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Abstract:
Deep seismic sounding provides important information on the seismic structure of the crust. Seismic experiments make use
of controlled sources (explosions, Vibroseis) or natural sources (earthquakes, ambient noise), or combinations of both types.
Seismic velocities are derived from modelling or tomographic inversion of diving waves and refractions. Particularly the combined
interpretation of compressional and shear velocities allows for insight into the lithological structure of the crust. Seismic
reflectivity is derived from imaging of waves reflected at geological boundaries. Typical patterns of crustal reflectivity are observed
for specific tectonic settings. Case studies are shown from two studies at plate boundary systems. (1) The old plate boundary
at the Namibian margin was formed by Cretaceous continental rifting and its interplay with the activities of the Tristan da
Cunha mantle plume. Traces of intensive magmatic overprinting of the crust at the landfall region of Walvis Ridge can be seen
in the derived velocity model and also in the reflectivity image. (2) The Dead Sea transform marks the active boundary between
the African and Arabian plates. The Dead Sea basin was formed by pull-apart in response to step-over of the fault system. Results
from tomography reveal a deep asymmetric basin structure. An anomalous body was found under the basin, between 13
and 18 km depth, which is interpreted as pre-basin sediments. Our results are supported by the distribution of earthquakes.
The results provide new constraints for the modeling of plate boundary processes.
