Anisotropic characterization of macroseismic fields

A methodology for the anisotropic characterization of macroseismic fields is proposed, in order to evaluate seismic hazard, based on the real geometry of the isoseismals of the field. The proposed methodology, independent of the macroseismic intensity attenuation law, allows both for a single field and for several fields in the same source zone, the determination of minimum and maximum attenuation values and of the relative directions.     

利用大地电磁测深法识别EDA地震电性变化前兆

论述了起因于ＥＤＡ现象的地球介质电导率各向异性特征，分析研究了地球介质是忸率各向异性变化引起的大地电磁响应函数变化形态特征，简述了大地电磁测深法对ＥＤＡ电性变化前兆的可识别性及大地电磁资料解释中考虑电导率各向异性的必要性，研究了对称各向异性层状介质大地电磁资料的反演问题。     

3-D seismic velocities calculated from lattice-preferred orientation and reflectivity of a lower crustal section: examples of the Val Sesia section (Ivrea zone, northern Italy)

To quantify the seismic properties of lower crustal rocks and to better constrain the origin of the lower crustal seismic reflectivity, we determined the complete 3-D seismic properties of a lower crustal section. Eight representative samples of the main lithologic and structural units outcropping in the Val Sesia (Ivrea zone) were studied in detail. The seismic velocities were calculated using the single crystal stiffness coefficients and the lattice preferred orientation (LPO) of each mineral in all samples. The 21 stiffness coefficients characterizing the elastic behaviour of each rock are determined. Mafic and ultramafic rocks such as pyroxenite and pyroxene-bearing gabbros display complex shear wave properties. These rocks are weakly birefringent (maximum 0.1 kms⁻¹) and it is difficult to find consistent relationships between the seismic properties and the rock structure. On the other hand, seismic properties of deformed felsic rocks are essentially controlled by mica. They display strong S -wave birefringence (0.3 km s⁻¹) and relatively high V _p anisotropy (7.6 per cent). Amphibole also strongly influences the rock birefringence patterns. For both kind of rocks, the foliation is highly birefringent and the fast polarized shear wave is systematically oriented parallel to the foliation. We show that the number of mineral phases in the rock strongly controls the anisotropy. The seismic anisotropy has a complex role in the P -wave reflectivity. Compared to the isotropic case, anisotropy enhances the reflection coefficient for about 60 per cent of the possible lithological interfaces. For 40 per cent of the interfaces, the reflection coefficient is much lower when one considers the medium as anisotropic.     

岩体强度的各向异性研究

本文运用断裂力学理论推导了含节理岩体的初裂强度公式,定量地讨论了节理方向、长度对岩石强度的影响,阐明了节理岩体强度各向异性的实质。通过证实,本文所建立的公式具有很强的适用性和较高的可靠性,可用于实际的工程岩体稳定性评价。     

The scattering of shear-waves in the crust

The two major sources of scattering for shear-waves in the crust, interactions with the topography at the surface and the effective anisotropy of aligned cracks throughout the rockmass, introduce first-order changes to the shear-wave particle-motion. At the surface, shear-waves are scattered by the topography within a wavelength or two of the recording site so that, unless the effective incidence angle is less than the critical angle sin^–1 V _S/V _P, the recorded waveforms may bear little relationship to the waveforms of the incident wave. Within the rockmass, shear-waves are scattered by extensive-dilatancy anisotropy (EDA), the distribution of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore-space pervading most rocks in the crust. Analysis of this shear-wave splitting yields new information about the internal structure of thein situ rockmass which is not otherwise available.     

Elastic constants and velocity surfaces of indurated anisotropic shales

The velocities of two Devonian-Mississippian shales have been measured to confining pressures of 200 MPa in a laboratory study of anisotropy and wave propagation. Both samples were found to be transversely isotropic at elevated pressures with the main symmetry axis perpendicular to bedding. The elastic constants of the shales were used to calculate phase and group velocity surfaces as a function of angle to the bedding normal. Multiple velocity measurements in non-symmetry directions, not undertaken in previously published studies of shales, have been used to confirm features observed on calculated velocity surfaces. It is demonstrated that velocities measured in non-symmetry directions are phase velocities. Group velocities were found to be significantly lower than the corresponding phase velocities of the shales due to their high anisotropies. Shear wave splitting was found to be negligible for propagation directions within approximately 30° of the bedding normals.