摘要:

An acoustic (3 MHz) backscattering profilometer has been developed for study of sediment transport in the bottom boundary layer. Laboratory experiments indicate reasonable correlations between backscattering intensity and mass concentrations for well‐sorted fine to coarse quartz sands typical of those found in marine sediments. Departure of the calibration curves from their expected values is thought to be due to the non‐spherical shape of the quartz grains. Field experiments on the inner shelf using the profilometer combined with velocity profiles show individual profiles (taken at 1 Hz) during erosion events are characterized by highly variable concentrations. Surface wave groups rather than individual waves appear to be correlated with groups of profiles with high concentrations extending 25‐100 cm above bottom. Averaging produces smooth profiles which fit well log‐normal distributions predicted by steady flow suspended load t

ISSN:0094-8276    

DOI:10.1029/GL009i003p00175

年代:1982

数据来源: WILEY

摘要:

Global great circle measurements of regionalized mantle Love wave phase velocities are interpreted in terms of regional models. The same study had been made by J.J. Lévêque (1980) for Rayleigh waves, and the resulting models for the two oceanic regions of different ages are used as a basis for comparison : the observed Love wave dispersion cannot be explained with these models if isotropic. The models obtained by inversion of Love wave data are compared with the models mentioned; the discrepancy appearing in the 250km depth range between the velocities βHand βVof respectively SH and SV waves is indicative of polarization anisotropy. Moreover, we put forward a significant variation from young to old oceans : the difference between βHand βVis of the order of 1 % for the former, compared to 3% for the latter. This variation can bring information about the behaviour of upper mantle materials in connection with the motion of oceanic p

ISSN:0094-8276    

DOI:10.1029/GL009i003p00179

年代:1982

数据来源: WILEY

摘要:

We measure the seismic moment of five great (Ms≥ 7.8) earthquakes from 1928 to 1942 along the Mexican portion of the Middle American Trench directly from recorded seismograms, and obtain considerably lower moment values than had previously been estimated fromMs. Such direct moment determinations are inherently more reliable.As the average seismic slip rate along this portion of the arc appears to be essentially constant, these measurements of the past seismic moment are used to estimate that the current (end of 1980) moment deficiency, is 46 × 1027dyne‐cm, or an average slip of 97 cm along the arc. Although this is much lower than past estimates, at the current rate of increase, 1.6 × 1027dyne‐cm/yr (3.4 cm/yr of seismic displacement), the moment deficiency by 1985 will be the equivalent of five magnitude 8 earthquakes; and by 1990, six such events. The slip along this portion of the Middle America Trench arc appears to fit Shimazaki and Nakata's "slip predictable" model, in which the size, but not the time of occurrence, of future large events may be es

ISSN:0094-8276    

DOI:10.1029/GL009i003p00182

年代:1982

数据来源: WILEY

摘要:

The fracture energy of Sioux quartzite in air and water measured with the wedge‐loaded double cantilever beam fracture specimen is found to depend on the angle of the wedge used. The measured fracture energy is greater when the wedge is sharper. This is ascribed to the decrease in the compressive stress acting on the specimen. It is proposed that the compressive stress aids in the formation of the microcracks necessary for crack extension. Measurements of the width of the microcrack zone (visible with the scanning electron microscope) adjacent to the primary crack support this interpretatio

ISSN:0094-8276    

DOI:10.1029/GL009i003p00186

年代:1982

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL009i003p00190

年代:1982

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL009i003p00193

年代:1982

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL009i003p00195

年代:1982

数据来源: WILEY

摘要:

Major solar flares during 1967‐1970 are significantly more likely to occur in active regions whose inferred overlying large‐scale (∼100,000 km) magnetic flux is oriented preferentially north‐to‐south than for south‐to‐north or indeterminant orientations. This purely solar effect may be the dominant cause of previously reported correlations between southward solar active region magnetic fields and enhancements in geomagnetic disturbances, solar wind velocities and (newly reported here) solar flare proton fluxes. If this effect may be generalized to other portions of the solar cycle, it could offer a fundamental clue to the flare mechanism which should also prove useful in the prediction of solar‐terrestri

ISSN:0094-8276    

DOI:10.1029/GL009i003p00199

年代:1982

数据来源: WILEY

摘要:

Reconnection of magnetic field lines at a neutral line that forms in the near‐earth region of the plasma sheet at substorm onset creates, from some longitudinal sector of the plasma sheet, a plasmoid that is then free to flow out of the magnetotail into the magnetosphere's wake. The magnetic field in the departing plasmoid frequently displays a strong positive or negative YSMcomponent that has been difficult to reconcile with the reconnection geometry as it is traditionally depicted in two dimensions. We show that this deviation of the magnetic field is a manifestation of a draping of the newly reconnected field line loops toward the tail's central (midnight) meridian and that the draping is a consequence of the three‐dimensional plasma flow associated with the reconnection proc

ISSN:0094-8276    

DOI:10.1029/GL009i003p00203

年代:1982

数据来源: WILEY

摘要:

The electric field experiment on board ISEE‐1 had a special mode of operation in which bursts of very high time resolution measurements were stored and slowly read‐out. This mode of operation allows us to study in detail the waveforms of the large amplitude electrostatic waves observed at the Earth's bow shock in a frequency range (10‐500 Hz) not well covered previously. These waves appear to be correlated with two streams in the ion angular distribution. Our observations indicate, therefore, that the ion‐ion stream instability is the dissipation mechanism of a collisionless fast quasi‐perpendicular s

ISSN:0094-8276    

DOI:10.1029/GL009i003p00207

年代:1982

数据来源: WILEY