Journal of geomagnetism and geoelectricity
Online ISSN : 2185-5765
Print ISSN : 0022-1392
ISSN-L : 0022-1392
Volume 34, Issue 5
Displaying 1-3 of 3 articles from this issue
  • R. THOMPSON, D. R. BARRACLOUGH
    1982 Volume 34 Issue 5 Pages 245-263
    Published: 1982
    Released on J-STAGE: March 12, 2010
    JOURNAL FREE ACCESS
    Historical observations of the geomagnetic field have been combined with archaeomagnetic results in order to produce a model of geomagnetic field changes since 1600 A. D. Cross validation of the historical data indicated the most suitable level of truncation of spherical harmonic modelling of the data to be at the fourth degree. Weighted least squares cubic splines have been used to combine our spherical harmonic analyses carried out at 50 year intervals and to model smooth changes of the historical magnetic field throughout the world. Our model is in broad agreement with previous calculations of the historical main field but differs in some of the details of the non dipole field and the axisymmetric coefficients. An area of sustained anticlockwise looping of the local magnetic field vector has been revealed for the first time in the Indian ocean. The present region of strikingly small secular change and low non dipole field in the Pacific appears to have formed during the last 200 years. Changes in local field intensity have also been modelled throughout the world for the last 400 years. Even the largest of these local intensity changes are significantly smaller than the high frequency palaeointensity changes reported by archaeomagnetists. The balance of energy between the magnetic dipole field and the higher order fields suggested to have been occurring since 1900 A. D. is not found from our analyses for before 1800 A. D.
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  • Long-Sheng GAO, Ming-Fa XU, Ping-Zhang HUANG
    1982 Volume 34 Issue 5 Pages 265-281
    Published: 1982
    Released on J-STAGE: March 12, 2010
    JOURNAL FREE ACCESS
    Piezomagnetic measurements have been performed on seven different kinds of rocks collected in the Beijing-Tangshan area of China. The measurements involve examination of the variations in the magnetic susceptibility before and after fracture of the rocks under high stress conditions. The results show that changes in the magnetic susceptibility depend upon the stress applied and are very weakly influenced by changes of volume and the processes of microfracturing and dilatation. In these experiments the range of the stress coefficient of susceptibility was approximately (0.4-2.75)×10-4 bar-1. Piezomagnetic curves were also obtained subsequent to fracture.
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  • W. W. SAGER, G. T. DAVIS, B. H. KEATING, J. A. PHILPOTTS
    1982 Volume 34 Issue 5 Pages 283-305
    Published: 1982
    Released on J-STAGE: March 12, 2010
    JOURNAL FREE ACCESS
    Nagata Seamount, located in the northern Line Islands at 12.5°N, 167.0°W, was surveyed and dredged by the Hawaii Institute of Geophysics in the fall of 1979. The seamount is 3.7km in height, approximately 45km at maximum width, and has an estimated volume of 590km3. Analysis of dredge rocks indicates that the seamount consists of alkali basalts and hyaloclastite breccias. Both acoustic reflection records and magnetic modeling studies indicate that the seamount is draped with a large volume of volcaniclastic debris about its base. At some time in its history Nagata Seamount may have been at or near sea level.
    The bulk of the seamount is made up of lavas of normal magnetic polarity; however, the uppermost 1, 000m consists of reversely polarized rock. The virtual geomagnetic pole determined from the seamount falls at 61.6°N, 4.0°E not far from the average Cretaceous Pacific seamount pole. Rock magnetism studies of the dredged basalts indicate a stable magnetization. Three dimensional gravity modeling determined an average density of 2.5g/cm3 for the seamount. This value of the density agrees with other similar data from Pacific and Atlantic seamounts; although, unlike some other seamounts studied gravitationally, no large density variations were revealed on Nagata Seamount.
    K/Ar radiometric ages of 70Ma and 86Ma were obtained from the dredged basalts in agreement with the paleomagnetic pole. The younger age may result from alteration, thus the older date is considered more representative. This age is similar to that of nearby Kapsitotwa Seamount.
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