摘要:

Cylinders of Barre and Westerly granites have been compressed to failure in static fatigue tests under dry and wet conditions at 1 Kb confining pressure and temperatures of 24° and 200°C. The effect of increasing temperature and water content is, as expected, to weaken the rocks appreciably and to reduce the time to failure, at constant stress, by about two orders of magnitude. The activation energy estimated for dry Barre granite deformed in the temperature range 24° to 200°C is 10.2 Kcal/mol which seems reasonable and is consistent with other work at higher temperatures. Extrapolation of the static fatique equations obtained for specimens deformed at 200°C, both wet and dry, to 108years yields maximum stress differences near 3 Kb. While such extrapolations may be reasonable for intact material over time‐spans of interest to national concerns, the stresses obtained seem unreasonably high for tectonic deformations. The discrepancy probably results from the very short durations of the tests which do not permit effective operation, at these low temperatures, of the several thermally‐activated weakening effects of H2O on s

ISSN:0094-8276    

DOI:10.1029/GL009i001p00001

年代:1982

数据来源: WILEY

摘要:

Experimental results at ultrasonic frequencies in brine‐ or water‐saturated samples cut perpendicular to bedding demonstrate that compressional velocities vary linearly with porosity and volume‐fraction clay in detrital silicate rocks characterized by pores with low aspect ratios. P‐wave velocity is more sensitive to porosity than clay content, obeying the empirically derived expression Vp = −2.4C − 8.6ϕ + 5.8 at 800 bars confining pressure and 400 bars pore pressure, where the units of Vp are kilometers per second, ϕ is the volume fraction pores, and C is the volume fraction clay. Deviations from this equation are less than ± 2% with the exception of the high pore aspect ratio St. Peter sandstone, which has a compressional velocity that is 17% higher than the predicted value. The equation is insensitive to the chemical compostion of the dominant clay mineral and apparently insensitive to the location of the clay grains with respect to the mineral framework, e.g., in free pore space or in contact zones between mineral grains of lower com

ISSN:0094-8276    

DOI:10.1029/GL009i001p00005

年代:1982

数据来源: WILEY

摘要:

We point out first the inadequacy of the two widely used approaches in calculating the amplitude attenuation of seismic waves in a random medium, the formulation of mean field attenuation and that of the scattering coefficient under the single‐scattering approximation. Then we calculate the average amplitude attenuation due to scattering in an infinite random slab. We slice the random slab into layers of thickness a correlation length and use the Born approximation for each slice to calculate the scattered field. To include the effect of multiple scattering, we take the back‐halfspace integration of the scattered energy as the energy loss and do energy correction for each successive slice. Taking scalar wave approximation for seismic waves, we get a formula for average amplitude attenuation essentially valid for high frequency range (ka»1). The attenuation depends strongly on the form of correlation function of the random inhomogeneities. We derive the formulas for Gaussian, exponential and Von Kárman correlation functions. The frequency dependence of attenuation by our formulation agrees well with experi

ISSN:0094-8276    

DOI:10.1029/GL009i001p00009

年代:1982

数据来源: WILEY

摘要:

A seismic refraction survey using ocean bottom seismometers with one vertical and two horizontal component geophones was carried out off western Canada. This report presents one feature of a comprehensive interpretation of the data set, that dealing with shear‐wave velocity anisotropy in the uppermost mantle. Amplitude‐and traveltime‐corrected SV record sections for two profiles, EX2 approximately parallel to the spreading ridge and EX3 approximately parallel to the direction of spreading, were compiled. From an interpretation based on comparisons with synthetic seismograms, the shear wave velocity in the upper mantle is 4.5 km/s for EX2 and 4.6 km/s for EX3. The corresponding compressional wave velocities are 8.3 km/s and 7.5 km/s. The significant difference in P‐wave velocities is attributed to azimuthal velocity anisotropy of the type normally found in the Pacific basin. In contrast, the small difference in S‐wave velocities suggests a very low degree of shear wave anisotropy. The combination gives a prominent anisotropy in values of Poisson's ratio for the upper mantle, the first in‐situ evidence for this feature. These results are consistent with laboratory measurements on ultramafic rock samples from ophiolit

ISSN:0094-8276    

DOI:10.1029/GL009i001p00013

年代:1982

数据来源: WILEY

摘要:

A multichannel seismic reflection profile across the East Pacific Rise near 9°N reveals an approximately 400 m thick layer at the top of the oceanic crust with seismic velocity of about 2.8 km/s at the Rise crest. This velocity increases to about 3.8 km/s, 210 km east of the Rise crest. These low velocities suggest that this layer is probably a porous lava flow layer (layer 2A of seismic refraction studies) generated by volcanic activity at the spreading center. Reflections from the base of the layer imply a relatively sharp transition between the lava flows and the underlying layer (presumably a sheeted dike complex). The underlying layer is a zone about 1.4 km thick (with velocity about 5.7 km/s) whose base may coincide with the top of a low velocity magma chamber from which the crust develops

ISSN:0094-8276    

DOI:10.1029/GL009i001p00017

年代:1982

数据来源: WILEY

摘要:

Lithospheric plates are subducted episodically, a few meters at a time, during earthquakes. These earthquakes recur over intervals ranging from decades to centuries. Within weeks to perhaps years after such an earthquake, elastic stresses induced in the asthenosphere relax, allowing displacement and stress to propagate away from the subduction zone into the adjacent plates. Finite element modelling of this process has shown that the propagation is asymmetric, the subducted slab serves to buffer the subducting plate against extensive motion. We have constructed a simple analytic model which incorporates the effect of he subducted slab in a fundamental way. The slab's effect is modelled by a Maxwell viscoelastic element which is attached to the subducting plate. Both plates obey Elsasser's displacement propagation equations. The elastic response of the Maxwell element at first restrains the motion of the subducted plate. The plate eventually moves on the timescale of stress relaxation in the Mesosphere (hundreds to thousands of years).The model predicts that the trench moves toward the subducting plate at an average speed of one half of the convergence rate. A strong extensional pulse is propagated into the overthrust plate shortly after the earthquake, whereas the stress pulse is small in the subducting plate. Although this extension changes into compression before the next earthquake in the cycle, the period of strong extension following the earthquake may be responsible for extensional tectonic features in the back‐arc regio

ISSN:0094-8276    

DOI:10.1029/GL009i001p00021

年代:1982

数据来源: WILEY

摘要:

On March 14, 1979 a sizeable earthquake (Ms = 7.6, Richter scale) occurred on the continental shelf adjacent to S.W. Mexico, near Petatlan in the state of Guerrero. This earthquake generated a small tsunami that was recorded in deep water, 1000 km away, thus providing for the first time a glance at a tsunami traveling in the open ocean. The same sea floor pressure record displays conspicuous signals associated with vertical sea floor motions generated at the passage of the first Rayleigh seismic wave, R1. Seismic and tsunami travel velocities are in agreement with our present understanding of the phenomena, and tsunami detectability in deep water is demonstrated to be well within present day state of the art in the design of sea floor pressure transducers. As calculations anticipate, the E.M. signals associated with the passage of the tsunami were too faint to be detected.

ISSN:0094-8276    

DOI:10.1029/GL009i001p00025

年代:1982

数据来源: WILEY

摘要:

The geoid is becoming increasingly important in interpretation of global tectonics. Most of the topography of the earth is isostatically compensated, so removal of its effect from the geoid is appropriate before tectonic modeling. The oceanic trenches, however, are dynamically depressed features and cannot be isostatically compensated in the classical way. Continental topography compensated at 35 km gives intracontinental geoidal undulations of up to 15 m over mountain ranges in a spherical harmonic expansion to order and degree 22. Oceanic topography compensated at 40 km, reasonable for the thermally supported long wavelengths, matches the +10 m difference between old continents and old oceans in a detailed NASA/GSFC geoid. Removing the assumed compensation for the oceanic trenches leaves negative anomalies of up to 9 m amplitude caused by their uncompensated mass deficit. This mass deficit acts as a partial "regional compensation" for the excess mass of the subducting slabs, and partly explains why geoidal (and gravity) anomalies over the cold slabs are less than thermal models predict.

ISSN:0094-8276    

DOI:10.1029/GL009i001p00029

年代:1982

数据来源: WILEY

摘要:

A first order analysis of current selenodetic and selenochronological data indicates that the present lunar figure was formed 3.0 ± 0.5 × 109years ago at an earth‐moon distance of 20.4 ± 2.3 earth radii. This datum and data from a variety of sources are used to define the time evolution of the earth‐moon distance. It is shown that there was a sudden increase in this distance about 2 × 109years ago when the Q of the earth must have decreased by a factor of 5. This change in Q correlates well with the onset of rapid continental growth and hence the expansion of shallow sea‐ and continental shelf waters as shown in the geological record for

ISSN:0094-8276    

DOI:10.1029/GL009i001p00033

年代:1982

数据来源: WILEY

摘要:

Limits on the selenotherm are estimated using (1) a preferred set of bounds on the lunar electrical conductivity profile; (2) published laboratory conductivity vs. temperature data for an olivine and several aluminous orthopyroxenes; and (3) estimates for the Al2O3content of the deep interior suggested by bulk composition models. The inferred limits are narrowest in the depth range 450 to 1350 km and are in accord with independent geophysical constraints. Thermal history models which yield present‐day selenotherms that are in best agreement with these limits are those which permit subsolidus convection at depths greater than 800 km in the moo

ISSN:0094-8276    

DOI:10.1029/GL009i001p00037

年代:1982

数据来源: WILEY