Passive Seismic Experiment Mosaic - A Pilot Study of Mantle Lithosphere Anisotropy of the Bohemian Massif

We present results of an array study of seismic anisotropy beneath the Bohemian Massif (BM) showing distinct lateral and directional variations of the fast shear-wave polarization and split-delay time consistent with variations in the P-residual spheres, in which directional means of relative residuals are subtracted. Our analysis of the S- and P-wave anisotropy suggests that the mantle lithosphere of the BM consists of at least three large domains with different orientation of the large-scale fabric separated by sutures cutting most likely the whole lithosphere. Boundaries of the units are characterised by a null or small shear-wave splitting, as well as by smaller values in the P-residual spheres. We present self-consistent 3D anisotropic models of the lithosphere domains of the BM resulting from joint inversion of anisotropic parameters searching for a 3D orientation of mantle fabric. While in the Saxothuringian and Sudetes parts of the BM the (a, c) foliations dip prevailingly to the N-NW, they dip to the S and SW in the southern and eastern parts of the Moldanubian.     

An approach to model the mechanical behavior of transversely isotropic materials

The present work proposes an approach to adapt existing isotropic models to transversely isotropic materials. The main idea is to introduce equivalence relations between the real material and a fictitious isotropic one on which one can take all the advantages of the well‐established isotropic theory. Two applications of this approach are presented here: a failure criterion and a damage model that takes into account the load‐induced anisotropy. In both cases, theoretical predictions are in agreement with the experimental data. In the present paper, the developed approach is applied to sedimentary rock materials; nevertheless, it can be generalized to any material that exhibits transverse isotropy. Copyright © 2015 John Wiley & Sons, Ltd.     

Body-wave dispersion in the Friuli focal area

The differential dispersion of P- and S- body waves is studied in the Friuli area. We estimate theoretically the respective contributions of the source, from dislocation models, and of the propagation, from the Futterman model. We demonstrate the possible existence of differential dispersion, which is interpreted as being due to attenuation. The quality factor Q_P deduced from this hypothesis shows a regional variation from 15 to 210. Results also indicate a possible dependence of Q_P on frequency in the range 5–40 Hz.     

U-series disequilibria in suspended river sediments and implication for sediment transfer time in alluvial plains: The case of the Himalayan rivers

²³⁸U-²³⁴U-²³⁰Th radioactive disequilibria were analyzed in suspended sediments (collected at different depths) from the Ganges River and one of its main tributaries: the Narayani-Gandak River. Results associated with bedload sediment data suggest that uranium-series (U-series) disequilibria in river sediments of the Ganges basin vary with grain size and sampling location. The range of observed U-series disequilibria is explained by a mixing model between a coarse-grained sediment end-member, represented by bedload and bank sediments, and a fine-grained end-member that both originate from Himalaya but undergo different transfer histories within the plain. The coarse-grained sediment end-member transits slowly (i.e. >several 100’s ky) in the plain whereas the fine-grained sediment end-member is transferred much faster (<20-25 ky), as indicated by the absence of significant variations in Th isotope composition of the fine-grained sediment end-members. These results show that U-series isotopes can be used to quantify the various transfer times of river sediments of different sizes and infer that there can be an order of magnitude of difference, or more, between the transfer time of suspended and bedload sediments. This underlines that a good knowledge of the proportion of suspended vs. bedload sediments transported in the river is required to accurately assess how fast erosion products are transferred in a catchment and how fast a catchment is likely to respond to external forcing factors.     

A closing Ligurian Sea?

Two earthquakes occurred in the Ligurian Sea in December 1989 and April 1990. Both were widely felt along the French and Italian Rivieras, thus reminding us of the seismic risk in this region. The significant increase in the number of seismic stations in the area facilitated the study of these two shocks and their related aftershocks. Using different techniques (absolute and relative hypocentral locations, doublet analysis and waveform modeling), we computed accurate hypocentral locations and estimated the location-error range for earthquakes in this area. We also computed the focal mechanisms for both mainshocks, and we present here a synthesis that integrates previous data. The reactivation in compression of the Ligurian Sea sphenochasm is confirmed, which would eventually result in the closing of an aborted oceanic domain. As the seismic activity is clearly restricted to the northern margin, we suggest it locally results from the lateral expulsion of the south-western Alps along the Apulian indenter.     

Special Contribution: BOHEMA 2001-2003: Passive Seismic Experiment to Study Lithosphere-Asthenosphere System in the Western Part of the Bohemian Massif

Studia Geophysica et Geodaetica -     

Anisotropic Measurements in the Rhinegraben Area and the French Massif Central: Geodynamic Implications

—As part of an integrated seismic study, polarization of shear waves has been analyzed for teleseismic events recorded at a set of permanent broadband, semi-permanent long- and short-period and temporary short-period seismological stations located in two geodynamically important areas in western Europe, namely the Rhinegraben-Urach area and the French Massif Central volcanic field.¶While for the semi-permanent and the permanent stations there is a good azimuthal coverage of teleseismic earthquakes which allowed us to investigate the azimuthal dependence and the spatial variation over short distances of an anisotropy direction, no even azimuthal distribution of teleseismic recordings with a clear elliptical (or linear) polarization of the S phases could be obtained in the case of the temporary stations.¶While the mean values of the splitting parameters φ and δt are geographically coherent for adjacent stations, our results show a large scatter of the individual splitting parameters for the set of events used. The magnitude of the splitting time suggests that the deformation extends below the lithosphere and that the thickness of the anisotropic structure is at least 100–200 km.¶For some stations located in the Rhinegraben-Urach area (ECH, RG-N, RG-S, RBG), the variations of φ are consistent with a two-layer anisotropic model as suggested by Vinnik et al. (1994) for the South German Triangle. For the stations ECH (Vosges mountains), RG-N and RG-S (Rhinegraben proper), the resulting estimates of fast direction are around N10°E–N30°E and N80°E–N100°E for the upper and lower layers, respectively. For the station RBG (Urach), the results are N60°E–N70°E and N125°E–N135°E, respectively.¶In the Rhinegraben-Urach area, the estimates of the effective fast direction for a one-layer model show a rotation from a graben-related (30°) pattern to an Alpine belt-related pattern in the eastern part (≈ E–W). In the French Massif Central region, the results reveal two distinct fast polarization patterns. While to the west of the Sillon Houiller, φ is parallel to this late-variscan transformlike fault zone and perpendicular to the variscan belt, it is to the east rather perpendicular to the Alpine belt. The results suggest a mixture of both a lithospheric and an asthenospheric component of the seismic anisotropy for the Rhinegraben-Urach as well as for the French Massif Central areas.