Geophysical Evidence of the Olivine-spinel Transition Hypothesis in the Earth's Mantle

Assuming olivine-spinel transition of Mg2SiO4 at the depth of 400 km in the earth, the difference in free energy between sub-stances composing the upper and lower mantle is estimated. The difference obtained here is approximately the same as that deduced by Ringwood from experimental results. The influence of the transition on the electrical conductivity is also studied. The steep increase of the electrical conductivity at the depth as inferred from the studies of geomagnetic variations seems to be justified by the transition hypothesis.最近Ringwoodは,Ni2GeO4-Mg2SiO4固溶体を調べ,地球中間層内に於てMg2Si04がオリビンスピネル転移を起すことが可能であることを推論した.彼の研究によれば,転移に伴なう自由エネルギーの変化は1,500℃に於て70,000±10,000Joule/mole,密度変化は約11%,スピネルMg2SiO4の格子常数は7.99Åとなる

Thermo-magneto-hydrodynamic Oscillations in the Earth's Core

A linearized treatment of the simultaneous equations of electro-magnetism, hydrodynamics and heat transfer in a conducting fluid sphere is described. It turns out that a sort of thermo-magneto-hydrodynamic oscillations seems likely to occur through the couplings between magnetic fields, fluid motions and thermal fields. With some drastic simplifications, the eigen-period of the oscillation, which superposes on the steady state of the earth's dynamo is estimated at 20 years.地磁気ダイナモに重畳する微小振幅の磁場,流体運動および温度場の相互結合を近似的に調べた.その結果,S10磁場については周期約20年程度の振動が起る可能性が見出された.地磁気永年変化の一部はこのようなメカニズムで発生することが可能であると推測される

13. A Theory of Frequency-Dependent Effect of an Island on Geomagnetic Variations

Self-induction effect of electric current flow around a circular hole in a thin conducting sheet is studied in an approximate way. It turns out, however, that the self-induction is not so large that marked frequency dependence of the vertical field component as observed, for example, on Hachijo-shima Island can be attributed to this origin. A phase lag amounting to several degrees seems likely to have arisen by the self-induction effect provided a periodic variation having a period of 30min. and an island of which the diameter is 10km are assumed.|離島上で観測される地磁気短周期変化が,いわゆる離島効果を示すことは,最近の観測によつて確立された.磁場が北向きに変化する場合には,鉛直成分対水平成分比(⊿Z/⊿H)は通常島の北端で負,南端で正となる.しかし,たとえば八丈島の観測点でみられるように,⊿Z/⊿Hが変化の周期によつて変化し,その符号が変化する場合もある.従来の理論では自己感応の影響が省略されていたが,この論文では,近似的に自己感応の影響を議論し,⊿Z/⊿Hの周期依存性が説明できるか否かをみた.その結果自己感応の影響は比較的小さく,そのような依存性は説明不可能で,たとえば周期によつて海中に流れる電流の場所が異なるというような原因を考えざるを得ないという結果が得られた

A Possible Cause of Earth-Current Anisotropy

地磁気変動のさい,日本の地下に巨大な電流が集中して流れることが明らかにされている.この電流変化が地表付近に誘導作用をおよぼし,地電流が特定の方向に卓越することがわかつた.地電流の異方性は地表付近の不均－ばかりでなく,このようにマントル内に起因する原因によつても発生するものと考えられる

Electrical State and Seismicity beneath Japan.

A hypothetical model of the upper part of the earth's mantle beneath Japan is proposed. In order to account for the low electrical conductivity as has been derived from the study on geomagnetic daily variation, it is assumed that the substance, which usually consists of the layer between the earth's surface and the depth of 400km, wedges into the lower part under Japan as deep as 700km, the width of the wedge being taken to be 200km. By this model, we can roughly explain the low conductivity there as well as the occurrence of deep focus earthquakes, the latter being likely to be caused by the shearing stresses due to the buoyancy force.地磁気日変化の解析より推定された日本地下の電気伝導度が低いという結果に基づいて,通常地表から深さ400kmまでをしめている物質が日本中部および東北部に於て,約700kmの深さまではいりこんでいるという模型を考えた.地球内の温度および圧力分布を考慮する時,この模型は要求される電気伝導度を与えまたこの貫入部分に働く浮力を計算すると,地球中間層を構成している物質の限界強度に近い数値が求められる.したがつて,この貫入部分の境界に沿つて深発地震が起りやすいと考えられる

Studies of the Thermal State of the Earth. The Second Paper: Heat Flow associated with Magma Intrusion

On the assumption that hot magma intrudes suddenly in a spherical cavity in the earth's crust, the conduction of heat inside and outside the magma is discussed. The changes in the geothermal gradient and consequently the heat flow at the earth's surface right up the magma are then calculated. It is assumed 1) that the initial temperatures are constant and zero respectively inside and outside the sphere and 2) that the temperature at the earth's surface is always kept at zero.地下の球状部分が突然高温になつて,その後熱伝導によりどのように冷却するかを理論的に調べた.地表温度が常に零に保たれるとした場合,地表の地温勾配はだんだんと増加して極大に達し,以後徐々に減少する.高温部分の半径を2km,中心の深さを5km,初期温度を1300℃とすれば,地温勾配の極大は6×104年後に起り,40℃/kmの値をとる.この計算は通常の岩石に対する熱拡散率を仮定した結果で,地表の熱流量に換算すれば2×10-6cal/cm2 secということになる.この値は平均の熱流量の倍にすぎないので,熱流量の測定からこの程度の大きさおよび深さの岩漿溜を検出することは困難であると予想される

Forced Oscillations of the Earth's Dynamo

A theory, based on magneto-hydrodynamics, that the geomagnetic secular variations are caused by forced oscillations of small amplitude superposing on the steady state of the earth's dynamo is tried. With drastic simplifications, the driving force due to buoyancy is obtained for hypothetical variations of a 100-year period. The time-dependent thermal field responsible for such a force field is also obtained. It is shown that the amplitude of the field would be 10-4℃ in order to account for the S10 field whose amplitude is assumed as 1000 gammas at the pole.地球回転のコリオリ力を考えると,地球核内において,電磁流体力学的自由振動は起りにくいことが筆者によつて指摘されている.したがつて地磁気永年変化に対応する核内の電磁流体力学的過程を考えるためには,時間的に変化する駆動力を考慮する必要がある.本報においては,その駆動力は湿度差に起因する浮力によるという場合を考え,極において振幅1000ガンマ,周期100年の双極子磁場の永年変化を想定すると,核内には定常状態に重畳して10-4℃程度の振幅をもつ温度差の分布があればよいという結論が得られた