Within Project Tor. which is about Teleseismic Tomography across the Tomquist Zone in Germany-Denmark-Sweden, we have confirmed very significant deep lithosphere differences And modeling is substantiated via completely independent methods. In 1996-1997 our 130 seismographs constituted the largest seismic antenna ever in Europe. The Tor area was chosen along a well studied crustal profile of an earlier project, and the modeling efforts were concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km The Tor data have been subjected to P-wave travel time tomography. surface wave and receiver function analysis as well as anisotropy and scattering measurements An important goal of the project was to make several independent inversions of the tomography data. and compare the results in an attempt to evaluate uniqueness, resolution and accuracy of these inversions. The comparisons of this paper involve more diversity in methods than any previous comparison. The geological outcome is a substantiation of earlier statements that, "The transition is interpreted to be sharp and steep in two places It goes all through the lithosphere at the northern rim of the Tornquist Zone near the border between Sweden and Denmark, and here the lithosphere difference is large to depths more than 200 km. The other lithosphere difference. of smaller scale, is found near the southern edge of the Ringkobing-Fyn High near the border between Denmark and Germany Also this transition is sharp and steep. and goes all through the lithosphere to depths around 120 km. These two sharp transitions divide the Tor region into 3 different lithosphere structures distinguishable in P-wave travel time tomography. surface wave dispersion. P- and S-wave anisotropy and partly in P-wave scattering" The mentioned broad-scale features are judged to be unambiguously determined, with well-described resolution and accuracy Unfortunately a detail like the slope of the subcrustal lithosphere transition right under the Tronquist Zone cannot be constrained even if this is where the resolution is best. and the curiosity largest. (c) 2009 Elsevier B V All rights reserved.

The main aim of the TOR project is to study the lithospheric–asthenospheric boundary structure under the Sorgenfrei–Tornquist Zone, across northern Germany, Denmark and southern Sweden. Relative arrival-time residuals of teleseismic P and S phases from 51 earthquakes, recorded by 150 seismic stations along the TOR array, were used to delineate the transition zone in the studied area. The effects of crustal structures were investigated by correcting the teleseismic residuals for travel-time variations in the crust based on a 3D crustal model derived from other data. The inversion was carried out for S phases. The results were then compared with the corresponding P-wave models. As expected, the derived models show that the relatively old and cold Baltic Shield has higher velocity at depth than the younger lithosphere farther South. The models show two sharp and distinct increases in depth to velocities which are low compared to our reference model, as we move from South to North. The location and sharpness of these boundaries suggests that the features resolved are, at least partially, compositional in origin, presumably related to mantle depletion. A sharp and steep subcrustal boundary is found roughly coincident with the southern edge of Sweden. This is below where the edge of the Baltic Shield is usually placed, based on surface geological evidence (the Sorgenfrei–Tornquist Zone). Another less significant transition is recognised more or less beneath the Elbe-lineament. Relatively high d(Vp/Vs) ratios under the central part of the profile (Denmark) indicate relatively low S-velocity in an area where a gravity high supports the hypothesis of extensive mafic intrusions.

From July 1996 to August 1997 the TOR project operated 130 seismographs in North Germany, Denmark and South Sweden, with the aim of collecting signals from local, regional and teleseismic earthquakes. This data set is particularly interesting since the seismic antenna crosses the most significant geological boundary in Europe, the Tornquist Zone, which in the northern part is the border between the Baltic Shield and the younger European lithosphere. Previous studies have shown significant physical changes in the crust and upper mantle across this transition zone, including two independent teleseismic tomographic studies of the TOR data set. But these two studies disagree on the orientation of the slope of the transition. Both studies used an iterative linearized inversion method. We will in this work Preprint submitted to Elsevier Science 27 July 2005 present an inversion based on Bayesian statistics, where the solution space is examined in order to study a very large number of tomographic solutions and to examine the solution uniqueness and uncertainty. The method is applied to measurements of 3345 relative teleseismic P-phase travel times from 48 teleseismic earthquakes with good azimuthal coverage with respect to the great circle arc of the TOR array. We find the lithospheric transition to be a north east inclination of around 30° to 45° off vertical.

Two passive seismic experiments have been carried out across the Trans European Suture Zone (TESZ) from northern Germany to southern Sweden (TOR) and across the Proterozoic-Archaean suture in Finland (SVEKALAPKO) to improve our understanding of the processes involved in the creation of the European continent. Teleseismic earthquakes recorded by the two networks and stations of the GRSN and GEOFON permanent networks have been used for studies of the crust-mantle, and upper mantle seismic discontinuities with the receiver function method. Along the TOR network the depth to the Moho increases from 30 km at the southern edge of the profile to 40 km at the Elbe Line. Between the Elbe Line and TESZ the Moho branches off and whereas the deeper branch continues at 40 km depth to the TESZ a second branch appears at 30-35 km depth. The upper branch descends north of the TESZ to below 55 km under the northern end of the TOR profile. The crustal thickening north of the TESZ is accompanied by an increase in average Vp/Vs values, appearance of intracrustal conversion zones and north dipping features which we interpret as remnants of the subduction and subsequent collision between Avalonia and Baltica. In southern Finland beneath the SVEKALAPKO network the Moho starts in the south at the depth of 40-45 km, plunges to about 65 km depth south of the Archaean-Proterozoic suture. This deepening of the Moho is coincident with a north dipping intracrustal structure apparently related to the subduction and collision and of the Proterozoic and Archaean provinces in Proterozoic. North of the line of the suture the Moho rises smoothly to 45-50 km depth in the Archaean province. Along the TOR profile, 410 and 660 discontinuities were hard to detect. However, manyfold stacking of receiver functions revealed that the conversions from the two discontinuities arrive more or less according to IASP91 predicted time. Across the SVEKALAPKO network 410 and 660 discontinuities arrive markedly earlier than IASP91 theoretical arrival times. In particular north of the Archaean-Proterozoic suture in Finland the 410 and 660 km conversions arrive about 2 s earlier, indicating about 5 per cent higher average upper mantle velocities and lower temperatures than what IASP91 global model predicts.

The TOR project investigates the lithosphere-asthenosphere structure under the Tornquist Zone between Denmark and Sweden. Around 150 seismic stations (108 short-period, 28 broadband and 14 other permanent stations) were employed in a rectangular array along a 900 km long by 100 km wide strip across the Tornquist Zone from July 1996 to August 1997. The results obtained based on a non-linear teleseismic tomography algorithm reveal significant P-velocity variations (up to 5 per cent) along the TOR array from northern Germany to southern Sweden. Distinct lithospheric blocks are also recognized in the inversion results. Two inverse methods (singular value decomposition and a quadratic programming method) were implemented in order to investigate whether or not the lithospheric blocks and major boundaries in the inversion are required by the data or are artefacts of the inversion. According to the results, the lithosphere under northern Germany is thin but reaches to intermediate thicknesses of about 120 km in the Tornquist Zone area. Farther north in the Baltic Shield, more than 200 km of continental lithosphere is recognized in the model. Abrupt lateral P-velocity changes (maximum 5 per cent) are seen at both the southern and northern sides of the Tornquist Zone.

This work is a part of the TOR1 project (1996–1997) and is devoted to determining the lithospheric structure across the Sorgenfrei–Tornquist Zone in Northern Europe. For the first time in Europe, a very dense seismic broadband array has offered the possibility of determining very sharp lateral variations in the structure of the lithosphere at small scales using surface wave analysis. We measure phase velocities for Rayleigh waves with periods ranging between 10 and 100 s, both within arrays with apertures of 40–50 km (small compared to the wavelength), and along long profiles of at least 100 km. Dispersion curves are then inverted and shear-wave velocity models down to the depth of 200 km are proposed. We show that the Sorgenfrei–Tornquist Zone is a major tectonic feature within the whole lithosphere. North–east of this feature, in Sweden beneath the Baltic Shield, no lithosphere–asthenosphere boundary is observed to exist to depths of 200 km. South–west of the Sorgenfrei–Tornquist Zone, beneath Denmark, we find a lithospheric thickness of 120±20 km. The transition across the Sorgenfrei–Tornquist Zone is sharp and determined to be very steeply dipping to the south–west. We also demonstrate the existence of a sharp discontinuity between the lithospheres beneath Denmark (120±20 km thick) and beneath Germany (characterized by thicknesses of 50±10 km in the northernmost part and 100±20 km in the southwest). This discontinuity is most likely related to the Trans-European Fault at the surface.

Project Tor (Teleseismic Tomography across the Tornquist Zone in Germany–Denmark–Sweden) is now producing results. We are able to detect very significant deep lithosphere differences, and we can now discuss the sharpness laterally. In 1996–1997, our 120 seismographs constituted the largest seismic antenna ever in Europe. The Tor area was chosen along a well-studied crustal profile of an earlier project, and the inversion efforts are concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km. The Tor investigation can be called two-and-a-half-dimensional, as it has a 900-km profile length with 100 km width plus a few seismographs off the profile. The Tor data have been subjected to P-wave travel time tomography, surface wave and receiver function analysis as well as anisotropy and scattering measurements. Through ray tracing in a compiled crustal model and subtraction of the modelled travel time anomalies, the influence of the lower lithosphere/asthenosphere on the seismic rays from distant earthquakes is being established. Travel time tomography results confirm very large lateral lithosphere differences of 4–6% in P-wave velocity. For several events of the large data base, it is demonstrated that the observed P-wave travel time anomalies of 1–2 s can be divided almost equally between known crustal effects and lower lithosphere/asthenosphere differences, which then must account for about 1 s of the travel time differences. The transition is interpreted to be sharp and steep in two places. It goes all the way through the lithosphere at the northern rim of the Tornquist Zone near the border between Sweden and Denmark, and here the lithosphere difference is large. A smaller lithosphere difference is found near the southern edge of the Ringkøbing-Fyn High just north of the border between Denmark and Germany. Also, this transition is sharp and steep, and goes all through the lithosphere. These two sharp transitions divide the Tor region into three different lithosphere structures distinguishable in P-wave travel time tomography, surface wave dispersion, P- and S-wave anisotropy, and partly in P-wave scattering.

A passive teleseismic experiment (TOR), traversing the northern part of the Trans-European Suture Zone (TESZ) in Germany, Denmark and Sweden, recorded data for tomography of the upper mantle with a lateral resolution of few tens of kilometers as well as for a detailed study of seismic anisotropy. A joint inversion of teleseismic P-residual spheres and shear-wave splitting parameters allows us to retrieve the 3D orientation of dipping anisotropic structures in different domains of the sub-crustal lithosphere. We distinguish three major domains of different large-scale fabric divided by first-order sutures cutting the whole lithosphere thickness. The Baltic Shield north of the Sorgenfrei–Tornquist Zone (STZ) is characterised by lithosphere thickness around 175 km and the anisotropy is modelled by olivine aggregate of hexagonal symmetry with the high-velocity (ac) foliation plane striking NW–SE and dipping to NE. Southward of the STZ, beneath the Norwegian–Danish Basin, the lithosphere thins abruptly to about 75 km. In this domain, between the STZ and the so-called Caledonian Deformation Front (CDF), the anisotropic structures strike NE–SW and the high-velocity (ac) foliation dips to NW. To the south of the CDF, beneath northern Germany, we observe a heterogeneous lithosphere with variable thickness and anisotropic structures with high velocity dipping predominantly to SW. Most of the anisotropy observed at TOR stations can be explained by a preferred olivine orientation frozen in the sub-crustal lithosphere. Beneath northern Germany, a part of the shear-wave splitting is probably caused by a present-day flow in the asthenosphere.

The TOR experiment (Teleseismic TOmography TORnquist) carried out in winter 1996/97 across the Trans-European Suture Zone (TESZ) in Germany, Denmark and Sweden has collected new data to investigate the transition zone between Precambrian and Palaeozoic Europe. In this study, seismograms of teleseismic earthquakes recorded by the broad-band TOR stations have been used to calculate the receiver functions. The time-domain inversion method has been applied to the receiver functions to compute S -wave velocities in the crust and uppermost mantle beneath each station. The results of inversion down to 60 km depth provide new, independent information about the distribution of S -wave velocity in this area. Beneath the Swedish stations on Baltica, the thickness of the crust varies from about 45 to 50 km with mostly gradually increasing S -wave velocity and no sharp discontinuities while for Danish and German stations the crust is thinner (29-38 km) with a sharp Moho discontinuity. A very distinct S -wave low velocity layer was found at depth of 8-16 km in the upper crust of Baltica, supported by the results of refraction/deep seismic sounding experiments using both P and S waves. The map of V-p /V-s ratio beneath the c . 1000 km long TOR 'profile' was obtained using the V-p velocity model from previous investigations. The values of V-p /V-s = 1.73 were found in the uppermost crust of Baltica and upper Avalonian crust. The low velocity layer in the upper crust of Baltica is characterized by high value of V-p /V-s = 1.85. Relatively low S -wave velocities are observed in the lower crust of Variscides (V (p) /V (s) = 1.79), Baltica (V-p /V-s = 1.83) and Avalonia (V-p /V-s = 1.91); in the uppermost mantle V-p /V-s values are 1.77, 1.79 and 1.82, respectively.

Data from the passive teleseismic experiment TOR across the Trans-European Suture Zone (TESZ), which took place in 1996-1997, are used to study the random structure of the lithosphere NE of the TESZ (the Baltic Shield beneath southern Sweden) and beneath the north German/Danish Basin. P coda waves from deep teleseismic events give information on the small-scale heterogeneity of the near-receiver lithosphere on scales of one wavelength. Using an energy flux model, the time and frequency behaviour of the P coda has been interpreted in terms of scattering (Q(s)) and anelastic attenuation (Q(i)). Estimates of Q(s) were obtained for the frequency range 0.5-3 Hz for the north German/Danish Basin and 0.5-7 Hz for the Baltic Shield. The two areas show a different scattering behaviour. In Denmark/northern Germany the scattering (Q(s) approximate to 125-200) is stronger than in southern Sweden (Q(s) approximate to 275-500) and the peak of Q(s)(- 1) occurs at higher frequencies NE of the TESZ. Anelastic attenuation is negligible for the Baltic Shield whereas for the north German/Danish Basin anelastic attenuation is present but still much weaker than scattering attenuation.With a modified energy flux model for depth-dependent scattering, and assuming an exponential autocorrelation function, the depth variation of correlation length a and rms velocity fluctuation epsilon can in principle be resolved. For the Baltic Shield the scattering is mostly confined to the crust with a approximate to 1 km and epsilon approximate to 4 per cent. The subcrustal lithosphere only shows weak fluctuations. The data from the north German/Danish Basin cannot be explained by strong scattering within the crust only. Crustal parameters are a = 5-10 km and epsilon approximate to 8 per cent. For the subcrustal lithosphere, correlation lengths of 10-20 km and rms velocity fluctuations of 6-8 per cent are found. Additional tests confirmed that the thick sedimentary cover in this area has no significant effect on these results for the deeper structure. Correlation lengths for the sediments are smaller than 5 km and rms velocity fluctuations epsilon are 7-8 per cent.

An international, interdisciplinary project, which 2 years ago deployed the largest dense seismic antenna ever in Europe, expects in the next 2 years to present important findings on the lithosphere and asthenosphere of a portion of the Trans‐European Suture Zone (TESZ). Final processing is currently under way of the data from the array of 120 seismographs along a 900‐km‐long by 100‐km‐wide strip from Gottingen, Germany, in the south, through Denmark, to Stockholm, Sweden in the north, across the northwestern part of the TESZ (Figure 1).Project Tor is a teleseismic tomography experiment with interdisciplinary data exploitation. It extends across the broad TESZ boundary between two markedly different lithospheric domains.These are (1) Proterozoic Europe, with Precambrian crust in Sweden and eastern Europe, and (2) Phanerozoic central Europe, with most of the crust influenced by the Caledonian and Variscan orogenies and only small areas of relic Precambrian crust. The project is designed to investigate the deep lithosphere traces of the broad‐scale geology of the TESZ area, including the Tornquist Zone, from which Project Tor has its name. It is part of EUROPROBE, a major Earth science program of the European Science Foundation, which is run by a regional committee of the International Lithosphere Program.

The Tor project makes use of teleseismic tomography across the Sorgenfrei-Tornquist 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.

The temporary seismic station array (TOR) was designed to study the lithosphere-asthenosphere system across the northwestern part of the Trans-European Suture Zone (TESZ) by teleseismic tomography. Teleseismic wavefronts, when propagating through complex crustal structure, undergo severe distortion that may result in travel time residual anomalies of significant amplitude. The inversion of teleseismic travel time residuals for deep structures without accounting for such crustal-related anomalies may erroneously map these travel time anomalies into features at greater depth. In this study we apply a three-dimensional (3-D) technique to estimate effects of a priori known 3-D crustal structure on travel times of teleseismic waves observed at the TOR seismic array across the TESZ to correct for these effects in future tomographic studies.A uniform 3-D crustal model is developed by use of published two-dimensional crustal models from previous active seismic surveys. The parameterization of this 3-D crustal model is designed to adequately represent those crustal structures that mostly influence the propagation of teleseismic wavefronts. The 3-D model includes lateral variation in velocity structure, Moho topography, and large and deep sedimentary basins. The teleseismic forward problem for this local 3-D model is solved by calculation of travel times to the base of the model using a standard whole Earth model and by subsequent propagation of spherical wavefronts using finite difference methods. Travel time calculations for an event near Japan reveal significant lateral variations in the range between -0.3 s and + 0.5 s due to crustal structures. Being able to obtain the full travel time held at the surface of the model has the additional advantage of improving the identification and timing of seismic phases observed at the TOR seismic array. (C) 1999 Elsevier Science B.V. All rights reserved.