Simulated envelopes of non-isotropically scattered body waves as compared to observed ones: another manifestation of fractal heterogeneity

Envelopes of scalar waves are simulated at various distances from an instant point source embedded in a random uniformly scattering medium by means of direct Monte-Carlo modelling of wave-energy transport. Three types of scattering radiation pattern ('indicatrix') are studied, for media specified by (1) a Gaussian autocorrelation function of inhomogeneities, (2) a power-law ('fractal', k -^α) inhomogeneity spectrum and (3) the mix of case (1) and the isotropic indicatrix (very small + large inhomogeneities). We look for a model that can qualitatively reproduce the two most characteristic features of real S-wave envelopes of near earthquakes, namely (1) the broadening of the 'direct' wave group with distance and (2) the monotonously decaying shape of the coda envelope that does not deviate strongly from that expected in the isotropic scattering case. Both properties are observed for any band over a wide frequency range (1-40 Hz). The well-studied isotropic scattering model realistically predicts the appearance of codas but fails to predict pulse broadening. The model of large-scale inhomogeneity realistically predicts the mode of pulse broadening but fails to predict codas. We have found that, for a particular frequency band, within each class of inhomogeneity studied, both requirements can be qualitatively satisfied by a certain choice of parameters. In the Gaussian-ACF case, however, this match can be obtained only for a narrow frequency range. In contrast, the fractal case (with a value of exponent a of about 3.5-4) reproduces qualitatively the observed wide-band behaviour, and we consider it a reasonable representation of the gross properties of the earth medium.     

The nature of particle motion in regional seismograms and its utilization for phase identification

The particle motion of regional arrivals is frequently treated in automatic phase-recognition schemes as that appropriate to simple P or S waves incident on an elastic, laterally homogeneous half-space. This model implies that the motion in ' P -type' phases can be described in terms of a single, generalized signal process and ' S -type' phases in terms of two independent processes ( SV and SH ) and thus, all regional arrivals could be fully characterized by three components of motion. In this paper, we present anlyses of the particle-motion patterns of various regional arrivals recorded at the ARCESS array from closely spaced events in the Kola Peninsula. We have found that only Pn -particle motion, described in terms of two independent signal processes, can be reliably characterized by three-component recordings. On the other hand, the various regional arrivals following Pn , such as Pg and Sn and Lg , can only be poorly characterized on the basis of three-component recordings alone. The reason is that these arrivals must be described in terms of more than two independent generalized signal processes, at least three for Pg and Sn , and possibly up to five for Lg . Recognition of these phases will thus require the use of more sensors than signal processes in the observing sensor configuration, such as three-component sensors combined with a small tripartite array. We have investigated the feasibility of adaptive, automatic recognition of regional arrivials by a wavefield extrapolation scheme utilizing such a mini-array. The process, which appear to be promising, adaptively learns the particle-motion patterns of individual arrivals, including complex site-response functions, from examples of closely located regional events.     

Length scales, patterns and origin of azimuthal seismic anisotropy in the upper mantle as mapped by Rayleigh waves

We measure the degree of consistency between published models of azimuthal seismic anisotropy from surface waves, focusing on Rayleigh wave phase-velocity models. Some models agree up to wavelengths of ∼2000 km, albeit at small values of linear correlation coefficients. Others are, however, not well correlated at all, also with regard to isotropic structure. This points to differences in the underlying data sets and inversion strategies, particularly the relative 'damping' of mapped isotropic versus anisotropic anomalies. Yet, there is more agreement between published models than commonly held, encouraging further analysis. Employing a generalized spherical harmonic representation, we analyse power spectra of orientational (2Ψ) anisotropic heterogeneity from seismology. We find that the anisotropic component of some models is characterized by stronger short-wavelength power than the associated isotropic structure. This spectral signal is consistent with predictions from new geodynamic models, based on olivine texturing in mantle flow. The flow models are also successful in predicting some of the seismologically mapped patterns. We substantiate earlier findings that flow computations significantly outperform models of fast azimuths based on absolute plate velocities. Moreover, further evidence for the importance of active upwellings and downwellings as inferred from seismic tomography is presented. Deterministic estimates of expected anisotropic structure based on mantle flow computations such as ours can help guide future seismologic inversions, particularly in oceanic plate regions. We propose to consider such a priori information when addressing open questions about the averaging properties and resolution of surface and body wave based estimates of anisotropy.     

The Aegean region: deep structures and seismological properties

3-D P -wave velocity anomaly patterns, the geometry of the Hellenic Wadati-Benioff zone and of the seismically active fracture zones in the overlying continental wedge, and the depth distribution of seismogenic properties (seismic activity, energy and b value) for the Aegean region are examined in this study. The western and eastern parts of the area studied differ significantly in terms of the depth distribution of the considered seismological parameters and the velocity structures. The results indicate different physical conditions in the western and eastern parts of the region in question. Data for the surface heat flow and the evolution of the volcanism in the Aegean region since the early Eocene are employed to interpret the results of the present study. Possible geodynamic processes at the plate boundaries between Africa and Eurasia are discussed.     

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.