摘要:

If plate tectonics exists on Venus, the rate of topographic dropoff from spreading centers should be about half that on Earth due to greater rock:fluid density contrast and lower temperature differential between the surface and interior. Using 1° × 1° mean elevations, 21,000 km of ridge were identified on Venus and 33,000 km on Earth. Heights h at distances up to ∼ 1000 km to either side of the ridges were analyzed to make least squares determinations of the parameters ho, Δh, and n in:where s is off‐crest distance. The Venus ridges have a much less well‐defined mode in crest heights hothan the Earth ridges, and a somewhat smaller exponent n, on the average, indicating greater concavity. Fixing n to the theoretical value of 0.5, plate velocities were inferred from the Δh’s, using ∂h/∂(t½) from North Atlantic and North Pacific data. On Earth, these velocity solutions were compared to those from plate rotations, which indicate the ∂h/∂(t½) is probably about 40% too high for a global mean. The resulting plate creation rates on Venus (assuming comparable total heat generation and taking into account the density and temperature differences) are 0.7 km²/yr or less, and indicate that not more than 15% of Venus’s energy is delivered to the surface by plate tectonics, in contrast to 2.9 km²/yr and 70

ISSN:0094-8276    

DOI:10.1029/GL008i012p01187

年代:1981

数据来源: WILEY

摘要:

A number of studies of seismic refraction data have resulted in suggestions that the oceanic crust thickens with age, perhaps over a long time span. It is difficult to reconcile notions of crustal thickening with many of our ideas about crustal structure and formation. In this study we have re‐evaluated data compiled from a number of sources to obtain crustal thickness and age parameters for 105 different refraction profiles in the Pacific. An attempt was made to exclude data which might have poorly determined values of crustal thickness. The crust does appear to thicken with age to 30 or 40 m.y., but the effect is very subtle. Once probable biases are removed from the data, crustal thickening is not significant to a 95% confidence limit if ten m.y. samples are used. However, if one takes sample intervals of 0‐30 m.y. and 30‐100 m.y. there appears to be some difference in the average crustal thickness. Even in the latter case, the amount of thickening is far less than previous studies have suggested, only .45 km. Thus evidence for the long‐term addition of mass to the crust remains

ISSN:0094-8276    

DOI:10.1029/GL008i012p01191

年代:1981

数据来源: WILEY

摘要:

We present a new iterative method for finding the normal modes of a laterally heterogeneous body. The number of operations used by our method is proportional to the square of the number of basis functions used, while the effort required by the variational method increases as the cube of the number of basis functions. A numerical example shows that, at least for lower order modes, our iterative method yields results which are as accurate as the variational method.

ISSN:0094-8276    

DOI:10.1029/GL008i012p01195

年代:1981

数据来源: WILEY

摘要:

The principle of maximum entropy with a χ² constraint is developed as a stacking technique for seismic data. The formalism is applied to the stacking of single correlated Vibroseis traces. The computational results can differ significantly from those of a mean stack. They also illustrate the effectiveness of the method in the reduction of noisy amplitudes compared to those that are less nois

ISSN:0094-8276    

DOI:10.1029/GL008i012p01199

年代:1981

数据来源: WILEY

摘要:

We systematically applied a retrospective interview approach to the study of unusual animal behavior prior to seven earthquakes in North, Central and South America. Data taken at some distance from two of the earthquakes provided a baseline rate of unusual animal behavior. In comparison to this baseline one earthquake was preceded by a significant increase in unusual animal behavior and four were not.

ISSN:0094-8276    

DOI:10.1029/GL008i012p01203

年代:1981

数据来源: WILEY

摘要:

Both inward slumping and outward ejecta sliding are inferred to have occurred in four 10‐ton explosive cratering events in Gulf coastal plain sediments. These inferences result from a comparison of displacements obtained by integration of real‐time velocity data from subsurface gauges with displacements based on post‐shot surveys. The slumping did not occur immediately after crater excavation; it was delayed by more than 5 seconds but less than one day. This delay probably resulted from slow recompaction and resaturation of the ejecta and crater walls on a scale of minutes to hours after the excavation and attendant dilation of the media. Ejecta sliding, probably at a few meters per second, apparently moved concrete blocks on the surface up to 2 meters outward relative to gauges buried at the same initial range. The ejecta sliding was not uniform and may occasionally have included portions of the underlying g

ISSN:0094-8276    

DOI:10.1029/GL008i012p01207

年代:1981

数据来源: WILEY

摘要:

New drilling techniques were recently used to drill and core the melt zone of Kilauea Iki lava lake to a depth of 93 m. A partial melt zone was found to exist at depths between 58 m and 89 m consisting of 40 volume percent melt. Downhole seismic shots detonated in and below the melt zone resulted in the first in situ measurements of seismic velocity directly through a well characterized partial melt zone. Periodic seismic sources were used to effectively penetrate the highly fractured hydrothermal zone of the lava lake crust. Low velocity P‐wave layers (≤ 2.0 km/s) were found at the surface, at 40 m depth, and at 90 m depth.Thermal convective experiments in the melt zone resulted in the first controlled in situ measurements of the interaction of water with a basaltic melt zone. Transient energy rates of 900 kW (980 kW/m²) and steady rates of 85 kW (93 kW/m²) were observed. The full water recovery (100%), high downhole steam temperatures (670 C), and high energy transfer rates (93 to 980 kW/m²) observed in these thermal experiments are consistent with a closed cavity model where the injected water/steam directly contacted basaltic melt or near melt.In addition to understanding lava lakes, these seismic and thermal experiments have applications for the location of magma bodies in the crust and for the efficient extraction of energy from these

ISSN:0094-8276    

DOI:10.1029/GL008i012p01211

年代:1981

数据来源: WILEY

摘要:

Compressional wave attenuation, Qp−1, and velocity, Vp, for an alkalic olivine basalt have been measured from 1250° to 1450°C and from 5 to 10 MHz by an ultrasonic interferometric technique. Vpshows no significant frequency or temperature dependence, and Qp−1increases with increasing frequency and decreasing temperature. For the conditions of this study, the melt can be modeled as a viscoelastic material with a single thermally activated structural relaxation mechanism having an activation energy of 2.2 eV and a relaxation time on the order of 1 ns. The data presented here fall in the region of the relaxation spectrum where the frequency, f, is much smaller than the relaxation frequency, and the absorption coefficient is proportional to f². Analysis of the data suggests that (1) compressional wave losses will be negligible in a basalt melt at seismic frequencies, and (2) the volume viscosity of basalt melt is comparable in magnitude to shear vis

ISSN:0094-8276    

DOI:10.1029/GL008i012p01215

年代:1981

数据来源: WILEY

摘要:

A linear least‐squares fit of a finite‐strain equation of state to pressure‐density measurements is possible even when the initial density is unknown. This provides an objective extrapolation of data on a high‐pressure phase to zero pressure, and the zero‐pressure density and elastic properties (as well as their uncertainties) can be directly estimated. For typical uncertainties in ultrahigh pressure diamond‐cell experiments, compression data extending over strains of the high‐pressure phase of at least 0.04 and 0.11 (i.e., compression ranges, Δρ/ρ, of at least 12% and 35%) are required in order to constrain, respectively, third‐ and fourth‐order contributions to t

ISSN:0094-8276    

DOI:10.1029/GL008i012p01219

年代:1981

数据来源: WILEY

摘要:

Measurements of NO2in spectra of the rising and setting sun from Mauna Loa, Hawaii, gave a mean of 30 ppt for NOxat 3 ± 0.5 km altituge in the mid‐Pacific troposphere. Natural variations of ∼ 50 percent occurred about this mean together with the systematic nocturnal decay expected from the reaction of NO2with ozone. The tropospheric column abundance of NO2exhibited a correlation with the variation of NO2density at 3 km; the changes in both appear related to the strength of the trade winds and may reflect an overall rising or sinking motion in the lower troposphere. The measured NOxappears to be free of any anthropogenic influence and low enough in density that in the lower mid‐Pacific troposphere the source of ozone is downward diffusion from above and not local photochemical prod

ISSN:0094-8276    

DOI:10.1029/GL008i012p01223

年代:1981

数据来源: WILEY