摘要:

A method of calculating the past states of the earth‐moon system is developed. The method is based on the existence of three distinct time scales for dynamical change. The short time scale is determined by the revolution periods of the sun and moon about the earth or, equivalently, by the year and current month. The intermediate time scale is set by the precessional motions of the lunar orbit plane and the earth's equator plane. The rate at which tidal friction alters the state of the earth‐moon system defines the long time scale. The equations of motion governing the earth‐moon system are successively averaged over the short and then the intermediate time scales. These averaged equations are then integrated back a short interval on the long time scale. The equations of motion appropriate to this new state of the earth‐moon system are then re‐averaged on the short and intermediate time scales, and once again the averaged eqations are stepped back on the tidal time scale. The first step in this procedure (i.e. averaging on the short time scale) is performed analytically, whereas the calculations on the intermediate and long time scales require the use of a large computer.At present, the inclination of the lunar orbit plane to the ecliptic remains nearly constant during the precessional motion. On the other hand, if the moon's semimajor axis were ever less than 10R⨁, the inclination of the lunar orbit plane would have maintained a fixed value with respect to the earth's equator plane. The current investigation shows that this inclination could never have been less than 10° and, therefore, that the moon could never have moved on an equatorial orbit. This result contradicts theories that postulate fission of the earth to form the moon and also those which propose that the moon formed by accretio

ISSN:8755-1209    

DOI:10.1029/RG004i004p00411

年代:1966

数据来源: WILEY

摘要:

The subject of natural tritium is reviewed from the inception of the search for this isotope in 1932 until the present. Three sources of natural tritium are considered: production in the atmosphere by galactic cosmic rays, production in the atmosphere by solar flare accelerated particles, and accretion from the sun. A recalculation of the cosmic‐ray production rate utilizing experimental data for the last solar cycle yields a worldwide average of 0.20 ± .09 triton/cm² sec during solar minimum and 0.16 ± .09 triton/cm² sec during solar maximum. Production of tritium by interaction of solar flare accelerated particles with the atmosphere is found to be less than 3% of the production by galactic cosmic rays. Calculations of the tritium decay rate from material balance, utilizing measured tritium concentrations of rain and ocean water corrected for synthetic tritium, are consistent within the limits of error with the production rate by galactic cosmic rays. The best estimate of the pre‐bomb inventory corresponds to a decay rate of 0.5 ± .3 triton/cm² sec as compared with the estimated production rate of 0.19 ± .09 triton/cm² sec. In view of the large errors it is not possible to determine whether appreciable amounts of tritium are accreted f

ISSN:8755-1209    

DOI:10.1029/RG004i004p00441

年代:1966

数据来源: WILEY

摘要:

Known mechanisms of internal friction in crystalline materials are examined to find which are likely sources of seismic attenuation in the earth's mantle. Presently available laboratory data on rocks are not useful for this purpose because of the low temperatures and pressures at which they have been determined, but experiments on sintered oxides indicate the important damping mechanisms at high temperature. The sources of seismic attenuation in the order of their probable importance are viscous grain boundary damping, stress‐induced ordering, and dislocation damping. If any parts of the mantle are partially melted, stress‐induced flow of fluid through inter‐granular channels may also cause attenuation. These damping mechanisms characteristically result in internal friction showing a strong frequency dependence. Scattering is almost certainly not an important source of seismic attenuation. The variation with depth of internal friction due to a thermally activated relaxation mechanism is found, and it is shown that such a source of internal friction can account for existing data on the anelasticity of the earth. The low attenuation observed at depths below about 500 km can result from the effect of pressure in decreasing atomic mobility; the observed attenuation decrease is therefore not in itself evidence of a phase change. The relation between damping and the strength of the mantle is developed, and it is shown that seismic attenuation data do not put useful bounds on this qua

ISSN:8755-1209    

DOI:10.1029/RG004i004p00457

年代:1966

数据来源: WILEY

摘要:

The ocean can be regarded as a horizontally stratified waveguide for the transmission of acoustical signals. The use of transducer arrays in the ocean presents special problems that are not present in an infinite homogeneous medium. For example, the signal at the receiver is the sum of many arrivals and each arrival is dispersive. In the normal mode formalism, the solution for a single frequency source can be expressed as the product of spectrum functions. Each spectral component is the horizontal component of wave number κ. The responses of waveguide transmission function, transmitting arrays, and receiving array can be expressed as functions of κ. Because of the horizontal stratification, horizontal and vertical arrays have quite different properties and are considered separately. The horizontal array is much less critical in design and operates more as it would in homogeneous space. The vertical array operates as a mode filter and is extremely sensitive to the depth and phase adjustment of the transducers. The signal output of the receiving array filter is the product of source, transmission, and receiver functions of κ. The maximum signal‐to‐noise ratio is obtained when the receiver array amplitude and phase response are the complex conjugate of the signal field at the receiver divided by the noise power. The condition is analogous to the matched filter condition of electrical circuit theory. The effect of fluctuations of the stratification of the ocean on sound transmission is included in an approximate treatment. The radiation in a mode is assumed to be trapped, and the radiation scattered by inhomogeneities can be ignored. The average signal transmission was found to depend on the correlation functions and distribution functions of the irregularities. The maximum signal‐to‐noise ratio is obtained when all modes or arrivals can be added coherently; i.e., no phase fluctuations exist. The performance of the arrays decreases as the phase fluctuation increases. Upper and lower limits of the signal‐to‐noise ratio are given for small and large phase fluctuations. The degree of coherence of the modes has been related to the reproducibility of the signal transmission between a single source and a s

ISSN:8755-1209    

DOI:10.1029/RG004i004p00475

年代:1966

数据来源: WILEY

摘要:

The Cenozoic structures of the western United States are interpreted here as being products mostly of horizontal motion of the crust. The distribution of strike‐slip faulting, tensional fragmentation of the brittle upper crust or rupturing of the entire continental crust, and compression define a pattern of northwestward motion increasing irregularly southwestward toward coastal California. Hans Becker, in 1934, and S. W. Carey, in 1958, are among those who have suggested such a tectonic system.The aggregate Cenozoic right‐lateral displacement of Cretaceous and older rocks and structures by the northwest‐trending strike‐slip faults of coastal California is about 500 km. The greater part of this movement has occurred along the San Andreas fault, but many other faults share in it. At least six earthquakes within the past century have been accompanied by lateral displacements at the surface along faults of the San Andreas system. Successively greater offsets of successively older geologic terranes demonstrate continuing motion throughout Cenozoic time. Late Miocene materials have been displaced at least 160 km; Oligocene, at least 260 km. The present velocity of regional shear strain, about 6 cm/yr, demonstrated by geodetic resurveying in southern and central California, is about 8 times faster than the average needed to account for the total movement within the Cenozoic. The faults are in general associated with structures formed by oblique tension south of Los Angeles and with structures due to oblique compression north of that city. The opening of the Gulf of California and the Salton Trough by the oblique rifting of Baja California and the Peninsular Ranges away from mainland Mexico is the greatest of the tensional effects.The strike‐slip faults may be confined to the crust. Earthquake foci extend no deeper than 16 km. The faults end to the south in the Gulf of California, whose crustal structure is oceanic. To the north, the San Andreas turns seaward as the north‐facing Gorda scarp, west in line of which in deeper water is the south‐facing Mendocino escarpment, produced apparently by an inactive left‐lateral oceanic fault. The continental sliver of coastal and Baja California, west of the faults of the San Andreas system, may be drifting northwestward independently over the ocean floor and the mantle, and the leading point of the sliver may have been deflected westward when it hit the Mendocino scarp on the sea floor.East of this coastal movement system is the Basin and Range province, whose obvious Cenozoic structures are dominated by block faulting. The present ranges have formed mostly since early Miocene time, similar older ranges having been destroyed by erosion and deformation. The normal faulting, which is not associated within the region with any complementary tectonic compression, requires crustal extension as its basic cause. If the faults maintain their average 60° dips at depth, extension is half the dip‐slip amount; but probably the major faults flatten downward, and the amount of extension about equals that of shallow dip‐slip. Total Cenozoic extension in northern Nevada and Utah may have been 300 km. Concurrent volcanism much augmented the thinned and fragmented crust, and the volcanic terranes in turn have been fragmented by block faulting.Right‐lateral strike‐slip faults trend northwestward in lanes between normal‐fault maintain blocks in the southwestern part of the Basin‐Range province. Cenozoic displacements reach 50 km on the Las Vegas fault and 80 km on the Death Valley‐Furnace Creek faults. Northeast of the strike‐slip faults, ranges and basins trend north‐northeastward in tension‐gash orientation. Within the belt of lateral faulting, ranges undergoing active normal faulting mostly trend north‐northwestward in oblique pull‐apart orientation. The Sierra Nevada and Klamath Mountains have moved northwestward and rotated counterclockwise, thus moving away from the continental interior more in the north than in the south, and the extension distributed behind them has formed the Basin‐Range province.The narrow block‐fault Rio Grande valley system of New Mexico and southern Colorado is structurally and topographically similar to the rift valleys of East Africa and reflects localized crustal extension. The Idaho batholith, like the Sierra Nevada batholith, is drifting northwestward as an unbroken plate. Extension east of the Idaho batholith is taken up by normal‐fault fragmentation in south‐central Idaho and southwestern Montana, whereas extension south of the batholith has produced a rift through the continental crust, the Snake River Plain, filled deeply by lava. Seismic velocities indicate granitic crust to be lacking in at least the western part of the plain. Right‐lateral faults of the Osburn system bound the batholithic plate on the north, and the motion they represent is taken up north of them by extension forming fault troughs.Integration of geologic and geophysical information shows that large regions of the Northwest are lava accumulations of continental crustal thickness, not old continental crust covered by lava. The volcanic terrane of northwestern Oregon and southwestern Washington forms new volcanic crust in a region which was oceanic before Cenozoic time. The volcanic terrane of southeastern Oregon, northeastern California, and northwestern Nevada fills an irregular tension rift through the Mesozoic continental crust. This rift resulted from the westward motion of the Klamath Mountains region, which was sundered from a position south of the Mesozoic terrane of northeastern Oregon and which was bent oroclinally as it moved westward in post‐middle Eocene time. The Mesozoic terrane of northeastern Oregon pivoted away from the Idaho batholith to form a smaller orocline and left a triangular rift since filled by lava. Independent motion of continental crust over mantle and oceanic crust seems to be indicated. Inertial forces due to redistribution of rotational momentum among crustal frag

ISSN:8755-1209    

DOI:10.1029/RG004i004p00509

年代:1966

数据来源: WILEY