ALBERT

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Description: Abstract: Teleseismic recordings of SKS waves at 15 stations in Central Europe revealed shear-wave splitting indicative for seismic anisotropy. The polarization direction alpha of the fast split wave varies between about N40 degree E in the west and N120 degree E in the east. The average delay-time between the two split waves is delta t = 0.83 plus minus 0.31 s. This is too large to explain the splitting as an effect of crustal anisotropy only. On the other hand the trend of alpha follows closely that of Hercynian fold axes. But even when assuming the preservation of frozen Hercynian rock fabric and structural trends throughout the whole lithosphere the latter is not thick enough beneath Central Europe to explain to explain the large delay-times. Therefore, lattice-preferred horizontal orientation (LPO) of the a-axis of olivin in the asthenosphere is assumed to be the prime or even sole cause of the observed anisotropy parameters. LPO may be caused by drag-induced shear flow below the West European plate. According to the model GFZ95 (Montag et al. 1995) the latter moves with absolute velocities between about 2 to 3 cm/y towards roughly N57 degree E plus minus 5 degree. This direction is close to the average alpha = 62 degree observed at the western seismic stations, i.e. for areas characterized by low topography of the Moho and lithosphere-asthenosphere boundary. But the latter deepens down to 150-200 km beneath the Alps and the western boundary of the East European Platform. These "roots" border the relatively thin lithosphere beneath Central Europe. They trend with about N40 degree E to N100 degree E in the south and about N110 degree to N130 degree E in the north and east. This corresponds to the observed variations in alpha. We hypothesize, therefore, that the observed anisotropy is mainly due to plate-drag induced LPO in the asthenosphere. The latter follows closely the direction of absolute plate motion in areas of low topography of the lithosphere-asthenosphere boundary but more closely the trend of the latter in case of strong topography. Zusammenfassung SKS-Wellen-Beobachtungen an 15 seismischen Stationen in Mitteleuropa ergaben eine Aufspaltung der Scherwellen in 2 orthogonal polarisierte Anteile, die sich mit unterschiedlicher Geschwindigkeit ausbreiten. Eine solche Aufspaltung tritt bei Anisotropie im Untergrund auf. Die Polarisationsrichtungen alpha der schnelleren SKS-Welle variieren systematisch von etwa N40 Grad E im Westteil und N120(E im Nord- und Ostteil des Untersuchungsgebietes. Die langsamere Welle setzt im Mittel um (t = 0,83 plus minus 0,31 s spaeter ein (delay-time). ( folgt auffallend dem mittleren Streichen der variszischen Faltenachsen. Dennoch lassen sich die Beobachtungen nicht allein durch strukturelle Anisotropie in der Kruste erklaeren. Letztere verursacht nach weltweiten Erfahrungen im Mittel nur (t ( 0,1 s. Auch wenn man annimmt, dass variszische Struktur- und Texturtrends noch in der gesamten Lithosphaere "eingefroren" sein koennen, dann reicht die Maechtigkeit der Lithosphaere unter Mitteleuropa nicht aus, um Werte (t 〉 0,3 s zu erklaeren. Es wird deshalb vermutet, dass der ueberwiegende oder auch gesamte beobachtete Effekt durch eine sub-horizontale Vorzugsausrichtung (LPO - lattice preferred orientation) der a-Achsen von Olivin in der Asthenosphaere bedingt ist. Letztere ist unter Mitteleuropa gut ausgepraegt. LPO kann z.B. durch viskose Reibung bei Bewegung der darueberliegenden Lithosphaerenplatte entstehen. Das Modell GFZ95 liefert nach Montag u.a. (1995) fuer die absolute Plattenbewegung in Mitteleuropa Geschwindigkeiten zwischen 2 und 3 cm/Jahr und eine mittlere Bewegungsrichtung von etwa N57(E plus minus 5(. In Gebieten mit geringer Topographie der Moho und Lithosphaeren-Asthenosphaeren-Grenze folgt ( weitgehend dieser Bewegungsrichtung der Westeuropaeischen Platte. Dagegen passt sich ( in Richtung Alpen und Westrand der Osteuropaeischen Tafel dem Trendverlauf der Lithosphaerenunterkante an, die dort bis zu etwa 100 km tief in die Asthenosphaere hereinragt. Additional keywords: lattic preferred orientation of olivin, asthenosphere, absolute plate motion direction, viscous drag, drag induced flow, lithosphere-asthenosphere boundary topography, lithosphere roots, modification of asthenosphere flow, Vorzugsausrichtung des Olivin-Kristallgitters, Asthenosphaere, absolute Plattenbewegung, viskose Reibung, reibungsinduzierte Stroemung, Topographie der Lithosphaeren-Asthenosphaeren-Grenze, Lithosphaerenwurzeln, Modifizierung der Asthenosphaerenstroemung, Variszisches Gebirge, strukturelle Anisotropie

Description: Fast polarization directions α of split SKS waves in Central Europe change from NE/ENE in the western part to dominatingly E/ESE orientation towards north and east. This coincides strikingly well with the dominating trend of Hercynian deformational crustal features. It hints to frozen anisotropy related to paleo-crustal fabric. But when considering plausible anisotropy values of about 2–3% then only a small fraction (δt 〈 0.3 s) of the rather large observed average delay-times (δt = 0.83 ± 0.31 s) between the two split waves could be attributed to structural anisotropy in the relatively thin Central European crust. Therefore, the main “anisotropy signal” has to be associated with lattice-preferred orientation (LPO) of olivine below the crust. It may be either frozen in the subcrustal lithosphere since Hercynian times or have developed more recently in the asthenosphere. The thickness of the lithosphere varies significantly beneath Europe and the depth contours show systematic changes in trend. The latter varies from dominatingly NE in the southwest to SE in the north and east. The polarization directions α of the fast split SKS waves observed at seismic stations in proximity to the southern and northeastern boundaries of Central Europe are subparallel to the trends of these strong anomalies in lithosphere topography. A causal relationship is assumed and a new model proposed to explain the observations in α and δt. It takes into account the possible effects of paleo-deformational events. They may have produced both anisotropic crustal fabric and probably still preserved and similarly trending frozen LPO in the subcrustal lithosphere. The model also considers the influence of recent absolute motion of the West European lithospheric plate towards NE and the effect of its pronounced lower boundary topography on the formation and trend of LPO in the asthenosphere. Accordingly, the effects of anisotropy of different nature and age at different depth levels but with similar trend may superimpose constructively. This could explain the rather large delay-times observed at Central European stations which are too large to be attributed to frozen anisotropy in the lithosphere alone. The model would even permit the total effect observed to be attributed to asthenosphere flow controlled by absolute plate motion direction and lithosphere-asthenosphere boundary topography.

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Description: Additional keywords: national evenings, follow-up studies, photo documentation, certificate, press releases, zirkular, Programm, Hintergrund-Informationen, Teilnehmer, Lektoren, Kursauswertung, Schlussfolgerungen, Empfehlungen, Finanzabrechnung, Workshops, nationale Abende, Nachfolgestudien, Zertifikate, Vulkanausbruch, Pressemitteilungen, Fotodokumentation

Description: Fast polarization directions ( of split SKS waves in Central Europe change from NE/ENE in the western part to dominatingly E/ESE orientation towards north and east. This coincides strikingly well with the dominating trend of Hercynian deformational crustal features. It hints to frozen anisotropy related to paleo-crustal fabric. But when considering plausible anisotropy values of about 2 to 3% then only a small fraction ((t 〈 0.3 s) of the rather large observed average delay-times ((t = 0.83 plus minus 0.31 s) between the two split waves could be attributed to structural anisotropy in the relatively thin Central European crust. Therefore, the main "anisotropy signal" has to be associated with lattice-preferred orientation (LPO) of olivine below the crust. It may be either frozen in the subcrustal lithosphere since Hercynian times or have developed more recently in the asthenosphere. The thickness of the lithosphere varies significantly beneath Europe and the depth contours show systematic changes in trend. The latter varies from dominatingly NE in the southwest to SE in the north and east. The polarization directions ( of the fast split SKS waves observed at seismic stations in proximity to the southern and north-eastern boundaries of Central Europe are sub parallel to the trends of these strong anomalies in lithosphere topography. A causal relationship is assumed and a new model proposed to explain the observations in ( and (t. It takes into account the possible effects of paleo-deformational events. They may have produced both an isotropic crustal fabric and probably still preserved and similarly trending frozen LPO in the subcrustal lithosphere. The model also considers the influence of recent absolute motion of the West European lithospheric plate towards NE and the effect of its pronounced lower boundary topography on the formation and trend of LPO in the asthenosphere. Accordingly, the effects of anisotropy of different nature and age at different depth levels but with similar trend may superimpose constructively. This could explain the rather large delay-times observed at Central European stations which are too large to be attributed to frozen anisotropy in the lithosphere alone. The model would even permit to attribute the total effect observed to asthenosphere flow controlled by absolute plate motion direction and lithosphere-asthenosphere boundary topography. Additional keywords: lattice preferred orientation of olivine, viscous drag induced flow, boundary topography, lithosphere roots, modification of asthenosphere flow, Vorzugsausrichtung des Olivin-Kristallgitters, Asthenosphaere, absolute Plattenbewegung, viskose Reibung, reibungsinduzierte Stroemung, Topographie der Lithosphaeren-Asthenosphaeren-Grenze, Lithosphaerenwurzeln, Modifizierung der Asthenosphaerenstroemung, Polarisationsrichtung der schnellen S-Welle