Distribution of Mantle Up-welling Determined from Plate Motions: A Case for Large-scale Bénard Convection

—?The number and geometric distribution of putative mantle up-welling centers and the associated convection cell boundaries are determined from the lithospheric plate motions as given by the 14 Euler poles of the observational NUVEL model. For an assumed distribution of up-welling centers (called “cell-cores”) the corresponding cell boundaries are constructed by a Voronoi division of the spherical surface; the resulting polygons are called “Bénard cells.” By assuming the flow-kinematics within a cell, the viscous coupling between the flow and the plates is estimated, and the Euler poles for the plates are computed under the assumption of zero-net-torque. The positions of the cell-cores are optimized for the HS2-NUVEL1 Euler poles by a method of successive approximation (“subplex”); convergence to one of many local minima occurred typically after ～20,000 iterations. Cell-cores associated with the fourteen HS2-NUVEL1 Euler poles converge to a relatively small number of locations (8 to 10, depending on interpretation), irrespective of the number of convection cells submitted for optimized distribution (from 6 to 50). These locations are correlated with low seismic propagation velocities in tomography, uniformly occur within hotspot provinces, and may specifically be associated with the Hawaiian, Iceland, Reunion/Kerguelen (Indian Ocean), Easter Island, Melanesia/Society Islands (South Pacific), Azores/Cape Verde/Canary Islands, Tristan da Cunha (South Atlantic), Balleny Islands, and possibly Yellowstone hotspots. It is shown that arbitrary Euler poles cannot occur in association with mantle Bénard convection, irrespective of the number and the distribution of convection cells. Nevertheless, eight of the observational Euler poles – including the five that are accurately determined in HS2-NUVEL1 (Australia, Cocos, Juan de Fuca, Pacific, and Philippine) – are “Bénard-valid” (i.e., can be explained by our Bénard model). Five of the remaining six observational poles must be relocated within their error-ellipses to become Bénard-valid; the Eurasia pole alone appears to be in error by ～115°, and may actually lie near 40°N, 154°E. The collective results strongly suggest Bénard-like mantle convection cells, and that basal shear tractions are the primary factor in determining the directions of the plate motions as given by the Euler poles. The magnitudes of the computed Euler vectors show, however, that basal shear cannot be the exclusive driving force of plate tectonics, and suggest force contributions (of comparable magnitude for perhaps half of the plates) from the lithosphere itself, specifically subducting slab-pull and (continental) collision drag, which are provisionally evaluated. The relationship of the putative mantle Bénard polygons to dynamic chaos and turbulent flow is discussed.     

Screening model for volatile pollutants in dual porosity soils

This paper develops mass fraction models for transport and fate of agricultural pollutants in structured two-region soils. Mass fraction index models, based on a semi-infinite domain solution, are derived that describe leaching at depth, vapor losses through soil surface, absorption, and degradation in the dynamic- and stagnant-water soil regions. The models predict that leaching is the result of the combined effect of the upward vapor-phase transport relative to downward advection, residence time relative to half-life, dispersion, and lateral diffusive mass transfer. Simulations show that leached fraction of volatile compounds does not always decrease monotonically with increased residence time relative to the pollutant half-life, as a result of complex interactions among the different physical and biochemical processes. The results show that leaching, volatilization, and degradation losses can be affected significantly by lateral diffusive mass transfer into immobile-water regions and advection relative to dispersion (i.e. Peclet number) in the mobile-water regions. It is shown that solute diffusion into the immobile phase and subsequent biochemical decay reduces leaching and vapor losses through soil surface. Potential use of the modified leaching index for the screening of selected pesticides is illustrated for different soil textures and infiltration rates. The analysis may be useful to the management of pesticides and the design of landfills.     

Probing mid-mantle heterogeneity using PKP coda waves

We investigate the utility of PKP coda waves for studying weak scattering from small-scale heterogeneity in the mid-mantle. Coda waves are potentially a useful probe of heterogeneity in the mid-mantle because they are not preferentially scattered near the CMB, as PKP precursors are, but are sensitive to scattering at all depths. PKP coda waves have not been used for this purpose historically because of interference with other late-arriving energy due to near-surface resonance and scattering. Any study of deep mantle scattering using coda waves requires the removal of near-surface effects from the data. We have analyzed 3624 recordings of PKP precursors and coda made by stations in the Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network (GSN). To study the range and time dependence of the scattered waves, we binned and stacked envelopes of the recordings. We have considered precursors that arrive within a 20 s window before PKP and coda waves in a 60 s window after PKP. The PKP scattered waves increase in amplitude rapidly with range as predicted by scattering theory. At ranges below 125°, we predict and observe essentially no scattered energy preceding PKP. Coda amplitudes at these ranges are independent of range and provide an estimate of energy due to near-surface effects that we can expect at all ranges. We use the average coda amplitude at ranges from 120 to 125° to correct coda amplitudes at other ranges. PKP coda waves show a strong dependence on time and range and are clearly influenced by scattering in the lower mantle. PKP coda waves, however, do not provide a tighter constraint on the vertical distribution of mantle heterogeneity than is provided by precursors. This is due, in part, to relatively large scatter in coda amplitudes as revealed by a resampling analysis. Modeling using Rayleigh–Born scattering theory and an exponential autocorrelation function shows that PKP coda amplitudes are not highly sensitive to the vertical distribution of heterogeneity in the mantle. To illustrate this we consider single-scattering in two extreme models of mantle heterogeneity. One allows heterogeneity just at the CMB; the other includes heterogeneity throughout the mantle. The amplitudes of precursors are tightly constrained by our stack and support our earlier conclusion that small-scale heterogeneity is uniformly distributed throughout the lower mantle. The best-fit model includes 8 km scale length heterogeneity with an rms velocity contrast throughout the mantle of 1%.     

Identification of Mining Blasts at Mid- to Far-regional Distances Using Low Frequency Seismic Signals

—?This paper reports results from two recent monitoring experiments in Wyoming. Broadband seismic recordings of kiloton class delay-fired cast blasts and instantaneous calibration shots in the Black Thunder coal mine were made at four azimuths at ranges from 1° to 2°. The primary focus of this experiment was to observe and to explain low-frequency signals that can be seen at all azimuths and should routinely propagate above noise to mid-regional distances where most events will be recorded by International Monitoring System (IMS) stations.¶The recordings clearly demonstrate that large millisecond delay-fired cast blasts routinely produce seismic signals that have significant spectral modulations below 10?Hz. These modulations are independent of time, the azimuth from the source and the orientation of the sensor. Low-frequency modulations below 5?Hz are seen beyond 9°. The modulations are not due to resonance as they are not produced by the calibration shots. Linear elastic modeling of the blasts that is guided by mine-blast reports fails to reproduce the fine detail of these modulations but clearly indicates that the enhanced “spectral roughness” is due to long interrow delays and source finiteness. The mismatch between the data and the synthetics is likely due to source processes, such as nonlinear interactions between shots, that are poorly understood and to other effects, such as variations of shot time and yield from planned values, that are known to be omnipresent but cannot be described accurately. A variant of the Automated Time-Frequency Discriminant (Hedlin, 1998b), which uses low-frequency spectral modulations, effectively separates these events from the calibration shots.¶The experiment also provided evidence that kiloton class cast blasts consistently yield energetic 2–10 second surface waves. The surface waves are strongly dependent on azimuth but are seen beyond 9°. Physical modeling of these events indicates that the surface waves are due mainly to the extended source duration and to a lesser extent to the slap-down of spalled material. The directionality is largely a path effect. A discriminant that is based on the partitioning of energy between surface and body waves routinely separates these events from the calibration shots.¶The Powder River Basin has essentially no natural seismic activity. How these mining events compare to earthquake observations remains to be determined.     

Radionuclides in the Natural–Anthropogenic System Comprising a Nuclear Power Plant and a Cooling Reservoir

The sources of technological radionuclides coming into the ecosystem of a nuclear power plant (NPP) cooling reservoir are considered and estimated. The information about the volumetric and mass activity in the biotic and abiotic components of the reservoir ecosystem is analyzed. The activity distribution between the components was determined. The activity reserve in the ecosystems of the cooling reservoirs in central European Russia has been estimated. The real radioactive input from NPPs to the reservoir is shown to be insignificant. It is concluded that cooling reservoirs can be used for any needs of the national economy and for the recreation (with respect to the radiation factor).     

Regime of Low-Frequency Wave Motion in the Coastal Zone of the Northwestern Caspian Sea

Regime characteristics of infragravitational and wind-induced waves for the northwestern Caspian Sea are given. The role of infragravitational waves in the dynamics of the northern Caspian coastal zone is determined based on empiric regional relationships, used to improve the conventional methods, and wind wave calculations.     

Analysis of Space and Time Variations in the Suspended Matter Distribution in Kaliningrad Bay of the Baltic Sea

The distribution of suspension, its composition, and particle size characteristics are studied in Kaliningrad Bay. Zoning of the bay according to the water turbidity is made. The main factors controlling the formation and space and time variations in the suspended matter are determined.     

Mantle Convection Modeling with Viscoelastic/Brittle Lithosphere: Numerical Methodology and Plate Tectonic Modeling

--The earth's tectonic plates are strong, viscoelastic shells which make up the outermost part of a thermally convecting, predominantly viscous layer. Brittle failure of the lithosphere occurs when stresses are high. In order to build a realistic simulation of the planet's evolution, the complete viscoelastic/brittle convection system needs to be considered. A particle-in-cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain-dependent yield stress. This is applied to a plate-deformation problem. Numerical accuracy is demonstrated relative to analytic benchmarks, and the characteristics of the method are discussed.