Showing papers in "Journal of Geophysical Research in 1978"

Finite strain isotherm and velocities for single-crystal and polycrystalline NaCl at high pressures and 300°K

Abstract: Ultrasonic, pressure-volume, and shock wave measurements of NaCl are reviewed, with the purpose of representing as much as possible within a single theoretical framework, A large portion of the data is consistent, to within reasonable uncertainties, with the Eulerian formulation of finite strain, in the BE2 form. This contains three parameters, of which two, K0 and K0′, are obtainable from single-crystal ultrasonic measurements, while the third, K0″, may be found with the aid of shock wave data. The combination of the values of Spetzler et al. (1972a) with the isotherm of Fritz et al. (1971) gives an equation of state consistent with the pressure-volume measurements to 300 kbar, to within a kilobar or two. There remain small but possibly significant discrepancies with the high-precision measurements of linear change at low pressures. Various two-parameter isotherms are also examined and found to fail either at low or at high compressions: While other three-parameter forms may be fitted to the data, they do not afford a satisfactory general treatment of wave propagation. Finite strain theory is applied to project the effective elastic coefficients of single-crystal NaCl to 270 kbar. The anisotropy index, 2B44(B11–B12), falls from 0.7 at P = 0 to 0.12 at 270 kbar, and the usual methods of averaging to find the properties of a quasi-isotropic aggregate diverge increasingly as pressure increases. The methods of Kroner and of Peresada give fair agreement with recent measurements of velocities in NaCl aggregates to 270 kbar.

Present‐day plate motions

Abstract: A data set comprising 110 spreading rates, 78 transform fault azimuths and 142 earthquake slip vectors was inverted to yield a new instantaneous plate motion model, designated RM2. The mean averaging interval for the relative motion data was reduced to less than 3 My. A detailed comparison of RM2 with angular velocity vectors which best fit the data along individual plate boundaries indicates that RM2 performs close to optimally in most regions, with several notable exceptions. On the other hand, a previous estimate (RM1) failed to satisfy an extensive set of new data collected in the South Atlantic Ocean. It is shown that RM1 incorrectly predicts the plate kinematics in the South Atlantic because the presently available data are inconsistent with the plate geometry assumed in deriving RM1. It is demonstrated that this inconsistency can be remedied by postulating the existence of internal deformation with the Indian plate, although alternate explanations are possible.

Efficient prediction of ground surface temperature and moisture, with inclusion of a layer of vegetation

Abstract: An efficient time-dependent equation for predicting ground surface temperature devised by Bhumralkar (1975) and Blackadar (1976) is tested against a 12-layer soil model and compared with five other approximate methods in current use. It is found to be generally superior if diurnal forcing is present and very much superior to the use of the insulated surface assumption. An analogous method of predicting ground surface moisture content is presented which allows the surface to become moist quickly during rainfall or to become drier than the bulk soil while evaporation occurs. These improved methods are not of much relevance unless the main influences of a vegetation layer are included. An efficient one-layer foliage parameterization is therefore developed that extends continuously from the case of no shielding of the ground by vegetation to complete shielding. It includes influences of both ground and foliage albedos and emissivities, net leaf area index, stomatal resistance, retained water on the foliage, and several other considerations. When it is tested against data for wheat measured by Penman and Long (1960), it appears quite adequate despite the many simplifying assumptions. The parameterization predicts that errors of up to a factor of 2 in evapotranspiration can be incurred by ignoring the presence of a vegetation layer.

Large‐scale characteristics of field‐aligned currents associated with substorms

Abstract: Characteristics of field-aligned currents have been determined during a large number of substorms from the magnetic field observations acquired with the Triad satellite. The statistical features of field-aligned currents include the following: (1) The large-scale regions of field-aligned currents determined previously by the authors (Iijima and Potemra, 1976a) persist during all phases of substorm activity, namely, region 1, located near the poleward boundary of the field-aligned current region, and region 2, located near the equatorward boundary. Field-aligned currents flow into region 1 on the morningside and away from region 1 on the eveningside. The current flow in region 2 is reversed to region 1 at any given local time except in the Harang discontinuity region (∼2000–2400 MLT), where the flow patterns are more complicated. (2) During active periods (|AL| ≥ 100 γ) the average latitude width of regions 1 and 2 increases by 20–30%, and the centers of these regions shift equatorward by 2°–3° with respect to the quiet time values. (3) The current density in region 1 is statistically larger than the current density in region 2 at all local times except during active periods and in the midnight to morning local time sector. In this region, where the westward electrojet is most active, the current density in region 2 can exceed the current density in region 1. (4) During relatively quiet conditions (|AL| < 100 γ) the largest field-aligned current densities occur in two areas of region 1 near noon (near ∼ 1030 MLT and ∼ 1300 MLT) with an average value of ∼1.6µA/m². During active periods (|AL| ≥ 100 γ) the regions of peak current density shift toward the nightside (the region near 1030 MLT shifts to ∼0730 MLT, and the region near ∼1300 MLT shifts to ∼1430 MLT), and the average current density increases to ∼2.2 µA/m². (5) The average total amount of field-aligned current flowing into the ionosphere always equals the current flow away from the ionosphere during a wide range of quiet and disturbed conditions. The average total current during quiet periods is ∼2.7 × 106 A and during disturbed periods is ∼5.2 × 106 A. (6) A three-region pattern of field-aligned current flow persists in the Harang discontinuity region (∼2000–2400 MLT) during undisturbed and disturbed periods, when the westward auroral electrojet does not intrude into this sector. This flow pattern consists of an upward flowing field-aligned current surrounded to the north and south by downward flowing currents. During periods when the westward auroral electrojet has intruded deeply into the evening sector the Triad magnetometer data exhibit complicated and fine-structured variations indicating the presence of complex field-aligned currents in this sector. (7) The alignment of current sheets is generally along the boundary of the auroral oval (rather than in the east-west direction), but noticeable distortions of this alignment occur during very disturbed periods. The alignment of field-aligned currents is different in region 1 and region 2 during active periods. The different behavior of field-aligned currents in region 1 and 2 during substorms actively suggests that they are controlled by different source regions in the magnetosphere or ionosphere. The region 1 field-aligned currents map to the outermost part of the magnetosphere and magnetotail region, whereas the region 2 currents map to regions of the plasma sheet closer to the earth.

Collision frequency of artificial satellites: The creation of a debris belt

Abstract: As the number of artificial satellites in earth orbit increases, the probability of collisions between satellites also increases. Satellite collisions would produce orbiting fragments, each of which would increase the probability of further collisions, leading to the growth of a belt of debris around the earth. This process parallels certain theories concerning the growth of the asteroid belt. The debris flux in such an earth-orbiting belt could exceed the natural meteoroid flux, affecting future spacecraft designs. A mathematical model was used to predict the rate at which such a belt might form. Under certain conditions the belt could begin to form within this century and could be a significant problem during the next century. The possibility that numerous unobserved fragments already exist from spacecraft explosions would decrease this time interval. However, early implementation of specialized launch constraints and operational procedures could significantly delay the formation of the belt.

Geomagnetic paleointensities from radiocarbon‐dated lava flows on Hawaii and the question of the Pacific nondipole low

Abstract: Radiocarbon ages have been published for nine basaltic lava flows on the island of Hawaii; the ages range from 2600 to somewhat older than 17,900 years B.P. By using the Thelliers' method in vacuum, geomagnetic paleointensity values were obtained from eight of the lavas; the ninth proved unsuitable. The paleointensities for the four youngest flows (2600–4600 years B.P.) yield virtual dipole moments (VDM's) that are 20% greater to more than twice the worldwide values for those times obtained by V. Bucha from archeomagnetic data. The dispersion of virtual geomagnetic poles for the eight lavas is 15.5°, appreciably larger than the average for older lava flows on Hawaii. These results contrast with the historic magnetic field in the region of Hawaii, in which both secular variation and nondipole components are very low. At about 10,000 years B.P. the measured VDM is not very different from the long-term worldwide average but differs considerably from a smooth extrapolation of Bucha's average curve. At about 18,000 years B.P. the measured VDM is very low and is associated with an unusually shallow paleomagnetic inclination for the latitude of Hawaii. These new paleointensity and paleodirectional data strongly suggest that sizable nondipole geomagnetic fields have existed in the vicinity of Hawaii at various times during the Holocene epoch and perhaps earlier.

Active tectonics of Tibet

Abstract: From an interpretation of Landsat imagery of Tibet the most recent structures appear to be normal faults that trend approximately north-south. Fault plane solutions of 14 earthquakes in the central part of the Tibetan plateau indicate large components of normal faulting. The solutions are not well constrained, but for the most reliable ones the T axes are oriented approximately east-west. Only on the margins of the high plateau, where elevations are lower, do fault plane solutions show active thrust faulting. These observations imply an east-west extension of most of Tibet at the present time. We relate this pattern to the collision of India and Eurasia and to deformation of an especially weak Tibetan crust and upper mantle. India applies a pressure to Eurasia that maintains Tibet at a high uniform altitude, and the hydrostatic head caused by this elevation transmits the pressure northward. Accordingly, Tibet is the pressure gauge of Asia. At the same time a small east-west strain (or flow) of the lower crust and upper mantle of Tibet would stretch the surface layer. Tibet's position relative to India at the present time is analogous to that of a ‘dead’ triangle in a plastic material indented by a rigid indenter (India).

The frontside boundary layer of the magnetosphere and the problem of reconnection

Abstract: Further Heos 2 plasma and magnetic field data obtained in the frontside boundary layers of the magnetosphere are presented. They reveal that the low-latitude extension of the entry layer is of a somewhat different nature. The most pronounced difference with respect to the entry layer in the cusp region is the substantial density jump at the magnetopause. Furthermore, the low-latitude boundary layer tends to be thinner and less turbulent, and the flow velocity inside the layer is always lower than that of the adjacent magnetosheath. This observation excludes large-scale reconnection at the front of the magnetosphere as the origin of the layer. It is suggested that diffusive entry of magnetosheath plasma and/or heating of detached plasma from the plasmasphere leads to the formation of the layer. It appears likely that reconnection is dominantly occurring as a transient process in the cusp region and accompanies the eddy convection inside the entry layer. As a consequence, magnetic flux is being eroded from the front of the magnetosphere. This is in agreement with the signature of short-term large-amplitude magnetic perturbations observed in the low-latitude boundary layer.

Energy exchange over young sea ice in the central Arctic

Abstract: A simple model of heat transport through young sea ice is combined with climatological data on air temperatures and incoming radiation in the central Arctic to predict how each component of the surface heat balance is affected by changes in ice thickness. Results indicate that during the cold months the net heat input to the atmosphere from ice in the 0- to 0.4-m range is between 1 and 2 orders of magnitude larger than that from perennial ice. Once the ice exceeds a meter in thickness, there is little change in any of the heat fluxes as the ice thickens. Although both the amount of absorbed shortwave radiation and the emitted longwave radiation depend on ice thickness, it is the turbulent fluxes which undergo the largest changes. The rate of heat exchange over thin ice is shown to be extremely sensitive to snow depth and assumed boundary layer temperatures. It is concluded that with the present ice thickness distribution in the central Arctic, total heat input to the atmospheric boundary layer from regions of young ice is equal to or greater than that from regions of open water or thick ice.

An analysis of isostasy in the world's oceans 1. Hawaiian-Emperor Seamount Chain

Abstract: Cross-spectral techniques have been used to analyze the relationship between gravity and bathymetry on 14 profiles of the Hawaiian-Emperor seamount chain. The resulting filter or transfer function has been used to evaluate the state of isostasy along the chain. The transfer function can be best explained by a simple model in which the oceanic lithosphere is treated as a thin elastic plate overlying a weak fluid. The best-fitting estimate of the elastic thickness of the plate is in the range 20–30 km. Analysis of individual profiles shows significant differences in the elastic thickness along the seamount chain. Relatively low estimates of the elastic thickness were obtained for the Emperor Seamounts north of 40°N, and relatively high estimates for the Emperor Seamounts south of 40°N and the Hawaiian Ridge. These differences cannot be explained by a simple model in which there is a viscous reaction to the seamount loads through time. The best explanation is a simple model in which the elastic thickness depends on age and hence temperature gradient of the lithosphere. The low values can be explained if the Emperor Seamounts north of 40°N loaded a relatively young hot plate, and the high values can be explained if the Emperor Seamounts south of 40°N and the Hawaiian Ridge loaded a relatively old cold plate. These estimates of the elastic thickness along with determinations from other loads on the Pacific lithosphere suggest that the elastic thickness corresponds closely to the 450 ± 150°C isotherm, based on simple cooling models. Thus the large deformations and associated flexural stresses (>1 kbar) at seamount loads do not appear to change appreciably through time. This conclusion is in agreement with subsidence data along the seamount chain and with some gravity observations in the continents.

Mantle convection and the thermal structure of the plates

Abstract: Though a simple thermal model of plate creation matches the observed bathymetry and heat flow with considerable accuracy, it only does so because a constant temperature is imposed as a lower boundary condition. A more realistic model is developed here, based on the idea that the thermal structure of the plate becomes unstable and leads to the development of small-scale convection. Convective heat transport then supplies the heat flux needed to match the observations rather than an artificial constant temperature boundary condition. The temperature dependence of the rheology is represented in a simple manner. Below a given temperature the material is assumed to move rigidly, defining an upper mechanical boundary layer. Beneath this rigid layer, where the temperatures are greater, the material is assumed to have a constant Newtonian viscosity. The part of the viscous region where there are significant vertical temperature gradients, immediately below the mechanical boundary layer, forms a thermal boundary layer. As the plate cools, both the mechanical and thermal boundary layers increase in thickness. A local critical Rayleigh number criterion is used to test the stability of the thermal boundary layer. On this basis a convective instability is predicted, its occurrence coinciding with the breakdown of the linear dependence of the depth of the ocean floor on the square root of age. Though the small-scale convection which develops from this instability modifies the thermal structure, the basic observational constraints are shown to be satisfied. The stability criterion is further tested in two different laboratory experiments. These experiments also illustrate a possible form for the instability, with cold dense material breaking away from the base of the plate and being replaced by hotter material from below. The effects of the cooling of the oceanic plate as it moves away from a midocean ridge have received a good deal of attention, both theoretical and observational. Forsyth [1977] recently reviewed the different measurements that have been made. Variations in mean heat flow, depth, and velocity of ! - 

Initiation of intracontinental subduction in the Himalaya

Abstract: Independent arguments based on topographic stress and crustal strength give upper limits of 200 bars and 300 bars, respectively, for the average shear stress on the intracontinental thrust fault that formed the Himalaya. According to either a one-dimensional or a two-dimensional fault model, such stresses could not have produced the Himalayan granites by friction, unless overthrusting velocity exceeded 30 cm/yr. More probably, Himalayan metamorphism was caused by exposure of continental crust to hot asthenosphere prior to the formation of the intracontinental thrust. Crust was exposed by peeling away of Indian subcrustal lithosphere in response to the force and moment exerted by the Tethyan slab. This detachment of buoyant crust from dense lithosphere better explains the metamorphic pattern and also explains why the distributed crustal shortening at the beginning of the collision orogeny was replaced by localized thrusting or intracontinental subduction.

Light hydrocarbons in recent Texas continental shelf and slope sediments

Abstract: The distributions of the concentrations of methane, ethene, ethane, propene, and propane in twelve l-to 2-m-long gravity cores for two transects from nearshore to midslope off the southwest Texas Gulf Coast are reported. Methane profiles exhibit maxima in the top 40 cm of sediment on the shelf, in contrast to downward increasing gradients in the slope region. Nearshore surface methane concentrations ranging from 50 to 400 tl (normal temperature and pressure) per liter pore water are apparently due to microbial production ijsulfate-free microenvironments such as fecal pellets in a near-seawater sulfate environment. A decrease in sediment methane levels to less than 5 tl/l pore water in downslope sediments is attributed to reduced microbial activity due to lower organic contents and temperatures. Profiles of the saturated and unsaturated C: and Ca hydrocarbons suggest that these gases are also microbia!ly produced. plastic liner, was removed from the core barrel and sectioned at specific depths. Five-centimeter sections were immediately extruded into 0.5-1 containers holding 125 ml of sodium-azide- poisoned hydrocarbon-free seawater. The containers were capped, and the headspaces flushed with helium or nitrogen through septa in the lids. The hydrocarbon gases dissolved in the interstitial water were equilibrated with the gas phase by agitation for 5 min with a high-speed shaker. The shaker also dispersed the sodium azide throughout the sediment to inhibit microbial activity. The headspace gases were then analyzed, or the containers were inverted to form liquid seals around the lids and stored in darkness at near-freezing temperatures until analysis.

Asthenosphere readjustment and the earthquake cycle

Abstract: A simple two-dimensional model of the earthquake cycle (preearthquake strain accumulation, coseismic strain release, and postseismic readjustment) has been constructed from the Nur-Mavko solution for a screw dislocation in an elastic plate (lithosphere) overlying a viscoelastic substrate (asthenosphere). The deformation at the free surface is calculated for an earthquake cycle imposed by prescribed slip on a transform fault. This deformation is compared to that produced by a similar cycle in an elastic half space so that the effects of viscoelastic relaxation in the asthenosphere may be isolated. The following conclusions are drawn: (1) The surface deformation produced by viscoelastic relaxation in the asthenosphere can be duplicated identically by a reasonable distribution of slip at depth on a vertical fault in an elastic half space. Thus differentiation of two possible modes of postearthquake readjustment will be difficult. (2) The effect of asthenosphere relaxation is important only if the depth of the seismic zone is comparable to the thickness of the lithosphere. If the seismic zone is 15 km deep and the lithosphere is 75 km thick, as commonly estimated for the San Andreas fault zone, asthenosphere relaxation is not particularly significant in determining surface deformation. (3) In a periodic sequence of earthquakes the principal observable effects of viscoelasticity in the asthenosphere are to produce a rapid postearthquake deformation and to concentrate strain accumulation and relaxation even closer to the fault than in the elastic half-space model.

Tidal fronts on the shelf seas around the British Isles

Abstract: A numerical model is used to derive the Simpson-Hunter stratification parameter on the shelf seas surrounding the British Isles. Positions of predicted fronts are compared with structures observed in infrared satellite images and the measurements of sea surface temperature recorded on a cruise around the British Isles. The numerical model predicts the stability of the frontal systems, and baroclinic instability is suggested as the main candidate for cross-frontal mixing.

Island subsidence, hot spots, and lithospheric thinning

Abstract: Drilling results from several western Pacific atolls indicate the long-term subsidence of these islands is much more than would be expected from the cooling and thickening of the underlying lithosphere. This excess subsidence cannot be satisfactorily explained by isostatic adjustments to the weight of the volcano or the coral reef cap. It appears to be related to island formation atop unusually shallow areas of sea floor, like the Hawaiian swell, associated with midplate hot spots. The excess subsidence is caused by the gradual return of these shallow areas to normal depths. Several authors have suggested that the Hawaiian swell is supported by upward flow in the asthenosphere. However, this model offers no reasonable explanation for the shape of the swell or the observed rates of atoll subsidence. The regional gravity anomaly over the Hawaiian swell indicates an average depth of compensation within the lower half of the lithosphere, not within the asthenosphere, as would be expected if the swell were maintained by asthenospheric flow. While the compensating mass may extend to greater depths, most of the density changes appear to occur within the lithosphere. We propose that the Hawaiian swell is formed by lithospheric thinning over the Hawaiian hot spot. Since the asthenosphere is less dense than the lithosphere, replacement of the lower portion of the lithosphere by asthenospheric material causes isostatic uplift of the surface of the plate. After the lithosphere moves away from the hot spot, it cools and thickens, and the swell subsides. The subsidence histories of the Hawaiian swell and several Pacific atolls are in quantitative agreement with this mechanism. The main problem with this model is that it requires extremely fast rates of lithospheric heating.

The Control of Volcanic Column Heights by Eruption Energetics and Dynamics

Abstract: The height reached by a volcanic eruption column, together with the atmospheric wind regime, controls the dispersal of tephra. Column height is itself a function of vent radius, gas exit velocity, gas content of eruption products, and efficiency of conversion of thermal energy contained in juvenile material to potential and kinetic energy during the entrainment of atmospheric air. Different heights will be attained for the same total energy release depending on the style of the eruption: a discrete explosion produces a transient plume, whereas a prolonged release of material forms a maintained plume. A maintained eruption plume will also be formed if discrete explosions occur within a few minutes of one another, and eruptions producing large volumes of tephra commonly lead to maintained plume formation. Observed eruption columns from eight eruptions with cloud heights in the range 2-45 km and volume rates of magma production in the range l0 to 2.3 X l05 m3/s are compared with predicted values deduced from theoretical relationships for fluid convection. Theoretical model heights were calculated in two ways: first, for a wide range of eruptive conditions by using a dynamic model of eruption column formation and second, by using a theoretical formula relating height to rate of thermal energy release. Results from the two calculations were found to agree well and furthermore showed satisfactory agreement with the eight observations. Expected cloud heights can be usefully expressed as a function of heat release rate, expressed as the equivalent volume eruption rate of magma, for three different values of the efficiency of heat use. The results imply that many eruptions involve highly efficient use of the released heat, which indicates that the particle sizes in these eruptions are sufficiently small to allow rapid heat transfer to air entrained into the column. For certain combinations of vent radius, gas exit velocity, and gas content, column collapse to form pyroclastic flows should occur. Cloud heights have been calculated for a wide range of permutations of these parameters corresponding to the onset of collapse. The maximum theoretical height expected for a stable maintained plume is about 55 km, corresponding to a volume eruption rate of 1.1 X 1tY m3/s.

Self‐consistent particle and parallel electrostatic field distributions in the magnetospheric‐ionospheric auroral region

Abstract: The variation of the self-consistent electrostatic potential along the magnetic field is calculated by application of the principle of quasi-neutrality to the plasma components distributed along an auroral field line. The equilibrium plasma consists of hot anisotropic magnetospheric plasma, ionospheric plasma evaporated or extracted upward by the parallel electrostatic field, and backscattered electrons. It is shown that the above charged particle populations can support a potential difference of up to several Kilovolts between the equator and the ionosphere along an auroral field line. Moreover, the corresponding parallel electric field has the proper signature to account for electron precipitation characteristics. Comparisons between theoretical and observed electron precipitation fluxes lead to estimates for the various physical parameters in the model.

Prehistoric large earthquakes produced by slip on the San Andreas Fault at Pallett Creek, California

Abstract: Late Holocene marsh deposits composing a terrace about 55 km northeast of Los Angeles, California, contain geologic evidence of many large seismic events produced by slip on the San Andreas fault since the sixth century A.D. I excavated several trenches into the deposits in order to study this evidence. The principal indicators of past events are (1) sandblows and other effects of liquefaction, (2) the termination of secondary faults at distinct levels within the stratigraphic section, and (3) sedimentary deposits and relationships along the main fault. The effects upon the marsh deposits of the eight prehistoric events are comparable to those of the great (MS=8.25+) 1857 event, which is the youngest of the nine events that disturbed the strata and is associated with about 4.5 m of right-lateral slip nearby. Radiocarbon dates indicate that the events occurred in the nineteenth, eighteenth, fifteenth, thirteenth, late twelfth, tenth, ninth, seventh, and sixth centuries A.D. Recurrence intervals average 160 years but vary from century to about 3 centuries. The dates may indicate a fairly systematic pattern of occurrence of large earthquakes.

Seismological aspects of the Guatemala Earthquake of February 4, 1976

Abstract: Detailed analyses of teleseismic surface waves and body waves from the Guatemala earthquake of February 4, 1976, show the following: (1) Left lateral displacement along a vertical fault with a strike varying from N66°E to N98°E is consistent with the teleseismic data. (2) The seismic moment was 2.6 × 10^27 dyn cm. The directivity of the surface wave radiation indicates an asymmetric (1:2.3) bilateral faulting with a total length of 250 km. In modeling the displacement a rupture velocity of 3 km/s was used, and the fault curvature was included. (3) If a fault width of 15 km is assumed, the average offset is estimated to be about 2 m. This value is about twice as large as the average surface offset. (4) Although the observed directivity suggests a uniform overall displacement along the fault, the body wave analysis suggests that the earthquake consists of as many as 10 independent events, each having a seismic moment of 1.3–5.3 × 10^26 dyn cm and a fault length of about 10 km. The spatial separation of these events varies from 14 to 40 km. This multiple-shock sequence suggests that the rupture propagation is jagged and partially incoherent with an average velocity of 2 km/s. (5) The average stress drop estimated from surface waves is about 30 bars, but the local stress drop for the individual events may be significantly higher than this. (6) The complex multiple event is a manifestation of a heterogeneous distribution of the mechanical properties along the fault, which may be caused by either asperities, differences in strength, differences in pore pressure, differences in slip characteristics (stable sliding versus stick slip), or combinations of these factors. (7) This complexity has important bearing on the state of stress along transform faults and is important in assessing the effect of large earthquakes along other transform faults like the San Andreas.

On isostatic geoid anomalies

Abstract: In regions of slowly varying lateral density changes, the gravity and geoid anomalies may be expressed as power series expansions in topography. Geoid anomalies in isostatically compensated regions can be directly related to the local dipole moment of the density-depth distribution. This relationship is used to obtain theoretical geoid anomalies for different models of isostatic compensation. The classical Pratt and Airy models give geoid height-elevation relationships differing in functional form but predicting geoid anomalies of comparable magnitude. The thermal cooling model explaining ocean floor subsidence away from mid-ocean ridges predicts a linear age-geoid height relationship of 0.16 m/m.y. Geos 3 altimetry profiles were examined to test these theoretical relationships. A profile over the mid-Atlantic ridge is closely matched by the geoid curve derived from the thermal cooling model. The observed geoid anomaly over the Atlantic margin of North America can be explained by Airy compensation. The relation between geoid anomaly and bathymetry across the Bermuda Swell is consistent with Pratt compensation with a 100-km depth of compensation.

Solar wind stream interfaces

Abstract: Results are presented for a superposed epoch analysis of discontinuous solar wind interfaces. The average time-space profiles of stream interfaces are discussed with reference to fluid properties (flow speed, pressure ridge, density, electron and proton temperatures) and kinetic properties (electron core and halo, flow speed fluctuations, electron heat flux, alpha particles). Other aspects of stream interfaces are described, such as the persistence of individual interfaces, shock associations, the sector boundaries of the interplanetary magnetic field, and sudden impulses in the geomagnetic field. Interface position is considered in terms of the observed temperature jump. A conceptual model of high-speed stream evolution is proposed.

Observations of the interplanetary sector structure up to heliographic latitudes of 16°: Pioneer 11

Abstract: In the past, observations of the sector structure of the interplanetary magnetic field have been limited to a heliographic latitude of +/- 7.25 deg because spacecraft trajectories have consistently been in or near the ecliptic. The postencounter trajectory of Pioneer 11, which is now en route from Jupiter to Saturn, takes this spacecraft to much higher latitudes than have been previously explored. A study of the sector structure has in this connection been conducted during the ascent of Pioneer 11 to a maximum heliographic latitude of 16 deg. The Pioneer 11 observations demonstrate the presence in interplanetary space of an equatorial current sheet tilted at an angle of about 15 deg to the solar equatorial plane. Thus there is actually only one sector boundary in interplanetary space, not several, as would have been the case if the orientation had been meridional.

On the statistical distribution of wave heights in a storm

Abstract: There has been recent controversy over how well the Rayleigh distribution matches the observed distribution of wave heights. Most of this controversy stems from comparisons based on different definitions of the significant wave height. Once consistent definitions are used, all available data support the conclusion that the Rayleigh distribution overpredicts the heights of the higher waves in a record. Analysis of 116 hours of hurricane-generated waves in the Gulf of Mexico permitted the empirical fitting of the data to a Weibull distribution. Statistics developed from the empirical distribution include the prediction that the highest wave in 1000 is only 0.907 times the height predicted by the Rayleigh distribution.

Theoretical modeling of the generation, movement, and emplacement of pyroclastic flows by column collapse

Abstract: Models are presented for the physical processes that occur in the formation of pyroclastic flows generated by the gravitational collapse of a vertical eruption column. The main controlling parameters are considered to be the vent radius (R), gas content (N), and initial velocity (W) of the gas. For the ranges R = 50 to 600 m, N = 0.5 to 5% gas, and W = 200 to 600 m/s, column collapse occurs at heights between 0.6 and 9.0 km above the vent, and the initialvelocities of the flows range from 60 to 310 m/s. The eruption column collapse is modeled as an inverted turbulent jet, modified by including expressions for the efficiency of heat transfer between air and small pyroclasts. Entrainment of air during the collapse can result in initial cooling of the flow by up to 350°C. The variation in the amount of cooling is considered toaccount for the considerable ranges of emplacement temperatures and the widely differing degrees of welding observed in different ignimbrites. High emplacement temperatures are favored by eruption columns with low gas contents and low gas velocities, whereas low emplacement temperatures are favored by eruption columns with high gas velocities and high gas contents. The initial stages of flow are modeled as a highly turbulent, low-particle-concentration density current. Numerical solutions of the turbulent stages of flow are presented assuming uniform radial spreading from a central source (the vent). Flows from large eruptions may still have velocities of up to 100 m/s at distances of tens of kilometers from the source. The methods have also been applied to uphill flow and demonstrate that flows produced at high volume rates of eruption can surmount topographic barriers of several hundred meters at distances of several tens ofkilometers from the vent and explain the spectacular mobility of some large pyroclastic flows.Turbulent suspension in a gas flow with a low concentration of particles is not a viable mechanism of particle transport, as many of the clasts (about 1 mm) found in the deposits have terminal velocities well above the shearing stress velocities of even a rapidly moving gas flow. (v* = 1 to 12 m/s). The flows are deduced to segregate into a high-concentration basal zone within a few kilometers of the vent, as larger clasts settle to the base of the flows. Fines are thought to be generated by crushing within the high-concentration basal zone and are fluidized by exsolving gases to produce a pyroclastic flow with high concentrations of fluidized particles. The upper dilute part of the flow and the fine ash elutriated by fluidization contribute to the formation of widely dispersed ash fall deposits which are as voluminous as theassociated ignimbrite. The flows are capable of transporting clasts of several centimeters or tens of centimeters to tens of kilometers distance. The motion of the dense lower part (the pyroclastic flow) disassociates itself from the upper turbulent cloud of fine ash and gas, whicheventually mixes with the atmosphere sufficiently to form a convective plume.

Some evidence for boundary mixing in the deep Ocean

Abstract: Profiles of salinity and potential temperature in the deep ocean are presented which suggest the characteristic signature of two complementary mixing processes: vertical mixing within approx.50-m-thick layers at boundaries and topographic features and lateral advection and eventual smearing of these mixed layers along iopyenal surfaces. The combined effect of these two processes is often parametrically disguised as a vertical eddy diffusivity in one-dimensional models. An estimate shows that the two processes can account for all the vertical mixing in the deep ocean without any vertical diffusion in the interior.

Nonconvective and convective electron cyclotron harmonic instabilities

Abstract: We present a systematic parameter study of the spatial growth rates of electrostatic waves in an electron plasma consisting of a cold component and a hot component with a weak ‘loss cone’ perpendicular velocity distribution. The cold electron density controls which harmonic band can be excited. When the upper hybrid frequency based upon the cold electron density alone is between 1 and 2 times the electron cyclotron frequency, only the first harmonic band is unstable; when the upper hybrid frequency is between 2 and 3 times the cyclotron frequency, the first and second harmonic bands are unstable; and so on. Sufficiently large cold density eventually stabilizes the low harmonic bands. The cold electron temperature Tc controls the spatial amplification; when 0 < Tc/TH < a few times 10−2, where TH is a characteristic energy of the hot electrons, the instability is nonconvective. The first and second harmonic bands can be simultaneously nonconvective. The nonconvective property has been found for a wide range of hot and cold electron densities. It persists even when the hot electron loss cone is almost completely filled. Sufficiently large Tc/TH changes the instability from being nonconvective to being convective. Since cold electrons which come from the ionosphere would be confined to the loss cone if they were not turbulently scattered, the ionospheric source velocity distributions induce a nonconvective instability. However, cold electrons can be heated by electrostatic wave turbulence. Until Tc/TH = 5 × 10−2, nonresonant, quasilinear diffusion heats the cold electrons more rapidly than resonant diffusion heats or scatters the hot electrons into the loss cone. We propose that cold electron heating removes the nonconvective property, so that the spatial amplification of the instabilities can be reduced to a magnitude consistent with their hot and cold electron sources in steady state.

Rodriguez, Darwin, Amsterdam, ..., A second type of Hotspot Island

Abstract: Darwin and Wolf Islands are about 200 km north of the main group of Galapagos Islands and are aligned on a 315° trend Rodriguez Island is on a long 090° trending ridge about 900 km east of Reunion Island These trends are very different from the fracture zone or hotspot trends in these areas The Reykjanes ridge has been interpreted as the result of channeled asthenosphere flow from the Iceland hotspot (which produces new asthenosphere) down the rise axis (where asthenosphere is converted into lithosphere) We propose similar channeling for hotspots not exactly on a rise crest—a ‘pipeline’ from a hotspot to a nearby rise crest, with this second type of island forming at the intersection of the rise and pipeline Predicted trends for these features may be found by vector addition of the absolute motion of a plate over a hotspot and the relative motion of a rise crest away from a plate

Relations between transverse electric fields and field-aligned currents

Abstract: A model for the field-aligned propagation of transverse electric fields and associated field-aligned sheet currents is presented which takes into account the wave nature of the process. The model is applied to the separate cases of ionospheric and magnetospheric sources, and the resulting ionospheric electric field to field-aligned sheet current ratios are determined for comparison with experimental observations. It is found that the magnetospheric wave 'conductivity' for shear mode Alfven waves is small with respect to typical values of the height-integrated ionospheric Pedersen conductivity. For plasma convecting across a stationary disturbance a dynamic equilibrium is achieved in which field-aligned currents flow continuously away from the source on convecting field lines. Consistency with typical ionospheric electric fields requires that the field-aligned sheet currents are limited to around 0.1 A/m for ionospheric polarization sources, while magnetospheric sources are easily capable of 1 A/m or more.

Necking of the lithosphere and the mechanics of slowly accreting plate boundaries

Abstract: Data on ridges with slow spreading rates (1–3 cm/yr), obtained through detailed studies with Deep-Tow instruments and manned submersibles (French-American Mid-Ocean Undersea Study), or where the active structures are not submarine (Afar triangle) yield a precise picture of the axial region. There is evidence for a thinned lithosphere to be present at the axis with a thickness of 4–5 km. The thermal structure and composition of this solid layer can be estimated from seismic data and thermal and petrologic models. Extensional tectonics prevails in a belt some 10 km wide and is expressed at the surface by normal faults and fissures. In most previous mechanical models the existence of the lithosphere at the axis is neglected, and the rise of viscous asthenospheric material in a narrow vertical cleft beneath the axis is considered the main cause for the steady state presence of an axial valley and the development of normal faults. In contrast with these models we suggest here that these features depend on the rheological behavior of the lithosphere at the axis: the lithosphere is continually thinned by tectonic strain but also thickened by cooling at the base and by volcanism at the surface. In the steady state this process can be viewed as a succession of tectonic ‘neckings’ in the central active part of the axial valley (10 km) followed by doming of the lithosphere over the whole width of the axial valley (30 km), in response to isostatic disequilibrium. Creep is likely to control the process at depth and because of the high temperature and large strains will be of the steady state type. The application of experimental flow laws for material constituting layer 3 and the uppermost mantle to this problem, where both temperature profile and strain rate can be estimated, allows an order of magnitude of the ‘strength’ of the lithosphere at a given strain rate to be calculated. With this strength, isostatic recovery in response to vertical shear stresses will occur at a distance of about 8–15 km from the axis. The resulting ‘simple shear’ strain progressively ‘levels off’ the mean topographic slope until it becomes horizontal in the rift mountains. Whereas for rifted ridges or slowly accreting plate boundaries the behavior of the lithosphere controls the mechanics of the axial region, where only small discontinuous transient magma chambers exist, we suggest that in the case of nonrifted ridges the behavior of the asthenosphere is more important, with axial crust in isostatic equilibrium over a large continuous permanent magma chamber.