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Magnetotelluric observations across the Juan de Fuca subduction system in the EMSLAB project
A magnetotelluric (MT)transect has been obtained near latitude 45ÞN from the active Juan de Fuca
Spreading center, across the subduction zone and Cascades volcanic arc, and into the back arc Deschutes
Basin region. This paper presents the MT data set and describes its major characteristics as they pertain to
the resistivity of the subduction system. In addition, we discuss the measurement and processing
procedures employed as well as important concerns in data interpretation. Broadband audiomagnetotelluric
(AMT)/MT soundings( approx. 0.01-500 s period) were collected on land with considerable redundancy in
site location, and from which 39 sites were selected which constrain upper crustal heterogeneity but sense
also into the upper mantle. Fifteen long-period MT recordings (about 50-10,000 s) on land confirm the
broadband responses in their common period range and extend the depths of exploration to hundreds of
kilometers. On the Juan de Fuca plate offshore, 33 out of 39 sea floor instruments at 19 locations gave
good results. Of these locations, five magnetotelluric soundings plus two additional geomagnetic
variation sites, covering the period range 200-10^(5) s approximately, constitute the ocean bottom segment
of our profile. The feature of the land observations which probably relates most closely to the subduction
process is a peak in the impedance phase of the transverse magnetic mode around 30-50 s period. This
phase anomaly, with a corresponding inflection in the apparent resistivity, is continuous eastward from
the seacoast and ends abruptly at the High Cascades. It signifies an electrically conductive layer in
otherwise resistive lower crust or upper mantle, with the layer conductance decreasing eastward from the
coast to a minimum under the Coast Range but increasing suddenly to the east of the central Willamette
Basin. The higher conductance to the east is corroborated by the vertical magnetic field transfer function
whose real component shows negative values in the period range 100-1000 s over the same distance. The
transverse electric mode apparent resistivity and phase on the land display a variety of three-dimensional
effects which make their interpretation difficult. Conversely, both modes of the ocean floor soundings
exhibit a smooth progression laterally from the coastal area to the spreading ridge, indicating that the
measurements here are reflecting primarily the large-scale tectonic structures of interest and are little
disturbed by small near-surface inhomogeneities. The impedance data near the ridge are strongly
suggestive of a low-resistivity asthenosphere beneath resistive Juan de Fuca plate lithosphere.
Approaching the coastline to the east, both impedance and vertical magnetic field responses appear
increasingly affected by a thick wedge of deposited and accreted sediments and by the thinning of the
seawater
1. Stability and Non-Steady State of Self-Exciting Dynamos 1
Time-dependent behaviours of two conducting spheres rotating in an infinitely extended conductor are studied. Although it has been proved by Herzenberg that such a system works as a selfexciting dynamo, the steady state of it turns out to be unstable for small disturbances. In some special cases, a paradoxical result that the magnetic field continues its growth in spite of non-rotation of the spheres is obtained by the numerical integration of the nonlinear simultaneous differential equations with the aid of a relay computer. This difficulty is caused by the crudeness of approximation for the electromagnetic coupling between the two spheres. If we improve this point, it seems likely that oscillatory fields and velocities could be found. It is intended to apply this study to investigations of the magneto-hydrodynamic actions which are supposed to be in the earth's core.|æè¿,Herzenbergã«ãã€ãŠ,å°äœçã®äžã§å転ãã2ã±ã®å°å°äœçã,self-exciting dynamoç³»ãšããŠ,ç£å Žãç¶æã§ããããšã蚌æããã.ããã§ã¯,ãããšé¡äŒŒã®modelãçšã,ãã®å®å®æ§ãšæéçå€åã調ã¹ã.åŸæ¥,éå®åžžãã€ãã¢ã®ç 究ã¯,åç€ãã€ãã¢ã䜿ã€ãŠãªãããŠãã.åæŠã¯,2ã±ã®åç€ãã€ãã¢ãçµåãããŠ,é»æµãè§é床ã®åãã,é転ããããšã瀺ããŠãã.é»æµãããã¯ç£å Žã®é転ã®å¯èœæ§ããã®ãããªç°¡åãªmodelã«å¯ŸããŠç€ºãããçŸåšã§ã¯,è¿äŒŒã®åºŠãããã,ããçŸå®çãªmodelãçšããŠ,éå®åžžãã€ãã¢ãç 究ããããšãæãŸãã.ããã§ã¯,å°çæ žå
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27. Stability and Non-Steady State of Self-Exciting Dynamos 2
The variation of the magnetic field and the angular velocity with time are studied for two conducting spheres rotating in an infinitely extended conductor. Induction equations are approximated by those expanded up to the second time-derivatives of the magnetic field. In some special cases, the magnetic field was found to change its polarity with diminishing amplitudes. The periods of the damping oscillation are about 1.3x104 years when the spheres of 500 km in radius are supposed to be rotating about 1100 km apart from each other in the earth's core. If the work done by the external torque is assumed to be 10-9 erg/sec.cm3, the induced mgnetic field and the surface velocity of the sphere become about I gauss and 0.7 cm/sec. respectively.|åå ±ã«åŒãç¶ã,å°äœäžã§å転ãã2ã±ã®å°äœçã«ãã€ãŠèªå°ãããç£å Žãšè§é床ãšã®æéçå€åã調ã¹ã.æ¬å ±ã§ã¯èªå°æ¹çšåŒãšããŠ,2次å°åœæ°ãŸã§ãå«ãè¿äŒŒåŒãæ¡çšããã,åŸãããçµæã¯,åå ±ãšåæ§,倧å¥ããŠ2çš®é¡ã®è§£ã«åé¡ã§ãã.äžã€ã¯ç£æãæžè¡°ã,å€ããã®é§ååã®ããã«å転ã次第ã«ã¯ããŸãå Žåã§,ä»ã®äžã€ã¯,ç£å Žãæ¥æ¿ã«å¢å€§ã,å転é床ãæ¥éã«æžå°ããå Žåã§ãã.2ã±ã®çã«é¢ããçŸè±¡ã®èµ·ãæ¹ãå
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14. Archaeomagnetic Study on Volcanic Rocks in Oshima Island, Japan
Variation in the geomagnetic field for the past one thousand years has been inferred from the direction of the remanent magnetization of rocks ejected at periods known from historical records. The results show an agreement with those of instrumental observations which have been conducted since the sixteenth century. Periodical changes are found both in declination and in inclination: inclination varies rather slowly within a period of about 600-700 years, while declination changes within a 400-500 year period.|æè¿,äžææ°ã«ãã€ãŠäŒè±å€§å³¶ã®ããããå°è³ªèª¿æ»ãé²ããã,ç岩ã®åŽåºå¹Žä»£ãçžåœæ£ç¢ºã«ç¥ããããã«ãªã€ã.ãããã®ç岩ã®æ®çç£æ°ã枬å®ããŠ,éå»çŽ1000幎éã®å°çç£å Žã®å€åãæšå®ããæããããåŸã.æŽå²æ代ã®å°çç£å Žã®å€åãæ±ããã«ã¯,é«ç²ŸåºŠã®æž¬å®ãèŠæ±ããã.è³ææ¡åã®éã«,ã¯ãªãã¡ãŒã¿ãŒãçšããããšããã,ããã¯ã³ãœãã¹ãé æ¹ã®æé é¡ã§èŠéããŠ,岩ã®æ¹åã¥ãããããªã€ã.ç£åæ¹åã®æž¬å®ã«éããŠã,çš®ã
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The Westward Drift of the Magnetic Field of the Earth
Secular variations in the geomagnetic field are studied with the intention of making a clear-cut separation of the variation due to the westward drift of the non-dipole field from that due to the growth and decay of the said field. By making use of a few different methods, the most probable velocity of the westward drift is determined as 0.20°/year on average. It turns out that the major parts of the observed field variation are caused by the drift. It is not reasonable, however, to ignore the residual variation in the field which cannot be explained by the drift. Growth and decay in the non-dipole field seems to be responsible for the residual field. The field variation in historical time is also likely to be approximately accounted for by the drift, so that it is suggested that the non-dipole field has a life-time as long as several hundred years or more.å°ç£æ°æ°žå¹Žå€åäž,å°çç£å Žãžã®è¥¿æ¹ç§»åã«ãã£ãŠã²ããããããå€åãš,ç£å Žã®æ¶é·ã«äŒŽãå€åãšãåé¢ããç®çã§ãã®ç 究ã¯ãªããã.第1ç« ç£å Žãå°çæ žãšå
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Attenuation of Geomagnetic Secular Veriation through the Conducting Mantle of the Earth
On the assumption that a time-dependent electric current sheet, at the core boundary, of which the magnetic potential is equivalent to that due to magnetic dipoles distributed at random, is the source of geomagnetic secular variations, the decrease in amplitudes of the secular variation through the mantle is discussed by taking the existence of the conducting core into account. The lower harmonic components damp out more conspicuously than the higher ones. The electrical conductivity of the mantle to cause an attenuation most compatible with that inferred from the observation ranges 4.5Ã10-10e.m.u.âŠÏâŠ2.0Ã10-9e.m.u. at the depth of 2500km.å°ç£æ°æ°žå¹Žå€åã¯,å°çæ žè¡šé¢ãæµããé»æµã®æéçå€åã«ãã€ãŠçãããšèããããŠãã.ç£æ°å極åãat randomã«æ žè¡šé¢ã«ååžããŠããã®ãš,ç䟡ãªé»æµç³»ãèã,ãã®æéçå€åãå°è¡šé¢ã§æ°žå¹Žå€åãšããŠèŠ³æž¬ããããšä»®å®ã,ãã®é»æµç³»ã«ãã€ãŠå°è¡šé¢ã«ã§ããç£æ°ããã³ã·ã£ã«ã,å°çäžéå±€ã®é»æ°äŒå°åºŠãçš®ã
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17. Synthesis of the Non-dipole Components of the Earth's Magnetic Field from Spherical Harmonic Coefficients
The non-dipole components of the earth's magnetic field are synthesized from particular sets of spherical harmonic coefficients for 1885 and 1945, and compared with those of charted data. It has been confirmed that the synthesized field can well approximate the observed field. The standard deviations of the differences between the synthesized and charted vertical components are obtained as 1360γ for 1885 and 778γ for 1945, indicating that the total approximation errors are about 19 percent of the root-mean-square intensity of the non-dipole field itself for 1885 and 11 percent for 1945. Gauss-Schmidt coefficients of different analyses are examined to estimate between-analyses errors. It follows that for most of the analyses in the 19th century the Gauss-Schmidt coefficients can be synthesized up to n=m=4 with an uncertainty of about 20%, whilst for those in the 20th century, they can safely be used at least up to n=m=6 with an uncertainty from several to ten percent.|çåœæ°è§£æçµæãéã«åæããŠ,å°çç£å Žãååãªç²ŸåºŠã§è¿äŒŒã§ããã°,ããªãå€ãæ代ã«ããã®ãŒã€ãŠå°çç£å Žã®æ°žå¹Žå€åã詳现ã«èª¿ã¹ãããšãã§ãã.1885幎ãš1945幎ã®ããã€ãã®è§£æããšããã,ã¬ãŠã¹ã»ã·ã¥ãããä¿æ°ããéå極åç£å ŽãåæããŠç£æ°å³ããèªã¿åã€ãå€ãšæ¯èŒãã.ãã®çµæåæç£å Žã§ããªãããè¿äŒŒã§ããããšãããã€ã.åæç£å ŽãšèŠ³æž¬ç£å Žã®å·®ããšã,ãã®æšæºåå·®ãæ±ãããš,1885幎ã«å¯ŸããŠ1360γ,1945幎ã«å¯ŸããŠ778γã§,éåæ¿åç£å Žèªäœã®èªä¹å¹³åã®ãããã19%ããã³11%ãšãªã.ç°ãªã解æéã®èª€å·®ãèŠç©ãããã«,åæ代ã®è§£æã§çžå¯Ÿå¿ããã¬ãŠã¹ã»ã·ã¥ãããä¿æ°ãæ¯èŒæ€èšãã.19äžçŽã®è§£æã«ã€ããŠã¯n=m=4ãŸã§åæããããšãå¯èœã§çŽ20%ã®èª€å·®ã§ç£å Žãè¿äŒŒã§ãã.ããã«å¯ŸããŠ,20äžçŽã®è§£æã§ã¯å°ããšãn=m=6è¿ãšãããšãã§ããŠ,æ°%ãã10%ã®èª€å·®ã«ãããŸãããšãæãã«ãªã€ã
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