Journal of Geophysics
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Stirred remanent magnetization: a laboratory analogue of post-depositional realignment
No full textThe remanent magnetization acquired by a slurry stirred in a magnetic field was measured as a function of the applied field, stirring rate and water content. The experimental results were fitted by a theoretical model in which the stirring process was approximated as a periodic randomization of the grains. The acquired remanence was proportional to the applied magnetic field and independent of the stirring rate only for weak fields ( <160 A/m) and slow stirring rates ( <10 rad/s). The remanent intensity decreased with decreasing water content. The implications for the laboratory modelling of post-depositional remanent magnetization are discussed.
ARK: https://n2t.net/ark:/88439/y004206
Permalink: https://geophysicsjournal.com/article/95
 
Gravity and a model of the median valley
Get PDFThe average median valley of the Mid-Atlantic Ridge between 12° and 18° N is described as a smooth depression flanked on both sides by a high. This applies both to the bathymetry and to the gravity anomalies. This picture of the median valley and its walls was obtained by stacking profiles across the valley in 50- to 70-km-wide bands. The reduced median valley can then be interpreted as the result of the parting of the lithosphere and the response of the asthenosphere as a viscous layer to repeated unloading. Fluid dynamic equations show that the response is in general broader than the original load disturbance. We describe this as a viscous lag of the shorter wavelength components. A steady-state solution was reached by numerical methods, showing a depression accompanied by a high on both sides. For the asthenosphere under the Mid-Atlantic Ridge at these latitudes a value followed for the kinematic viscosity of 1.5 x 1019 stokes. The model can be extended to other parts of the mid-ocean ridge system by adapting the time-dependent constants (viscosity and spreading rate). If the viscosity is a factor 5 lower, no median valley results. Rising to isostatic equilibrium of a light body under the floor of the median valley then accounts for the existence of a median ridge like found at Reykjanes Ridge and at the East Pacific Rise. The coefficient of viscosity under the East Pacific Rise would be about 0.4 x 1018 stokes. The concept of a viscous lag of the short-wavelength components replaces Sleep's (1969) original notion of a 'loss of head'. The secondary valleys and ridges found in the median valley and on the flanks of the Mid-Atlantic Ridge crest cannot be explained by the model. They represent essentially a non-continuum process, in which presumably an episodic jumping of the inner valley plays an important role. Additional faulting occurs at the hinge line between the floor and the walls of the median valley.
ARK: https://n2t.net/ark:/88439/y006206
Permalink: https://geophysicsjournal.com/article/106
 
The simulation problem for broad-band seismograms
Get PDFA fundamental problem in the numerical data preprocessing of digital broad-band seismograms is the simulation of arbitrary analog seismograph systems, especially seismometer-galvanometer combinations. A special case of this simulation problem is the deconvolution or restitution problem as the realization of a wide-band seismograph system with a transfer function proportional to ground displacement, velocity or acceleration. The simulation problem can be solved by a digital cascade recursive filter using the bilinear z-transformation. Applications of the simulation filter are: a combined interpretation of digital broad-band and analog narrow-band recordings, a routine analysis of broad-band seismograms consistent with ordinary analog stations, the determination of the local magnitude from simulated Wood-Anderson seismograms and the restitution of broad-band recordings. The relationship between bandwidth, fine structure and information content of seismograms can be demonstrated in an obvious way by comparing broad-band recordings of the Graefenberg-array with simulated seismograms for different standardized seismometer-galvanometer systems.
ARK: https://n2t.net/ark:/88439/y024704
Permalink: https://geophysicsjournal.com/article/108
 
Anhysteretic remanent magnetization of small multidomain Fe3O4 particles dispersed in various concentrations in a non magnetic matrix
Get PDFThe anhysteretic remanent magnetization (ARM) and hysteresis parameters of synthetic, small, multidomain particles (0.2–0.4 μ) of Fe3O4 , dispersed at various concentrations in a non magnetic matrix (packing factors p = 0.01–0.3), have been measured. The coercive force Hc is weakly dependent on p. At higher concentrations the shape of the sample has a strong influence on the magnetization. There appears to be an additional concentration dependence of the ARM. However, the independence of partial ARMs also holds for larger p.
ARK: https://n2t.net/ark:/88439/y043772
Permalink: https://geophysicsjournal.com/article/194
 
Ground-based observations of an onset of localized field-aligned currents during auroral breakup around magnetic midnight
Get PDFThe substorm on 2 March 1978 was selected for study as a relatively weak substorm, starting at about local magnetic midnight, that could be observed with instruments in Northern Scandinavia. The analysis is based on a comparative study of data from the IMS magnetometer network, all-sky cameras, pulsation magnetometers, and riometers in the Scandinavian area. In addition other data are used to support the results, e.g., a photograph from the DMSP-F2 satellite, showing the auroral situation over Scandinavia, and further west, immediately after the substorm onset. The substorm was preceded by a weak activation of aurora and magnetic disturbance about 3 min before the onset. After a fading that lasted for 20 s and could be observed only in optical aurora, the substorm onset led to a strong brightening of the aurora, an enhancement of the westward electrojet, a sudden rise in the ionospheric D-layer absorption, and Pi B type pulsations. Immediately after the onset, the ground magnetic data suggest the appearence of a pair of oppositely directed, localized, field-aligned currents (FACs). The main development of the signatures of the downward FAC was clearly delayed by about 3 min. There were significant correlations between the magnetic signatures of the two FACs and different features and spectra of the optical aurora, both in time and location. The observed Pi B type pulsations lasted as long as a growth in the local onset-connected FACs could be inferred. Within the first three minutes the localized three dimensional current system developed into a more sheet-like configuration. An expansion to the west, possibly accompanied by a westward travelling surge, was traced with riometers and magnetometers on Iceland and Greenland.
ARK: https://n2t.net/ark:/88439/y084828
Permalink: https://geophysicsjournal.com/article/208
 
Seismic structure of the Icelandic crust above layer three and the relation between body wave velocity and the alteration of the basaltic crust
Get PDFSeismic refraction profiles from Iceland are studied with the aid of synthetic seismograms. The classical layered model of the Icelandic crust is shown to be an unacceptable interpretation of the available data. This is because the layered model does not satisfy the observed amplitude variation. On the other hand, a model which assumes continuously increasing velocity with depth does not contradict the observations and is therefore acceptable although it is not the only possible interpretation. The model represented here shows that the surface value of the P-velocity is variable from 2.0 km/s to 5.0 km/s, depending primarily on the degree of metamorphism. The P-velocity increases rapidly with depth in the velocity interval 2.0-3.5 km/s followed by an approximately constant gradient of about 0.57 s-1. This constant gradient continues down to the 6.5 km/s isovelocity surface below which the P-velocity becomes nearly constant. In view of this, it is more reasonable to divide the Icelandic crust into two parts: the upper crust with velocity continuously increasing with depth (corresponding to layers 0, 1, 2 in the layered model) and the lower crust with almost constant velocity (corresponding to layer 3 in the layered model). The depth to the lower crust is variable and depends on how deep the crust is eroded. A typical depth to the lower crust is 5-6 km for an uneroded basalt pile but can be considerable less where the basalt pile is deeply eroded, especially below extinct central volcanoes.
ARK: https://n2t.net/ark:/88439/y091317
Permalink: https://geophysicsjournal.com/article/84
 
A late Cretaceous 40Ar-39Ar age for the Lappajarvi impact crater, Finland
Get PDFWe report on a 40Ar — 39Ar study of karnaite from the ⁓ 17 km Lappajarvi impact crater, Finland. Four samples from a 3,000 m profile across the crater center give rather well defined age plateaux and indicate complete degassing at the time of the impact event. The mean age is 77 m.y., much younger than geologically derived age estimates.
ARK: https://n2t.net/ark:/88439/y062490
Permalink: https://geophysicsjournal.com/article/186
 
Palaeomagnetic secular variation curves extending back to 13,400 years B.P. recorded by sediments deposited in Lac de Joux, Switzerland
Get PDFPalaeosecular variation records have been obtained from three cores from Lac de Joux, Switzerland ( 46°37' N, 6°16' E) on which a parallel biostratigraphy has been constructed. All three cores show a well developed Late-glacial sequence which extends to beyond 13,400 years B.P. and the recorded palaeomagnetic declination and inclination variations can be correlated between these cores. The longer core (no. 3) also shows a well developed Post-glacial sequence and the declination and inclination records can be correlated in detail with the independently dated United Kingdom records which extend back to about 10,000 years B.P. Ages along Lac de Joux core 3, obtained by palaeomagnetic correlation with U.K. cores, are compared with ages based on the local palynology and thus a timescale has been attached to the Lac de Joux record back to beyond 13,400 years B.P. during which time there is no evidence of any geomagnetic excursion or short event.
ARK: https://n2t.net/ark:/88439/y079999
Permalink: https://geophysicsjournal.com/article/247
 
A model of electrical resistivity beneath NE-Iceland, correlation with temperature
Get PDFShort 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
 
High precision measurement of the frequency of Mode 0S0
Get PDFMode 0S0 was excited by the Indonesian earthquake of 19 August 1977 and recorded at the South Pole and at UCLA. Phase estimates from 16 independent time lapse spectra were used to determine the frequency of this mode to high precision. The result is f (0S0) = 2.932851 cph with a standard deviation of 11 ppm.
ARK: https://n2t.net/ark:/88439/y062410
Notes: Erratum
Permalink: https://geophysicsjournal.com/article/36