Showing papers in "Journal of Geophysical Research in 1972"

Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the western Pacific

Abstract: A model for oblique convergence between plates of lithosphere is proposed in which at least a fraction of slip parallel to the plate margin results in transcurrent movements on a nearly vertical fault which is located on the continental side of a zone of plate consumption. In an extreme case of complete decoupling only the component of slip normal to the plate margin can be inferred from underthrusting. Recent movements in the western Sunda region provide the most convincing evidence for decoupling of slip, which in this region is thought to be oblique to the plate margin. A speculative model for convergence along the margins of the Philippine Sea is constructed from an inferred direction of oblique slip in the Philippine region. This model requires that the triple point formed by the junction of the Japanese and Izu-Bonin trenches and the Nankai trough migrate along the Sagami trough.

Oxygen isotope exchange between quartz and water

Abstract: Equilibrium constants for oxygen isotope exchange between quartz and water have been measured from 195°C (1000 ln α = 12.0) to 750°C (1000 ln α = 0.4). Over limited temperature ranges the behavior of fractionation with temperature can be approximated by 1000 ln α = 3.38 (106 T−2) − 3.40 for 200°–500°C and by 1000 ln α = 2.51 (106 T−2) − 1.96 for 500°–750°C. The results of measurements in the quartz-water system can be combined with analogous results from other mineral systems to make mineral-pair isotopic thermometers for application to problems of petrogenesis.

Cohesive force across the tip of a longitudinal‐shear crack and Griffith's specific surface energy

Abstract: The cohesive force across the fault plane is considered in order to understand the physical mechanism of rupture at the tip of a longitudinal-shear crack. The elastic field around the tip of a crack and the condition of rupture growth are systematically derived from the assumption that the cohesive force is given as a function of the displacement discontinuity. This assumption is more physically meaningful than those originally used by G. I. Barenblatt in 1959 and 1962. The stress field around the tip is calculated for several models of cohesive force, and is shown to be nonsingular even at the tip. The condition of rupture growth that is used to determine the rupture velocity turns out to be equivalent to the Griffith criterion and the relation employed by B. V. Kostrov in 1966, but the specific surface energy is defined more clearly in this paper.

Pitch-angle diffusion of radiation belt electrons within the plasmasphere.

Abstract: Study of the formation of the quiet-time electron slot, which divides the radiation belt electrons into an inner and an outer zone. The pitch-angle diffusion of radiation belt electrons resulting from resonant interactions with the observed plasmaspheric whistler-mode wave band is quantitatively investigated. The effects of wave propagation obliquely to the geomagnetic field direction with the resulting diffusion at all cyclotron-harmonic resonances and the Landau resonance are evaluated along with the effects of interactions occuring at all geomagnetic latitudes. The results obtained account for the long-term stability of the inner radiation zone, the location of its outer edge as a function of electron energy, and the removal of electrons to levels near zero throughout the slot. Computed pitch-angle distributions and precipitation decay rates are in good agreement with slot-region observations.

Time‐dependent friction in rocks

Abstract: Friction experiments have been conducted on porous sandstone, quartzite, graywacke, and granite in the 20- to 850-bar normal stress range. Sliding on clean rough-ground surfaces is initially stable for this range. However, as powered rock debris accumulates on the slip surface, stick slip becomes the dominant mode of sliding. The coefficient of static friction of surfaces with gouge exhibits a highly time-dependent behavior. Static friction increases with the logarithm of the time that adjacent blocks remain in stationary contact. Over the entire range of normal stresses the static friction for 105-sec intervals between stick-slip events is greater than the static friction for 15-sec intervals by 6 to 10%. This behavior may be significant in understanding the mechanisms of earthquake foreshocks, aftershocks, and fault creep.

Vertical and areal distribution of Saharan dust over the western equatorial north Atlantic Ocean

Abstract: Aerosol measurements were made as a part of the Barbados Oceanographic and Meteorological Experiment (Bomex) during May, June, and July 1969. Maximum dust concentrations occurred between the altitudes of 1.5 km and 3.7 km, a region which we call the Saharan air layer. The average concentration of mineral aerosol within this layer was 61 μg m−3; in contrast, the average concentration in the low-level air was 22 μg m−3. These dust concentrations are comparable to those found in continental surface air. Because of the presence of a strong inversion at the base of the Saharan layer, sea salt was confined to te lower altitudes where the average concentration was 10 μg m−3. Thus, sea salt appears to be a relatively minor constituent of the trade wind aerosol during much of the year. On the basis of these measurements and of a model describing the movement of Saharan air outbreaks, we estimate that 25 to 37 million tons of dust are transported through the longitude of Barbados each year. This quantity of dust is sufficient to supply all the material required to maintain the present rate of pelagic sedimentation across the entire northern equatorial Atlantic Ocean.

Satellite observations of energetic heavy ions during a geomagnetic storm.

Abstract: Large fluxes of energetic heavy ions (M / q ≈ 16) were observed in the inner magnetosphere during the geomagnetic storm of December 17, 1971. The observations were made by a set of energetic-ion mass spectrometers covering the energy range 0.7–12 kev on board the polar-orbiting satellite 1971-089A (800-km altitude, ≈0300 LT). Significant heavy-ion fluxes were observed for a period of approximately 48 hours during the main phase of the storm. The heavy-ion fluxes frequently exceeded the proton fluxes in the 0.7- to 12-kev energy range. The heavy-ion spectrums were highly variable and frequently contained a peak of several kev. The heavy ions were observed over a wide latitudinal range (2.4 ≲ L ≲ 9) and generally extended to somewhat lower latitudes than the protons. The peak energy flux of these ions was approximately 0.4 erg/cm² sec ster, which is substantial in terms of expected observable ionospheric effects. They may also contribute significantly to the storm-time magnetic-field depression (Dst), since at the same flux they represent an energy density greater by a factor of 4 than that of the protons.

Thermal conductivity of Earth materials at high temperatures

Abstract: The total thermal conductivity (lattice plus radiative) of several important earth materials is measured in the temperature range from 500 to 1900 K. A new technique is used in which a CO2 laser generates a low-frequency temperature wave at one face of a small disk-shaped sample, and an infrared detector views the opposite face to detect the phase of the emerging radiation. Phase data at several frequencies yield the simultaneous determination of the thermal diffusivity and the mean extinction coefficient of the material. The lattice, radiative, and total thermal conductivities are then calculated. Results for single-crystal and polycrystalline forsterite-rich olivines and an enstatite indicate that, even in relatively pure large-grained material, the radiative conductivity does not increase rapidly with temperature. The predicted maximum total thermal conductivity at a depth of 400 km in an olivine mantle is 0.020 cal/cm/sec/deg C, which is less than twice the surface value.

Rupture zones of large South American earthquakes and some predictions

Abstract: This study attempts to forecast likely locations for large shallow South American earthquakes in the near future by examining the past space-time pattern of occurrence of large (M ≥ 7.7) earthquakes, the lateral extent of their rupture zones, and, where possible, the direction of rupture propagation. Rupture zones of large shallow earthquakes generally abut and do not overlap. Patterns of rupture propagation appear to follow certain trends. These facts, plus the nonrandom behavior of the space-time history of seismic activity, present consistencies that may permit prediction, in a gross sense, of future events. By mapping the rupture zones of large earthquakes (in contrast with plotting only epicenters), it is possible to identify segments of the shallow seismic zone that have not ruptured in many decades. Limited experience elsewhere indicates that these gaps between rupture zones tend to be filled by large-magnitude earthquakes. In certain places it is possible to make approximate estimates of the time of occurrence of the next large earthquake. For at least 300 or 400 years, the entire fault segment near the Central Valley province of central and southern Chile (about 32°–46°S) has fractured about once each century from a generally N-S progression of several large (M ≥ 8) earthquakes. Large earthquakes in this region have almost always occurred to the south of a previous large earthquake. In addition, it is possible to infer a direction of rupturing for two large earthquakes in this century (1928 and 1960). Both these earthquakes fractured southward away from the rupture zone of an earlier earthquake. It would be consistent with these observations if a new series started about the end of this century near Valparaiso (33°S) and progressed southward. In other sections of South America there are several extensive segments of the active seismic belt that have not ruptured during this century. Northern Chile and southernmost Peru (about 17°–25°S) have been relatively aseismic for about 100 years. South of Lima (about 12.5°–14°S), between the rupture zones of the 1940 and 1942 Peruvian earthquakes, there is another significant gap in recent activity. Both these regions are probably areas of relatively high earthquake risk. The northern Peru and southern Ecuador region (about 9°–1°S) has also been relatively aseismic during this century. However, this region differs from the two previously mentioned gaps in that this coastal zone was a region of moderate seismicity during historic times. Perhaps aseismic creep is an unusually important factor in relieving tectonic strain along this particular segment of the shallow seismic zone. Another possibility is that large shallow earthquakes in this region have an extremely long recurrence time. Much of the shallow seismic zone of northern Ecuador and southwestern Colombia has ruptured twice during this century. During large earthquakes in this region, the rupturing tends to be directed toward the north or NE. The data for this region suggest that the area to the NE of the 1958 Colombian earthquake may be a region of relatively high earthquake risk.

Polar-cap electric field distributions related to the interplanetary magnetic field direction

Abstract: The correlations between the azimuthal direction of the interplanetary magnetic field and the most simple polar cap signatures are discussed. Only the spatial distribution of the dawn-dusk polar cap field is considered. For each OGO 6 traverse across the northern or southern polar cap, the simultaneous values of the interplanetary magnetic field in solar-equatorial coordinates were recorded by the Explorer 33 magnetometer. Histograms of these values are presented and are discussed. The high degree of correlation with the longitudinal angle indicates that the relative geometry of the interplanetary magnetic field and magnetospheric magnetic fields must be fundamental to explaining the distribution of polar cap electric fields. The sign of the solar-equatorial component perpendicular to the sun-earth line appears to be a more critical parameter than the sign of the component toward the sun. The Svalgaard-Mansurov correlation and the correspondence between fast convection and parallel magnetospheric and interplanetary magnetic fields are described.

Alaskan Earthquake of 1964 and Chilean Earthquake of 1960: Implications for Arc Tectonics

Abstract: The 1964 Alaskan earthquake (Ms ≈ 8.4) involved a segment of the eastern Aleutian arc 800 gm long; the 1960 Chilean earthquake sequence (Ms ≈ 8.5) affected roughly 100 km of the southern Peru-Chile arc. These two major events are strikingly similar in that (1) seismicity was shallow (<70 km), the earthquake focal regions and most of the associated tectonic deformation being between the oceanic trenches and volcanic chains of the two arcs; (2) regional vertical displacements were characterized by broad asymmetric downwarps elongate parallel to the arcs with flanking zones of marked uplift on the seaward sides and minor, possibly local, uplift on the landward sides; and (3) horizontal displacements, where determined by retriangulation, involved systematic shifts in a generally seaward direction and transverse tensile strains across the zones of subsidence. Surface displacements and seismicity for both events are compatible with dislocation models involving predominantly dip-slip movement of 20 meters or more on major complex thrust faults (megathrusts) inclined at average angles of about 9° beneath the eastern Aleutian arc and perhaps 20° beneath the Peru-Chile arc. The thrust-fault mechanism deduced for both the Alaskan and Chilean earthquakes is broadly consistent with the concept that the sectors of the Pacific rim in which they occurred are major zones of convergence along which the oceanic plates progressively underthrust the less mobile America plate. Directions of convergence between lithospheric plates at these arcs as deduced primarily from paleomagnetic data are in reasonably good agreement with the observed earthquake-related deformation; the deduced rates of convergence, however, appear to be too high in the eastern Aleutian arc and too low in the southern Peru-Chile arc. Despite gross similarities in tectonic setting and the present style of earthquake-related deformation, the geologies of the continental margins in the eastern Aleutian arc and southern Peru-Chile arc differ significantly. This difference suggests that Mesozoic and Cenozoic sediments and volcanic rocks conveyed into the eastern Aleutian trench have progressively accreted to the Alaskan continental margin, whereas most or all of the material carried into the southern Peru-Chile trench has disappeared beneath the Chilean continental margin.

Electromagnetic instabilities produced by neutral-particle ionization in interplanetary space

Abstract: This article examines the consequence of the ionization (e.g., photoionization) of a small population of neutral atoms (hydrogen or helium) in interplanetary space. It is found that, even if the density of the newly ionized particles is only a very small fraction of that of the solar wind, these particles can efficiently excite electromagnetic waves by means of a new collective instability. The instability is driven by an anisotropy in kinetic energy of the newly ionized particles. The typical linear growth rate is γ ≃ (ωin/2½) (υ0⊥/c) where ωin is the plasma frequency of the newly ionized ions, and υ0⊥ is the characteristic speed of the newly ionized ions perpendicular to the ambient magnetic field in the frame of the solar wind.

Use of the three‐dimensional covariance matrix in analyzing the polarization properties of plane waves

Abstract: A technique for analyzing the polarization properties of plane waves that offers a number of advantages over methods currently used in the analysis of both ground and satellite observations of waves is developed. This technique reduces the computations required to find the wave normal vector, is less sensitive to common noise sources, and is amenable to analog implementation. The technique here is applied specifically to the analysis of a proton whistler, but it can also be used in most studies of ULF, ELF, and VLF magnetic-wave phenomena.

Spacecraft charging at synchronous orbit.

Abstract: Observation that ATS 5 can charge to potentials as high as 10,000 volts during eclipse, and as high as 200 volts in sunlight. This charging has been used to infer the presence of the plasmasphere at synchronous altitude in the midnight region, and its prompt disappearance when hot plasma is injected in association with substorms. Although injected plasma has a density of 1.0 per cu cm the remaining cold plasma (less than 50 eV) has a density of 0.01 per cu cm. A mathematical model of ATS 5 has been constructed that can be used to predict spacecraft potentials. In addition, parts of ATS 5 can charge to several hundred volts without affecting the overall spacecraft potential appreciably.

Measurements of magnetotail plasma flow made with Vela 4B

Abstract: The flow of plasma in the earth's magnetotail has been measured with an electrostatic analyzer on Vela 4B at geocentric distances of ∼18 RE. The analyzer on the rotating (64-sec period) satellite measures proton energy spectra from 79 ev to 19 kev, and the plasma, flow is detected and measured by the substantial spin modulation that it often causes in the measured proton fluxes. The satellite's spin axis is kept directed radially outward along a radius vector from the earth, and so the analyzer, whose aperture is in the satellite's equatorial plane, most effectively senses flows in the direction perpendicular to the radius vector. Some results of the measurements are that (1) plasma flow speeds of several hundred km/sec are frequently measured in the plasma sheet, particularly during substorms, and these sometimes approach 1000 km/sec; however, evident flow in a given direction seldom persists for more than a few minutes; (2) these rapid substorm-related flows are usually directed generally sunward; (3) flow in the anti-sunward (tailward) direction is observed early in some substorms as the plasma sheet thins down; this may suggest the formation of a neutral line at geocentric distances <18 RE;, (4) the magnetotail is separated from the surrounding magnetosheath by a boundary layer a few thousand kilometers thick in which magnetosheath-like flow occurs but at reduced particle density and velocity; and (5) averaging of all flow measurements made in the plasma, sheet over many months does not reveal any distinct pattern of flow either sunward or anti-sunward; an average of the flows observed during periods of a few minutes of clearly evident flow, however, does reveal a flow in the general direction of the sun. It appears that the plasma sheet may often be in turbulent motion with turbulence-cell dimensions no greater than a few RE.

The aa indices: A 100-year series characterizing the magnetic activity

Abstract: After the 100-year series of the aa indices is described, two results are set forth. The general trend of the activity variation permits us to forecast a very-low level of intensity (3–4 in Ap units for the yearly values) during the next solar minimums (this finding would be of interest in magnetospheric studies), and the annual variation of the activity is definitely proved.

Focal mechanisms and plate tectonics of the southwest Pacific

Abstract: Ninety-six new focal mechanisms were determined for earthquakes on the belt of seismic activity separating the Pacific and Australian plates. The direction of convergence of these plates varies from NE-SW to E-W. The Australian plate underthrusts the Pacific plate to the ENE under the Solomon and New Hebrides islands and overthrusts the Pacific to the east along the Tonga-Kermadec arc and the North Island of New Zealand. The data for the Macquarie ridge concur with the idea that the pole of rotation for the Pacific and Australian plates is nearby and to the east of this feature. The data also suggest a NNE-SSW convergence of the Pacific and Australian plates in northwestern New Guinea. The relative motions of the plates near the Bismarck Archipelago are complex because of the presence of at least three additional small plates. The south Bismarck plate, the best defined, underlies the southern part of the Bismarck Sea. It is bounded on the north by an E-W belt of seismicity at about 3°S defining a left-lateral strike-slip fault. The New Britain arc forms the southern boundary, where the Solomon Sea floor underthrusts the south Bismarck plate to the NNW. There is some evidence for SW convergence of the south Bismarck and Australian plates in northeastern New Guinea. Small plates, less well-defined seismically, are also proposed under the northern part of the Bismarck Sea and under the Solomon Sea. The plate underlying the Solomon Sea floor is bounded by the Solomon and New Britain arcs and by eastern New Guinea. The southern boundary is not sharply defined by seismic data. The Solomon Sea plate is moving approximately NW with respect to the Australian plate and underthrusting the Pacific plate to the NE along the Solomon arc. The consistent pattern of relative motions of these three small plates allows quantitative estimates of relative rates of motion between them. These data demonstrate that plate tectonics is applicable even for regions with dimensions of only a few hundred kilometers. Geologic data from New Guinea are used to speculate about earlier plate motions in that area.

Detailed studies of frictional sliding of granite and implications for the earthquake mechanism

Abstract: DETAILED LABORATORY MEASUREMENTS HAVE BEEN MADE ON THE FRICTIONAL CHARACTERISTICS OF WESTERLY GRANITE WITH GROUND SURFACES AND TO NORMAL STRESSES OF 1 KB. THESE MEASUREMENTS SHOW THAT FOR THIS TYPE OF SURFACE THERE ARE THREE TYPES OF SLIDING: CONTINUOUSLY STABLE, EPISODIC STABLE, AND STICK- SLIP. ALL THREE OF THESE MODES HAVE ALSO BEEN OBSERVED ON THE SAN ANDREAS FAULT IN CENTRAL CALIFORNIA. IN OUR LABORATORY EXPERIMENTS, THERE IS A TRANSITION FROM FIRST TO THIRD MODE AT FROM 5 TO 15 BARS NORMAL STRESS, ABOVE WHICH STICK-SLIP PREDOMINATES. STICK-SLIP, HOWEVER, IS ALWAYS PRECEDED BY A SMALL AMOUNT OF STABLE SLIP. IF THIS OCCURS ON A LARGE SCALE ON FAULTS, IT MAY BE A PROMISING PREMONITORY EFFECT FOR EARTHQUAKE PREDICTION. LOADING RATE IS FOUND TO INFLUENCE THE FRICTIONAL STRENGTH INVERSELY, WHICH MAY BE AN EXPLANATION FOR STICK-SLIP. STRESS DROPS WERE FOUND TO INCREASE WITH SHEAR STRESS, BUT NOT LINEARLY, SUGGESTING THAT RADIATION EFFICIENCY MAY INCREASE WITH STRESS DROP.

Interaction of solar and galactic cosmic-ray particles with the Moon

Abstract: Calculation of the rates of formation of radionuclides as a function of depth in the moon for bombardments by galactic-cosmic-ray particles and by solar protons. The fluxes and spectra of galactic-cosmic-ray particles and of solar protons as a function of depth in the moon are first determined semiempirically. For galactic cosmic rays, the model emphasizes the production of secondary particles and the attenuation of particles by nuclear interactions. Solar proton calculations cover a range of observed spectral parameters, and only ionization energy losses need be considered. The excitation functions for the nuclear reactions used in these calculations are presented. The calculated production rates are given for a range of depths in the moon and are compared with experimental results and with earlier calculations. The model can also be applied to other effects of particle bombardment.

Filtering marine magnetic anomalies

Abstract: When marine magnetic anomalies can be adequately modeled by two-dimensional magnetic structures within one or more plane layers, many interesting manipulations of both models and anomalies are linear filtering operations [Dean, 1958; Bott, 1967; Black and Scollar, 1969; Schouten, 1971]. Linear filters can be applied quickly and accurately by using the fast Fourier transform algorithm [Cooley and Tukey, 1965]. We have applied this algorithm to two broad classes of physically meaningful filters. The first class consists of the filters that together can synthesize theoretical magnetic anomalies, given a magnetic model. The second class consists of filters that are the inverse of the filters in the first class and can be used to resolve the source of an observed anomaly. We shall describe in some detail the two classes of filters and their application to synthesizing theoretical magnetic anomalies and ultimately to resolving two correlating magnetic anomaly sections of the Eltanin 41 track across an asymmetrically spreading area south of Australia [Weissel and Hayes, 1971, 1972].

Field observation of nearshore circulation and meandering currents

Abstract: Field observations were made of wave-induced nearshore circulations and meandering longshore currents on an undulatory surf-zone bed, under the action of uniform incident waves. Circulations were associated with normal-wave incidence; meandering currents were associated with oblique-wave incidence. The transport in the observed circulations generally agreed with Bowen's (1967) linear theory based on the concept of radiation stress (Longuet-Higgins and Stewart, 1962, 1964), provided that a friction coefficient C = 0.014 was assumed. The longshore current near the shore line moved from a shoal to a depression as predicted, but this movement was also directed from an area of high waves to one of low waves, which is different from the case of a circulation driven by nonuniform breaker heights on the bar. Spilling breakers over a shoal underwent greater energy dissipation than plunging breakers in the rip current. Observed streamlines were narrow in the outflow and broad in the inflow, a characteristic that was probably associated with a nonlinear mechanism arising from a steep depression in the rip channel, as previously explained by Arthur (1962). These circulations were pulsational, unlike the circulations of a steady-state solution. Occasional strong outflows at beat frequencies caused water to escape from the circulation. For a given surf-zone undulation, breaking over the inner bar was essential to the formation of a circulation, and the intensity of breaking, controlled by tide, corresponded with a proportionally stronger circulation. Thus, circulations were generally stronger during low tide than during high tide. Low rip-current velocities at high tide fluctuated with incoming swells, whereas high velocities at low tide tended to fluctuate at surf beat frequencies. In proportion to increasing rip velocities, the rip pulsation tended toward lower intervals. Mean surface slopes caused by wave set-up and set-down agreed with trajectories of neutral-density balls released in the circulation. Meandering currents associated with oblique-wave incidence could be explained as a combined effect of circulation cells and parallel longshore flows.

Measurement of single-crystal elastic constants of bronzite as a function of pressure and temperature

Abstract: The nine single-crystal elastic constants of orthopyroxene, Mg0.8Fe0.2SiO3, have been measured as a function of temperature from 25° to 350°C and at 25°C as a function of pressure to 10 kb by means of the ultrasonic pulse superposition technique. It was found that the shear constants exhibited a distinctly nonlinear pressure dependence in addition to the usual linear terms. Owing to the difficulty in obtaining precise data for the longitudinal modes above approximately 4.5 kb, where curvature might be observed, only a linear pressure dependence was found for the on-diagonal longitudinal constants. Because the second pressure derivatives of the on-diagonal longitudinal constants (c11, c22, and c33) enter the expressions required for the evaluation of the second pressure derivatives of the off-diagonal constants (c12, c23, and c13), the second derivatives of the off-diagonal constants are probably considerably in error. The second pressure derivatives of the on-diagonal shear constants and of the unprocessed data for the cross-coupling moduli, however, have been precisely and consistently measured and represent the first observations of curvature for noncubic oxide materials. The dimensionless quantities K(∂cμv/∂P2) (where K denotes the bulk modulus, cμv denotes the elastic constants, and P denotes the pressure) for the on-diagonal shear moduli are about ten times larger than the corresponding quantities for the eight alkali halides for which these quantities are known. The isotropic bulk and shear moduli and their pressure and temperature derivatives calculated from the single-crystal data by means of the Voigt-Reuss-Hill (VRH) approximation are KS = 1.035 Mb, G = 0.749 Mb, (∂KS/∂P)T = 9.59, (∂G/∂P)T = 2.38, (∂KS/∂T)p = −0.268 kb°C−1, and (∂G/∂T)p = −0.119 kb°C−1. Owing to the large values of the pressure derivatives of the longitudinal elastic constants c11, c22, and especially c33, the pressure derivative of the bulk modulus of orthopyroxene is approximately twice as large as that for most other materials normally considered to be of importance in the earth's mantle. The ultrasonic equation of state calculated from the first-order Birch equation agrees well with static-compression data and, below about 150 kb, with shock-wave data. The elastic Grueneisen parameter calculated from the VRH approximation is found to be 30% larger than the thermal Grueneisen parameter.

Interaction of the solar wind with the neutral component of the interstellar gas.

Abstract: A model is constructed to represent the interaction between the solar wind and the neutral component of the interstellar gas. It is found that the neutral gas has several important effects on the solar-wind expansion beyond the orbit of the earth and that it should be possible to infer the presence of the neutral gas from observations of the solar wind made by a space probe traveling into the outer solar system. The effects include a deceleration and heating of the supersonic solar wind, a cooling of and pressure reduction in the subsonic solar wind, and a tightening of the spiral magnetic field in the supersonic solar wind.

Vlf hiss and related plasma observations in the polar magnetosphere.

Abstract: This paper presents a study of auroral-zone vlf hiss and low-energy charged-particle observations with the Injun 5 satellite. The results of this study provide a direct verification of the association between auroral-zone vlf hiss and intense fluxes of low-energy electrons with energies on the order of 100 eV to several keV. On the dayside of the magnetosphere, these low-energy electrons are identified with the dayside polar-cusp region observed at higher latitudes with the Imp 5 satellite. At other local times, through the dawn and dusk regions and into the nightside of the magnetosphere, the vlf hiss and low-energy electron precipitation regions are believed to correspond to the extension of the dayside polar cusp into the distant plasma sheet and downstream magnetosheath on the nightside of the magnetosphere. Intense fluxes of upgoing electrons are often observed in a narrow latitudinal band near the low-energy electron precipitation bands. These upgoing electrons are believed to be associated with another type of vlf emission called a saucer, which is frequently observed with Injun 5.

Relation of corner frequency to fault dimensions

Abstract: The spectrum of the far-field displacement radiation from a conventional fault model has been calculated for comparison with the spectrum calculated from the recent Brune model. The spectrums are generally similar, but some of the details are attributed to different effects in the two models. For example, where the spectrum decays as ω−1 at intermediate frequencies, the effect is attributed to partial stress drop by the Brune theory but to rupture surface shape (width very much less than length) by the conventional theory. Both theories yield an inverse proportionality between the fault dimensions and the corner frequency in the displacement spectrum. The constants of proportionality in the two theories are similar.

Effect of pore pressure on the velocity of compressional waves in low‐porosity rocks

Abstract: The velocity V sub p of compressional waves has been measured in rock samples of low porosity to confining pressures P sub c of 2 kb for a number of different constant pore pressures P sub p. An effective pressure defined by P sub e = P sub c-nP sub p, n less than or equal to 1, is found to be the determining factor in the behavior of V sub p rather than an effective pressure defined simply by the differential pressure Delta P = P sub c-P sub p. As pore pressure increases at constant effective pressure, the value of n increases and approaches 1, but as effective pressure increases at constant pore pressure, the value of n decreases. These observations are consistent with Biot's theory of the propagation of elastic waves in a fluid-saturated porous solid.

Static fatigue of quartz

Abstract: Nearly 200 samples of single-crystal quartz were broken in uniaxial compression. The quartz was observed to exhibit strong static fatigue, the mean fracture time 〈t〉 being related to stress σ, concentration of water CH,O, and temperature T by 〈t〉 = t0 CH2O−;α exp [(u′/RT) − κ′σ]. Microfracturing associated with crack growth and spalling occurred continuously during the experiments. No creep strains could be detected. The rate of microfracturing n followed the expression n = a0 CH2O α exp [(−u′/RT) + κ′σ]tγ. These and other results suggest that static fatigue of quartz is due to stress corrosion cracking. The most likely mechanism is a silica hydration reaction. Static fatigue is suggested as the mechanism of brittle creep of rocks and of time-dependent earthquake phenomena.

Prediction of shallow‐water spectra

Abstract: Significant changes occur in the sea-surface spectrum as it propagates from deep to shallow water. These changes are a combined result of nondissipative forces (refraction, shoaling) and dissipative-generative forces (bottom friction, wind generation, wave breaking). In this study two prediction schemes for these changes are developed. Numerical predictions of spectral changes in shoaling water are made and compared with observed data.

Lead 210, bismuth 210, and polonium 210 in the atmosphere: Accurate ratio measurement and application to aerosol residence time determination

Abstract: Procedures developed for the sequential chemical separation and radiochemical analysis of 210Pb, 210Bi, 210Po, and 90Sr are described. Results from the measurements of these radioisotopes in surface air and precipitation are presented and discussed in relation to their sources and their application as tracers for the estimation of the residence times of particles in the lower atmosphere. It is concluded that the simple steady-state model does not apply to these long-lived radon daughters and that published residence time estimates based on 210Po/210Pb ratios are incorrect and too long. On the basis of 210Bi/210Pb activity ratios, correcting for the presence of an assumed secular equilibrium component of 210Pb, 210Bi, and 210Po, we estimate a mean atmospheric residence time of about 4 days for particles in the lower troposphere and about 1 week for particles in precipitation. Further tests of the steady-state assumption and other factors affecting such estimates are in progress.