Journal of Physics of the Earth
Online ISSN : 1884-2305
Print ISSN : 0022-3743
ISSN-L : 0022-3743
Volume 45, Issue 3
Displaying 1-4 of 4 articles from this issue
  • N. Purnachandra Rao, M. Ravi Kumar
    1997 Volume 45 Issue 3 Pages 167-176
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Different views prevail regarding the uplift and tectonics of the Shillong Plateau in northeastern India. In light of these, we discuss the cause of the current uplift, the time of initiation of the uplift, and the current tectonic scenario. Various geophysical results indicate that neither a thermal anomaly nor isostatic compensation could have caused the uplift as suggested by many workers in the past. Several coinciding factors point towards a combination of tectonic forces in this unique thrust environment, comprising the Himalayan thrust and the Burmese thrust, to be responsible instead. Stratigraphic evidence suggests that the initiation of the current uplift was during Mio-Pliocene. While most tectonic models attribute the Shillong Plateau uplift to a N-S compressive stress regime, it appears that an E-W compressive stress owing to the Indo-Burman subduction active during that period also aided the uplift by providing the necessary and timely impetus. Even at present, a small component of stress in the E-W direction seems to be operational as inferred from a computation of strain rates and velocity vectors. The predominance of thrust type focal mechanisms and their P axis orientations possibly indicate that the uplift is sustained by compressive forces acting on the plateau from all sides.
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  • Masato Furuya, Yozo Hamano, Isao Naito
    1997 Volume 45 Issue 3 Pages 177-188
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    We explored the atmospheric contribution to the excitation of Chandler wobble (CW), which has spanned about 11 years beginning from September 1983. The atmospheric angular momentum (AAM) function presented by the Japan Meteorological Agency (JMA) and the wobble data set (SPACE93) are employed. We devised a wobble domain method of analysis which enables us to quantify the narrow band power of AAM. The AAM-induced wobble closely resembles the observed wobble, and wind contribution turns out to dominate over atmospheric pressure contribution in the vicinity of the Chandler frequency. When only pressure contribution is taken into account, it is insufficient, as shown in previous studies.
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  • Tomoeki Nakakuki
    1997 Volume 45 Issue 3 Pages 189-201
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    Numerical simulation of 2-D mantle convection with variable viscosity is performed to examine the effect of plate subduction on cooling of the Earth's mantle. We employ temperature-, pressure-, and stress-dependent viscosity in order to realize plate-like motion and subduction of a cold thermal boundary layer at the surface. Our model predicts that the velocity of surface plates strongly depends on the viscosity of the mantle interior, at least when the viscosity of the mantle strongly depends on the pressure. As a result, the heat transfer of the Earth's mantle strongly couples with the temperature, and the Earth cools slowly. This is consistent with past mantle temperatures derived from petrological data. It also suggests the possibility that the Earth has a higher Urey ratio than that inferred from measurements of terrestrial heat flow combined with geochemical data.
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  • Toshiro Tanimoto
    1997 Volume 45 Issue 3 Pages 203-226
    Published: 1997
    Released on J-STAGE: April 30, 2009
    JOURNAL FREE ACCESS
    A global shallow Earth structure is retrieved from short-period surface wave data (40-200 s) using a data set assembled from global digital networks. Phase velocity maps for both Love and Rayleigh waves are retrieved by spherical harmonic inversion. A comparison with results by block inversion is also presented, and the similarity of the results by the two different methods supports the belief that the data are capable of resolving most details in final velocity maps. Strong effects of crustal structure are evident in the patterns of short-period Love wave phase velocity variations, although depth resolution kernels show that the complete resolution of crustal structure is not possible from this period range of data. Two models of S-wave velocity maps are presented, one with an assumed crustal structure and topography and the other without such an assumption. Major differences between the two models occur just below the moho; the one with an assumed crustal structure shows the beginning of tectosphere-like structure immediately below the moho under cratons, while the model without it shows the beginning depths of tectosphere-like structure between 50 and 80 km. It is obvious that the short-period cut-off in this study (40 s) is not sufficiently short to resolve shallow structures in detail. A few interesting tectonic features in the models include: 1) S-wave velocity under ridge axes is slow in the upper 200 km and is very slow in the upper 100-120 km, due possibly to partial melting at shallow depths; and 2) depth slices under the Himalaya-Tibet region show fast velocity anomaly extending from India to the underside of Tibet. While it is difficult to distinguish between a subducted oceanic lithosphere and a thickened continental lid, the shape of the contour lines seems to favor the interpretation that this fast velocity anomaly is the underthrusting Indian ocean lithosphere.
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