Eystein S. Husebye

Seismic image of the hypothesized Icelandic hot spot

Abstract P-wave travel-time data from a total of 61 events as recorded by the Icelandic seismograph network of altogether 39 stations have been subjected to 3-D inversion analysis. Major results obtained are as follows: for level 1 (depth range 0–75 km) a dominant low-velocity zone is nearly coincident with the Iceland rift zone with extremal values for the active Krafla volcanic area. Velocity highs are found both to the northwest and southeast. In level 2 (depth range 75–175 km) the low-velocity zone is shifted southeastward as compared to level 1 and with the strongest anomalies in the Surtsey-Hekla area. At greater depths, level 3 covering the depth range of 175–275 km, velocity low is found south of the Kolbeinsey Ridge and the central part of the Iceland rift zone is still negative. In level 4 (depth range 275–375 km) the seismic structural image is dominated by a pronounced velocity low (up to 4%) beneath central Iceland. Vertical cross-sections of the above four level anomaly patterns are indicative of a hot spot or mantle plume whose depth extent beneath central Iceland amounts to at least 375 km. The magma ascent rate was estimated to approximately 1.7 m/yr on the basis of Stokes law and assuming material viscosity typical of the asthenosphere. In parallel to the hot spot hypothesis elaborations, we may interpret the southeastern extension at the rift zone as due to channeling effects of the hot spot ascending magma “surplus”.

The seismicity of the norwegian and Greenland Seas and adjacent continental shelf areas

Abstract The seismic activity of the Norwegian and Greenland Seas and adjacent areas has been examined in view of the tectonic evolution of the North Atlantic. The 529 earthquakes used covered the period 1955–1972, and for fifteen of these events fault-plane solutions were available. An analysis was made of the location precision which turned out to be better than 20 km in most cases. Expectedly, little new evidence was obtained at the midoceanic ridges and major fracture zones, with possible exceptions of the Knipovich Ridge showing a well-defined seismicity belt supporting the idea of an active spreading ridge, and the Spitsbergen Fracture Zone, which seems to be a system of en-echelon faults. Most interesting is a weak linear event pattern in the Lofoten Basin, possibly giving evidence of unknown structures parallel to the Greenland and Senja Fracture Zones, although sediment loading also may be important. Earthquakes along the shelf edge off Norway are located at or near isostatic gravity belts which may act as hinge lines for the marginal subsidence, thus implying stress release caused by differential subsidence of the continental crust. Part of the seismicity of eastern Greenland and western Norway appears to be related to zones of weakness of pre-Cenozoic age. The seismic activity along the edges of the Norwegian Channel is very limited.

Identification of Seismic Sources — Earthquake or Underground Explosion

Seismic Source Identification: A Review of Past and Present Research Efforts.- Earthquake Source Modelling.- The Nature of the Earthquake Source.- Dynamics of Seismic Sources.- Explosion Source Modelling.- Inelastic Processes in Seismic Wave Generation by Underground Explosions.- Anomalous Rayleigh Waves from Presumed Explosions in East Kazakh.- P Wave Coupling of Underground Explosions in Various Geologic Media.- Seismic Source Parameter Estimation.- Seismic Moment Tensors.- Determination of Source Mechanism and Hypocentral Coordinates from Waveform Data.- The Effect of Greens Functions on the Determination of Source Mechanisms by the Linear Inversion of Seismograms.- Simplified Bodywave Source Terms with One Application in Moment Tensor Recovery.- The Interpretation of Moment Tensor Inversions.- Seismic Wavefield Synthesis.- Coupling Near Source Phenomena into Surface Wave Generation.- On Seismic Synthesis.- Some Recent Extensions of the Reflectivity Method.- Isochronal Formulation of Seismic Diffraction.- High Frequency Toroidal Modes.- Elastic and Electromagnetic Wave Propagation in Horizontally Layered Media.- Calculation of Wave Fields in Mantle Velocity Models.- The Effect of Focal Depth and Source Type on Synthetic Seismograms.- Seismic Wave Analysis.- Surface Wave Propagation Across Different Tectonic Regions.- Lg Wave Propagation in Eurasia.- Seismograms of Explosions at Regional Distances in the Western United States: Observations and Reflectivity Method Modeling.- Phase Identification and Event Location at Regional Distance Using Small-Aperture Array Data.- P and S-Velocity Jump at the Inner-Core Boundary from PKP Amplitudes.- Conversion Phases from Mantle Transition Zones.- The Excitation and Attenuation of Seismic Crustal Phases in Turkey.- Comparison of Waveform Inversion Schemes for Horizontally Stratified Media.- Scattering and Earth Heterogeneities.- Attenuation and Scattering of Short-Period Seismic Waves in the Lithosphere.- Source Location in Laterally Varying Media.- Optimum Approaches to Magnitude Measurements.- Three-Dimensional Seismic Velocity Image of the Upper Mantle Beneath Southeastern Europe.- Tau Inversion of Upper Mantle Array Data with Application to the Problem of the Scattering of Seismic Waves in the Lithosphere.- Observation of Scattering in the Lithosphere.- Lateral Variations in the Earths Crust and their Effect on Seismic Wave Propagation.- Signal Analysis.- Fundamentals of Multidimensional Time-Series Analysis.- The Instantaneous Amplitude, Phase and Frequency in Seismic Event Detection, Timing and Identification.- Seismic Source Discrimination.- Multidimensional Discrimination Techniques - Theory and Application.- Seismic Discrimination Problems at Regional Distances.- Linear Discrimination for Samples of Limited Size.- Developments in Seismic Instrumentation.- The Global Digital Seismograph Network: A Status Report.- Broad-Band Seismometry - A Unified Approach Towards a Kinematic and Dynamic Interpretation of Seismograms.- Seismic Data Centers.- Automatic Processing Methods in the Analysis of Data from a Global Seismic Network.- International Seismological Data Center. Demonstration Facilities in Sweden.- Design and Development of a Seismic Data Center.- Evaluation of Seismic Network Capability for Event Location.- Appendix Scientific Directors, Lecturers, Participants.- Name Index.

Multi-channel seismic reflection measurements in the Eurasian Basin, Arctic Ocean, from U.S. ice station Fram-IV

Abstract We present the first multi-channel seismic reflection data ever collected from the Eurasian Basin of the Arctic Ocean. The 200 km data set was acquired by a 20 channel sonobuoy array deployed at U.S. ice drift station FRAM-IV and operated for 34 days about 370 km north of Svalbard in April–May 1982. Cross array drift and ice floe rotation which may constitute the most serious obstacle to the advantage of multi-channel data acquisition did only occur to a minor degree during the experiment and render most of the data set suitable for processing using common mid-point binning. A 0.7–1.4 s (two-way traveltime) thick sedimentary section has been deposited over oceanic crust of mid-Oligocene age below the Barents Abyssal Plain. In the deepest part, sediments are infilling topographic lows which indicate predominantly turbidite deposition. Erosional truncations are only locally present in the central part of the section. Conformable bedforms deposited over gentle basement highs indicate a relatively stable bottom current regime since mid-Oligocene time. Thus the establishment of a deep water connection between the Arctic Ocean and lower latitude water masses appear to have had only minor effect on Eurasian Basin bottom current circulation. Extensive submarine slide scars on the north slope of Yermak Plateau show that mass waste have been a sediment source to the Barents Abyssal Plain.

Seismicity of the Norwegian Sea: The Jan Mayen Fracture Zone

Abstract The seismicity of the Jan Mayen Island region is re-examined, and a new and improved seismicity map is presented. These data, together with four previously published and one new and very precise fault-plane solution, have been interpreted in the light of other geophysical information bearing on the area. The data support the hypothesis that the transform portion of the Jan Mayen Fracture Zone consists of a system of en-echelon faults. A separate seismicity area immediately northeast of the Jan Mayen Island itself is quite pronounced on the new seismicity map. Moreover, from the available earthquake data the suggested Iceland—Jan Mayen ridge offset is located at 70.75°N.

Seismic imaging of upper crustal basement faults in the Skagerrak Sea

Abstract The continental upper crystalline crust appears almost non-reflective on many deep seismic reflection profiling sections. Typically, only the major faults and lithological boundaries are imaged, whereas smaller-scale faults are usually not identified. A deep seismic survey (0–16 s TWT) covering the Skagerrak Sea was undertaken in 1987. Although the northeastern part of the Skagerrak has only thin or no sediment cover, the data in the depth range 0–6 s TWT were reprocessed to commercial exploration standards. Close inspection of these high quality sections provides evidence of abundant, moderately steep dipping (20–50°) faults that can be traced to depths of 10–13 km. Fault geometry, which can be inferred at profile intersections, is in accordance with the general tectonic fabric observed on land and beneath the Skagerrak Sea. These areas were deformed during the Proterozoic Sveconorwegian orogeny and during the formation of the Permian Oslo Rift. Due to post-Permian erosion of the Skagerrak Sea proper, the relative age and movements of the basement faults could not be determined. This study shows that under favorable conditions, faults in the upper crystalline crust are mappable by seismic means. Such measurements have the potential for resolving variations in block geometry with depth and also recent seismotectonic movements.

Lg Wave Propagation in Eurasia

WWSSN records for explosions and earthquakes in Eurasia are analyzed for propagation characteristics of regional seismic phases, especially Lg. An overall prominence of the commonly reported phases Sn, Lg and Rg is not evident in view of the large scatter in our observed group velocities from reading all clear, wavelike onsets in the seismograms. When only the most energetic secondary arrival for each seismogram is included, however, Lg stands out as a reasonably stable and consistent phenomenon. Still the propagation efficiency of Lg is less than that reported for eastern U.S., even for the Western Russia/Baltic Shield region. Propagation characteristics of Sn, Lg and Rg phases are complex and the associated amplitude scatter is of the order of one mb magnitude unit even for nearly identical travel paths. The tectonic barrier concept occasionally introduced for explaining strong Lg attenuation observations is not consistently valid even across the Himalayas. The potential of regional phases in a source identification context is not found particularly promising.

Seismic mapping of heterogeneities in the Fennoscandian lithosphere and asthenosphere

Abstract A synopsis is presented of that part of the NORSAR research efforts which has been aimed at investigating lateral inhomogeneities in the lithosphere and asthenosphere in Fennoscandia and adjacent areas. In particular the NORSAR siting area, overlying the northern part of the Oslo graben has been seismically mapped in very great detail, using novel P-wave time and amplitude analyzing techniques. The results obtained here in terms of two- and three-dimensional seismic anomaly maps will be discussed in detail. The estimated thickness of the lithosphere in southern Scandinavia is of the order of 230 km based on S-P converted wave observations. Also, the subcrustal structures of the Caledonides appear to be more complex than those of the Baltic Shield. Lateral variations in the upper mantle down to a depth of around 700 km have been observed between western Russia and central Europe.