scratching technique for the study and analysis of soil surface abrasion mechanism aeolian abrasion is the most fundamental and active surface process that takes place in arid and semi arid environments its nature is a wear process for wind blown grains impinging on a soil or sediment surface which causes particles and aggregates to fracture from the soil surface through a series of plastic and brittle cracking deformation such as cutting ploughing and brittle fracturing using a universal micro tribometer umt a scratching test was carried out on six soil surfaces sandy soil sand loam silt loam loam silt clay loam and silt clay the results indicate that traces of normal and tangential force vs time show a jagged curve which can reflect the plastic deformation and brittle fracturing of aggregates and particles of various sizes fractured from the soil surfaces the jagged curve peaks and the area enclosed underneath may represent the bonding forces and bonding energies of some aggregates and grains on the soil surface respectively connecting the scratching test with an impact abrasion experiment furthermore demonstrates that soil surface abrasion rates are proportional to the square of speeds of impacting particles and to the 2 6 power of mean soil grain size and inversely proportional to the 1 5 power of specific surface abrasive energy or to the 1 7 power of specific surface hardness

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

changes in seismic anisotropy shed light on the nature of the gutenberg discontinuity the boundary between the lithosphere and asthenosphere is associated with a platewide high seismic velocity lid overlying lowered velocities consistent with thermal models seismic body waves also intermittently detect a sharp velocity reduction at similar depths the gutenberg g discontinuity which cannot be explained by temperature alone we compared an anisotropic tomography model with detections of the g to evaluate their context and relation to the lithosphere asthenosphere boundary lab we find that the g is primarily associated with vertical changes in azimuthal anisotropy and lies above a thermally controlled lab implying that the two are not equivalent interfaces the origin of the g is a result of frozen in lithospheric structures regional compositional variations of the mantle or dynamically perturbed lab

seismic evidence for a chemical heterogeneity in the midmantle a strong and slightly dipping seismic reflector beneath the mariana subduction zone 1 a clear later phase 80 s after the direct p wave is observed in most of individual seismograms recorded by a short period seismometer network in japan j array from a cluster of deep earthquakes that occurred at the northern mariana subduction zone this phase 1 shows a p wave particle motion 2 arrives later from earthquakes with shallower focal depths 3 has a steeper incident angle than that of p wave and 4 shows a deviation of a few degrees in the arrival azimuth from that of p wave we interpret it as an s to p converted wave which takes off downward from the source and is reflected at a velocity discontinuity reflector below the earthquakes travel time inversion shows that the seismic reflector dips southwest by 20 at 24 25 n 144 75 e and at a depth of 1115 km with a lateral extension at least 100 100 km the location corresponds to the lower edge of a high velocity anomaly in global tomographic models amplitude and waveform analyses suggest a decrease of s wave velocity by 2 6 and an increase of density by 2 9 within the reflector there is almost no difference in p wave velocity 1 between the reflector and the surrounding mantle the estimated thickness of the reflector is 12 km these observations indicate that the observed seismic structure is more likely to be a chemical reservoir rather than a purely thermal anomaly the seismic reflector might be a piece of subducted oceanic crust as suggested by a previous study it also could be related to the break down of the d phase of dense hydrous magnesium silicates at midmantle pressure condition reported by recent mineral physics studies both scenarios imply that mechanical chemical segregation might occur within the subducted slab at midmantle condition

an experimental study of wind abrasion rocks were abraded by a sand laden air jet in the laboratory to examine the quantitative relationships between wind abrasion and wind velocity quantity and grain size of windblown sand particles and physical properties of the target rocks the results of 101 runs are expressed by an experimental equation with dimensionless constant which indicates that the rate of abrasion is directly related to the kinetic energy of the sand projectiles and inversely proportional to the compressive strength of the target rocks a tentative extension of the experimental equation to field condition leads to an equation for the wind abrasion in fields an application of the latter equation to the natural settings is preliminarily examined by comparing it with sharp s 1964 field experiments and a plausible result is obtained

the emission and vertical flux of particulate matter 10 mum from a disturbed clay crusted surface arid and semi arid environments are important sources for the atmospheric loading of pm10 particulate matter 10 lm although the emission of this material is often limited by surface crusts this study investigates the emission and vertical flux of pm10 from a clay crusted playa with and without saltating grains to abrade the surface using a portable field wind tunnel it was found that despite disturbance to the surface the emission of pm10 decays rapidly without abrasion only in the presence of saltating grains was pm10 continuously liberated from the surface such that the emission rate the total amount of pm10 emitted from the surface expressed as a horizontal flux varied linearly with the saltation transport rate although the emission of pm10 was found to depend on saltation abrasion past studies have tended to focus on the relationship between the vertical flux of pm10 the amount of pm10 being transported vertically through the boundary layer and the shear velocity in this study the vertical flux of pm10 was found to vary with the shear velocity to the power of 2ae14 although the vertical pm10 flux is a proportion of the emission rate the horizontal flux no statistically significant relationship was observed between the emission rate and the shear velocity the disparity of these results is explained by the lack of a consistent relationship between the shear velocity and the saltation transport rate in this supply limited environment this suggests that the observed relationship between the vertical pm10 flux and the shear velocity is a spurious correlation resulting from the use of shear velocity to calculate the vertical dust flux it is thus concluded that shear velocity is not an appropriate variable for emission modelling in supply limited environments and that improvements in dust emission modelling will only be realized if the abrasion process is the focus of a concerted research effort

multiplicity of the 660 km discontinuity beneath the izu bonin area abstract the relatively simple subducting slab geometry in the izu bonin region provides a valuable opportunity to study the multiplicity of the 660 km discontinuity and the related response of the subducting slab on the discontinuity vertical short period recordings of deep events with simple direct p phases beneath the izu bonin region were retrieved from two seismic networks in the western usa and were used to study the structure of the 660 km discontinuity after careful selection and pre processing 23 events from the networks forming 32 pairs of event network records were processed related vespagrams were produced using the n th root slant stack method for detecting weak down going sdp phases that were inverted to the related conversion points from depth histograms and the spatial distribution of the conversion points there were three clear interfaces at depths of 670 710 and 730 km these interfaces were depressed approximately 20 30 km in the northern region in the southern region only two layers were identified in the depth histograms and no obvious layered structure could be observed from the distribution of the conversion points

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

pervasive seismic wave reflectivity and metasomatism of the tonga mantle wedge subduction zones play critical roles in the recycling of oceanic lithosphere and the generation of continental crust seismic imaging can reveal structures associated with key dynamic processes occurring in the upper mantle wedge above the sinking oceanic slab three dimensional images of reflecting interfaces throughout the upper mantle wedge above the subducting tonga slab were obtained by migration of teleseismic recordings of underside p and s wave reflections laterally continuous weak reflectors with tens of kilometers of topography were detected at depths near 90 125 200 250 300 330 390 410 and 450 kilometers p and s wave impedances decreased at the 330 kilometer and 450 kilometer reflectors and s wave impedance decreased near 200 kilometers in the vicinity of the slab and near 390 kilometers just above the global 410 kilometer increase the pervasive seismic reflectivity results from phase transitions and compositional zonation associated with extensive metasomatism involving slab derived fluids rising through the wedge

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

multiple discontinuities near 660 km beneath tonga area 1 we study the discontinuities beneath tonga subduction zone using the deep earthquakes recorded by the pacific northwest seismic network pnsn the multiple discontinuities around 660 km beneath tonga subduction zone are found using converted phase sdp the amplitude ratios of the converted phase with the direct p wave ac ap are also determined the largest number of robust converted phases with large ac ap comes from the depth range of 660 690 km its peak is at 680 km the second one appears in the depth range of 700 745 km its peak is at 740 km the two discontinuities are obviously depressed by the subduction slab according to experimental and numerical studies a possible explanation of the observation is that the first discontinuity is formed by the transition of spinel to perovskite and magnesiowustite and the second is formed by the transition of ilmenite to perovskite

an analysis of soil crust strength in relation to potential abrasion by saltating particles wind erosion depends on the ease with which particles can be detached from the soil surface but suitable tests to characterize this property are not available two possible methods to determine surface soil strength in the field were therefore compared on a range of artificially crusted surfaces these were made by spraying or tension wetting aggregates 10 2 2 0 5 and 0 5 mm from a structurally unstable sandy loam followed by drying each test involved measuring the force exerted on a probe driven at a steady rate into the surface using either a flat tipped 0 6 mm diameter penetrometer or a flat ended cylindrical punch with inner and outer diameters of 5 and 6 mm respectively both probes showed that crusts could be produced reproducibly depending on the probe and aggregate size penetration mainly occurred either as a result of aggregates being deflected out of the pathway of the probe or by genuine rupture of aggregates or of the crusted surface the penetrometer because it was comparable to the size of sand grains gave results that can be used to characterize surface erodibility to saltating particles the punch gave results that would be unsuitable for this purpose as would other strength tests that are on too large a scale penetrometer results were analyzed to calculate the energy required for penetration it was thus possible to demonstrate that only the spray wetted fine aggregates had a surface that could undergo large scale rupture by saltating sand grains for all other surfaces saltating particles would be unable to supply sufficient energy to rupture aggregates or the crusted surface erosion could only occur by a slower process of abrasion in which smaller particles or aggregates are chipped away from the surface however it is shown that saltating particles could rupture the interaggregate bonding in the 2 0 5 mm aggregate surfaces thus permitting creep an alternative and potentially simpler way of characterizing surface erodibility by using a surface modulus of elasticity is also discussed our results demonstrate that the small diameter penetrometer is a promising technique for characterizing erodibility of aggregated and crusted surfaces 1997 by john wiley sons ltd

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab slab derived fluids play an important role in heat and material transfer in subduction zones dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow nevertheless direct observations of the fluid chemistry and pressure temperature conditions of fluids are few this report describes co2 bearing saline fluid inclusions in spinel harzburgite xenoliths collected from the 1991 pinatubo pumice deposits the fluid inclusions are filled with saline solutions with 5 1 1 0 wt nacl equivalent magnesite crystals co2 bearing vapor bubbles and a talc and or chrysotile layer on the walls the xenoliths contain tremolite amphibole which is stable in temperatures lower than 830 c at the uppermost mantle the pinatubo volcano is located at the volcanic front of the luzon arc associated with subduction of warm oceanic plate the present observation suggests hydration of forearc mantle and the uppermost mantle by slab derived co2 bearing saline fluids dehydration and decarbonation take place and seawater like saline fluids migrate from the subducting slab to the mantle wedge the presence of saline fluids is important because they can dissolve more metals than pure h2o and affect the chemical evolution of the mantle wedge

imaging the seismic lithosphere asthenosphere boundary of the oceanic plate 1 the seismic lithosphere asthenosphere boundary lab or g discontinuity a seismologically characterized abrupt drop in wave speed in the uppermost mantle is one of the key issues in current geodynamics although plate tectonics started as a theory for the ocean reports on lab for normal oceanic regions are scarce due to paucity of seismic data and whether or not the oceanic lab grows with age is the key issue to be resolved we conduct a systematic survey for the oceanic lab using s to p converted seismic waves along three margins of oceanic plates whose crustal age ranges from 10 myr to 130 myr and we observe laterally continuous oceanic lab images the thickness of the oceanic plate estimated from labs increases with the plate age though scattered suggesting that the evolution of oceanic lithosphere is predominantly governed by temperature and that the oceanic seismic lab represents a boundary that grows with age

wind erosion of crusted soil sediments saltating particles increase the rate of dust release from sediments in arid and semi arid areas they also break interparticle bonds in aggregated and crusted soils thereby increasing the number of particles available for entrainment this pilot study examines rates of erosion in relation to the flux of saltating grains for three crusted sediments of different strengths dislodgement of surface particles decreases with increasing crust strength as measured by a cylindrical flat ended penetrometer in addition initial dust release from craters formed by single impactors in unaggregated soil is examined in relation to the associated saltator the volume of material removed depends linearly on the kinetic energy of the abraders

changbaishan volcanism in northeast china linked to subduction induced mantle upwelling the changbaishan volcanic complex in china cannot be easily explained as the consequence of a mantle plume seismic images from the region identify buoyant mantle material that may have been entrained and dragged downwards by the subducting pacific plate but is now escaping upwards through a gap in the plate and producing the intraplate volcanism

trans pacific upper mantle shear velocity structure 1 we use 50 tonga fiji events recorded at the broadband trinet array southern california to develop a pure path upper mantle shear velocity model across the pacific at the epicentral distances of 70 95 multibounce s waves up to s5 are observed including their triplicated branches which become particularly clear after stacking since these s wave multiples turn at various depths simultaneously modeling their differential traveltimes and waveforms provides strong constraints on the radial velocity structure we parameterize the velocity model according to a priori information from the previous oceanic models so that we can take a grid search approach to fully investigate possible interdependencies among the model parameters we construct synthetics with a reflectivity code and study both the sh and sv components by modeling the whole recordings from events at different depths with different mechanisms we are able to separate shallow low velocity zone lvz features from deeper structure our preferred model pac06 contains a fast lid vsh 4 78 km s 1 vsv 4 58 km s 1 with a thickness of 60 km the underlying lvz is prominent with the lowest velocities vsh 4 34 km s 1 and vsv 4 22 km s 1 occurring at a depth of 160 km these velocities are below the estimates of solid state lvz suggesting the presence of partial melt the anisotropy vsv vsh of pac06 extends to 300 km depth which is constrained by s5 turning at this depth besides the 406 km and 651 km discontinuities pac06 also has a small 1 velocity jump at 516 km we consider these main features of pac06 to be well determined since pac06 explains a large data set from various events therefore it is ideally suited for comparing with mineralogical models

crust strength a wind tunnel study of the effect of impact by saltating particles on cohesive soil surfaces a wind tunnel study examined the effect of distributions of saltating particles on sediment surfaces which were characterized by distributions of their tensile strength the sediments consisted of varying proportions of large sand sized particles with a fine particle cement the energies of the impacting particles and the surface strengths were compared with the mass of material lost from the surface it is important to consider distributions of parameters rather than mean values only since abrasion and erosion may occur from surfaces not predicted from average strength and saltation velocities at the impact velocities used in this study mean velocity 4 4 m s 1 with standard deviation of 0 51 surfaces containing less than 12 per cent fine material were easily eroded but insignificant erosion occurred when the fine particle content exceeded 60 per cent small amounts of cementing material were easily ruptured allowing the large sand grains to be moved largely in creep by the bombarding particles a significant amount of energy was lost to the bed as the percentage of fine material increased the surface became more difficult to break up and less energy was lost to the bed the probability that erosion will occur for known energy distributions of impacting particles and surface strength can be calculated and the mass loss increases exponentially with a decrease in the percentage of fine cementing particles copyright 2001 john wiley sons ltd

seismological structure of the upper mantle a regional comparison of seismic layering abstract we investigate seismic layering i e discontinuities regions of anomalous velocity gradients and anisotropy and its lateral variability in the upper mantle by comparing seismic models from three tectonic regions old 100 ma pacific plate younger 40 ma philippine sea plate and precambrian western australia these models were constructed by combining two data sets scs reflectivity profiles which provide travel times and impedance contrasts across mantle discontinuities and observations of frequency dependent travel times of three component turning s ss ss sss sss sa and surface r1 g1 waves which constrain the anisotropic velocity structure between discontinuities the models provide a better fit to observed seismograms from these regions than the current generation of global tomographic models the australian model is characterized by high shear velocities throughout the upper 350 km of the mantle with no low velocity zone lvz in the isotropically averaged shear velocities in contrast the oceanic models are characterized by a thin high velocity seismic lid underlain by a distinct lvz with a sharp boundary the g discontinuity separating them the g is significantly deeper beneath the western philippine sea plate than beneath the older pacific 89 and 68 km respectively implying that thermal cooling alone does not control the thickness of the lid we interpret this discontinuity as a compositional boundary marking the fossilized base of the melt separation zone msz active during sea floor spreading no discontinuity is detected at the base of the lvz in the oceanic models the s velocity gradient between 200 and 410 km depth is much steeper in the oceans than beneath australia this high oceanic gradient is probably controlled by a decrease in the homologous temperature over this depth interval the relative depths of the transition zone tz discontinuities are consistent with clapeyron slopes expected for an olivine dominated mineralogy the 660 km discontinuity displays variability in its amplitude that appears to correlate with its depth shallow and bright beneath the pacific deep and dim beneath australia and the philippine sea such behavior is possibly caused by the juxtaposition of the olivine and garnet components of the phase transition radial anisotropy extends through the upper 250 km of the mantle in the australia model and through the upper 160 km of the two oceanic models the magnitude of anisotropy is consistent with that expected for models of horizontally oriented olivine and the localization of anisotropy in the shallowest upper mantle implies that it reflects strain associated with past or present tectonic events

seismic evidence of the lithosphere asthenosphere boundary beneath the tonga area southwestern pacific abstract we study the lithosphere asthenosphere boundary lab in the tonga subduction zone using four deep focal events recorded by the capital seismic network csn of china and the northeast china extended seismic array necessarray the existence and depths of the lab beneath the lau ridge are clearly revealed by the linear slant stacking of observed sp precursors sdp as illustrated by the reflected points of robust sdp phases from lab the depth of lab in the northern lau ridge is about 63 km with a range of 63 64 km in the northwest is about 77 km with a range of 76 78 km and in the south is about 72 km the uncertainty of determined depths is no more than 4 km given the effects of the picking errors of arrival times and velocity heterogeneities our results suggest that the oceanic lithosphere of the lau ridge is thinner in the northern part than in the northwestern and the southern parts according to the previous geodynamic and petrological studies the oceanic lithospheric thinning in the northern lau ridge should be related to the strong erosion of the active mantle convection in the back arc mantle wedge with the enrichment of the volatiles and melts

the gutenberg discontinuity melt at the lithosphere asthenosphere boundary the lithosphere asthenosphere boundary lab beneath ocean basins separates the upper thermal boundary layer of rigid conductively cooling plates from the underlying ductile convecting mantle the origin of a seismic discontinuity associated with this interface known as the gutenberg discontinuity g remains enigmatic high frequency ss precursors sampling below the pacific plate intermittently detect the g as a sharp negative velocity contrast at 40 to 75 kilometer depth these observations lie near the depth of the lab in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base i propose that the g reflectivity is regionally enhanced by dynamical processes that produce melt including hot mantle upwellings small scale convection and fluid release during subduction

the grain bed collision in sand transport by wind in the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain bed collision has been identified as having a significant role the grain bed collision has been studied by both physical experiments and numerical simulation a principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system thus the study of the exact physical mechanism of the collision has to some extent been neglected the transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension thus the precise physics of the grain bed collision become highly significant with regard to the physics of dust release from a surface this paper re examines previous collision data and compares them with a simple collision model this model proposed that the impinging grain strikes a single surface grain and rebounds an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding the collision data are shown to support the model first because the duration of the collisions observed on the high speed film supports a short contact time with the surface and second because the data fit well with the model

poisson s ratios of the upper and lower crust and the sub moho mantle beneath central honshu japan abstract poisson s ratios of the upper and lower crust and the sub moho mantle beneath central honshu japan are investigated using three independent methods that are based on s to p ratios of apparent velocities the wadati diagrams and an inversion of p and s arrivals shallow earthquakes at distances of 200 500 km from the nagoya university telemeter network are used for the apparent velocity ratio method crustal and subcrustal earth quakes under the network are used for the other two methods the network consists of wide band seismometers with three components which are particularly suitable for detecting s waves the three different methods give a consistent result for poisson s ratio that is 1 0 23 0 01 in the upper crust 2 0 26 0 28 in both the lower crust and in the sub moho mantle the result indicates a sharp contrast in between the upper and the lower crust rather than at the moho the low in the upper crust can only be explained by the presence of a substantial amount of free quartz indicating granitic rocks a higher in the lower crust suggests that this portion is presumably less saturated in silica and may be even undersaturated pointing to intermediate to mafic rocks the sub moho is almost equal to the averaged over the entire upper mantle that has been estimated from the wadati diagrams of deep shocks beneath japan but is higher than those calculated from pn and sn velocities in oceanic and stable continental regions

a conceptual model of wind erosion of soil surfaces by saltating particles a conceptual model is described for the prediction of wind erosion rates dependent on the distribution of impact energy delivered to the surface by saltating grains p ei and the distribution of local surface strength p es methods are presented for the measurement of both distributions and consequent loss of material from the bed it is concluded that saltating sand grains can rupture weak crusts under even moderate wind conditions and that the rate of erosion will depend on the shape of the distribution tails copyright 1999 john wiley sons ltd