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The Krafla rifting episode in North Iceland has had 11 main tectonic events during the period December 1975 to May 1979. Each event has lasted from a few hours to several weeks. The first and eighth events affected to some extent the same part of the Krafla fissure swarm. These two tectonic events in the fissure swarm were characterized by down-faulting of a central area of the fissure swarm about 5-km-wide E-W and 20-km-long N-S. The resulting graben was boarded on both sides by an intensely faulted and fractured zone. These fracture zones showed spreading of 1.5 m in the first event and 2.66 m in the eighth one along the same reach. Elastic contraction on both sides of the fissure swarm added up to 1.4 m in the eighth event. The graben floor is estimated to have subsided about 1 m in the first event while a further subsidence of 1.1 m was observed in the 8th event. The flanks of the graben rose about 0.5 m during the latter event. The rise diminished away from the fissure swarm.
ARK: https://n2t.net/ark:/88439/y080508
Permalink: https://geophysicsjournal.com/article/83
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The Kramers-Kronig relations for magnitude and phase of a linear causal filter are used to derive an exact general expression for the viscoelastic modulus M, corresponding to power laws for the quality factor, Q ≈ ωγ. The exponent γ varies from -1 to + 1, such that the spectrum of rheologies extends from a Kelvin-Voigt to a Maxwell body. High- and low-frequency approximations for M(ω) are derived, and in the special cases γ = ±1, ±1/2, ±1/3, ±1/4,... closed-form solutions are given which apply for arbitrary frequencies. With M(ω) at hand, both high-frequency phenomena such as velocity dispersion and low-frequency phenomena such as creep and stress relaxation can be investigated. Results for phase-velocity dispersion are given as well as short- and long-time-scale approximations of the creep and relaxation functions. Simple dissipation operators are derived which can be convolved with theoretical seismograms in order to correct these for the influence of absorption. Some results on relaxation spectra for the case 0 ≤ γ ≤ 1 are summarized in an appendix. Taken together, the results of this paper suggest that media with 0 < γ < 1 should be considered as generalized Maxwell bodies and media with -1 < γ < 0 as generalized Kelvin-Voigt bodies. Application of the Kramers-Kronig relations to the viscoelastic modulus is better than the use of those relations in conjunction with the wavenumber of a plane wave, which is the procedure that has been employed so far.
ARK: https://n2t.net/ark:/88439/y076559
Permalink: https://geophysicsjournal.com/article/82
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In this paper we present the results of a 10-year period (1971-1980) of research on the seismicity of the Upper Rhinegraben. Our investigations are exclusively based on instrumentally recorded earthquakes with local magnitudes between 0.5 < ML < 5. The increase in the number of high-gain seismic stations during the past 2 decades improved the quality of the observations considerably, thus allowing detailed recognition of the spatial distribution of the earthquake loci in focal areas deduced from the analysis of historical events. No region, regarded up to now as aseismic, revealed itself as seismic, not even at the level of microearthquakes. Excluding the focal area of the Swabian Jura, the northernmost and southernmost parts of the Upper Rhinegraben show the highest degree of seismic activity. The middle part of the Rhinegraben, between Strasbourg and Karlsruhe, reveals only modest activity, somewhat in contrast to the historical record. The number of earthquakes increases towards the east of the river Rhine relative to the west. An even more pronounced asymmetry is shown in the southern graben by different maximum focal depths perpendicular to the strike of the Rhinegraben. In the Vosges mountains and in the graben proper, depths of 13 and 16 km, respectively, are not exceeded. Maximum depths down to about 20 km are found in the Black Forest. No earthquake was detected in the lower gabbroic crust or in the mantle. The maximum focal depth seems to be governed by variations in the temperature-depth distribution. Fault plane solutions of more than 30 earthquakes demonstrate that the seismic dislocations take place predominantly as strike slip mechanisms in the southern graben area whereas normal faulting prevails in the north. In the northern graben, most of the seismic dislocations occur on fault segments striking N30°W whereas in the south a strike ofN20°E or N60°W (the conjugate direction) is dominant. Furthermore, the fault plane solutions indicate a clockwise rotation of the principle stress directions from north to south by about 40°.
ARK: https://n2t.net/ark:/88439/y060010
Permalink: https://geophysicsjournal.com/article/81
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Knowledge of the crustal structure of the Iceland-Faeroe Ridge is based on results from the North Atlantic Seismic Project of 1972 supplemented by earlier short refraction lines and reflection, gravity and magnetic surveys. The main 5. 7 km/s upper crustal layer is locally overlain by lower velocity layers of variable thickness. The upper crust is interpreted as being predominantly basaltic, comprising lavas, regions of pyroclastic rock and intrusives including ring complexes. A 6.7 km/s lower crustal layer underlies the upper crust at a depth of between about 4 and 8 km along the Ridge; this layer is present also beneath the Icelandic shelf but not beneath the Faeroe shelf. A deeper 7.8 km/s refractor interpreted as the Moho occurs at about 30-35 km depth beneath the south-eastern part of the Ridge, shallowing to about 28 km towards the north-western end of it. A significant increase in velocity with depth within the main 6.7 km/s layer has not been detected but may occur, in which case the Moho would be somewhat deeper. The seismic crustal results are consistent with a gravity profile across the Ridge, which indicates approximate Airy isostatic equilibrium. The crust beneath the Ridge, which is of a thickness more typical of the continents than the oceans, is believed to have been formed by sea-floor spreading during the period 55 to 40 Ma ago.
ARK: https://n2t.net/ark:/88439/y051071
Permalink: https://geophysicsjournal.com/article/80
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A new method for the calculation of theoretical seismograms, which is suitable for a wide class of inhomogeneous media, is suggested. The method is based on a combination of partial separation of variables with finite-difference techniques. Different variants of the method, based on the application of the Fourier-Bessel transform, finite integral transforms, expansions in Legendre polynomials, etc., are discussed in detail. Examples of theoretical seismograms for various simple structures are presented.
ARK: https://n2t.net/ark:/88439/y045452
Permalink: https://geophysicsjournal.com/article/79
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Recently, a formalism for three-dimensional surface-wave scattering in a plane geometry was derived. Since teleseismic surface-wave data are generally recorded at epicentral distances large enough to be influenced by the sphericity of the Earth, it is necessary to find the effects of a spherical geometry on surface-wave scattering. The theory of surface-wave scattering relies heavily on a dyadic decomposition of the Green's function, and a new derivation is given for the (dyadic) Green's function of a spherically symmetric Earth. This new derivation employs Poisson's sum formula and is more rigorous than previous derivations. Using the dyadic Green's function, a relation is established with the scattering theory in a flat geometry. This finally leads to a linearized formalism for three-dimensional surface-wave scattering on a sphere. Even for shallow surface waves the effects of sphericity are important and necessitate a modification of the propagator terms in the expression for the scattered surface waves.
ARK: https://n2t.net/ark:/88439/y032183
Permalink: https://geophysicsjournal.com/article/78
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Short period magnetotelluric measurements (15 s–1 h) were made at 19 sites in NE-Iceland, distributed over the neovolcanic zone and the adjoining older Tertiary flood basalt areas. With model-calculations of one- and two-dimensional resistivity distributions a characteristic model was found for the lower crust and upper mantle. Beneath a thin surface layer the resistivity is 100 Ωm except within the active neovolcanic zone where it is 50 Ωm. This layer extends to a layer with low resistivity of 15 Ωm. The low-resistivity layer is about 5 km thick. The depth of its upper boundary increases from 10 km to about 20 km with increasing distance from the rift axis. The resistivity beneath the low-resistivity layer is about 100 Ωm down to at least 100 km. Comparison of field data with laboratory measurements on conductivity at high temperatures indicates that the low-resistivity layer consists of partially molten basalt at a temperature of 1,000° –1,100° C. The underlying layer very probably consists of partially molten ultramafic rocks and is presumably the uppermost part of the mantle beneath Iceland. The basaltic low-resistivity layer is interpreted as the base of the crust formed by upward movement of the basaltic melt fraction from the mantle.
ARK: https://n2t.net/ark:/88439/y021104
Permalink: https://geophysicsjournal.com/article/77
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In the inflation-deflation sequence of the Krafla magma chamber since its beginning in 1975, 12 deflation or subsidence events have been identified until May, 1979. The tilt and distance measurements relating to these subsidence events are discussed in some detail. All of these subsidence events are associated with horizontal magma flow along the N-S trending fissure zone, which goes through the central part of the Krafla caldera, to form a dike 3 to 5 m wide, 80 km long, and 1.0 to 2.5 km high from the bottom to the top. The total volume of magma, which flowed out of the Krafla magma chamber during these 12 events, is estimated as 481 x 106 m3, whereof 407 x 106 m3 are estimated to have flowed northwards, 72 x 106 m3 southwards and about 2 x 106 m3 came to the surface as basaltic lava.
ARK: https://n2t.net/ark:/88439/y011015
Permalink: https://geophysicsjournal.com/article/76
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The reflectivity method for the computation of theoretical seismograms is extended for the case of a point source buried in a layered medium. Two sources are considered, an explosive source and a vertical single force. Appearing accuracy problems are solved. Poles of Rayleigh waves are shifted away from the real axis of the wavenumber plane by introducing attenuation, in order to allow numerical integration along the real axis. The results of several computations are discussed. This method allows the computation of complete seismograms including surface waves, leaking modes and all body wave phases, including depth phases like pP.
ARK: https://n2t.net/ark:/88439/y005596
Permalink: https://geophysicsjournal.com/article/75
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Detailed low-level aeromagnetic data between Iceland and 70° N are combined with published bathymetric, seismic reflection, and other data to yield a new tectonic synthesis of this region of anomalously shallow sea floor. On Kolbeinsey Ridge short transform faults have repeatedly formed and disappeared over the last 7-8 Ma. Spreading from Kolbeinsey Ridge began about anomaly 6C time (24 Ma); total opening rates increased from 1.5 cm/a to 2 cm/a about 12-13 Ma ago. The Intermediate-Iceland- Plateau extinct axis thus does not exist, and oceanic crust must underlie much of the Greenland margin, perhaps up to the coast itself. This in turn implies locally over 100 km prograding, much of which probably occurred during Plio-Pleistocene glacial periods. The Iceland shelf has been prograded locally 25 km or more. The plate acceleration a 12-13 Ma ago correlates with time-transgressive basement ridges or escarpments previously found on Reykjanes Ridge and here identified north of Iceland as well. The features are proposed to reflect an abrupt mid-Miocene increase in discharge from the Iceland plume. Other time-transgressive basement structures are found on younger Kolbeinsey Ridge crust. Lower Tertiary anomalies 13 to at least 22 are identified in the Denmark Straits, ruling out the hypothesis that the Iceland platform resulted from a westward jump of the spreading center at anomaly 7 time. The magnetic smooth zones being formed where the Kolbeinsey and Reykjanes Ridges enter Iceland have a multiple origin: degassing at depths less than 500 m, coupled with crustal reheating as a result of burial by sediment may be the most important processes.
ARK: https://n2t.net/ark:/88439/y093477
Permalink: https://geophysicsjournal.com/article/74
