The Tor project makes use of teleseismic tomography across the SorgenfreiTornquist Zone and has now revealed significant variations in the deep lithosphere under northern Germany, Denmark and southern Sweden. Here we present the first interpretations of P-wave traveltime anomalies from the Tor project. The project utilised 120 seismographs placed in a rectangular array, the largest seismic antenna so far used in Europe, for half a year in the period 1996-1997. The present investigation establishes a 3D crustal/upper mantle model of the P-wave velocity based on existing data. A picture of the crustal influence on the seismic P-wave rays is established by ray tracing through the model. When this is subtracted from that observed by the Tor array, a picture of the influence of the lower lithosphere/asthenosphere system emerges. For several earthquakes it is shown that the observed P-wave traveltime anomalies of nearly 2 seconds can be divided almost equally between known crustal effects and lower lithosphere/ asthenosphere differences. The transition appears gradual from most directions but for rays coming from the north-east direction the transition appears sharper. This means that the broad scale deep lithosphere transition is gradual with the sharpest discontinuity plane dipping down steeply in a north-easterly direction from the Sorgenfrei-Tornquist Zone. Based on existing knowledge of the area we conclude that the transition from thin to thick lithosphere occurs within a short distance, and that the lithosphere/asthenosphere boundary dips steeply down from the surface expression of the Sorgenfrei-Tornquist Zone.
- Programområde 3: Energiressourcer