Anisotropy across the Sorgenfrei-Tornquist Zone from shear wave splitting

During the TOP-1 passive seismic experiment in 1996/97, a maximum of 139 te mporary seismograph stations were operating over the Sorgenfrei-Tornquist Z one (STZ) in an area extending from northern Germany through Denmark to cen tral Sweden. One of the objectives was to study horizontal anisotropy direc tions in the subcrustal lithosphere and asthenosphere across the Trans-Euro pean Suture Zone. To achieve this goal, broad-band and intermediate-period (5 s) data of the TOP-1 stations and additional stations of permanent netwo rks (GRSN, GEOFON) were analysed for splitting of SKS and SKKS phases. As a result of the relatively dense station spacing, the method offers good lat eral resolution of anisotropy. Preliminary results suggest that the directi ons of the fast horizontal S wave velocity are affected by the STZ. In cent ral Europe and southern Sweden, far away from the STZ, fast S wave directio ns are approximately E-W while they turn more northerly closer to the STZ w here they are approximately parallel to the trend of the STZ. No significan t shear wave splitting was observed north of 57 degrees N and east of 14 de grees E. Small delay times between 0.2 and 0.5 s observed at the northernmo st TOP-1 station T40S and T60S may be controlled by anisotropy in a thicken ed crust. The mantle contribution of horizontal anisotropy within the STZ i s probably constrained to an approximately 60-km-thick zone in the depth ra nge between 70 and 300 km. The observations are consistent with a model whe re azimuthal anisotropy is not governed by present-day mantle flow in the a sthenosphere, but rath:er is frozen into the subcrustal lithosphere during the last episode of tectonic activity. (C) 1999 Elsevier Science B.V. All r ights reserved.