Journal of Geophysics
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Teleseismic evidence for velocity heterogeneity beneath the Rhenish Massif
Get PDFObservations of teleseismic P wave residuals for 56 stations in the vicinity of the Rhenish Massif show that arrivals within the Massif may be up to 0.6 s later than those immediately outside. Stations within the Massif also tend to have delays which are strongly azimuthally dependent (up to 1 s variation) in marked contrast to those outside (maximum 0.3 s variation). The strongest variation and delays are associated with the area of the Massif west of the Rhine, and preliminary modelling suggests they are caused by a low velocity region in the uppermost mantle (ca. 50–150 km depth) centred beneath the West Eifel volcanic field. Delays of up to 0.8 s, but with little azimuthal variation, are also found within the Vogelsberg volcanics, and are attributed to a shallower (≲60 km) low velocity region.
ARK: https://n2t.net/ark:/88439/y073559
Permalink: https://geophysicsjournal.com/article/187
 
The solution of dynamic problems of elastic wave propagation in inhomogeneous media by a combination of partial separation of variables and finite-difference methods
Get PDFA 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
 
Geodetic measurements and horizontal crustal movements in the rift zone of NE-Iceland
Get PDFBased on the special geodetic network set up by Niemczyk and Emschermann in 1938 in order to determine horizontal movements of the Earth's crust in the fissure area of the neo-volcanic zone of NE-Iceland, a number of repeated measurements were carried out from 1965 to 1977. In this article, modern geodetic measuring techniques and the (constantly improving) positional accuracy achieved are described; this is followed by an evaluation showing significant crustal movements in the riftzone. In the period of 1965 to 1971, movements in the total area of approximately 110 km from east to west were restricted to compressions, whereas afterwards, and in particular since 1975, expansions of up to 2 m/km have occurred in the central rift area. At present, this expansion is largely compensated by compression in the peripheral areas of up to 0.05 m/km, so that between 1971 and 1977 a total east-west expansion of the neovolcanic zone of only about 0.4 m has resulted over a distance of about 90 km.
ARK: https://n2t.net/ark:/88439/y096107
Permalink: https://geophysicsjournal.com/article/115
 
Statistical analysis of damaging earthquakes and volcanic eruptions in Iceland from 1550-1978
Get PDFRecords of all known eruptions and damaging earthquakes in Iceland since 1550 were produced containing 82 and 44 events, respectively. Neither record exhibits a significant trend. Analysis of the second order properties of the records reveals no indication of clustering or periodicity in either record. There is a significant relationship between the two kinds of events. Some of this may be due to triggering effects, but there is also a more long term relationship with eruptions leading the earthquakes. This relationship was examined further by classification of the events geographically and by types of the eruptions. The relationship seems to embody the whole area rather than local effects.
ARK: https://n2t.net/ark:/88439/y071809
Permalink: https://geophysicsjournal.com/article/269
 
Geomagnetic induction studies in Scandinavia — I. Determination of the inductive response function from the magnetometer array data
Get PDFData from the Munster IMS Magnetometer Array (Kuppers et al. 1979) have been analysed in the frequency domain to derive the inductive response function, C(ω, 0), from the ratio of the vertical magnetic field to the spatial gradient of the horizontal magnetic field. The response function was best determined by statistical frequency analysis techniques after the spatial gradients had been derived by least-squares fitting of two-dimensional second-order polynomials to the observations, with the constraint imposed that the solutions be curl-free. The derived response function was found to obey two different causality requirements, and most of the inequality constraints imposed on it (Weidelt, 1972). A preliminary model, in which conductivity is a function of depth only and which explains the major details of the observed response, is presented. It has a highly resistive uppermost layer of some 104 Ωm and of the order of 30 km thick, underlain by a layer of about 125 Ωm to a depth of around 140 km, where a transition takes place to a highly conducting (3 Ωm) half space. The effect of various non-uniform sources on the observations of C(ω, k) for this 1D model is illustrated.
ARK: https://n2t.net/ark:/88439/y091007
Permalink: https://geophysicsjournal.com/article/169
 
K-Ar age determinations on tertiary volcanic rocks: 5. Siebengebirge, Siebengebirge-Graben
Get PDFSanidines from 16 tuffs of the first volcanic phase in the Siebengebirge and the Siebengebirge-Graben were dated by the K/Ar-method. Ages range from 24.1 (Nachtigallental) to 22.9 M.y. (core Rott). Three mineral ages from surface outcrops in the Central Siebengebirge (23.9 ± 0.5 M.y.) are statistically indistinguishable from those of the sub-surface samples Stieldorf-1, Stieldorf-2, and Rott (23.0 ± 0.5 M.y.). These ages confirm the Uppermost Oligocene biostratigraphic age of the 'Blatterkohle Rott' which is interlayered with the trachyte tuff. Sanidine and biotite mineral ages from five trachyte samples give similar apparent ages from 26.4 to 24.6 M.y. These results show the trachytes to be older than the tuffs, in disagreement with the geologically established sequence. The sequence of eruptions in the Siebengebirge area however (trachyte-tuff, trachyte, latite and alkaline-basalt) was confirmed wherever outcrops allow observations. This discrepancy is discussed. The apparent ages of three dated latites are in the same range (26.2 to 25.1 M.y.), in agreement with the geological sequence. The latite ages show that the time span between the eruptions of the trachytes and the latites must have been very small. Isolated basalts belonging to a fourth phase yield K/Ar ages from 25 to 19 M.y. Pliocene volcanism could not be verified in the Siebengebirge. In addition, stratigraphically well-dated glauconites from the Tertiary of the Niederrhein area were used to correlate our isotopic age date with the Cenozoic time scale.
ARK: https://n2t.net/ark:/88439/y070559
Permalink: https://geophysicsjournal.com/article/259
 
A seismic study of the rift zone in Northern Iceland
Get PDFThe complex geological-geophysical expedition of the Russian Academy of Sciences jointly with the National Energy Authority (Orkustofnun) of Iceland conducted in 1977 a detailed seismic investigation of the flood basalt and rift zones in northern Iceland, similar to studies in southwestern Iceland. Continuous seismic profiling with 4–6 shot points was carried out across a total length of 90 km. In the flood basalt zone shallow refracting horizons were observed with Vp = 4.3–4.5 km/s and tilted toward the rift zone. They closely correspond to dense sheets of flood basalts mapped geologically. Refracting and reflecting horizons at 2 to 12 or 15 km depth are tilted more gently in the same direction. A slight depression filled with neovolcanic formations with VP < 3 km/s exists near the surface in the rift zone. Reflectors with steep tilt can be traced to depths of 15 km. The deepest part of the depression is west of Myvatn in the region of current volcanism and rifting. A seismically homogeneous body without reflectors was detected under this region at 10–15 km depth. On the whole, the structure of the rift zone in northern Iceland is similar to, but more complex than, that of southwestern Iceland.
ARK: https://n2t.net/ark:/88439/y050551
Permalink: https://geophysicsjournal.com/article/267
 
Gravity and elevation changes caused by magma movement beneath the Krafla Caldera, Northeast Iceland
Get PDFPrior to the present activity of the Krafla volcano, which started in 1975, levelling and gravity surveys had been carried out in the area. The network has since been extended, and measurements are carried out about every second month. The measurements reveal a quasi-periodic behaviour of the tectonic activity, characterized by slow inflation of the caldera for several weeks or months at a rate of 6–10 mm/d, interrupted by sudden subsidence events lasting for one or a few days. Elevation changes within the caldera are described by a deflation-inflation swelling with its apex near the center of the caldera. Calculations. using a model of a spherical chamber with varying pressure at 3 km depth, show good agreement with the measured elevation values. Comparison of gravity and levelling data, using a Bouguer type relationship, suggests that inflation and deflation of the floor of the caldera is caused entirely by in- and outflow of magma. Some discrepancies between the levelling and gravity data immediately after subsidence events can be explained by additional mass flow of groundwater.
ARK: https://n2t.net/ark:/88439/y037843
Permalink: https://geophysicsjournal.com/article/99
 
Surface deformation of the Krafla Fissure Swarm in two rifting events
Get PDFThe 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
 
Age and crustal structure of the Canary Islands
Get PDFThe postulation of a Mesozoic age for the shield-building basaltic series of Gran Canaria and Tenerife by Storevedt et al. (1978), based upon paleomagnetic data from these islands, is inconsistent with abundant and concordant K/Ar-ages from several laboratories. These latter data leave no doubt that no rocks older than Mid-Miocene have been found on Gran Canaria and Tenerife. Moreover, no volcaniclastic layers older than Miocene were found in Deep Sea Drilling cores near the Canary Islands. Also, the volcanic apron around at least Gran Canaria appears to be Miocene in age judging from seismic reflectors that extend to well-dated drilled sections. There is no evidence for a sialic crustal layer extending beneath all or any of the Canary Islands.
ARK: https://n2t.net/ark:/88439/y002516
Permalink: https://geophysicsjournal.com/article/283