Søren Gregersen

Depth to Moho in Greenland: receiver-function analysis suggests two Proterozoic blocks in Greenland

Abstract The GLATIS project (Greenland Lithosphere Analysed Teleseismically on the Ice Sheet) with collaborators has operated a total of 16 temporary broadband seismographs for periods from 3 months to 2 years distributed over much of Greenland from late 1999 to the present. The very first results are presented in this paper, where receiver-function analysis has been used to map the depth to Moho in a large region where crustal thicknesses were previously completely unknown. The results suggest that the Proterozoic part of central Greenland consists of two distinct blocks with different depths to Moho. North of the Archean core in southern Greenland is a zone of very thick Proterozoic crust with an average depth to Moho close to 48 km. Further to the north the Proterozoic crust thins to 37–42 km. We suggest that the boundary between thick and thin crust forms the boundary between the geologically defined Nagssugtoqidian and Rinkian mobile belts, which thus can be viewed as two blocks, based on the large difference in depth to Moho (over 6 km). Depth to Moho on the Archean crust is around 40 km. Four of the stations are placed in the interior of Greenland on the ice sheet, where we find the data quality excellent, but receiver-function analyses are complicated by strong converted phases generated at the base of the ice sheet, which in some places is more than 3 km thick.

Seismic investigations of the Nansen Ridge during the FRAM I experiment

Abstract As part of the FRAM I ice station experiment in the Arctic Ocean during the period March 11-May 15, 1979, a sonobuoy array was kept in operation for about 6 weeks. During the last two days of the experiment the array drifted over the crestal mountains of the western flank of the Nansen Ridge. Here microearthquakes were recorded at a rate of 1–2 events per hour, with epicentral distances from 15 to 25 km. Two parallel epicenter lineations are found, a main one that coincides with the median valley and another less active one that follows the northwestern rift-valley wall. This activity may be related to the uplift of a fault block from the median valley onto the rift valley wall. A crustal model for the crestal mountain area was obtained from two unreversed refraction profiles, giving preliminary compressional velocities of 4.6 km/s at the sea floor, a 6.1 km/s refractor at 1.5 km depth, and a 7.9 km/s refractor at 6 km depth, while in the median valley a 7.2 km/s refractor is found at a depth of 3 km.

Important findings expected from Europe's largest seismic array

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.

Fennoscandian dynamics: Present-day earthquake activity

Abstract The Fennoscandian earthquake catalogue is constantly updated and revised, and we present the best available map of earthquake activity. It exhibits significant differences between areas of high and low seismicity. The focal mechanisms have been determined for many of the earthquakes. The total set of these focal mechanisms shows that the stress pattern is very consistent. The horizontal deviatoric stresses are characterized by a grouping in the NW-SE directions for the maximum compression. This stress field is not distorted even by the significant crustal inhomogeneity in the Fennoscandian Border Zone. This orientation of the maximum horizontal compression is precisely that expected from ridge push forces from the North Atlantic Ridge. It is not consistent with the stress field expected from postglacial uplift after the last Ice Age. However, recently, large neotectonic faults have been identified and studied in northern Scandinavia, and these are taken as indicators of large-scale displacements shortly after the end of the last Ice Age. Seen in parallel with similar studies in Scotland, this shows that the stress field has changed drastically with time. The present-day uniform stress field and the relatively scattered distribution of observed earthquakes lead to the conclusion that the earthquakes concentrate in specific zones of brittle, weak crust. We claim that arguments based on tectonics must be dependent on the specific tectonic characteristics of each area separately, and this remains the outstanding problem of Fennoscandian earthquake activity as observed presently.

Seismological evidence for a very sharp Sorgenfrei-Tornquist Zone in southern Sweden

A passive seismological experiment across the northern boundary of the Tornquist Fan was carried out from December 1994 until April 1995 along a 100 km long profile in Denmark and Sweden. Observed PS converted seismic waves indicate an abrupt increase in Moho depth of 4–5 km within 5 km horizontal distance at about 50 km to the northeast of the Oresund. This is a much sharper jump than that inferred from the wide-angle data of EUGENO-S line 1. Preliminary analysis of seismic shear wave anisotropy indicates asthenospheric mantle flow parallel to the Sorgenfrei-Tornquist Zone. These parallel directions are already observed between the Elbe Line and the Sorgenfrei-Tornquist Zone while further south in central Europe the flow directions follow the trend of the Alpine belt.

Stress change over short geological time: the case of Scandinavia over 9000 years since the Ice Age

Abstract Palaeoseismological investigations are used in many regions of the world to extend back in time the earthquake statistics of historical written or oral records as well as instrumental information. This is very valuable for discussions of earthquake hazard, but it only applies to areas of stable stress regime. Although the intraplate areas of Scandinavia and Greenland have experienced only rather small earthquakes within the human timescale, they serve as a clear warning on the application of palaeoseismology for hazard studies in regions where the stresses have changed. In a small part of Scandinavia, where recent earthquake activity is not significantly different from that of its surroundings, large faults have been discovered and several have been investigated via palaeoseismology. They are interpreted to show the occurrence of large earthquakes about 9000 years ago. Signs of this are coincident landslides as well as liquefaction in loose sediments, which are well dated through varve-counting. In contrast to this the present-day stress release in earthquakes and in surface rock deformations is mainly caused by plate motion. Regional investigations in Scandinavia and Greenland/North America, as well as those included in the World Stress Map Project of the 1990s, have shown compression within the plate, mainly in the direction of absolute plate motion. The ice cap influence has disappeared. So stress reorganization is clearly indicated over the short geological timespan of 9000 years. Into this argument goes the observation from Greenland and Antarctica, that no earthquakes occur under the ice caps. For Scandinavia the argument is that no earthquakes occurred under the ice sheet during the Ice Age, and that the stored stresses were released when the ice sheet melted 9000 years ago. This does emphasize a warning. There are regions of the globe where palaeoseismological investigations can give a fantastic extension of the short-term historical earthquake records. But in some regions stress reorganization has changed this condition.

Crustal structure across the tornquist zone (southwestern edge of the Baltic Shield): A review of the Eugeno-S geophysical results

Abstract A seismic refraction and wide-angle reflection experiment was carried out in the summer of 1984 in southern Sweden, Denmark and northernmost Germany. The EUGENO-S project (European GEotraverse Northern segment—Southern part) was part of the European Geotraverse. Observations were made along five profiles reaching a total length of 2100 km. In addition to 51 explosions, an airgun array was operated along the offshore parts of the profiles, giving signals to distances greater than 200 km. Three profiles cross the southwestern edge of the Baltic Shield, and two run along the edge, one on each side. The results show that in this area the Moho discontinuity is a sharp boundary. The crustal thicknesses vary significantly, between 26 km and 47 km. In some places the transition zones are narrow, in others they are broad. The variations are mostly in the upper crystalline layer and in the sediments. The sedimentary thicknesses are between 0 and 10 km. The general relationship is such that thick sediments correlate with thin upper crystalline crust and shallow Moho depths. The thickness of the lower crust (approximately 20 km) is quite uniform throughout the region of the edge of the Baltic Shield. A well-known gravity anomaly in Denmark around Silkeborg is explained by high-density and high-velocity material anomalously close to the bottom of the sediments. A number of regional lineaments known from geological studies and reflection profiles can be followed through the crust into the upper mantle. Some pieces of shield crust have become detached, and are now found in the Ringkobing-Fyn basement high in the middle of Denmark.

Crustal inhomogeneities and seismicity near the margins of the North Atlantic Ocean

Abstract Some very pronounced crustal inhomogeneities have been found in the North Sea and in Greenland, in regions which were adjacent to each other before the opening of the North Atlantic Ocean by sea-floor spreading. Some of these regions of inhomogeneities are zones of relatively high seismicity, while others are aseismic. In the North Sea, the Viking Graben has earthquakes, while its southern continuation, the Central Graben, is almost aseismic. In eastern Greenland, there are only few earthquakes near the Mesozoic graben, while there is a prominent earthquake zone at the coast line in north-eastern Greenland. It is concluded that, superposed on the apparent similarities in the two regions, some differences in stress conditions or crustal weakness patterns must exist.