摘要:

Volcanic activity in the Cascades in historic times suggests that the Juan De Fuca plate is underthrusting aseismically at about 1.8 cm/yr. This rate of underthrusting is identical to the rate computed from sediment studies.

ISSN:0094-8276    

DOI:10.1029/GL008i011p01123

年代:1981

数据来源: WILEY

摘要:

Observations of lithospheric Q for shear waves in the range 1‐30 Hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres. For the single scattering approximation I have assumed 1/Q = 1/Qi+ gv/ω, where Qiis the intrinsic Q due to anelasticity, v is the velocity of shear waves, ω is angular frequency and g is the turbidity. g is taken as a constant in this paper; since g = ∫ n(a)σda, where n(a)da is the number of scattering spheres of radius a/unit volume and σ is the scattering cross‐section for a sphere, this suggests that geometrical scattering dominates. This will be true for ka>1, where k is wavenumber. Based on this criterion and the observation that Q is large (i.e. g small) at a frequency of 0.05 Hz, a is of the order of magnitude 1‐10 km. Fits of observations of shear wave Q from Japan and Central Asia indicate that Qi= 2000 ± 500 for the lithosphere of both regions; g = 0.01 km−1for the Japan observations and g = 0.005 km−1

ISSN:0094-8276    

DOI:10.1029/GL008i011p01126

年代:1981

数据来源: WILEY

摘要:

Simple theoretical results are obtained for the excitation of seismic waves by an indigenous seismic source in the case that the source volume is intersected by a structural discontinuity. In the long wavelength approximation the seismic radiation is identical to that of a point source placed on one side of the discontinuity or of a different point source placed on the other side. The moment tensors of these two equivalent sources are related by a specific linear transformation and may differ appreciably both in magnitude and geometry. Either of these sources could be obtained by linear inversion of seismic data but the physical interpretation is more complicated than in the usual case. A source which involved no volume change would, for example, yield an isotropic component if, during inversion, it were assumed to lie on the wrong side of the discontinuity. The problem of determining the true moment tensor of the source is indeterminate unless further assumptions are made about the stress glut distribution; one way to resolve this indeterminancy is to assume proportionality between the integrated stress glut on each side of the discontinuity.

ISSN:0094-8276    

DOI:10.1029/GL008i011p01129

年代:1981

数据来源: WILEY

摘要:

Remotely estimating the properties of subsurface fracture zones is important in characterizing the structure of the shallow earth. We present a vertical seismic profiling (VSP) technique to make this fracture zone estimation and discuss the results of a VSP experiment performed in the upper 770 m of the Michigan Basin. Both P and SH waves were used to observe an explosively‐fractured volume of Antrim shale. The experiment was divided into two parts: a "before" survey run on the unaltered rock, then an identical "after" survey executed across the fractured volume. A seismic velocity structure of the basin was calculated from the "before" survey. Comparison of the "after" observations to the "before" data, elucidated the fracture volume and its effective elastic parameters. From travel‐time delays, amplitude attenuation, converted and scattered waves, we estimated the depth (395 m), shape (ellipsoidal), size (10 m × 20 m × 30 m) and porosity (20%) of the fracture

ISSN:0094-8276    

DOI:10.1029/GL008i011p01132

年代:1981

数据来源: WILEY

摘要:

A new method of fusing samples for40Ar/39Ar dating using a continuous argon‐ion laser is presented. Samples of hornblende, biotite, muscovite and slate were fused with a continuous laser and the results obtained agree with acres determined using our radio‐frequency (RF) fusion system. By varying the laser beam power level, it was possible to perform a step‐heating laser fusion of the slate which closely matched two RF step‐heating runs on the same material. Thus, not only does the continuous laser give an age‐microprobe capability, but it can routinely perform multi‐step heating experiments which are crucial to40Ar/39Ar age int

ISSN:0094-8276    

DOI:10.1029/GL008i011p01136

年代:1981

数据来源: WILEY

摘要:

Grooves on Ganymede have been interpreted as fractures that result from extensional tectonics which, accompanied by some type of resurfacing, changed relatively low‐albedo, heavilycratered terrainto higher‐albedo, less crateredgrooved terrain. The formation of grooves involved initial fracturing of cratered terrain, resurfacing confined within these fractures, and subsequent fracturing of resurfacing material. Crosscutting and terminating, or "T" structural relationships among grooves, groove sets, and cratered terrain indicate that grooved terrain developed in three stages. The formation of primary grooves marks the initial breakup of cratered terrain. Secondary grooves generally terminate against primary grooves and complete the breakup of cratered terrain into polygons that are mechanically isolated from adjacent polygons. In the final stage (3), polygons of cratered terrain are fractured, resurfaced, and the resurfacing material is subsequently fractured by a set of closely spaced, parallel to subparallel grooves that terminate against the older primary and/or secondary grooves. In each of these stages, the groove‐forming process occurs in the same sequence: fracturing, resurfacing, and subsequent fracturing of resurfacing material, but most of the area of grooved terrain is resurfaced during stage 3. The process may stop during any of the three stages yielding areas of fractured cratered terrain, polygons of cratered terrain, polygons which have been resurfaced but not subsequently fractured (smooth terrain), or completely grooved terrain. Regardless of place or time of development, grooved terrain appears to have evolved according to the same sequential order of stages. Because the development of grooves within each stage created smaller isolated polygons out of larger areas, this process has resulted in the break‐up of the surface of Ganymede into progressively smaller and more equidimensional mechanica

ISSN:0094-8276    

DOI:10.1029/GL008i011p01139

年代:1981

数据来源: WILEY

摘要:

To examine whether the long‐term averaged solar flare Ne is "planetary" type, as observed in contemporary solar flares, we have studied etched feldspar separates of several lunar soils and three samples from different depths of two well‐documented lunar rocks. Our results of noble gas elemental and isotopic ratios suggest that the long‐term average solar flare Ne composition is more similar to "solar" type than the "planetary"

ISSN:0094-8276    

DOI:10.1029/GL008i011p01143

年代:1981

数据来源: WILEY

摘要:

Information on solar irradiance at wavelengths below 185 nm, observed by the EUVS experiment on the AE‐E satellite over the entire development of the present sunspot cycle 21, is important to many investigations of planetary thermospheres and ionospheres.Strictly observational informationis generally lacking in both the completeness and the spectral detail required by the more advanced study programs. Therefore, it has been necessary also to develop computermodelsin connection with fully detailed compilations of an appropriatereference spectrum. Our selection of appropriate forms of effective publication has been difficult for various reasons. Recognizing that full reproduction of our various lists of observational, reference, and model data in scientific journals would be impracticable, we started a more or less informal procedure of timely release of information to a limited number of particularly interested colleagues. With the present letter, we hope to mitigate at least some of the dissatisfactory aspects of this procedur

ISSN:0094-8276    

DOI:10.1029/GL008i011p01147

年代:1981

数据来源: WILEY

摘要:

The accumulation of interplanetary magnetic fluxes around the Venus ionopause is examined in the light of the PVO magnetometer and plasma wave measurements. It is argued that convective electric currents within the ionosphere are not sufficient to account for the observed geometry of the magnetic profiles. It is suggested that turbulent flow conditions forced in the ionosheath by the interaction process with the ionospheric plasma should produce an anomalous resistivity in the local flow and an effective lag in the transport of the interplanetary magnetic fluxes around the ionospheric obstacle. The enhanced magnetic fluxes seen above the ionopause should result from effective turbulent currents generated throughout the region of interaction.

ISSN:0094-8276    

DOI:10.1029/GL008i011p01151

年代:1981

数据来源: WILEY

摘要:

Energetic (>24 keV) ion distributions are used to remotely sound an ion trapping boundary at the dayside magnetopause for two inbound crossings of ISEE‐2 on days 347 and 312, 1977, at 0830 and 1100 LT. By applying single particle trajectory models to the observed temporal relationships between trapping boundary distance and orientation parameters, we conclude that longitudinal standing waves occur on the boundary, having typical wavelengths of 1500‐3000 km, amplitudes of 500 km, and periods of 200‐400 sec. The standing wave nodes move along the node axis with velocities 1.5 to 6 km/sec. The data are inconsistent with models of traveling waves moving tailward along the bou

ISSN:0094-8276    

DOI:10.1029/GL008i011p01155

年代:1981

数据来源: WILEY