propagation and stress transfer between locked and unlocked fault segments. This model provides a simple interpretation of the shallow depth of the seismogenic layer observed in several areas of the world and lends itself to modelling creep processes during either post‐seismic rebound or pre‐seismic stress buildup. Stress transfer is accomplished mostly by the slow extension of the creeping section. During a seismic cycle it is envisaged that different regimes dominate over deep, intermediate and shallow sections of faults: (i) slow pre‐seismic stress build‐up accompanied by creep and stress migration toward intermediate depths; (ii) brittle fracture over shallow and intermediate sections of faults; (iii) post‐seismic rebound over intermediate and deep sections of faults. The present crack model, while providing finite‐stress solutions, allows a better understanding of how stress may accommodate at different depths over a fault plane during a seismic cycle
Selection rules for bulk piezomagnetic coefficients of isotropic titanomagnetite dispersions
No full textA thermodynamic approach is developed for piezomagnetic phenomena, linked to reversible variations of magnetic susceptivity in isotropic stressed media. In spite of the problematic interpretation of rock piezomagnetism at the microscopic level, a simple macroscopic parameterization of antiferromagnetic media is obtained, based on invariance arguments. In intrinsically isotropic media, linearity in the stress-strain relation is shown to imply linearity of the relation between induced magnetization and stress. The macroscopic piezomagnetic behaviour of a sample in the linear stress-strain regime is described by a fourth rank tensor with 2 independent coefficients only. These coefficients can be measured in the laboratory by simple uniaxial compression tests. Comparisons with experimental data on titanomagnetite bearing rocks and synthetic dispersions show that the linear approach is valid in so far as the applied stress does not exceed a few 100 bars
Stress drop and slip vector on a dislocation in an elastic space due to localized force distributions
No full textSome earthquake models based on the elastic theory of dislocations are presented. Earthquake occurrence is modelled as the opening of a crack in an infinite elastic medium triggered by the action of localized stress distributions. The fracture is modelled as a continuum of infinitesimal dislocations. This approach allows us to have complete information about the stress field and the displacement field and to make a first step towards understanding the relation between source mechanism and stress distributions
A porous flow model of flank eruptions on Mt. Etna: second-order perturbation theory
No full textA porous flow model for magma migration from a deep source within a volcanic edifice is developed. The model is based on the assumption that an isotropic and homogeneous system of fractures allows magma migration from one localized feeding dyke up to the surface of the volcano. The maximum level that magma can reach within the volcano (i.e., the «free surface» of magma, where fluid pressure equals the atmospheric pressure) is reproduced through a second-order perturbation approach to the non-linear equations governing the migration of incompressible fluids through a porous medium. The perturbation parameter is found to depend on the ratio of the volumic discharge rate at the source (m3/s) divided by the product of the hydraulic conductivity of the medium (m1/s) times the square of the source depth. The second-order corrections for the free surface of Mt. Etna are found to be small but not negligible; from the comparison between first-order and second-order free surfaces it appears that the former is higher near the summit, slightly lower at intermediate altitudes and slightly higher far away from the axis of the volcano. Flank eruptions in the southern sector are found to be located in regions where the topography is actually lower than the theoretical free surface of magma. In this sector, modulations in the eruption site density correlate well with even minor differences between free surface and topography. In the northern and western sectors similar good fits are found, while the NE rift and the eastern sector seem to require mechanisms or structures respectively favouring and inhibiting magma migration
On the interpretation of slow ground deformation precursory to the 1976 Friuli earthquake
No full textDuring three years preceding the 1976 Friuli earthquake, a continuous southward ground tilt was recorded by a tiltmeter placed near Tolmezzo, 15 km north-west of the epicentre of the impending earthquake. The cumulative ground tilt amounted to as much as 3 minutes of arc. Since the tiltmeter was placed in the proximity of an active fault, such a tilt can be explained if the fault slipped aseismically on its shallower section during the same three year period. Aseismic slip on the fault might have been caused by the same mechanism which concentrated stress in the region and eventually produced the 1976 earthquake
Fault slip beyond a barrier on a transform plate boundary
No full textA strike‐slip plate boundary is considered with non‐uniform strength. Depth‐dependent slip on the fault surface is assumed to occur aseismically in the lower fault section, in response to a basal shear stress, and seismically in the brittle upper section. The upper fault section acts as a barrier, preventing aseismic slip from reaching shallower depths. If, however, friction decreases in the proximity of the Earth's surface, brittle fracture may occur beyond the barrier. It is found that, when friction in the uppermost fault section exceeds the applied shear stress, fault slip occurs there with a smaller amplitude than at larger depth. This may explain the discrepancies, which have been sometimes reported, between fault offsets observed at the Earth's surface and fault offsets inferred from seismograms by using simpler dislocation models
Displacement and stress fields produced by a centre of dilation and by a pressure source in a viscoelastic half‐space: application to the study of ground deformation and seismic activity at Campi Flegrei, Italy
No full textAnalytical 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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