摘要:

Woodlark basin extends from the eastern tip of New Guinea to New Georgia, in the central solomon Islands. The basin is characterized by extremely rugged bathymetry, with most lineations paralleling its long axis. The bathymetric pattern, seismicity, and upper mantleP‐wave velocity, as well as the petrology of rocks exposed on the surrounding islands, suggest that seafloor spreading is taking place within the basin. This spreading may account for the marked discontinuity in the solomon Islands' marginal trench and for the lack of deep earthquakes near New Georgi

ISSN:0148-0227    

DOI:10.1029/JB075i035p07335

年代:1970

数据来源: WILEY

摘要:

The Red Sea, Gulf of Aden, and African rift systems meet at the Ethiopian Afar. The Afar depression is generally agreed to represent oceanic crust, part of the Red Sea–Gulf of Aden floor produced by drift of Arabia from Africa. This simple thesis faces several difficulties: the fact that most of Afar lies anomalously above sea level; the existence of two large sialic horsts separating Afar from the Red Sea and the Gulf of Aden; the parallel development of spreading axes in the Red Sea and northern Afar; the complex and intense fault‐belt virgations of the Afar floor, and the absence of ridge‐transform structures; the concentration of seismic‐energy release along the western margin of Afar, and not along the axis of the depression; and the contrasting gravity situation of Afar with that of the Red Sea. From the known data on Afar, a new analysis in terms of plate tectonics shows how the sialic horsts have played a fundamental role in deter‐mining the existence of Afar. A concept of bifurcated spreading zones is introduced that does not require the existence of transform faults and leads to rotational structures and longitudinal shear within the Red Sea basin. The influence of the Gulf of Aden spreading on Afar has become significant only in the Quaternary, and the dynamic evolution of the pattern of spreading at the triple‐rift junction is emphasized, contrary to a static picture inferred from D. P. McKenzie and W

ISSN:0148-0227    

DOI:10.1029/JB075i035p07340

年代:1970

数据来源: WILEY

摘要:

Velocities of both the compressional and the shear waves measured on seven synthetic olivines with different Fe/Mg ratios are reported. The olivine compositions studied were 100% Fo, 95% Fo, 90% Fo, 85% Fo, 80% Fo, 50% Fo (where Fo stands for forsterite), and 100% fayalite. Effects of the Fe/Mg ratio on the compressional, shear, and bulk velocities, the bulk modulus, and the seismic parameter are examined for olivine at ambient conditions. An iron substitution in the olivine lattice results in a systematic decrease in these velocities, the seismic parameter, and the bulk modulus such that, for fayalite as compared with forsterite, there is a 22% decrease inVp, a 30% decrease inVs, a 16% decrease inVΦ, a 30% decrease in seismic Φ, and only about a 4.7% decrease inKs. The iron substitution increased Poisson's ratio of olivine by 19% for fayalite relative to forsterite. It was also found that the bulk modulus of olivine decreases almost linearly with increases in the iron content in the olivine lattic

ISSN:0148-0227    

DOI:10.1029/JB075i035p07353

年代:1970

数据来源: WILEY

摘要:

Evidence for a velocity increase at shallow depth within the upper mantle was found during a seismic refraction experiment in the Gulf of Mexico. The shots were fired in the Gulf of Mexico and observed in Mexico and Florida. This paper reports on the analysis of the travel times to the Mexican stations and on their interpretation in terms of crust and upper‐mantle structure. A break in apparent velocity from about 8 km/sec (corresponding toPn) to a velocity of 8.77 km/sec occurred at a distance of about 360 km. This change in apparent velocity is interpreted as arising from a discontinuity, or rapid increase of velocity with depth, at a depth of 57 km. It is suggested that the discontinuity is due to a phase transition. ThePamplitudes decrease appreciably beyond 700 km. The reduction in amplitude is interpreted as being due to penetration of the rays into a layer of decreasing velocity. Several models with low‐velocity zones between 100 and 200 km depth are consistent with these observati

ISSN:0148-0227    

DOI:10.1029/JB075i035p07362

年代:1970

数据来源: WILEY

摘要:

Theoretical source spectra that are closely tied to empirical evidence from explosions and earthquakes are presented. The effects of the transfer functions of a layered medium on these source spectra are shown and a theoretical earthquake‐explosion spectral ratio is defined for Rayleigh waves. By using spectral estimates on many widely distributed events, it has been found that explosions and earthquakes can be separated by splitting Rayleigh‐wave energy between 10 and 50 sec into two period bands and calculating their ratio. Explosion ratios are confined to a narrow range. Earthquake ratios have a large scatter because they depend on depth and source mechanism paramet

ISSN:0148-0227    

DOI:10.1029/JB075i035p07382

年代:1970

数据来源: WILEY

摘要:

The ancient geomagnetic field intensity was obtained by comparing the magnitude of natural remanent magnetization with that of thermoremanent magnetization induced in a known field. Throughout the study the step‐wise heating method was used; measurements of both partial thermoremanent magnetization and the change in natural remanent magnetization were made for each temperature interval. The material studied was pottery from eastern Japan covering the period from 9300 to 2500 years ago. It was concluded that the geomagnetic field at 2000 B.P. was about 1.6 times as large as that at present. The field intensity showed the minimum of about half the present value at nearly 5500 B.P. Assuming a simple periodic change of the field intensity for the convenience of further calculations, the secular variation gives a sinusoidal curve as a first approximation. The secular variation of atmospheric C14concentration calculated for the observed variation in field intensity is in good agreement with values obtained from tree‐ring samples. TMs supports the suggestion that the variation of the geomagnetic field intensity plays an important role in the control of the concentration of C14in the atmosph

ISSN:0148-0227    

DOI:10.1029/JB075i035p07403

年代:1970

数据来源: WILEY

摘要:

During 1968 about 7500 km of new magnetic data Were recorded by the USNSJ. W. Gibbsbetween the Canary and Cape Verde islands, from the continental shelf to approximately 30°W. These data, together with an equal amount of data from other sources, reveal major magnetic features. The magnetic boundary between relatively undisturbed and disturbed magnetic zones is delineated near the middle of the northwest African continental rise. A sequence of linear north‐south‐trending anomalies immediately seaward of the magnetic boundary comprise a band about 300 km wide and can be correlated from about 15°N at the Cape Verde Islands to about 26°N just south of the Canary Islands. The band of magnetic anomalies appears to have a right lateral offset of about 100 km near 25°N where intersected by a west‐northwest‐trending fault near the eastward projection of the Atlantis fracture zone from the mid‐Atlantic ridge. At least one prominent positive magnetic anomaly is associated with the northwest, African continental shelf. The magnetic disturbance boundary and the associated band of linear magnetic anomalies are nearly mirror images of similar anomalies associated with the continental margin off eastern

ISSN:0148-0227    

DOI:10.1029/JB075i035p07412

年代:1970

数据来源: WILEY

摘要:

The Wachtman‐Anderson relation has consistently shown itself to be an accurate interpolation equation for the experimental isobaric relationship between the isentropic bulk (Bs) or Young's (Ys) modulus and the temperatureT. Here the equation is analyzed in some detail, and it is shown that, although the equation is an approximation, a high degree of consistency is maintained between the relation itself and the Grüneisen equations from which it has been derived. In particular it is shown that the relation may be used to obtain reasonable estimates of the derivative (∂2Bs/∂T2) along the zero‐pressur

ISSN:0148-0227    

DOI:10.1029/JB075i035p07421

年代:1970

数据来源: WILEY

摘要:

It has been suggested that the seismograph recording of the Apollo 12 lunar module (LM) impact was the result of a spray of secondary ejecta around the seismometer rather than the result of seismic waves propagated through the moon. We have made a theoretical study of the ballistic trajectories, plausible angular distributions, and seismic signals to be expected from such a spray. Secondary ejecta cannot account for signals arriving earlier than 45 seconds, unless there was a considerable elevation difference between the seismometer and the point of LM impact, but could explain the remaining portion of the signal, provided that the angular distribution of the secondary ejecta is assumed to peak sharply in the vertical direction. Hence it is concluded that one can neither prove nor disprove the hypothesis that a substantial portion of the signal is due to secondary ejecta.Any future controlled impact at a significantly greater distance may show a more discernible difference between the onset signal, corresponding to a seismic propagation velocity of 3–4 km/sec measured by Latham, and a later signal whose arrival corresponds to a flight velocity of 1.68 km/sec. The proposed model also indicates that the time of occurrence of the broad peak should scale as the square root of the distance. Since meteoroid impacts can be expected to give a more peaked angular distribution of secondary ejecta, the model may be more applicable to meteoroids or meteoroid‐like impacts than to the

ISSN:0148-0227    

DOI:10.1029/JB075i035p07426

年代:1970

数据来源: WILEY

摘要:

Elastic properties of the upper few centimeters of the lunar surface material are estimated from frequency and decay rate of vertical oscillations of Surveyor spacecraft on landing. Data from tests using a duplicate spacecraft resting on a rigid surface and on test soils provided the elastic properties of the spacecraft and the expected changes in resonant frequency of oscillations when going from a rigid to a compliant surface. The surface stiffness is obtained by matching the output of an analog computer model with the test and lunar data. From the stiffness of the lunar material a range of possible seismic velocities is obtained. Compressional and shear velocities are estimated to be about 45 m/sec and 23 m/sec, respectively. These results appear to be consistent with data from the Apollo program.

ISSN:0148-0227    

DOI:10.1029/JB075i035p07439

年代:1970

数据来源: WILEY