IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS013 Oral Presentation 2230
Layering of seismic anisotropy and the past and present deformation of
the lithosphere and asthenosphere beneath Germany.
Dr. Sergei Lebedev
Earth Sciences Utrecht University IASPEI
Brigitte Endrun, Monika Bischoff, Thomas Meier
Dynamics of the lithosphere and asthenosphere has shaped and continues to re-shape the surface of
Europe. We perform anisotropic array tomography with data from the German Regional Seismic
Network and map--for the first time--stratification of seismic anisotropy beneath Germany within the
entire lithosphere-asthenosphere depth range. Employing a new implementation of the two-station
method, we measure interstation, surface-wave dispersion curves in broad period ranges (10-15 s to
250-400 s). We then invert the data for azimuthally anisotropic phase-velocity maps. Because surface
waves at different periods sample different depths within the Earth, variations of anisotropy patterns
with period (10--300 s) translate into tight constraints on the layering of seismic anisotropy within the
lithosphere and asthenosphere. In both the lower crust and mantle lithosphere beneath the bestsampled
western and central Germany, shear-wave fast-propagation directions are approximately NE,
parallel to the strike of the Variscan suture. The lithospheric anisotropy is likely due to the fabric that
has been created during the Variscan orogeny and frozen-in within the lithosphere ever since. The
measured present thickness of this anisotropic layer thus constraints the thickness of the lithosphere at
the end of the orogeny---important new information regarding the geodynamics of the continental
collision. Between approximately 100 and 300 km depths, the anisotropic fast-propagation direction is
distinctly different, striking East-West. This pattern is likely to reflect the strain in the asthenosphere
due to the current and recent lithosphere-asthenosphere relative motion. The asthenospheric fastpropagation
directions that we measure match those inferred from shear-wave splitting measurements;
this shows that most of the splitting originates in the asthenosphere. Interestingly, the measured fastpropagation
directions are also parallel to the published most conductive directions inferred from a
magnetotelluric dataset, both in the lower crust (NE) and in the asthenosphere (E). The match of fastpropagation
and high-conductivity directions has important implications for the origin of electrical
anisotropy in the Earth.
Keywords: anisotropy, dynamics, evolution