Showing papers in "Journal of Geophysical Research in 1968"

A survey of low-energy electrons in the evening sector of the magnetosphere with OGO 1 and OGO 3.

Abstract: Observations of electrons of energy 125 ev to ∼2 kev with the OGO 1 satellite and of electrons of energy 40 ev to ∼2 kev with OGO 3 (by means of modulated Faraday cup detectors) are used to investigate the low-energy electron population in the magnetosphere within the local-time range ∼17 to ∼22 hours. Intense fluxes of these electrons are confined to a spatial region, termed the plasma sheet, which is an extension of the magnetotail plasma sheet discovered by the Vela satellites and is identified with the soft electron band first detected by Gringauz. The plasma sheet extends over the entire local-time range studied in this investigation, from the magnetospheric tail past the dusk meridian toward the dayside magnetosphere. In latitude it is confined to within 4–6 RE of the geomagnetic and/or solar magnetospheric equatorial plane, in agreement with observations already reported from the Vela satellites; no electron fluxes are detected high above the equator, not even very near the magnetopause. In radial distance the plasma sheet is terminated by the magnetopause on the outside and by a well-defined sharp inner boundary on the inside. The inner boundary has been traced from the equatorial region to the highest latitudes investigated, ∼40°; during geomagnetically quiet periods, it is observed at an equatorial distance of 11 ± 1 RE and appears to extend to higher latitudes along magnetic field lines. Weak or no electron fluxes are found between the inner boundary of the plasma sheet and the outer boundary of the plasmasphere. Detection (by an indirect process) of the very high ion densities within the plasmasphere gives positions for its boundary in good agreement with other determinations. During periods of magnetic bay activity, the plasma sheet extends closer to the earth; the inner boundary of the plasma sheet is then found at equatorial distances of 6–8 RE. This is most simply interpreted as the result of an actual inward motion of the plasma during a bay. In one case, it was possible to associate the beginning of this motion with the onset of the bay and to estimate an average radial speed of ∼12 km/sec, from which an electric field corresponding to ∼48 kilovolts across the magnetospheric tail was inferred. Within the plasma sheet, the electron population is characterized by number densities from 0.3 to 30 cm−3 and mean energies from 50 to 1600 ev and higher, with a strong anticorrelation between density and mean energy, so that the electron energy density (∼1 kev cm−3) and energy flux (∼3 ergs cm−2 sec−1) show relatively little variation. The lower energies and higher densities tend to occur during periods of geomagnetic disturbance. The nonobservation of electrons in regions above the plasma sheet implies an upper limit on the electron number density of 5 × 10−2 cm−3 if their mean energy is assumed to be ∼50 ev (typical of the magnetosheath) and 10−2 cm−3 if the energy is ∼1 kev (typical of the plasma sheet). At the inner boundary of the plasma sheet there is a sharp softening of the electron spectrum with decreasing radial distance but apparently little change in the electron number density. The electron energy density decreases across the inner boundary roughly as ∼exp (distance/0.4 RE) during quiet periods; during times of magnetic bay activity the energy density decreases as ∼exp (distance/0.6 RE), and there is a more complicated spatial structure of density and mean energy.

Permeability of granite under high pressure

Abstract: The permeability of Westerly granite was measured as a function of effective pressure to 4 kb. A transient method was used, in which the decay of a small incremental change of pressure was observed; decay characteristics, when combined with dimensions of the sample and compressibility and viscosity of the fluid (water or argon) yielded permeability, k. k of the granite ranged from 350 nd (nanodarcy = 10−17 cm2) at 100-bar pressure to 4 nd at 4000 bars. Based on linear decay characteristics, Darcy's law apparently held even at this lowest value. Both k and electrical resistivity, ρs, of Westerly granite vary markedly with pressure, and the two are closely related by k = Cρs−1.5±0.1, where C is a constant. With this relationship, an extrapolated value of k at 10-kb pressure would be about 0.5 nd. This value is roughly equivalent to flow rates involved in solute diffusion but is still a great deal more rapid than volume diffusion. Measured permeability and porosity enable hydraulic radius and, hence, the shape of pore spaces in the granite to be estimated. The shapes (flat slits at low pressure, equidimensional pores at high pressure) are consistent with those deduced from elastic characteristics of the rock. From the strong dependence of k on effective pressure, rocks subject to high pore pressure will probably be relatively permeable.

Seismology and the new global tectonics

Abstract: A comprehensive study of the observations of seismology provides widely based strong support for the new global tectonics which is founded on the hypotheses of continental drift, sea-floor spreading, transform faults, and underthrusting of the lithosphere at island arcs. Although further developments will be required to explain certain part of the seismological data, at present within the entire field of seismology there appear to be no serious obstacles to the new tectonics. Seismic phenomena are generally explained as the result of interactions and other processes at or near the edges of a few large mobile plates of lithosphere that spread apart at the ocean ridges where new surficial materials arise, slide past one another along the large strike-slip faults, and converge at the island arcs and arc-like structures where surficial materials descend. Study of world seismicity shows that most earthquakes are confined to narrow continuous belts that bound large stable areas. In the zones of divergence and strike-slip motion, the activity is moderate and shallow and consistent with the transform fault hypothesis; in the zones of convergence, activity is normally at shallow depths and includes intermediate and deep shocks that grossly define the present configuration of the down-going slabs of lithosphere. Seismic data on focal mechanisms give the relative direction of motion of adjoining plates of lithosphere throughout the active belts. The focal mechanisms of about a hundred widely distributed shocks give relative motions that agree remarkably well with Le Pichon's simplified model in which relative motions of six large, rigid blocks of lithosphere covering the entire earth were determined from magnetic and topographic data associated with the zones of divergence. In the zones of convergence the seismic data provide the only geophysical information on such movements. Two principal types of mechanisms are found for shallow earthquakes in island arcs: The extremely active zone of seismicity under the inner margin of the ocean trench is characterized by a predominance of thrust faulting, which is interpreted as the relative motion of two converging plates of lithosphere; a less active zone in the trench and on the outer wall of the trench is characterized by normal faulting and is thought to be a surficial manifestation of the abrupt bending of the down-going slab of lithosphere. Graben-like structures along the outer walls of trenches may provide a mechanism for including and transporting sediments to depth in quantities that may be very significant petrologically. Large volumes of sediments beneath the inner slopes of many trenches may correspond, at least in part, to sediments scraped from the crust and deformed in the thrusting. Simple underthrusting typical of the main zone of shallow earthquakes in island arcs does not, in general, persist at great depth. The most striking regularity in the mechanisms of intermediate and deep earthquakes in several arcs is the tendency of the compressional axis to parallel the local dip of the seismic zone. These events appear to reflect stresses in the relatively strong slab of down-going lithosphere, whereas shearing deformations parallel to the motion of the slab are presumably accommodated by flow or creep in the adjoining ductile parts of the mantle. Several different methods yield average rates of underthrusting as high as 5 to 15 cm/yr for some of the more active arcs. These rates suggest that temperatures low enough to permit dehydration of hydrous minerals and hence shear fracture may persist even to depths of 700 km. The thickness of the seismic zone in a part of the Tonga arc where very precise hypocentral locations are available is less than about 20 km for a wide range of depths. Lateral variations in thickness of the lithosphere seem to occur, and in some areas the lithosphere may not include a significant thickness of the uppermost mantle. The lengths of the deep seismic zones appear to be a measure of the amount of under thrusting during about the last 10 m.y. Hence, these lengths constitute another ‘yardstick’ for investigations of global tectonics. The presence of volcanism, the generation of many tsunamis (seismic sea waves), and the frequency of occurrence of large earthquakes also seem to be related to underthrusting or rates of underthrusting in island arcs. Many island arcs exhibit a secondary maximum in activity which varies considerably in depth among the various arcs. These depths appear, however, to correlate with the rate of underthrusting, and the deep maxima appear to be located near the leading (bottom) part of the down-going slab. In some cases the down-going plates appear to be contorted, possibly because they are encountering a more resistant layer in the mantle. The interaction of plates of lithosphere appears to be more complex when all the plates involved are continents or pieces of continents than when at least one plate is an oceanic plate. The new global tectonics suggests new approaches to a variety of topics in seismology including earthquake prediction, the detection and accurate location of seismic events, and the general problem of earth structure.

Sea-floor spreading and continental drift

Abstract: A geometrical model of the surface of the earth is obtained in terms of rigid blocks in relative motion with respect to each other. With this model a simplified but complete and consistent picture of the global pattern of surface motion is given on the basis of data on sea-floor spreading. In particular, the vectors of differential movement in the ‘compressive’ belts are computed. An attempt is made to use this model to obtain a reconstruction of the history of spreading during the Cenozoic era. This history of spreading follows closely one previously advocated to explain the distribution of sediments in the oceans.

Rises, trenches, great faults, and crustal blocks

Abstract: The transform fault concept is extended to a spherical surface. The earth's surface is considered to be made of a number of rigid crustal blocks. It is assumed that each block is bounded by rises (where new surface is formed), trenches or young fold mountains (where surface is being destroyed), and great faults, and that there is no stretching, folding, or distortion of any kind within a given block. On a spherical surface, the motion of one block (over the mantle) relative to another block may then be described by a rotation of one block relative to the other block. This rotation requires three parameters, two to locate the pole of relative rotation and one to specify the magnitude of the angular velocity. If two adjacent blocks have as common boundaries a number of great faults, all of these faults must lie on ‘circles of latitude’ about the pole of relative rotation. The velocity of one block relative to the other must vary along their common boundary; this velocity would have a maximum at the ‘equator’ and would vanish at a pole of relative rotation. The motion of Africa relative to South America is a case for which enough data are available to critically test this hypothesis. The many offsets on the mid-Atlantic ridge appear to be compatible with a pole of relative rotation at 62°N (±5°), 36°W (±2°). The velocity pattern predicted by this choice of pole roughly agrees with the spreading velocities determined from magnetic anomalies. The motion of the Pacific block relative to North America is also examined. The strike of faults from the Gulf of California to Alaska and the angles inferred from earthquake mechanism solutions both imply a pole of relative rotation at 53°N (±3°), 53°W (±5°). The spreading of the Pacific-Antarctic ridge shows the best agreement with this hypothesis. The Antarctic block is found to be moving relative to the Pacific block about a pole at 71°S (±2°), 118°E (±5°) with a maximum spreading rate of 5.7 (±0.2) cm/yr. An estimate of the motion of the Antarctic block relative to Africa is made by assuming closure of the Africa-America-Pacific-Antarctica-Africa circuit and summing the three angular velocity vectors for the cases above.

Marine magnetic anomalies, geomagnetic field reversals, and motions of the ocean floor and continents

Abstract: This paper summarizes the results of the three previous papers in this series, which have shown the presence of a pattern of magnetic anomalies, bilaterally symmetric about the crest of the ridge in the Pacific, Atlantic, and Indian oceans. By assuming that the pattern is caused by a sequence of normally and reversely magnetized blocks that have been produced by sea floor spreading at the axes of the ridges, it is shown that the sequences of blocks correspond to the same geomagnetic time scale. An attempt is made to determine the absolute ages of this time scale using paleomagnetic and paleontological data. The pattern of opening of the oceans is discussed and the implications on continental drift are considered. This pattern is in good agreement with continental drift, in particular with the history of the break up of Gondwanaland.

Microfracturing and the inelastic deformation of rock in compression

Abstract: The cracking that occurs during the deformation of rock in compression was studied by detecting and analyzing the radiated elastic waves. A new experimental method which increased the sensitivity of detection by several orders of magnitude over that of previous studies was used. The pattern of crack occurrence for a wide variety of rocks and at confining pressures up to 5 kb was found to be closely related to the stress-strain behavior. Dilatancy in the region above half the fracture stress was found to be directly proportional to cracking. Microfracturing, i.e. cracking, of brittle rock was compared with the microfracturing observed in frictional sliding and in the deformation of ductile rock. Cataclastic deformation of marble was found to be similar to frictional sliding in this respect but distinctly different than brittle deformation. The marble was found to undergo a gradual transition from cataclastic to fully plastic flow as confining pressure was increased in tests up to 4 kb. A model of deformation of an inhomogeneous brittle material is introduced which predicts the observed microfracturing activity and inelastic stress-strain behavior of rock. According to this model, microfracturing events in the dilatant region below about 95% of the fracture stress can be considered randomly independent. At higher stresses, where a rapid acceleration of activity is observed, the events cannot be considered independent and are correlated with the formation of the fault.

Seismic moment, seismicity, and rate of slip along major fault zones

Abstract: A straightforward method for computing rates of slip from earthquakes in major fault zones is presented. The slip rate is calculated from the sum of moments for the earthquakes. Rates obtained are in approximate agreement with rates obtained from geodetic measurements or magnetic anomalies, provided that long time samples are considered and provided that adjustments are made in the vertical extent of the zone of earthquake generation. For some fault zones, particularly deep island arc shear zones, strain is perhaps being relieved by steady creep, whereas, in other fault zones, e.g., the San Andreas, strain is accumulating for a large earthquake. The zone of earthquake generation for oceanic transform faults may be as little as 5 km in vertical extent.

Brittle-ductile transition in rocks

Abstract: The deformational characteristics of 2 limestones, one gabbro, and one dunite have been investigated as a function of confining pressure. It was found, that friction of these rocks and friction of granite and serpentinite studied elsewhere are nearly identical, and that the brittle-ductile transition pressure is simply the pressure at which the stress required to form a fault is equal to the stress required to cause sliding on the fault. The transition pressure is higher in extension than it is in compression. This difference occurs because the frictional shear stress required to cause sliding is determined not by confining pressure but by the principal stresses and the angle of the fault. For the same frictional shear stress on a fault surface, the confining pressure is much higher in extension than it is in compression. (19 refs.)

Seismic moment, stress, and source dimensions for earthquakes in the California-Nevada region

Abstract: The source mechanism of earthquakes in the California-Nevada region was studied using surface wave analyses, surface displacement observations in the source region, magnitude determinations, and accurate epicenter locations. Fourier analyses of surface waves from thirteen earthquakes in the Parkfield region have yielded the following relationship between seismic moment, M0 and Richter magnitude, ML: log M0 = 1.4 ML + 17.0, where 3 < ML < 6. The following relation between the surface wave envelope parameter AR and seismic moment was obtained: log M0 = log AR300 + 20.1. This relation was used to estimate the seismic moment of 259 additional earthquakes in the western United States. The combined data yield the following relationship between moment and local magnitude: log M0 = 1.7 ML + 15.1, where 3 < ML < 6. These data together with the Gutenberg-Richter energy-magnitude formula suggest that the average stress multiplied by the seismic efficiency is about 7 bars for small earthquakes at Parkfield and in the Imperial Valley, about 30 bars for small earthquakes near Wheeler Ridge on the White Wolf fault, and over 100 bars for small earthquakes in the Arizona-Nevada and Laguna Salada (Baja California) regions. Field observations of displacement associated with eight Parkfield shocks, along with estimates of fault area, indicate that fault dimensions similar to the values found earlier for the Imperial earthquake are the rule rather than the exception for small earthquakes along the San Andreas fault. Stress drops appear to be about 10% of the average stress multiplied by the seismic efficiency. The revised curve for the moment versus magnitude further emphasizes that small earthquakes are not important in strain release and indicate that the zone of shear may be about 6 km in vertical extent for the Imperial Valley and even less for oceanic transform faults.

Oxygen isotope behavior in the sulfate-water system

Abstract: Fundamental data on oxygen isotope behavior in the sulfate-water system are given. Oxygen exchange rates between sulfate ions and water are very slow and show a marked pH dependence. Of the order of 10/sup +3/ to 10/sup +5/ years are required to closely approach isotopic equilibrium at earth surface conditions. Oxygen isotope fractionation experiments yielded the following relationships: Ln alpha = 3878/(t+2) -0.0034 anhydrite water (and) Ln alpha = 3251/(t+2) -0.0056 dissolved sulfate water. During progressive evaporation of saturated CaSO/sub 4/ solution the gypsum formed tends to concentrate oxygen-18 relative to dissolved sulfate by 2%. Sulfate reducing bacteria preferentially metabolize oxygen-16 by about 4.6% in both pure and natural cultures. Oxidation of sulfide to sulfate in natural environments is a complex process making use of both water oxygen and molecular oxygen.

Melting of a peridotite nodule at high pressures and high water pressures

Abstract: Melting of a natural peridotite (spinel-bearing lherzolite) which occurs as a nodule in the tuff of Salt Lake, Hawaii, has been studied at pressures between 1 atm and 50 kb under anhydrous conditions and at pressures between 20 and 60 kb under hydrous conditions with the tetrahedral-anvil type of high-pressure apparatus. Under anhydrous conditions the lherzolite begins to melt near the liquidus of some olivine tholeiites. Garnet is stable near the solidus at pressures higher than at least 30 kb. Under hydrous conditions, when sealed capsules are used, the solidus of the lherzolite is at about 1000°C at 26 kb and about 1150°C at 60 kb. It is 400–700°C lower than the solidus under anhydrous conditions. When unsealed capsules are used, the solidus is raised by 200–400°C from the solidus determined by using sealed capsules. From the present experiments it appears that under anhydrous conditions magmas of olivine tholeiite composition can be formed from lherzolite, but those of quartz-tholeiite composition cannot be formed by partial melting, at least in the pressure range 10–30 kb. Quartz-tholeiite magma, however, can be formed within a much larger pressure range under hydrous conditions. The solidus under hydrous conditions (water pressure is equal to total pressure) would give a possible lowest temperature of beginning of melting of the upper mantle. It is also suggested that the partial melting of the hydrous upper mantle may play an important part in the formation of the low-velocity zone.

Coherence of geomagnetic DP 2 fluctuations with interplanetary magnetic variations

Abstract: From the comparison of the worldwide geomagnetic data with IMP 1 magnetic records obtained in the interplanetary space, it is found that the DP 2 fluctuations, which are thought to be the geomagnetic counterpart of intensity fluctuations of the magnetospheric convective system, are coherent with variations in the north-south component of the interplanetary magnetic field. This coherence is observed irrespective of whether this component is directed northward or southward. Average time delay between the crossing of an interplantary magnetic structure across the nose of the bow shock and the associated magnetic variation on the ground is 7 minutes at the pole and 9 minutes at the midday equator. Applicability of the proposed models of the magnetospheric electric field to this phenomenon is critically examined, and the penetration of the interplanetary electric field into the magnetosphere is suggested as the origin of the DP 2 phenomenon.

Mineralogical and chemical relationships among enstatite chondrites

Abstract: Fifteen of the sixteen known enstatite chondrites were studied microscopically in reflected and transmitted light, and their modal compositions were determined by point-counting techniques. Compositions of clinoenstatite, orthoenstatite, plagioclase, kamacite, taenite, troilite, oldhamite, daubreelite, niningerite, ferroan alabandite, and schreibersite were determined with the electron microprobe X-ray analyzer. Chemical composition, mineral occurrence, and mineral composition were found to depend on degree of recrystallization of the chondrites as judged by, for example, distinctness of chondrules and coarseness of silicates. On the basis of these parameters, three groups of enstatite chondrites can be distinguished and are referred to as type I, intermediate type, and type II. Differences between types I and II are pronounced, whereas intermediate type is transitional. The suggestion of Van Schmus and Wood that type II enstatite chondrites originated from type I by reheating is reviewed in the light of the new data. It is concluded that, although many of the chemical-mineralogical parameters of type II chondrites could be explained as being the result of reheating of type I chondrites, there are some that would require rather stringent environmental conditions during reheating. For example, lower iron and sulfur contents in type II chondrites would presumably require reheating of type I chondrites to ≥975°C, the lowest temperature at which a melt would appear in the Fe-Ni-S system of type I composition and at which physical separation of the liquid from the silicates could occur. Differences in Si/Mg ratios would require reheating to even higher temperatures and fractionation in an open system. Furthermore, observed differences in nitrogen and sinoite contents between type I and type II are difficult to explain unless reheating took place in a closed system, or under oxygen fugacities low enough to allow nitrogen to react with SiO2 and Si to form Si2N2O. An alternative model to the one by Van Schmus and Wood is discussed; it assumes that major differences in chemical and mineralogical composition between type I and type II were essentially established before or during chondrule formation and agglomeration by, for example, igneous differentiation or fractionation during condensation from a solar nebula, and that differences in texture are due either to different cooling rates of type I and type II chondrites during and after agglomeration of chondrules or to mild reheating to temperatures ≤975°C. This model does not, however, readily explain why only enstatite chondrites of type II bulk chemical composition (i.e. low Fe, S) cooled slowly or were reheated, and why chondrites of type I composition (high Fe, S) were always quenched to temperatures low enough to prevent recrystallization and were not reheated.

Preliminary geothermal model of the Sierra Nevada

Abstract: Four heat-flow values from the central Sierra Nevada vary systematically from 1.3 μcal/cm2 sec near the crest to 0.45 μcal/cm2 sec at the eastern edge of the San Joaquin Valley. Corresponding heat-production values determined from drill cores by Harold Wollenberg and Alan Smith range from 0.9 μμcal/cm3 sec (higher than the average granite) to 0.07 μμcal/cm3 sec (lower than the average basalt) in these granitic and intermediate rocks. With Francis Birch and Robert Roy and their collaborators we find that heat flow q is related to surface heat production A0 by q = q0 + DA0, where q0 and D are empirically determined constants (0.4 μcal/cm2 sec and 10 km, respectively). This relation can be preserved under differential erosion (for which there is much evidence) only if the crustal heat sources are distributed vertically according to A(z) = A0 e−z/D. q0 is interpreted as the uniform contribution from the mantle. This exponential source-distribution law forms the basis for an idealized model by means of which it is possible to reconcile the observed distributon of heat flow, surface heat producton, seismic velocities, and crustal thickness, as well as the volume of sediments attributed to post-plutonic erosion of the Sierra and the generation of the Sierra Nevada batholith by anatexis in a great geosyncline as proposed by Paul Bateman and his associates. The model implies that in a period of remarkable activity ending in the early Tertiary the western Sierra was almost completely purged of heat-producing elements in a time interval of the order of 100 m.y.

Breaker type classification on three laboratory beaches

Abstract: Breaker type, for waves on smooth concrete slopes, depends on beach slope m, wave period T, and either deep-water or breaker height, H0 or Hb. For forty-three varied laboratory conditions, breaker type can be sorted fairly well by either of two dimensionless combinations of these variables, an offshore parameter, H0/(L0m2) or an inshore parameter, Hb/(gmT2). As either of these parameters increases, breaker type changes from surging or collapsing to plunging to spilling. For the offshore and inshore parameters, respectively, the surge-plunge transition values are about 0.09 and 0.003 and the plunge-spill transition values are about 4.8 and 0.068. The deep-water heights in the offshore parameter were computed from linear, wave-generator theory. Breaker type data were obtained from films of breaking waves for conditions that produced a dominant breaker type, free from interference by secondary waves.

The polar wind

Abstract: Polar wind, describing upward plasma expansion of topside polar ionosphere and acceleration of positive H and He ions

Wave ‘set-down’ and set-Up

Abstract: ‘Set-down’ and set-up, the negative and positive changes in mean water level due to the presence of a train of surface waves, was measured in a wave channel. Well outside the break point the experimental results are in good agreement with the theoretical relationship determined by Longuet-Higgins and Stewart. Near the break point, where the wave height is greater than predicted by first-order wave theory, the measured ‘set-down’ was consistently less than theory would predict from the deep water wave height. Inside the break point the bore height was found to be a linear function of the mean water depth. In this region, the gradient of the set-up, , was related to the beach slope tan β and the mean ratio of wave height to water depth by the equation .

The polar wind and the terrestrial helium budget

Abstract: Helium isotopes escape mechanism from earth atmosphere related to polar wind ionospheric plasma flow from earth

Seismic displacements near a fault

Abstract: Ground motion at only 80 meters from the San Andreas fault was recorded during the Parkfield earthquake of June 28, 1966, by the accelerographs of the U.S. Coast and Geodetic Survey. Seismic displacements obtained from the accelerograms are compared with synthetic seismograms calculated for a moving dislocation model. An excellent agreement was obtained between the observed and theoretical seismograms. The dislocation is the only important source parameter that controls the seismic motion at such short distances. Other parameters such as the fault length and depth show negligible effect of the motion. The rupture velocity is a significant but not dominant factor. From the comparison of observed and theoretical seismograms, it was concluded that the fault dislocation was about 60 cm. This value is an order of magnitude greater than the fault offset observed at the surface. This discrepancy was attributed to the decoupling of a thin surface layer (probably less than 100 meters from the surface). The fault depth was estimated by combining the estimated dislocation with the seismic moment obtained from long-period surface waves. The fault depth within the basement rock was found to be about 3 km, which is significantly smaller than the depth (15 km) of the aftershock zone. This discrepancy was attributed to the effect of increasing friction with depth which momentarily prevented downward extension of rupture during the main shock.

Lengths of geomagnetic polarity intervals

Abstract: Variations in the lengths of geomagnetic polarity, intervals are analyzed by means of a probability model based on the theory of Bernouli trials. Polarity reversals are assumed to occur as the result of the interaction between steady oscillations of the geomagnetic dipole and secular variations of the nondipole field. The particular cycle on which a polarity inversion occurs is determined by the magnitude of the nondipole field, which is assumed to vary randomly and independently of dipole variations. The reversal properties of the geomagnetic dynamo are characterized by the single parameter p, the probability that a polarity inversion will occur during one cycle of change in the geomagnetic dipole moment. From an analysis of polarity changes during the past 10 m.y., the value of p is estimated to be 0.05. During the Permian period, it was at least two orders of magnitude smaller. The analysis suggests that within the past 10 m.y. there have occurred hitherto undiscovered short geomagnetic polarity events with durations shorter than 0.05 m.y.

Experimental study of the fracturing process in brittle rock

Abstract: MANY SMALL CRACKING EVENTS OR MICROFRACTURES PRECEDE FRACTURE IN LABORATORY COMPRESSION TESTS ON ROCK. EACH EVENT RADIATES ELASTIC WAVES. DURING THE COMPRESSION OF GRANITE TO FRACTURE, TWENTY TWO OF THE LARGEST MICROFRACTURE EVENTS WERE LOCATED IN SPACE BY USING FIRST ARRIVALS OF S WAVES DETECTED WITH A MULTITRANSDUCER ARRAY. AT ABOUT 92% OF THE FRACTURE STRESS, A RAPID ACCELERATION OF MICROFRACTURING ACTIVITY OCCURRED. THE LOCATION OF THE EVENTS THAT OCCURRED BELOW THIS POINT SHOWED NO OBVIOUS RELATION TO THE EVENTUAL FAULT. AT STRESSES WITHIN THE REGION OF ACCELERATED ACTIVITY, HOWEVER, THERE WAS A MARKED CLUSTERING OF MICROFRACTURES ON THE EVENTUAL FAULT PLANE. THESE OBSERVATIONS SUGGEST THAT THE APPROXIMATE TIME AND PLACE OF FRACTURE COULD HAVE BEEN PREDICTED. IN THE LIGHT OF THE SIMILARITY OF MICROFRACTURES AND EARTHQUAKES, THIS SUGGESTS A POSSIBLE METHOD FOR PREDICTING EARTHQUAKES. /AUTHOR/

Thickness determinations of the lunar surface layer from lunar impact craters.

Abstract: Small, fresh lunar craters with normal, central-mound, flat-bottomed, and concentric geometry are widespread on maria surfaces. The same types of craters have been produced in the laboratory by impacting projectiles against targets consisting of loose, granular, noncohesive materials overlying cohesive substrates. The mechanics of formation of each laboratory crater type is described, and evidence is offered that the corresponding types of lunar craters are of impact origin. Extensive studies of the effects of lunar impact variables on the conditions of formation of these crater types show that a previously described statistical method can be used to determine the thickness of the lunar surfaces layer within narrow limits. Two independent methods for determining the layer thickness at specific points are presented. Thickness estimates of the Surveyor 1 site obtained previously from study of medium-resolution Orbiter 1 photographs are re-evaluated by using subsequently obtained high-resolution photographs, and thickness determinations of two additional areas are presented. The different areas examined have different surface layer thickness. The fragments of the surface are certainly partly of impact origin, but volcanic contributions may also be present. The maria substrates are probably composed of volcanic flow rocks with interbeds of fragmental material.

Geomagnetic Dp 2 fluctuations and associated magnetospheric phenomena

Abstract: A DP 2 fluctuation has a time scale of about one hour and appears coherently all over the world. It differs from the polar substorm (DP 1) in that it is not caused by the activation of the auroral electrojet. Its current system consists of twin current vortices and a zonal part. It is associated with fluctuations in the magnetospheric plasma convection, and it will provide a useful means for the identification of the ‘viscous-like’ interaction mechanism.

Power spectra and discontinuities of the interplanetary magnetic field - Mariner 4.

Abstract: Power spectra and fluctuations of interplanetary magnetic field from Mariner 4 data for solar active and quiet days

Magnetic anomalies in the Indian Ocean and sea‐floor spreading

Abstract: A reconnaissance survey of the magnetic anomaly pattern over the Indian Ocean is interpreted in terms of the spreading floor hypothesis. The recent movements of the oceanic crustal blocks and of the adjacent continents are deduced from an examination of the axial magnetic pattern over the mid-ocean ridge. An active branch of ridge runs SE-NW from south of Australia to the Gulf of Aden, and the spreading along its axis corresponds to a relative rotation of Africa away from Asia with the pole of rotation centered near 26°N and 21°E. A southwest branch of ridge running from Mauritius to south of Africa has not been the site of measurable spreading since the Mesozoic era. A description of the anomaly pattern over the flanks of the ridge leads to a hypothetical history of the fragmentation of the Gondwana continent.

Earth models obtained by Monte Carlo inversion.

Abstract: The problem of uniqueness of earth structures obtained by the inversion of geophysical data is still unsolved. Monte Carlo methods offer the advantage of exploring the range of possible solutions and indicate the degree of uniqueness achievable with currently available geophysical data. This procedure was applied to the earth by using the following data: 97 eigenperiods, travel times of compressional and shear waves, and mass and moment of inertia of the earth. Indirect use was made of dt/dΔ data obtained from arrays. Five million models have been examined, and six have passed all tests. Results are as follows: (1) The earth's core is inhomogeneous, consistent with Fe(15–25%)-Si for the outer core and Fe(20–50%)-Ni alloy for the inner core. (2) The radius of the core is increased by 5 to 20 kilometers. (3) Large density and velocity gradients are found in the transition region without prior assumption of an equation of state. The transition region is a compositional boundary as well as a zone in which phase changes occur. The lower mantle shows an increase of the FeO/MgO + FeO ratio by a factor of 2 compared with the upper mantle. This could inhibit mantle-wide convection. (4) Upper mantle models fit better than ‘standard’ models when they are more complex, showing large fluctuations in shear velocity and density. Such complexity might be expected if the mantle is chemically and mineralogically zoned, if there are high thermal gradients, and if partial melting takes place. However, the magnitude of the fluctuations suggests that the zoning is lateral in nature and that the mantle is variable in composition laterally ranging from pyrolite to eclogite.

Rare Earth abundances in some basic rocks

Abstract: Rare-earth (RE) abundances are reported for several basalts, diabases, and gabbros. Compared with the RE distribution in chondritic meteorites, continental basic rocks are characterized by an enrichment of the light lanthanides. Intrusive basic rocks have lower RE contents and less fractionated RE distributions than continental basalts. Chill zones of the Stillwater and Bushveld complexes have different distributions which are Eu enriched and light RE depleted. Oceanic island basalts have RE abundances similar to those of continental basalts. Abyssal subalkaline basalts dredged from the mid-Atlantic ridge and Eastern Pacific rise have RE distributions which are nearly chondritic. There is a depletion in La, Ce, Pr, and a broad maximum from Sm to Tb. Similar patterns are found in fresh basalts, slightly altered basalts, and in greenstones. Abyssal basalts that are more alkalic are not depleted in La, Ce, and Pr. The relationship of the RE data to hypotheses for the origin of ridge basalts is considered.