摘要:

The asthenosphere is assumed to be entirely at melting point temperature. This state can be steady owing to an upward migration of liquid dykes of magma through it. The vertical extension of these dykes must reach about 100 meters in order that the suggested process may become effective. The two models of ‘global tectonics’ suggested by Isacks et al. are criticized, and a new model is put forward. Lithospheric plates are pushed upslope by the ridges, where a deep crack filled with compressed magma persists in spite of a continuous symmetrical solidification on the walls. Underneath a counterflow involves the lower asthenosphere. The unconformity of the geoid undulations with the existing convection cells are explained by a time lag in the adjustment of this counterflow. Using the relative movements of the lithospheric plates given by Le Pichon, and using some simple mechanical requisites, the absolute velocities are calculated, as well as the velocities and shear stresses in the asthenosphere. For instance Africa and the Indian Ocean are drifting swiftly eastward, while South America drifts westward at only 0.4 cm/yr. It is shown that the counterflow of the asthenosphere must exactly balance the discharge of the lithosphere over it, as in a classical convection cell. The concept of ‘orogenic efficiency’ of such a cell is introduced and estimated. A new model of the sinks of lithosphere is put forward that assumes the existence of vertical faults within the sinking plate. The distance between the oceanic trenches and the first foci under the island arcs is submitted to calc

ISSN:0148-0227    

DOI:10.1029/JB074i027p06525

年代:1969

数据来源: WILEY

摘要:

A differentially connected pair of proton precession magnetometers has been operated on a baseline of 8 km, with one detector on the side of each of two neighboring active volcanoes, near to the middle of North Island, New Zealand. In April 1968, during and preceding an eruption, variations in the difference field up to 10 γ in amplitude were observed, greatly exceeding variations during the preceding months of inactivity and apparently correlated in detail with the volcanic activity. The variations were too rapid to be explicable in terms of thermal changes; the favored explanation is based on the piezomagnetic effect

ISSN:0148-0227    

DOI:10.1029/JB074i027p06541

年代:1969

数据来源: WILEY

摘要:

Ruapehu is an andesite volcano in the North Island of New Zealand whose activity is being monitored by two new instruments: a seismic power recorder described in this paper, and a differential magnetometer described in a companion paper. On April 5, 18, and 26, May 29, and June 9, 1968, episodes of activity commenced with volcanic tremor of frequency 2 c/s and radiated seismic power up to 100 kw. Between 10 hours and 7 days after the commencement, when the tremor power had decreased, strong hydrothermal eruptions occurred in the crater lake. The largest eruption, which occurred at 0749 NZST on April 27, 1968, had a peak seismic power of 9 Mw and a seismic energy of 1015ergs; it ejected nearly 105cubic meters of mud and water from the lake.

ISSN:0148-0227    

DOI:10.1029/JB074i027p06545

年代:1969

数据来源: WILEY

摘要:

A simple model consisting of volume backscattering from within the lunar regolith can explain the observed diffuse component of lunar radar echoes. At a wavelength of 68 cm, a good match of the model with the data yields a value of 2.5–3 for the relative permittivity of the regolith in good agreement with the quasi‐specular backscattering estimates. The fit at 23 cm is poorer but yields a value slightly less than 2. At 3.8 cm, the value is below 1.5, and both that value and the 23 cm value are in good agreement with the radiometrically determined values at their respective wavelengths. These lower values at short wavelengths are probably a result of the rapid density increase with depth just at the surface. A slope‐dependent reflectivity can reconcile the values of relative permittivity obtained radiometrically and from the proposed diffuse backscattering model to those obtained from radar cross‐section measurements. There is some evidence that the permittivity of the rocks forming the lunar highlands may be lower than that of the rocks underlying th

ISSN:0148-0227    

DOI:10.1029/JB074i027p06553

年代:1969

数据来源: WILEY

摘要:

Results from the study of the magnetism in serpentinized ultramafic rocks of the Red Mountain intrusion southeast of San Francisco, California, suggest that these rocks possess a chemical remanent magnetization acquired during serpentinization. The stability of magnetization decreases with serpentinization, owing to the growth of larger magnetic grains. Thus, in highly serpentinized peridotites (serpentinites), most of the CRM is destroyed and the NRM becomes mainly viscous remanent magnetization, rendering these rocks unsuitable for paleomagnetic work. On the other hand, in partially serpentinized peridotites, pyroxenites, and dunites, the CRM is highly stable and paleornagnetically reliable. A study of the stable directions of magnetization indicates that the Red Mountain ultramafic body was emplaced and serpentinized prior to the folding of the Franciscan formation. The stability of magnetization and removal of viscous components were verified by the results of a storage test and alternating‐field demagnetization experiments, by the divergence of the mean directions of magnetization from that of the present field, and, most important, by the convergence of the directions after tilt correction. Paleomagnetic pole positions calculated from the mean directions suggest that the magnetic pole rapidly migrated southward into the Atlantic Ocean during late Mesozoic, possibly more than once, as proposed previously by Grommé and Gluskot

ISSN:0148-0227    

DOI:10.1029/JB074i027p06567

年代:1969

数据来源: WILEY

摘要:

The method recently applied by P. M. Muller and W. L. Sjogren in mapping gravity anomalies of the moon employed a new principle of gravity measurement: time differentiation of Doppler‐tracked velocities. Utilization of this method offers a promising new area of planetary investigation yielding data that, up to present, would have required landed mobile instrumentation. Analysis of the method shows that, unlike conventional terrestrial gravity, Doppler gravity varies in direction, depending on the configuration of the Doppler system; other configurations are possible which may yield data of improved analytical quality. A comparison of the Mare Imbrium Doppler gravity anomaly with theoretical curves indicates that the source is plate‐like in shape. Topographic effects, however, must be evaluated before a complete analysis is possible. Comparison of anomalies and topography indicates that lunar surface features are not in isostatic equilibrium. Minor modifications of the present system that should yield data of improved analytical quality include more circular orbits and lower altitu

ISSN:0148-0227    

DOI:10.1029/JB074i027p06579

年代:1969

数据来源: WILEY

摘要:

The magnitude, colatitude, longitude, and distance from the center for radial dipoles were adjusted by a least‐squares procedure so as to reproduce the surface magnetic field synthesized from the Fineh‐Leaton (F.L.) 1955 Spherical Harmonic Coefficients and for the 1965 International Geomagnetic Reference Field (IGRF). The rms residual was reduced to 28 γ using 21 dipoles for the 1955 field (6th degree and order) and to 25 γ using 35 dipoles for the IGRF (8th degree and order). These residuals include all components of the surface field. The dipole parameters were then varied using a least‐squares method to approximate the secular change field. These solutions match theZcomponent of secular change field synthesized from Leaton's 1955 coefficients to a rms residual of 2.15 γ per year and the IGRF secular change field to a rms residual of 1.26 γ per year. In both cases the dipole sources were found to be approximately 0.2 earth radius from the center of

ISSN:0148-0227    

DOI:10.1029/JB074i027p06583

年代:1969

数据来源: WILEY

摘要:

In an earlier paper Bolt and Nuttli presented evidence that thePwave delay times, relative to Berkeley, at a number of seismographic stations in central and northern California, exhibit a significant dependence on azimuth of wave approach. Numerical analysis of these data indicates that their amplitudes are too large to be accounted for by crustal structure only. The data can be explained by variations in the depth to both the top and the bottom of the mantle low‐velocity channel forPwaves. The derived model which is preferred has a velocity of 7.9 km/sec at the top of the mantle, 7.2 km/sec in the channel, and 8.2 km/sec below the channel; calculation then shows that the channel is thickest between the coast and the Central Valley of California (average depth to top of 45 km and to bottom of 225 km) and thins eastward (depths to top of 90 and 95 km and to bottom of 200 and 165 km at Shasta and Mineral, respectively) below the Sierra Nevada. The data also suggest that the lower surface of the channel perhaps rises in a westward direction from the coast. The model is compatible with Bouguer gravity data if one includes a crustal root extending to depths of the order of 40 km under the Sierra Nevada. The model is also consistent with existing heat flow data, which indicate that heat flow is less than normal in the Sierra Nevada and higher than normal in the regions to the west and eas

ISSN:0148-0227    

DOI:10.1029/JB074i027p06594

年代:1969

数据来源: WILEY

摘要:

For a representative selection of spherical earth models compatible with seismic observations, there is a significant frequency range over which the fundamental and first higher Love mode group velocity curves approach each other closely or actually overlap. Higher Love modes can be excited comparably to the fundamental mode for both shallow and deep sources under a variety of circumstances, and thus higher mode interference is an important factor to be taken into account in the proper interpretation and analysis of Love waves. Simple theoretical computations reveal the nature of the effect of mode contamination on measured phase velocities, and biases in the selection and analysis of surface wave data make it appear likely that fundamental mode Love waves experiencing higher mode interference will exhibit anomalously high apparent phase velocities, as observed in the United States midcontinent and in Japan. Thus it is suggested that it may not be necessary to resort to complex or anisotropic models to explain these observations. Consideration of the effects of mode interference, as well as knowledge of source parameters (type, depth, orientation), are important in determining dependable fundamental and higher Love mode phase velocity dispersion over a broad frequency band. Phase velocity filtering across a large array could effectively separate the modes. Love waves traversing continental regions from the source will suffer little or no contamination from higher modes for wave periods of less than about 60 sec and thus are preferred to oceanic paths in obtaining reliable fundamental mode data in this restricted frequency range.

ISSN:0148-0227    

DOI:10.1029/JB074i027p06603

年代:1969

数据来源: WILEY

摘要:

In some circumstances the first higher mode of the Love surface wave can be significantly excited in the period range 30–90 sec by earthquakes and may travel at a group velocity comparable to that of the fundamental mode. This difficult‐to‐separate contamination will produce a large scatter, but no uniform bias, in phase velocities measured from an ensemble of events. The phase velocities measured from a single event may show a bias over a limited range of periods, but this perturbation may be positive or negative. These results argue against an explanation of observed inconsistencies of Love and Rayleigh wave phase velocities as being due to higher mode contamination. The results are also applicable to the effect of multipath interference of two similar‐mode waves traveling at an angle with respect to one

ISSN:0148-0227    

DOI:10.1029/JB074i027p06612

年代:1969

数据来源: WILEY