摘要:

Comparison of travel‐time and amplitude data from two Oblique Seismic Experiments located on 110 m.y. old oceanic crust in the western Atlantic and northwestern Pacific Oceans shows that compressional velocities within layer 2 are, within experimental error, identical at the two sites. This result implies that, at least for 110 m.y. old crust, the velocity structure in layer 2, where extrusive lavas grade downward into a sheeted dike complex, is independent of the spreading rate at which the crust was create

ISSN:0094-8276    

DOI:10.1029/GL012i005p00219

年代:1985

数据来源: WILEY

摘要:

Recent studies indicate three problems with the concept of continental drift as an incidental corollary of plate movemement: (1) Slab pull can not drive plates with continental leading edges, (2) There is no low velocity zone under shields, and (3) Continents have "roots" 400 to 700 km deep. These problems imply that if continental drift occurs, it must use mechanisms not now understood, or that it may not occur at all, plate movement being confined to ocean basins.

ISSN:0094-8276    

DOI:10.1029/GL012i005p00223

年代:1985

数据来源: WILEY

摘要:

Steady state axisymmetrical convection experiments in shells with external to internal radii ratios equal to those of the whole or the lower mantle of the Earth are performed using a finite difference method. The fluid is incompressible and can have either a constant or a temperature dependent viscosity. The Rayleigh number is less than 11 times the critical number, and the Prandtl number is infinite. A transition from a 3 or 4 cell to a 2 cell steady state structure with polar rising current is observed to occur systematically for a Rayleigh number about 7 to 8 times the critical number. In spite of the low Rayleigh number used, the structure we obtain is compatible with the strong axisymmetrical degree two pattern of the geoid anomalies or the seismic velocity anomalies.

ISSN:0094-8276    

DOI:10.1029/GL012i005p00227

年代:1985

数据来源: WILEY

摘要:

Surface wave raytracing calculations for periods of 150 to 250 s were performed using the two models of global phase velocity heterogeneity produced by Harvard and CIT, providing a comparison of predicted amplitude and phase anomalies. Theoretical R3/R2and G3/G2amplitude asymmetries resulting from focussing and defocussing have similar azimuthal distributions of the more pronounced anomalies for the two structures. The azimuthal patterns of long‐period Rayleigh and Love wave phase anomalies about a given source region also resemble one another for the two models. Travel time errors as large as 12 s are accumulated for 200 s period R3arrivals by neglecting the raypath deviations from great circles. Such errors may be significant when inverting for earth structure or when applying corrections for lateral heterogeneity in source studie

ISSN:0094-8276    

DOI:10.1029/GL012i005p00231

年代:1985

数据来源: WILEY

摘要:

Group velocity dispersion was determined for Rayleigh waves between periods of 0.5 and 2.0 sec (Rg) recorded from quarry blasts in southern New England. The paths of these surface waves traverse various geologic structures, and distinct dispersion regions have been identified. Normal dispersion was consistently observed in the period range studied, indicating a superficial layer of relatively low velocity (4.1

ISSN:0094-8276    

DOI:10.1029/GL012i005p00235

年代:1985

数据来源: WILEY

摘要:

Analyses of results of the National Geodetic Survey's leveling refraction tests indicate that the standard deviation about the mean (σ) for high‐scale minus low‐scale rod readings closely correlates with measured refraction error. Use of this relation in conjunction with values for σ obtained from routinely constrained surveys provides a basis for estimating the refraction error associated with levelings of stipulated order and

ISSN:0094-8276    

DOI:10.1029/GL012i005p00239

年代:1985

数据来源: WILEY

摘要:

Conflicting results on tidal variation of seismic travel times exist in the literature. With improved methods, we have conducted a seismic survey at a Massachusetts granite quarry. The survey was conducted in the intervals (230d 23h, 231d11h) and (231d22h, 233d10h), 1983 (U.T.) along a 148 m baseline situated in nearly flat topography. The source for the present experiment was an air gun placed in a mud‐filled pit. Travel times for the first five body wave extrema were analyzed. The results are: I. The 1st, 2nd, 4th, and 5th extrema show travel time variations; the 3rd extremum remains constant throughout the experiment. The magnitude of the fractional travel time variation, Δt/t, ranges from 0.5 to 0.9%. II. Changes in travel times of the 1st and 2nd extrema correspond to opposite changes in travel times of the 4th and 5th extrema. III. Two sets of nearly orthogonal joint systems are observed in the granite; the theoretical tidal strain in the direction perpendicular to the nearly vertical joints matches the travel time variations of the 1st and 2nd extrema whereas the tidal strain in the direction perpendicular to the nearly horizontal sheets matches the travel time variations of the 4th and 5th extrema, when a 4‐hr delay is introduced for all the tidal strains. These results are interpreted in terms of the velocity changes of seismic rays as the two joint systems open and close due to the tidal st

ISSN:0094-8276    

DOI:10.1029/GL012i005p00243

年代:1985

数据来源: WILEY

摘要:

The results of a first‐principles model for the equation of state of MgO from ambient to combined pressure‐temperature conditions assumed for the earth's lower mantle are presented. Ab initio electron‐gas pair potentials are used for the short‐range interactions. The thermal contribution to the equation of state is determined from the quasiharmonic phonons calculated from the pair potentials. The model successfully reproduces the room‐temperature compression curve measured to ∼1 Mbar and the thermal expansion at zero pressure. The volume coefficient of thermal expansion (298 K) is calculated to drop to 30% and 18% of the zero‐pressure value at 0.5 Mbar and 1.0 Mbar,

ISSN:0094-8276    

DOI:10.1029/GL012i005p00247

年代:1985

数据来源: WILEY

摘要:

Paleomagnetic results from late Quaternary sequences of lava flows, wet sediments, and dry sediments are used to investigate the long‐term statistical behavior of the Earth's magnetic‐field secular variation. Two statistical parameters, ΔI anomaly (average site inclination minus expected site axial‐dipole inclination) and virtual‐geomagnetic‐pole angular dispersion, are estimated from these data sets. Internal comparison of the late Quaternary statistical results indicates that the lava flows, wet sediments, and dry sediments all yield high quality statistics, and that there is no systematic "inclination error" associated with either the wet or dry sediment data. Comparison of the late Quaternary statistical results with similar statistical estimates for the last 5 million years, on average, indicates that the late Quaternary global pattern of ΔI anomaly is unchanged (self‐stationary) from its 5‐million‐year average, but that the late Quaternary global pattern of angular dispersion is significantly different (nonstationary) from its 5‐m

ISSN:0094-8276    

DOI:10.1029/GL012i005p00251

年代:1985

数据来源: WILEY

摘要:

Aerosol optical thicknesses calculated from solar spectral extinction measurements made with a sunphotometer on board a NASA Convair 990 aircraft in May, 1983 are presented. The measurements were obtained over the latitude range 71°N to 56°S.The latitudinal distribution of the optical thickness at 500 nm wavelength shows distinct minima of .01‐.02 at 25° latitude in both the hemispheres, a broad maximum of about 0.12 between 50°N and 60°N latitude and a maximum of about 0.1 around 10°S and 40°S latitude. The wavelength dependence of aerosol optical thickness is nearly neutral.The median radius of aerosol particles is estimated to be between 0.16µ and 0.18µ at latitudes between 12°S and 71°N, with the maximum of 0.18µ observed between the equator and 25°N latitude. A median radius in the range from 0.11µ and 0.15µ is found at latitudes south of 12°S.An aerosol mass density of .04‐.05 g/m² was estimated at latitudes between 50°N and 60°N and of .02‐.03 g/m² at other latitudes in the northern hemisphere. In the southern hemisphere it was .02‐.03 g/m² between 35°S and 40°S and .01‐.02 g/m² at other latitudes. Rough estimates of aerosol mass indicate that about 1.5 megatonnes of aerosol still persisted in the stratosphere between the equator an

ISSN:0094-8276    

DOI:10.1029/GL012i005p00255

年代:1985

数据来源: WILEY