Showing papers in "Journal of Geophysical Research in 1971"

Multiquadric equations of topography and other irregular surfaces

Abstract: A new analytical method of representing irregular surfaces that involves the summation of equations of quadric surfaces having unknown coefficients is described. The quadric surfaces are located at significant points throughout the region to be mapped. Procedures are given for solving multiquadric equations of topography that are based on coordinate data. Contoured multiquadric surfaces are compared with topography and other irregular surfaces from which the multiquadric equation was derived.

Large-amplitude Alfvén waves in the interplanetary medium, 2

Abstract: Dynamic nonshock properties of large amplitude microscale Alfven waves in interplanetary medium, using plasma and magnetic field data from Mariner 5

Some results from a time‐dependent thermodynamic model of sea ice

Abstract: A one-dimensional thermodynamic model of sea ice is presented that includes the effects of snow cover, ice salinity, and internal heating due to penetration of solar radiation. The incoming radiative and turbulent fluxes, oceanic heat flux, ice salinity, snow accumulation, and surface albedo are specified as functions of time. The model is applied to the central Arctic.

Origin and development of marginal basins in the western Pacific

Abstract: The semi-isolated basins and series of basins of intermediate to normal oceanic depths that lie behind island arc systems are termed marginal basins, and are believed to be of extensional origin. Marginal basins, although differing in size and amount of sediment fill, are all underlain by oceanic crust, and can be divided into actively spreading inter-arc basins and older inactive basins. The inactive basins are grouped on the basis of age as reflected by crustal heat flow; inactive basins with above-normal heat flow are apparently younger than those with normal heat flow. Volcano-tectonic rift zones with associated silicic tuffs are found in the tectonic position of inter-arc basins in some continental trench-arc systems and are thought to have been formed by similar extensional process. Large scale crustal extension and demonstratable trench migration imply displacement of the Benioff zone. However, the portion of the Benioff zone shallower than 150 km, and lying between the volcanic chain and the trench, remains relatively undeformed along the trend of arc systems in which the dip of the deeper segment of the Benioff zone changes markedly. Geometric relationships suggest that, as the inter-arc basin opens, the trench is forced to migrate and the Benioff zone is flattened, except in areas where crust can be consumed in a second trench behind the extensional zone. Active extension is restricted to arc systems with earthquakes deeper than about 350 km. The available data can be explained by a thermal diapir of shear-heated mantle material, bouyantly rising beneath the inter-arc basin from the upper surface of the under-thrust lithosphere. Hydrostatic forces associated with such a diapir are felt capable of over-coming the compressional forces in the mantle produced by shearing along the upper surface of the lithosphere and by the bending of the lithosphere. The existence of low density, high temperature upper mantle is indicated by the lack of a large gravity anomaly over the shallow oceanic crust of the inter-arc basin, by high heat flow there, and by anomalously high attenuation of shear waves passing through the upper mantle beneath the extensional zone.

Topside current instabilities

Abstract: Topside ionospheric instabilities of electrostatic ion acoustic and ion cyclotron waves to field aligned currents in single and multiion plasmas

Average and unusual locations of the Earth's magnetopause and bow shock

Abstract: Average and unusual locations of magnetopause and bow shock positions observed by IMP spacecraft

An exact effective stress law for elastic deformation of rock with fluids

Abstract: The exact expressions for the effective stress 〈σij〉 and, in particular, pressure 〈P〉 that cause elastic strain of material with pore fluids are, assuming only that Hook's law is valid, 〈σij〉 = σij - αPδij and 〈P〉 = Pc - αPp, where α = 1 - (K/Ks), Pc and Pp are confining and pore pressures, and K and Ks are the bulk moduli of the rock and grain, respectively The equation for 〈P〉 was first suggested by Geertsma (1957) and by Skempton (1960) on empirical grounds The expression does not depend directly on porosity, but when pores vanish the effective pressure 〈P〉 equals the confining pressure Pc, because then K=Ks Thus the strain of a porous solid with pore pressure can be completely determined from the elastic modulus of the solid without pore pressure, if the effective stress law in the equation for 〈σij〉 is used The exact expression for the effective stress describes quite accurately the measured strains in sandstone and granite samples at confining and pore pressures to 25 kb The results are not applicable to inelastic processes, such as fracture, or elastic processes other than strain

Elevation of ridges and evolution of the central Eastern Pacific.

Abstract: An empirical relationship between ridge elevation and age of the oceanic crust is presented for the Pacific, Atlantic, and Indian oceans. This relationship is accounted for by the thermal contraction of a cooling lithosphere as it moves away from a center of spreading, and thus is compatible with plate theory. Hence, it is possible to use topographic profiles to predict the age of the ocean floor. Detailed examination of profiles in different areas indicates that slow-spreading ridges (half-rate 3 cm/yr). However, all ridges appear to show a uniform subsidence rate near the ridge crest. This uniform rate makes it possible to use topographic profiles in regions of smooth topography to predict the age of oceanic crust, less than 40 m.y. old, to better than ±2 m.y. Topographic profiles will be particularly useful for predicting age where magnetic anomaly patterns are absent or difficult to interpret. Magnetic, topographic, seismic reflection, twenty-nine new, and all other heat-flow observations across the east Pacific rise and Mathematician and Clipperton seamount chains in the central eastern Pacific are examined. The magnetic and heat-flow observations are too inconclusive to enable a tectonic reconstruction of the area. However, topographic profiles at right angles to the rise between 20°N and the equator show that the Mathematician and Clipperton seamount chains are the old crest of the east Pacific rise. These parts of the rise crest terminated approximately 5 m.y. B.P. by the spreading center jumping 4° to the east. Seismic reflection profiles at right angles to the crest support this conclusion. The topography west of these two chains is used as the basis for a proposed evolution of the central eastern Pacific during the past 20 m.y.

Reykjanes ridge crest: A detailed geophysical study

Abstract: A geophysical survey employing satellite navigation was carried out over the Reykjanes submarine ridge southwest of Iceland. Water depth, sediment thickness, and the gravity and magnetic fields were continuously measured. In addition, bottom cores and measurements of sediment and water temperatures were obtained at stations. Expendable radio sonobuoys were used to make seismic refraction measurements. This paper combines these various geophysical data to obtain information about phenomena in the water layer, about details of crustal structure, and about mechanisms operating at the ridge axis. The satellite navigation results and water temperature data are used to deduce current directions and magnitude over the ridge. These currents play a role in the observed distribution of sediment. Variations in these currents are inferred from sediment temperature measurements. Magnetic profiles parallel to the ridge crest are used to demonstrate the presence of a thin, highly magnetized layer (termed layer 2A, since it constitutes the top of layer 2 of refraction seismology), as well as to directly infer the presence of normally and reversely magnetized rocks in bands on the ridge. Seismic refraction measurements reveal: (1) a 6.5-km/sec layer under the ridge; (2) a flankward increase in seismic velocity in the crust; and (3) evidence for a surface layer of relatively low velocity (about 3 km/sec) corresponding to layer 2A. Geothermal measurements revealed two zones of low heat flow, one within 10 km of the ridge axis and the other about 75 km from the axis. The maximum values of heat flow were observed in a zone from 15 to 50 km. The over-all average of heat flow over the ridge is not significantly different from that observed in the adjacent oceanic basins. Free-air gravity anomalies over the Reykjanes ridge range from +25 to +60 mgal. Compared to the mid-Atlantic ridge, the Bouguer anomalies over the Reykjanes ridge are about 60 mgal less, but the gradients are nearly the same. The narrow axial magnetic anomaly can be traced with minor offsets to the Reykjanes Peninsula. On Iceland, positive magnetic anomalies occur over much wider areas, implying that the active zone is much wider in Iceland than over the Reykjanes ridge.

Plasma clouds in the magnetosphere.

Abstract: Magnetospheric plasma clouds equatorial observation by ATS 5 satellite, revealing plasma injection during substorms and dispersion by earth magnetic and electric fields

Ozone production rates in an oxygen-hydrogen-nitrogen oxide atmosphere

Abstract: The distribution of the compounds NO, NO2, NO3, N2O5, and HNO3 has been calculated for different choices of relevant parameters, the values of which are uncertain. An appreciable part of the NO and NO2 is converted to NO3, N2O5, HNO3 and possibly HNO2 is the ozone layer. Reactions of odd oxygen with NO and NO2 may be the dominating reassociation processes for odd-oxygen particles in the region below 45 km which is very important for the global ozone budget. Several processes may lead to the presence of significant amounts of nitrogen oxides, nitrous acid, and nitric acid in the stratosphere. Reported variations during the solar cycle of ozone concentrations above 30 km (Dutsch, 1969) can be explained by corresponding variations in the stratospheric odd nitrogen oxide content. An artificial increase of the mixing ratio of the oxides of nitrogen in the stratosphere by about 1×10−8 may lead to observable changes in the atmospheric ozone level. Chains of reactions involving the constituents OH, H2O2, and HO2 also lead to the catalytic destruction of odd oxygen. The presence of nitric acid with a mixing ratio of about 3×10−9 in the ozone layer (Murcray et al., 1968; Rhine et al., 1969) indicates much larger OH and HO2 concentrations than can be explained solely by the reaction O(1D) + H2O → 2 OH. The reaction N2O5 + H2O → 2HNO3 followed by O + HNO3 → OH + NO3 may also be an important source of OH if the rate constants given by Jaffe and Ford (1967) are adopted. It is difficult to explain the measured nitric acid concentrations between 20 and 30 km with the reaction HO2 + NO + M → HNO3 + M. Laboratory data (Asquith and Tyler, 1969) indicate that the reaction H2O2 + NO2 → HNO3 + OH (Nicolet, 1970α) is unimportant in the atmosphere.

Observations of charged particle precipitation into the auroral zone

Abstract: Electron and proton precipitation observations in auroral, polar cap and outer radiation zones by electrostatic analyzers on earth satellite Injun 5

Penetration of magnetosheath plasma to low altitudes through the dayside magnetospheric cusps

Abstract: Daytime high-latitude fluxes of low-energy ( 107 cm−2 ster−1 sec−1 with typical energy fluxes in the range 0.01 to 0.1 ergs cm−2 ster−1 sec−1. It is concluded that solar wind plasma can penetrate to low altitudes through the high-latitude cusp in the magnetopause, which is often referred to as the neutral point. This flux is related to a number of geophysical phenomena, including magnetospheric surface currents, daytime auroras, VLF and LF emissions, ionospheric irregularities, and geomagnetic fluctuations.

Fracture and flow of rocks under high triaxial compression

Abstract: A NEW TRIAXIAL COMPRESSION TECHNIQUE HAS MADE POSSIBLE THE STUDY ON GENERAL LAWS OF FRACTURE AND FLOW OF ROCKS UNDER GENERAL TRIAXIAL STRESS STATES, IN WHICH ALL THREE PRINCIPAL STRESSES ARE DIFFERENT. IN THIS PAPER, THE EFFECTS OF THE STRESS STATES ON FRACTURE AND YIELDING OF ROCKS ARE EXPERIMENTALLY STUDIED BY THIS METHOD. THE EXPERIMENTAL PROCEDURE IS DESCRIBED, AND THE EQUATIONS FOR STRESS STATES PRODUCING FRACTURE AND YIELDING ARE GIVEN AS MONOTONIC INCREASING FUNCTIONS OF THE THREE PRINCIPAL STRESSES. THE NEW FAILURE CRITERIA CORRESPONDING TO THE GENERALIZED VON MISES CRITERIA ARE INTERPRETED. THE DUCTILITY DEFINED AS THE PERMANENT STRAIN JUST BEFORE FRACTURE IS DETERMINED BY SUBTRACTING THE ELASTIC AXIAL STRAIN FROM THE TOTAL AXIAL STRAIN. FROM THE RESULTS ON THE EFFECT OF STRESS STATES, FRACTURE AND FLOW PROPERTIES OF THE EARTH'S UPPER MANTLE ARE DEDUCED. /AUTHOR/

Thermal conductivity of rock‐forming minerals

Abstract: Thermal conductivities /K/ of rock forming minerals reveals K as linear function of density for constant mean atomic weight

Temperature Field and Geophysical Effects of a Downgoing Slab

Abstract: The factors affecting the thermal behavior of a lithospheric slab descending into the mantle are so numerous and complicated that only numerical methods can accurately account for them. A series of finite-difference calculations of the temperature field, including the effects of viscous dissipation, adiabatic compression, phase changes and radioactive heat generation are carried out, and the relative effects of different parameters are investigated. An analysis of the stability and convergence of the numerical method indicates that the errors are small and can be reduced to any desired level by varying the grid size and the time steps. At a crustal spreading rate of 8 cm/yr, with all heat sources, the slab reaches thermal equilibrium with the surrounding mantle at a depth of about 650 km. Among observable geophysical quantities, seismic travel times and amplitudes provide the most information about the slab. Surface heat flow is sensitive to subsurface conditions that are at relatively shallow depths, and gravity anomalies are broad and are masked by crustal effects. Three dimensional calculations predict strong bending of seismic rays near slabs, which causes strong focusing and produces shadow zones. Analysis of travel-time data for the Tonga-Fiji region indicates that waves propagating down the slab from shallow events are advanced by about 4 sec. Observations and theoretical travel-time anomalies based on calculated temperature fields are in general agreement.

Plate tectonic emplacement of upper mantle peridotites along continental edges

Abstract: Recently developed ideas of global tectonics haye provided a new framework within which to consider the origin of alpine-type peridotites In plate theory, compressional zones associated with island arcs are considered to represent plate boundaries where oceanic lithosphere is subducted The subduction zones are characterized by lithospheric underthrusting, andesitic volcanoes, and deep seismic activity that generally dips under the continental edge (the Benioff zone) The presence of large oceanic-mantle crustal slabs thrust over or into continental edges contemporaneously with blueschist metamorphism in New Caledonia and New Guinea establishes an important variant of plate tectonics in the zones of compression The ‘obduction’ zones are characterized by a complete lack of volcanic activity and by high-pressure metamorphism During formation, they can be represented by shallow seismic zones dipping oceanward The common association of peridotites and blueschists in these orogenic belts may result from the initial stage of compressional impact (or orogeny) between an oceanic and a continental lithospheric plate Disturbed zones combined with a lack of high-temperature contacts at boundaries between cold mantle-peridotite slabs and trench sediments provide geologic evidence of emplacement by obduction (tectonic overriding) Internal subsolidus plastic deformation of these peridotites can be attributed to deep-seated strain within the upper mantle during spreading Serpentinites represent alteration developed during tectonic emplacement into wet sediments of the continental plate, which produces a less dense and plastic envelope that facilitates further tectonic movement in these compressional zones Recognition of these peridotite-serpentinite-blueschist belts within exhumed subduction or obduction zones will allow delineation of ancient compressional impacts between moving lithospheric plates

Signature in the interplanetary medium for substorms.

Abstract: A detailed signature for individual substorms is sought in the interplanetary medium. Hourly values of interplanetary field and plasma parameters are correlated with hourly averages of the AE index. An interplanetary variable involving the southward component of the interplanetary field in the solar magnetospheric coordinate system is shown to be singularly important for the generation of substorms. The parameter best correlated with AE (0.8 correlation coefficient) is the integral or summation of Bz south over time for the hour preceding the AE hourly average. The magnitude of this integral appears to be linearly related to the hourly average of AE. The linearity suggests that the southward interplanetary field represents a continuing dynamic mechanism for the production of substorms rather than just being a trigger for the release of energy that has been stored in the magnetospheric tail. Furthermore, the additional energy that the southward component of the interplanetary field apparently puts into the tail is not accumulated for longer than about 1 hour before it appears as a substorm. A linear fit to AE that uses interplanetary parameters is obtained for two time intervals of data.

Origin and Emplacement of the Ophiolite Suite: Appalachian Ophiolites in Newfoundland

Abstract: The ophiolite suite is probably generated by axial plate accretion at oceanic ridges, and by diffuse slow spreading in marginal basins behind and within island arc complexes. The dunite/harzburgite component may represent the highest parts of a depleted upper mantle shell, about 16 km thick, from which a basaltic partial melt (approx. 30%) has been withdrawn to form the gabbros, dike swarms, and pillow lavas. Ophiolites have complex internal igneous, structural, and metamorphic relationships that are probably related to processes involved in their generation at ridges and in marginal basins, having no significance in terms of processes within the orogenic belt in which they are finally emplaced. Varying plate accretion rates may have a great influence on the structural and metamorphic patterns of ophiolites. The autochthonous sedimentary cap of ophiolites developed by axial accretion may, rarely, consist of continental margin non-volcanic flysch and salt, but usually comprises chert, argillite, and deep-water limestone: that of ophiolites generated in marginal basins probably consists, in addition to these facies, of andesitic flysch derived from adjacent island arcs. Ophiolites are probably emplaced, in orogenic belts, at consuming plate margins either beneath and behind oceanic trenches (subduction zones) or by thrusting onto continental margins (obduction zones) when a continental margin meets a subduction zone. Metamorphic relationships within and around ophiolite complexes are numerous, probably reflecting processes involved in both genesis and emplacement, and may involve (1) thermal and hydrothermal alteration at the site of origin, (2) blueschist metamorphism in subduction zones, (3) regional high-temperature metamorphism in island arcs, (4) high-temperature aureoles developed around peridotites injected into oceanic crust at time of origin, into the transition region between marginal basins and island arcs or into the regionally metamorphosed assemblages of island arcs, (5) garnet-amphibolites developed along the sole thrusts of obducted peridotites, which may represent transported high-temperature arc metamorphics, oceanic crust, or metamorphic rocks generated during obduction, (6) rodingites developed during low-temperature peridotite diapirism and serpentinization on ridge flanks or during ophiolite emplacement, and (7) amphibolites and eclogites, in subduction melanges, representing transformed oceanic crust ripped up from the downgoing plate beneath trenches. Obducted ophiolites often indicate a narrow time span between origin and emplacement, possibly due to the opening and closing of marginal basins. Evolving ridge/transform/trench and trench/trench/trench triple junctions have great significance for developing diachronous events associated with ophiolite emplacement along continental margins and island arcs. Early Ordovician ophiolites in the Newfoundland Appalachians have complex and variable structural relationships with rocks of the Paleozoic continental margin. Immense obducted ophiolite slices carry transported garnet-amphibolites, and are believed to have developed in a marginal basin behind early Paleozoic island arcs on the northwestern (present) margin of a proto-Atlantic Ocean.

Relativistic electron precipitation during magnetic storm main phase

Abstract: Relativistic electron precipitation during magnetic storms, showing cyclotron resonances with electromagnetic ion cyclotron waves

Aftershock zones of great earthquakes, seismicity gaps, and earthquake prediction for Alaska and the Aleutians

Abstract: Aftershocks of shallow earthquakes larger than magnitude 7 in the Aleutians, southern Alaska, southeast Alaska, and offshore British Columbia from 1920 to 1970 were relocated by computer in an attempt to delineate the rupture zones of large earthquakes. Plate tectonic theory indicates that gaps in activity for large earthquakes for the past 10's to 100's of years are likely sites of future large earthquakes. Three prominent gaps of this type are delineated: one in southeast Alaska; another in southern Alaska near the epicenters of the great earthquakes of 1899 and 1900; and one in the far western Aleutians. These gaps deserve high priority for study and instrumentation. Large earthquakes appear to be much more regular than smaller shocks in their distributions with respect to space, time, and size. Aftershock zones of events since 1930 that are larger than magnitude 7.8 are longer than 250 km and those less than 7.5 are shorter than 125 km. The rupture zones of events that occurred before 1930 could not be delineated from aftershock locations. Aftershock zones of large earthquakes tend to abut without significant overlap even for rupture zones as long as 1200 km. Nearly the entire Alaska-Aleutian zone from 145°W to 171°E has broken since 1938 in a series of large earthquakes. The rupture zones of five large events appear to form a space-time sequence that progressed from 155°W in 1938 to 171°E in 1965. This sequence is much like the well-known westward progression of activity since 1939 along the North Anatolian fault. Shocks with long rupture zones tend to occur along those parts of the Alaska-Aleutian zone that are relatively simple tectonically. The ends of many aftershock zones of large earthquakes are located at the intersection of major transverse features with the Aleutian arc. Large earthquakes rarely, if ever, reoccur along the same part of a fault zone in less than several tens of years, i.e. within a time less than that for substantial strain accumulation. Events of comparable magnitude that occur soon after some great earthquakes usually involve rupture in a region adjacent to but different from that of the main shock. The March 30, 1965, earthquake of magnitude 7.5, which involved normal faulting in the Aleutian trench, appears to have been triggered by thrust faulting along the adjacent inner margin of the trench in the magnitude 7.9 earthquake of February 4, 1965. Large events of the thrust type are commonly followed within ten years by events involving normal faulting in the adjacent part of the trench. Estimates of average displacements and of the repeat times of great earthquakes from measurements of 20-sec surface waves are systematically too small and do not agree with the meager historic record of great shocks. Other estimates of repeat times vary from 30 to 850 years, but neither of these extremes appears to be typical. The aftershock zone of the April 1, 1946, Aleutian earthquake, which generated one of the largest and most widespread seismic sea waves in the Pacific during this century, was very small. A large displacement of the ocean floor may be responsible for the generation of the large sea wave. An average displacement of 2.4 to 4.1 meters was calculated from amplitudes of 100-sec waves.

Effects of stress on velocity anisotropy in rocks with cracks

Abstract: The effective elastic compliance of rock that contains cracks is evaluated from energy considerations, as first proposed by Eshelby [1957]. The compliance of rock depends on the compliance of the solid matrix, the directional distribution of the cracks, an ‘inhomogeneity’ interaction tensor, and the shape distribution of cracks, which are assumed to be shaped like pennies. The effective compliance is linearly elastic for small-amplitude elastic waves. Anisotropic crack distribution causes elastic anisotropy, with associated acoustic birefringence. Nonhydrostatic stress causes stress-induced anisotropy, owing to anisotropic closure of cracks. Although velocities are uniquely determined from the distribution of cracks, the distribution cannot be determined uniquely from the velocities. The theoretical results compare favorably with measured compressional velocities and crack distribution in Salisbury granite and with measured stress-induced compressional and shear velocity anisotropy in Barre granite.

Thermal conductivity of rocks from measurements on fragments and its application to heat-flow determinations

Abstract: A series of measurements indicates that the thermal conductivity of a rock generally can be estimated adequately for geophysical purposes from simple laboratory measurements on fragments of the rock. The solid-rock conductivity is deduced from a divided-bar measurement of the conductivity of a cylindrical cell containing water-saturated rock fragments. All determinations fall within about ten per cent of conventionally measured solid-rock values for a variety of crystalline and sedimentary rocks with conductivities ranging from about 3 to 14 mcal/cm sec°C. Good agreement is found between values determined from drill cuttings and solid core from the same depth in deep boreholes. The method is illustrated by the determination of heat flow in a 3-km-deep borehole near Tehran, Iran (35°40′N latitude, 51°37′E longitude; elevation, 1710 meters). Contrasting gradients are compensated by conductivity variations determined from drill cuttings. Component heat flows show good agreement with a mean of 2.1±0.1 μcal/cm2 sec.

Marginal offsets, fracture zones, and the early opening of the North Atlantic

Abstract: A simplified model is proposed that can explain the evolution of marginal offsets, ridges, and fracture zones. Submarine features of the equatorial Atlantic margins are examined and are found to be in reasonable agreement with the model. The results reaffirm the suggestion that the South Atlantic opened in two stages beginning about 140 m.y. and 80 m.y. ago. The pole of rotation describing the relative motion of the South American and African plates changed radically when the constraints imposed by adjacent continental blocks were relaxed.

Plasma in the Earth's polar magnetosphere

Abstract: First observations of the plasmas in the dayside polar magnetosphere were obtained with the earth-satellite Imp 5 during July-August 1969. Several of the more important observational results are the following. (1) The polar neutral ‘points’ that appear at the high-latitude magnetopause in mathematical models for the shape of the geomagnetic cavity formed by the interaction of the solar wind with the geomagnetic field are observationally ‘bands’ with width ∼1 RE across the dayside high-latitude magnetopause (one band in the northern hemisphere and presumably a second in the southern hemisphere). (2) These two bands, or regions of the magnetopause through which the magnetosheath plasma has direct access to the magnetosphere, and the corresponding extension of these bands from magnetopause to auroral altitudes have been designated herein as the ‘polar cusps.’ (3) At all other positions of the dayside magnetopause, the magnetopause appears to be an effective barrier against the direct entry of magnetosheath plasma. (4) During periods of relative magnetic quiescence the intersection of the dayside polar cusp with the auroral zone is positioned at invariant latitude Λ=79° (±1°) and its latitudinal width is 20 to 400 km projected onto the auroral zone. (5) During periods of the relative magnetic disturbance the position of the polar cusp moves equatorward by several degrees in invariant latitude without a large increase in its latitudinal width, i.e., by factors ≲2. (6) The high-latitude termination of energetic trapped electron (E>45 kev) intensities in the high-latitude dayside outer radiation zone occurs coincident with the polar cusp, albeit these intensities are small and of irregular profile with radial distance in this region. (7) No measurable intensities of energetic electrons (E>40 kev), magnetosheath protons and electrons, and ring-current protons were observed at latitudes above the polar cusp, i.e., in the polar cap region. (8) The proton and electron differential energy spectrums as viewed in the solar direction in the distant polar cusp (within several earth radii of the magnetopause) are identical to those observed within the magnetosheath to within observational accuracy. (9) The bulk velocity of protons in the distant polar cusp as deduced from the angular distributions appears to be lower than that of the magnetosheath plasma near the magnetopause by factors ∼2 or 3. (10) In the midaltitude polar cusp at ∼4 to 5 RE geocentric radial distances, the proton spectrum differs from that at the magnetosheath in that protons with energies ≲500 ev are severely less than those observed in the magnetosheath. (11) The proton spectrums in the midaltitude polar cusp are similar to those in the distant plasma sheet with the exception that the proton number densities in the polar cusp are typically larger by factors ∼20 to 200. (12) The angular distributions of proton intensities in the midaltitude polar cusp are strongly peaked along the local magnetic field (i.e., down into the auroral zone); the dimensions of the atmospheric loss cone at these altitudes appear to be insufficiently large to account for the observed anisotropy; and (13) the magnetosheath plasma in the midaltitude polar cusp is observed to be separated into two thin sheets, one of magnetosheath proton intensities and the other populated with magnetosheath electrons; these sheets are immediately adjacent to each other with the electron sheet equatorward of the proton sheet; their latitudinal widths as projected into the auroral zone are roughly equal, ∼10 to 200 km. These observations, along with recent measurements from other earth satellites, have been interpreted in terms of a proposed magnetospheric model with several new features, among which are the following. (1) Plasma sheet protons gain access to the magnetospheric field lines via the dayside polar cusps. (2) All magnetic field lines threading the distant plasma sheet beyond ∼20 or 30 RE were convected from the polar cusps. (3) Magnetic field lines in the polar cap region of the magnetotail do not merge or pass through the plasma sheet.

On the alternating field demagnetization characteristics of multidomain thermoremanent magnetization in magnetite

Abstract: It is demonstrated that the stability against af demagnetization of TRM carried by multidomain magnetite increases with increasing strength of the inducing field. This multidomain field-dependence trend of stability against af demagnetization is opposite to that of single-domain particles. A simple test to distinguish multidomain and single-domain carriers of remanence is designed that makes use of the distinctive af demagnetization characteristics. The test was applied to a variety of igneous rocks. None of these showed truly multidomain characteristics. Thus, although some TRM is undoubtedly multidomain in these rocks, the TRM is dominated by single-domain carriers. These results are consistent with the growing evidence of the importance of very fine grained magnetite as a carrier of NRM in many igneous rocks.

Theory of water-filled crevasses in glaciers applied to vertical magma transport beneath oceanic ridges

Abstract: The theory describing the behavior of glacier crevasses that are partially filled with water is developed further, and the results are applied to the problem of vertical magma transport beneath oceanic ridges. It is assumed that lava collects beneath the center of a ridge. According to the theory, if a tensile stress acts across a ridge that is cold enough to behave like an elastic plate, a crack filled with liquid rock will nucleate at the bottom surface of the plate. This crack will increase in length and volume, pinch its lower end shut, and finally rise toward the top surface. (The immediate driving force causing the crack to rise is the Peach-Koehler force, which acts on dislocations. In the case of the liquid-filled crack the Peach-Koehler force produces a total, pseudo-Archimedian force that is identical in form to the true Archimedian buoyancy force on a solid within a liquid.) Eventually the crack will stop, and the lava in it will freeze and cause the oceanic ridge to spread somewhat. Lava will spill out onto the top surface only if an oversize crack reaches the upper surface. A crack that has been preceeded by other cracks on its upward journey will be trapped in the interior of the plate and its lava will freeze in place.

Elastic properties of marine sediments

Abstract: This report includes discussions of elastic and viscoelastic models for water-saturated porous media, and measurements and computations of elastic constants including compressibility, incompressibility (bulk modulus), rigidity (shear modulus), Lame's constant, Poisson's ratio, density, and compressional- and shear-wave velocity. The sediments involved are from three major physiographic provinces in the North Pacific and adjacent areas: continental terrace (shelf and slope), abyssal plain (turbidite), and abyssal hill (pelagic). It is concluded that for small stresses (such as from a sound wave), water-saturated sediments respond elastically, and that the elastic equations of the Hookean model can be used to compute unmeasured elastic constants. However, to account for wave attenuation, the favored model is ‘nearly elastic,’ or linear viscoelastic. In this model the rigidity modulus μ and Lame's constant λ in the equations of elasticity, are replaced by complex Lame constants (μ + iμ′) and (λ + iλ′), which are independent of frequency; μ and λ represent elastic response (as in the Hookean model), and iμ′ and iλ′ represent damping of wave energy. This model implies that wave velocities and the specific dissipation function 1/Q are independent of frequency, and attenuation in decibels per unit length varies linearly with frequency in the range from a few hertz to the megahertz range. The components of the water-mineral system bulk modulus are porosity, the bulk modulus of pore water, an aggregate bulk modulus of mineral grains, and a bulk modulus of the structure, or frame, formed by the mineral grains. Good values of these components are available in the literature, except for the frame bulk modulus. A relationship between porosity and dynamic frame bulk modulus was established that allowed computation of a system bulk modulus that was used with measured values of density and compressional-wave velocity to compute other elastic constants. Some average laboratory values for common sediment types are given. The underlying methods of computation should apply to any water-saturated sediment. If this is so, values given in this paper predict elastic constants for the major sediment types.

Geophysical and tectonic features of the Aegean Arc

Abstract: Several geophysical and tectonic features of the Aegean Island arc are described. An attempt has been made to interpret these features by means of a model of the area that includes a lithospheric plate that underthrusts the arc and moves from the Mediterranean to Europe. The upper boundary of this plate is determined by a zone of intermediate earthquakes. These earthquakes are located close to a surface of amphitheatrical shape that is dipping under the arc at a mean angle of about 30°. There is an aseismic zone in the upper mantle under the inner part of the arc. The absorption of short-period body waves in this zone explains the observed abnormal distribution of the seismic energy in the area. The results of this work are in general agreement with the ‘new global tectonics’.

Unified theory of SAR arc formation at the plasmapause

Abstract: Stable auroral red arcs generation at plasmapause from ion cyclotron wave turbulent dissipation of ring current proton energy