ALBERT

Description: The increasingly dense coverage of Europe with broad-band seismic stations makes it possible to image its lithospheric structure in great detail, provided that structural information can be extracted effectively from the very large volumes of data. We develop an automated technique for the measurement of interstation phase velocities of (earthquake-excited) fundamental-mode surface waves in very broad period ranges. We then apply the technique to all available broad-band data from permanent and temporary networks across Europe. In a new implementation of the classical two-station method, Rayleigh and Love dispersion curves are determined by cross-correlation of seismograms from a pair of stations. An elaborate filtering and windowing scheme is employed to enhance the target signal and makes possible a significantly broader frequency band of the measurements, compared to previous implementations of the method. The selection of acceptable phase-velocity measurements for each event is performed in the frequency domain, based on a number of fine-tuned quality criteria including a smoothness requirement. Between 5 and 3000 single-event dispersion measurements are averaged per interstation path in order to obtain robust, broad-band dispersion curves with error estimates. In total, around 63,000 Rayleigh- and 27,500 Love-wave dispersion curves between 10 and 350 s have been determined, with standard deviations lower than 2 per cent and standard errors lower than 0.5 per cent. Comparisons of phase-velocity measurements using events at opposite backazimuths and the examination of the variance of the phase-velocity curves are parts of the quality control. With the automated procedure, large data sets can be consistently and repeatedly measured using varying selection parameters. Comparison of average interstation dispersion curves obtained with different degrees of smoothness shows that rough perturbations do not systematically bias the average dispersion measurement. They can, therefore, be treated as random but they do need to be removed in order to reduce random errors of the measurements. Using our large new data set, we construct phase-velocity maps for central and northern Europe. According to checkerboard tests, the lateral resolution in central Europe is ≤150 km. Comparison of regional surface-wave tomography with independent data on sediment thickness in North-German Basin and Polish Trough confirms the high-resolution potential of our phase-velocity measurements. At longer periods, the structure of the lithosphere and asthenosphere around the Trans-European Suture Zone (TESZ) is seen clearly. The region of the Tornquist-Teisseyre-Zone in the southeast is associated with a stronger lateral contrast in lithospheric thickness, across the TESZ compared to the region across the Sorgenfrei-Tornquist-Zone in the northwest.

Description: The postglacial uplift/subsidence in Scandinavia is regular. And the phenomenon is similar in time scales of tens, hundreds and thousands of years studied via geodesy, seismology and geology. Searches for irregularities in the form of earthquakes claimed in the scientific literature have disclosed many earthquakes right after the Ice Age and some later cases for further evaluation. In a previous report the present author has mentioned doubts about the validity of some of the most significant claimed irregularities. In the present paper a review is made of these significant claimed irregularities in the southwestern flank of the Scandinavian postglacial uplift/subsidence via literature studies of geodetic and geological claims of earthquakes as well as discussions in the field. Geodetic observations exist for all of Scandinavia. Those describe the phenomenon in 10s–100s of years scale. Earthquake observations in seismology are of relevance in the same time scales. Geological studies of dated shore lines describe the postglacial vertical earth-surface motion in a quite different time scale of 100s–1000s of years. There is a need for integration of these observations geographically. This is happening in the various time scales in the DynaQlim project. The review finds the claims improbable about the following: (1) geodynamical motion in the Copenhagen area, (2) a paleo-earthquake in Læsø and (3) the recently proposed water level discrepancy in the southern part of Denmark. The assessment is less certain, but falls to improbable concerning (4) proposed paleo-earthquakes by Hallandsåsen in southwestern Sweden.

Description: The postglacial uplift and surrounding subsidence in Scandinavia is well described as close to regular, and the phenomenon is similar on timescales of tens, hundreds and thousands of years studied via geodesy, seismology and geology. Searches for irregularities in the form of earthquakes claimed in the scientific literature have disclosed many earthquakes right after the Ice Age, 9000 yr ago, and some later cases for further evaluation. In a previous report, the present authors have mentioned doubts about the validity of some of the most significant claimed irregularities. In the present paper, a review is made of these significant claimed irregularities in the south-western flank of the Scandinavian postglacial uplift and subsidence via literature studies of geodetic and geological claims of earthquakes as well as discussions in the field. Geodetic observations exist for all of Scandinavia that describe the phenomenon on a scale of 10s–100s of years. Earthquake observations in seismology are of relevance in the same timescales. Geological studies of dated shore lines describe the postglacial vertical earth-surface motion in a quite different timescale of 100s–1000s of years. There is a need for integration of these observations geographically. This is happening in the various timescales in the DynaQlim project. The review finds the claims unlikely to be earthquakes concerning the following: (1) geodynamical motion in the Copenhagen area, (2) a palaeo-earthquake in Læsø and (3) the recently proposed water level discrepancy in the southern part of Denmark. The assessment is less certain, but falls to improbable when concerning (4) proposed palaeo-earthquakes by Hallandsåsen in south-western Sweden.

Description: Within Project Tor, which is about Teleseismic Tomography across the Tornquist 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 Ringkøbing-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 Tornquist Zone cannot be constrained even if this is where the resolution is best, and the curiosity largest.

Description: Structure and dynamics of the upper mantle is important in understanding timing and mechanisms shaping prensent day topography and near surface geology. Debate persists regarding the geological age of the Scandes Mountains. We contribute by imaging upper mantle structure beneath southern Scandinavia using teleseismic P-wave travel time tomography (P-tomography). We include data from mobile stations deployed in projects CALAS, CENMOVE, MAGNUS, SCANLIPS and Tor. Permanent stations included are those available from the University of Uppsala, NORSAR and GEUS. P-wave arrival times generally show differences of up to 1 second across the study area. Upper mantle velocities are relatively high in southern Sweden and southern Norway east of the Oslo Graben. Lower velocities are observed in the Norwegian-Danish Basin southwest of the Sorgenfrei-Tornquist Zone(STZ) and in the southwestern part of Norway. We interpret the southwestern boundary of thick Baltic Shield lithosphere where we observe the highest horizontal P-wave velocity gradient. Thus, we find the boundary of thick lithosphere to more or less coinside with the STZ in the southeastern part of the study area, extending from southern Sweden into the northern part of Jutland. From here it turns north, passing through the Oslo Graben area to about 60_N then turning northwest, approaching the Norwegian west coast around 65_N. Thus, as compared to Baltic Shield, upper mantle velocity is significantly reduced beneath deep sedimentary basins of Denmark and northern Germany.