Deep-structural differences in southwestern Scandinavia revealed by P-wave travel time residuals
pp. 203-214

Much information is available on crustal structure in the area of southern Scandinavia, but little high-resolution information has been presented on structural variations in the upper mantle. We present relative P-wave travel time residuals (P-residuals) from distant earthquakes at 48 temporary seismological stations and 15 permanent seismological stations covering areas of southern Norway, northern Denmark and southwestern Sweden. We investigate whether high topography in southern Norway may be associated with an anomalous upper mantle, and we look for the southwestern boundary of thick shield lithosphere. Differences in station means of P-residuals corrected for topography and contributions from the crust are up to about 1 s across the study area. Early arrivals are observed to the east in Sweden in shield areas associated with thick lithosphere, and around the Oslo Graben (OG). Late arrivals southwest of the Sorgenfrei-Tornquist Zone in the Norwegian-Danish Basin are interpreted as being associated with thinner lithosphere related to the basin formation. Late arrivals are also observed in the southern part of Norway west of the Oslo Graben area. This indicates reduced P-wave velocity and reduced density in the upper mantle and perhaps some regional isostatic buoyancy from the upper mantle. However, a clear spatial correlation with areas of high topography is not observed. The combined information of mean P-residuals and the azimuthal dependence of single P-residuals indicate the presence of a narrow lithospheric transition around the Sorgenfrei-Tornquist Zone extending from the southeast into the northernmost part of Jutland. From here it may continue northwards and connect with the transition we observe in southern Norway in about the position of the Oslo Graben

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