A major shear zone within the Nagssugtoqidian of West Greenland

A regional lineament of highly parallelized planar and linear structures can be followed over a distance of 150 km from the coast to the Inland Ice at Nordre Strømfjord in central West Greenland. This shear zone is situated within the Nagssugtoqidian mobile belt and transects an area of intricate interference structures, from which it has been formed by high shear strain. By using data for the orientation of planar structures outside and within the zone, the shear strain is calculated to be approximately 6. This value applies to the shear zone close to the coast where its width is approximately 15 km. The zone is cut by the granulite/amphibolite facies boundary, and towards lower metamorphic grade the width of the zone decreases. At the same time the shear strain increases, so that the offset across the zone could be constant irrespective of width. At the coast, where the zone has been mapped, the fanning of planar structures shows the zone to be wedge-shaped, thinning upwards.It is therefore suggested that the zone represents a deep-seated ductile part of a major, transcurrent fault with a sinistral displacement of at least 100 km.     

The Uppsala seismograph array station

Summary The Seismological Institute at Uppsala, Sweden, has operated a triangular array station for one year, August 1969 to August 1970. The triangle is almost equilateral with around 100 km side, with one corner at Uppsala and the other two at Hedemora and Kungsör. Short-period vertical-component seismographs of type Grenet-Coulomb are used. The signals are transmitted over commercial telephone lines to the central recording unit at Uppsala. This paper describes all technical details of the system, with particular emphasis on the signal transmission. Experiences from the operation are given. In spite of only three points in the array, it serves its purposes very well, primarily because of its location on very homogeneous ground, the Baltic shield. Moreover, the installation and operation of such an array is generally possible within the limited budgets of a university Institute.     

Average crustal structure of Sweden

Summary Records obtained at the permanent stations of the Swedish seismograph network from explosions carried out in Scandinavian waters in June 1969 are evaluated. The study includes determination of velocities for all crustal phases observed, furthermore of layer thicknesses, Poisson ratios and amplitude ratios. The purpose of the study is partly to provide a first approximation to the crustal structure in Sweden, partly to provide regional data for location of earthquakes and explosions in the area in the future. Average velocities (km/sec) are forPn 7.88±0.05,Pg1 6.25±0.08,Pg2 5.70,Sn 4.58±0.04,S ^* 3.70±0.04,Sg1 (Lg1) 3.58±0.03,Sg2 (Sg) 3.40±0.03,Rg 3.02±0.07. The average thickness is 12 km for the granitic layer, and 23 km for the basaltic layer, thus making the average crustal thickness equal to 35 km. Relative amplitudes plotted versus distance complete the dynamical side of the study and they are useful for identification of waves. A regional travel-time table is presented for the distance range 0°–10° with entries for each 0.1° and including all crustal phases read.     

Variation in si4+-content of phengites through a three stage deformation sequence

Three important phases of deformation (D₁-D₃) affect the Precambrian metamorphic rocks of the Strathgordon region, S.W. Tasmania. Textural analysis has related phengite development to the deformation events in quartz + phengite phyllites. Phengite chemistry suggests a prograde metamorphic history at about 4 kb from 400°C at D₁ to 450°C at D₂. The significant feature of this work is that a sharp break in conditions, shown by a marked change in the composition of the phengites, occurs from D₂ to D₃, the latter taking place at about 250°C and at a pressure <4 kb. A pronounced hiatus in the deformation history is indicated for the D₂/D₃ interval. During D₃ water was introduced.     

The caustic and other properties of SKP

The caustic of SKP is found at an epicentral distance Δ_C = 129.5° for surface foci and at Δ_C = 128.9° for foci at 400 km depth, by means of amplitude-distance graphs based upon short-period time-domain measurements. These results are essentially confirmed by long-period time-domain measurements of SKP as well as by frequency-domain studies, even though the spectra are less accurate for such determinations. The average period of SKP is $\text{T}\text{= 1.45 ± 0.45}\text{sec}$ from short-period records, significantly different from the corresponding PKP-period of 1.00 ± 0.31 sec. Likewise, the long-period averages of SKP = 10.8 ± 4.5 sec and of PKP = 7.7 ± 3.0 sec are significantly different from each other. A travel-time table of SKP1 is deduced, covering the epicentral distance range of 130–143° and the focal depth range of 100–700 km. All results are based on measurements on seismograms of the Swedish network of stations, deriving almost exclusively from earthquakes in the southwest Pacific area.     

Power spectra and geographical distribution of short-period microseisms in Sweden

Summary Microseisms recorded by short-period vertical-component seismographs in a network of permanent and temporary stations in Sweden are investigated by means of their power spectra. The geographical distribution of the microseisms is explained in terms of models involving line sources. A good fit to the observations is obtained by locating the line sources along the surrounding oceanic coasts. The Norwegian west coast is of dominant importance for medium- and long-period microseisms, whereas for the short-period ones also coasts of smaller sea basins, as the Baltic Sea and the Skagerrak, are of significance. As a consequence, it is found that short-period microseisms are largest at stations on or near oceanic coasts, and that the smallest microseisms occur at inland stations.     

An earthquake with exceptionally strong higher-mode surface waves

Summary An earthquake in the Northern Sinkiang Province, China, on November 13th, 1965, produced exceptionally strong higher-mode surface waves recorded at the Swedish seismograph stations, especially by the Galitzin seismographs at Kiruna. Group-velocity dispersion curves are deduced for the second Rayleigh and the second and third Love wave modes. The energy content of the second Rayleigh and Love modes is computed, and it is found that the former amounts to about 70 times as much as in average. Various reasons for this exceptional behaviour are discussed, such as depth extent of focus, focal mechanism and properties of propagation path.     

Review of Ray Theory Applications in Modelling and Imaging of Seismic Data

Throughout the last twenty years, 3D seismic ray modelling has developed from a research tool to a more operational tool that has gained growing interest in the petroleum industry. Various areas of application have been established and new ones are under development. Many of these applications require a modelling system with flexible, robust and efficient modelling algorithms in the core. The present paper reviews the basic elements of such a system, based on the open model concept and the wavefront construction technique. In the latter, ervený's dynamic ray tracing is an intrinsic part. The modelling system can be used for generating ray attributes and synthetic seismograms for realistic 3D surveys with tens of thousands of shots and receivers. Moreover, some other types of application areas are illustrated: Production of Green's functions for prestack depth migration and hybrid modelling (combined ray and finite-difference modelling), attribute mapping and illumination analysis, both for survey planning and interpretation. Finally, the concepts of isochron rays and velocity rays related to seismic isochrons have been introduced recently, with very interesting future applications.     

Extending the limits of the Borrobol Tephra to Scandinavia and detection of new early Holocene tephras

Analyses of two infilled lakes in Blekinge, southeast Sweden, indicate the presence of at least three tephra horizons of Termination 1 and early Holocene age. Geochemical analyses confirm the presence of the Borrobol Tephra, the Askja Tephra (10,000 ¹⁴C yr B.P.), and one previously unreported tephra of Icelandic origin. Extending the limits of the Borrobol Tephra to Scandinavia illustrates that this ash is far more widespread than previously realized and is therefore, an important marker horizon for determining the rate and timing of the initial warming at the start of Greenland Interstade 1 (GI-1) within Europe. The relatively unknown Askja Tephra and the newly discovered Hässeldalen Tephra are stratigraphically placed at the Younger Dryas/Preboreal transition. This paper demonstrates the suitability and success associated with the extraction techniques for tracing microtephra horizons in areas distal to volcanic sources.