detection of lower mantle scatterers northeast of the marianna subduction zone using short period array data short period university of washington northern california and southern california seismic array records of a sequence of deep mariana subduction zone earthquakes show unexpected clear arrivals nearly 90 and 105 s following p we measure slowness arrival time and arrival azimuth of these phases relative to p using array techniques to find where they emanate in the mantle by combining slowness and back azimuth deviations and the move out with depth of the arrivals from different earthquakes we find their most probable origin is through s to p conversion at point scatterers in the mid to lower mantle at 25 7 n 148 2 e 1590 km and 25 5 n 151 0 e 1850 km respectively separated by 300 km these features indicate elastically distinct material in the middle of the lower mantle which if fragments of subducted slabs are not related to contemporary subduction they may represent upwelling convective material from the deep mantle or d but they are not purely thermal in nature as chemically distinct bodies they may represent potential reservoirs of the isotopic and trace element components recognized geochemically

small scale heterogeneity at the top of the lower mantle around the mariana slab abstract short period waveform data recorded at the western us seismic array for 14 mariana deep earthquakes near 19 n show two anomalous wave packets within about 30 s after the arrival of the direct p wave simply called p or p wave one at about 13 s called x1 phase and the other around 29 s called x2 phase we perform array analyses to locate the sources of these phases in the first step we measure arrival time slowness back azimuth and amplitude of these phases relative to p the amplitudes of the x1 and x2 phases correlate with each other for the individual events and vary more than an order of magnitude among the events which cluster in a region spanning less than 50 km the amplitude correlation suggests that the two waves have a similar origin and the large amplitude variation eliminates receiver side reverberation as the origin of the phases the x1 phase has a slowness and an arrival azimuth which are not distinguished from those of p waves across the entire array and has nearly constant delay times regardless of the focal depths from 567 to 605 km the x2 phase arrives as a more emergent wave packet with slowness and azimuth different from those of p in the second step we compute composite semblance coefficients for the cases of p to p and s to p single scattering near the foci the x1 and x2 phases are best interpreted as s to p scattered waves generated in the uppermost lower mantle north of the focal region based on three observations 1 highest composite semblance values 2 scatterer locations mutually consistent between the two event groups near the depths of 500 and 600 km and 3 a reasonable amount of elastic property anomalies required the x1 phase scatterer is determined at 19 8 0 3 n 145 7 0 3 e 710 30 km on the other hand the scatterer of the x2 phase appears to split into two objects one at 20 4 0 4 n 145 4 1 0 e 900 80 km and the other at 20 6 0 4 n 147 4 0 5 e 860 40 km although the geometries of the scatterers of x1 and x2 phases are not constrained horizontal or subhorizontal discontinuities are unlikely the changes in elastic properties associated with these heterogeneous objects probably occur within several kilometers according to their high efficiency at converting short period waves they are thus likely to represent sharp chemical variations in major element composition these objects are located within a thickened high velocity anomaly at the top of the lower mantle which has been determined by previous seismic studies a plausible tectonic interpretation of these objects is that they are fragments of former oceanic crust which are entrained in the pacific slab impinging on the more viscous lower mantle

small scale heterogeneity in the mid lower mantle beneath the circum pacific area abstract data from a western united states short period seismic network are investigated in order to detect anomalous later p wave arrivals within a time window from 50 s to 170 s after direct p waves for deep and intermediate depth earthquakes at circum pacific subduction zones we interpret the anomalous arrivals as s to p scattered waves from heterogeneities in the mid lower mantle the searched regions depths from 4800 km to 2200 km cover approximately 2 of the entire lower mantle the heterogeneous scattering objects are located by array processing of the seismic waves using semblance with careful investigations of arrival time slowness and azimuth of the later phases relative to direct p waves we usually observe two or three later phases which are separated in arrival time by 5 20 s and have similar arrival azimuth and slowness this observation suggests that a few scatterers often form a cluster with a typical spacing of 100 200 km vertically more than ten clusters or groups of s to p scattering objects are detected in the mid lower mantle beneath the circum pacific regions northern kuril izu bonin mariana northern fiji and west coast of south america the s to p scattering objects are found in a depth range from 1100 to 1800 km and the most prominent objects of each region tend to be located around the middle of the depth range the amplitudes of the scattered waves range from 1 to 10 of the direct p waves with exceptionally large values as large as 40 the associated magnitudes of the changes in elasticity is not tightly constrained but probably are nearly 1 and may occasionally exceed 4 of shear velocity considering their high efficiency at converting short period waves wavelengths of 5 10 km the changes in elastic properties at the heterogeneous objects appear to occur within several kilometers seismic tomography studies have not detected prominent seismic wave speed anomalies except for those beneath south america which appear to be located in the former farallon plate and those at north of fiji which appear to be located around the pacific superplume the mid lower mantle heterogeneity should represent sharp chemical variations in major element composition of the lower mantle rocks the most plausible tectonic explanation of the variations is remnant of subducted and folded former oceanic crust which preceded the current subduction of the pacific plate the extreme estimates of low shear velocity 4 may indicate the prominent drops in the shear modulus due to ferro elastic phase transition in stishovite sio 2 our observations imply that heterogeneity of kilometer scale persists for billions of years despite of stirring due to convection and therefore is ubiquitous in the lower mantle it is qualitatively consistent with some recent convection models

mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

a study of channel changes in a reach of the north nashwaaksis stream new brunswick canada repeated surveys of a short channelized reach of the north nashwaaksis stream near fredericton n b over the period 1971 1981 provide the basis for evaluating the type and magnitude of some physical and botanical changes in the study reach in 1967 a 200 m reach of the stream was shortened by about 10 per cent by cutting a new channel through a pasture the original channel was protected by a dense growth of shrubs along its banks the average width of the top of the channel in the channelized reach widened from 11 m to 17 m in response to a major flood in 1973 rough estimates indicate that as much as 78 per cent of the published suspended sediment load associated with the flood could reasonably have been derived from the banks in this short channelized reach during the 1973 flood a mid channel bar was formed in the portion of the channel which experienced the most widening subsequent to the flood the mid channel bar was first colonized by sedges and then by alders by 1981 the width of the low flow channel in the channelized reach was about the same as that for the unmodified upstream reaches this case study provides useful information for those planning channelization projects on small to intermediate size streams in humid temperate areas

unsuccessful initial search for a midmantle chemical boundary with seismic arrays compositional layering of the midmantle has been proposed to account for seismic and geochemical patterns van der hilst and karason 1999 and inferred radiogenic heat source concentrations kellogg et al 1999 compositional layering would require thermal boundary layers both above and below an interface we construct a minimal 1 d model of a mid mantle boundary consistent with the observed nearly adiabatic compressional velocity structure dziewonski and anderson 1981 and the proposed high heat flow from the lower mantle albarede and van der hilst 1999 kellogg et al 1999 ray tracing and reflectivity synthetic seismograms show that a distinct triplication is predicted for short period p waves although topography on a boundary would cause uncertainty in the strength and the range of the triplication many clear observations would be expected we examine data from the us west coast regional networks in the most likely distance range of 60 to 70 for a 1770 km depth boundary and find no evidence for p wave triplications

stirring geochemistry in mantle convection models with stiff plates and slabs abstract numerical models of mantle convection are presented that readily yield midocean ridge basalt morb and oceanic island basalt oib ages equaling or exceeding the apparent 1 8 ga lead isotopic ages of trace element heterogeneities in the mantle these models feature high viscosity surface plates and subducting lithosphere and higher viscosities in the lower mantle the formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component the models are run at the full mantle rayleigh number and take account of faster mantle overturning and deeper melting in the past more than 97 of the mantle is processed in these models including the expected excess density of former oceanic crust readily accounts for the depletion of morb source relative to oib sources a novel finding is of gravitational settling of dense tracers within the low viscosity upper mantle as well as at the base of the mantle the models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle these results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle

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

seismic scatterers at the shallowest lower mantle beneath subducted slabs abstract data from a western united states short period seismic network are analyzed in order to investigate anomalous later phases within a time window from about 10 s to 120 s after direct p waves for deep and intermediate depth earthquakes in circum pacific subduction zones the anomalous phases are best interpreted as s to p scattered waves wavelengths of 10 km from heterogeneities in the shallow lower mantle depths 950 km several s to p scatterers where elastic properties of the rocks must substantially change within several kilometers are detected in the shallowest 300 km of the lower mantle beneath four circum pacific regions kuril bonin fiji and peru around most of the observed scatterers the seismic tomography models have delineated high seismic velocity anomalies which are associated with recently subducted pacific slab or nazca slab the most likely origin of the scatterers would be basalt which used to form the oceanic crust interestingly enough the majority of the scatterers are located near the bottom boundaries of the slabs given rather moderate degrees of folding and or buckling of the pacific and nazca slabs in the shallowest lower mantle at the study areas the existence of the oceanic crust associated with the most recent slab subduction at the scattering sites is unlikely alternative and more likely explanations include the presence of ancient basaltic rocks localized dehydration of hydrous minerals or a sharp boundary between different rock fabrics such as isotropic and anisotropic lower mantle rocks associated with mantle flow dragged by the slab subduction

mixing by time dependent convection abstract two dimensional numerical flow models are employed to study the mixing of passive heterogeneities in the earth s mantle both simple kinematic models of time dependent flow and dynamic convection models are studied initially small blocks of tracers are inserted into the flow and their progressive dispersal is monitored through time some of the tracers also serve as infinitesimal strain markers three different regimes of mixing are observed a all heterogeneities are rapidly strained and stirred according to a turbulent mixing law where the size of a blob decreases exponentially with time b most heterogeneities are only slowly strained and possibly follow a laminar mixing law with a linear decrease with time of the size of a heterogeneity c the size reduction proceeds rapidly in part of the fluid but some poorly stirred islands persist for a long time all three regimes are found in various kinematic flow models while in the dynamic convection models only regime a is observed the effective strain rate in the exponential turbulent mixing law is more than an order of magnitude smaller than the average flow strain rate when the results from the dynamic convection models with variable viscosity are scaled to the upper mantle mixing over small length scales would be achieved after a few hundred million years for whole mantle convection with a significant increase of viscosity with depth mixing times could be in the order of 1 2 ga in the latter case it appears marginally possible that part of the isotopic heterogeneity observed in oceanic basalts has evolved within the convecting mantle rather than in physically separated reservoirs

global mineral distributions on mars 1 determining the mineralogy of mars is an essential part of revealing the conditions of the surface and subsurface a deconvolution method was used to remove atmospheric components and determine surface mineralogy from thermal emission spectrometer data at 1 pixel per degree ppd minerals are grouped into categories on the basis of compositional and spectral similarity and global concentration maps are produced all binned pixels are fit well with rms errors of 0 005 in emissivity higher rms errors are attributed to short wavelength particle size effects on dust covered surfaces significant concentrations 0 10 of plagioclase high ca pyroxene sheet silicates high si glass and hematite are detected and display distributions consistent with previous studies elevated concentrations of plagioclase and high ca pyroxene are consistent with basaltic surfaces and are located in low albedo highlands regions north of 45 s significant concentrations of plagioclase and sheet silicates high si glass and low concentrations of high ca pyroxenes are consistent with andesitic surfaces and are concentrated in both southern and northern high latitude low albedo regions andesitic surfaces in the southern hemisphere have a lower spectral contrast than northern surfaces an isolated surface located in solis planum is spectrally distinct but compositionally similar to other surfaces interpreted to be andesitic in composition concentrations of olivine below the detection limit correctly identify its presence in two of three locations potassium feldspar low ca pyroxene basaltic glass olivine sulfate carbonate quartz and amphibole are not detected with confidence at 1 ppd the results presented here indicate a predominance of volcanic compositions within martian dust free surfaces

dipping low velocity layer in the mid lower mantle evidence for geochemical heterogeneity data from western united states short period seismic networks reveal a conversion from an s to a p wave within a low seismic velocity layer greater than or equal to the 4 percent velocity difference compared to the surrounding mantle in the mid lower mantle 1400 to 1600 kilometers deep east of the mariana and izu bonin subduction zones the low velocity layer about 8 kilometers thick dips 30 to 40 southward and is at least 500 kilometers by 300 kilometers its steep dip large velocity contrast and sharpness imply a chemical rather than a thermal origin ancient oceanic crust subducted into the lower mantle is a plausible candidate for the low velocity layer because of its broad thin extent

bedload transport in two gravel bedded streams bedload transport measurements in two upland streams are considered as a function of the excess stream power exerted on the bed by the flow during low flows when the framework gravels remain undisturbed fine sediments are winnowed from the bed surface layer once a threshold of 3 4 w m 2 is exceeded and the transport rate is strongly supply limited however for stream powers in excess of 15 w m 2 framework gravels are mobilized and the efficiency of the transport process approaches a local maximum of about 1 per cent for discharges up to 2 3 of bankfull an inverse depth dependence in the efficiency of the transport process was noted but although bedload calibre increased as a function of discharge its influence on efficiency could not be demonstrated however it was suspected that the size sorting relationships of the bed material in a number of rivers in relation to the transport efficiency might profitably be examined further

long wavelength character of subducted slabs in the lower mantle abstract the high resolution seismic tomography provides the detailed information about the fate of the subducted plates in the mantle while the slab deformation in the transition zone is quite complex and differs among the individual subduction zones it could be generally concluded that most slabs seem to penetrate into the lower mantle the character of the fast seismic velocity anomalies however changes when the slabs cross the 670 km discontinuity while in the upper mantle tomography resolves relatively narrow plate like structures in the lower mantle it rather indicates thick blobs of seismically fast material such thickening of the downgoing slabs is traditionally attributed to the effect of the viscosity increase in the lower mantle here we study the deformation of the slabs in the lower mantle in a viscous model with a non linear composite rheology including the diffusion creep the dislocation creep and a power law stress limiter we perform numerical simulations of the subduction process in a 2d cartesian domain and concentrate especially on the effect of the change of the rheological parameters across the 670 km boundary on the slab deformation in the upper part of the lower mantle the thickening or the buckling of the plate is indeed observed in the models with relatively low yield stress 0 1 gpa of the stress limiting mechanism and with a viscosity increase at the 670 km discontinuity further we show that both the major mantle phase transitions and the strength of the crust layer have significant consequences for the creation of the buckling instabilities in the lower mantle

phase transition and density of subducted morb crust in the lower mantle abstract phase relations mineral chemistry and density of a natural mid oceanic ridge basalt morb composition were investigated up to 134 gpa and 2300 k by a combination of in situ x ray diffraction measurements and chemical analyses using transmission electron microscope tem results demonstrate that the morb composition consists of mgsio 3 rich perovskite stishovite casio 3 perovskite and cafe 2 o 4 type al phase in the upper part of the lower mantle the most abundant mineral of mgsio 3 rich perovskite undergoes phase transition to a cairo 3 type post perovskite phase above 110 gpa and 2500 k post perovskite phase is similar in composition to perovskite except considerably high na 2 o content stishovite transforms to cacl 2 type sio 2 phase above 62 gpa and 2000 k and further to pbo 2 type phase above 110 gpa pbo 2 type sio 2 phase includes large amount of al 2 o 3 which significantly expands its stability relative to cacl 2 type phase phase transition of casio 3 perovskite from tetragonal to cubic was also observed with increasing temperature cafe 2 o 4 type al phase is stable to the bottom of the mantle the density of morb crust was calculated using volume data combining with measured chemical compositions and calculated mineral proportions the former morb crust is denser than the average lower mantle at all depths greater than 720 km contrary to earlier predictions the subducted basaltic crust may have accumulated at the base of the mantle

river response to channel regulation case study of the raba river carpathians poland processes induced by the channelization of the raba river in the present century are examined to illustrate the response of a gravel bed stream following narrowing and straightening of its channel up to 3 m of incision has occurred the change from a slow and relatively steady degradation in the lower reaches to separate degradation events in the higher reaches is attributed to the differential rate of headcut retreat and to the control exerted by mid channel bars upon the rate of river energy dissipation progressive outwashing of finer grains from bed material has followed the diminishing sediment yield of the basin and the increase in stream power the ensuing growth in mean grain size and changes in sediment fabric have increased boundary resistance to flow and reduced particle susceptibility to entrainment downstream magnification of peak discharges has become increasingly pronounced with the advancing incision the decrease in flood plain storage and self acceleration of flows passing the relatively deep and straight channel has caused flood waves to become progressively more flashy in nature an increase in channel depth and reduction in gradient caused by downward and backward erosion as well as bed material coarsening has promoted the re establishment of an equilibrium conversely flow velocity increases due to flow concentration in the deepened channel reduction in grain mobility allows the river to attain a new equilibrium at flow velocity and stream power levels higher than before the channelization numerous disadvantages of the applied regulation scheme and its failure to reduce flood hazard raise the question of its maintenance to be successful any regulation design must take into account changes in sediment supply and flood hydrographs resulting from the simultaneous alterations in basin management

comment on mixing by time dependent convection by u christensen commentaire et reponse concernant l article de u christensen earth planet sci lett 95 382 394 1989

seismic evidence for small scale heterogeneity throughout the earth s mantle details in the amplitude structure of short period precursors to the seismic core phase pkp can be used to constrain the depth extent of mantle scattering the simplest model consistent with the observations invokes small 8 km weak r m s velocity perturbations of 1 random heterogeneities uniformly distributed throughout the mantle the data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle and instead place an upper limit on the amplitude of any short wavelength topography at the core mantle boundary

subparallel dipping heterogeneities in the mid lower mantle 1 data from western u s short period seismic networks show anomalous later phases which we interpret as s to p converted waves from heterogeneities in the mid lower mantle depths of 1000 1850 km northeast of the mariana subduction zone the geometry of the objects is mapped by a two step approach 1 computing the s to p scattering likelihood with the arrival time slowness and azimuth of the later arrivals measured by array techniques relative to direct p waves and 2 by direct use of the waveforms using semblance at least three subparallel objects are detected in the lower mantle study region with the dips of 30 to 40 southward and with a lateral extent larger than 500 km by 300 km the elastic properties associated with the heterogeneous objects appear to change within several kilometers considering their high efficiency at converting short period waves wavelengths of 5 10 km previous seismic tomography studies have not predicted significant velocity anomalies around the heterogeneities this is probably because they are below the resolution limit of present methods the three major heterogeneities are separated from each other nearly 300 km vertically and significant anomalies seem absent between each object nevertheless semblance cross sections with a finer grid size 0 1 0 1 10 km of the clearest intermediate object at 1400 1600 km show a hint that the object is not a single interface but is actually composed of double interfaces nearly 50 km apart purely thermal anomalies of kilometers scale must be transient features in the mantle and thus are highly unlikely to be the cause of the heterogeneities they should rather represent either sharp chemical variations in major element composition or sharp changes in preferred orientations of anisotropic lattice of the lower mantle minerals thermal anomalies however may be superimposed on such variations because of heat generation either by concentrated radioactive nuclei or by viscous shear heating due to deformations a plausible tectonic interpretation of these dipping objects is that they represent remnants of subducted lithospheric plates which preceded the current subduction of the pacific plate although we cannot make conclusive arguments on what the mechanism of creating such heterogeneities from these observations alone we suggest that they may represent stretched thinned and duplexed rocks within chaotic mantle flows

an analysis of large scale variations in small scale mantle heterogeneity using global seismographic network recordings of precursors to pkp high frequency precursors to the core phase pkp are caused by scattering off heterogeneities in the lowermost mantle and d regions and they provide a unique window into the small scale structure of the deep earth we study lower mantle scattering by analyzing 412 high quality pkp precursor records at ranges between 120 and 137 5 as obtained from the global seismic networks during the last 10 years to examine regional variations in scattering strength we compare individual records with the globally averaged pkp precursor stack of hedlin et al 1997 we identify strong differences in apparent scattering strength among specific source receiver paths inversion of these data for scattering source regions is complicated by ambiguity between source and receiver side scattering and the sparse and uneven data coverage synthetic tests however suggest that inversions with applied smoothness constraints can resolve large scale differences in scattering strength over significant parts of the lower mantle we use a conjugate gradient method based on an approximation to rayleigh born scattering theory to image differences in the average strength of scattering within the lowermost 1000 km of the mantle our results indicate particularly strong scattering beneath central africa parts of north america and just north of india whereas weaker scattering is seen beneath south and central america eastern europe and indonesia some regions of strong scattering correlate roughly with large scale anomalies revealed by seismic tomography including the african plume and the tethys trench these correlations are tentative rather than definitive because bootstrap resampling tests show that many details in our model are not reliably resolved and the network data alone do not permit complete resolution of the source receiver ambiguity in all areas further progress in this area will require integration of available network recordings with data collected by regional networks and arrays and consideration of the phase velocity of the precursors as well as their temporal variations

the influence of river training on mountain channel changes polish carpathian mountains the purpose of this paper is to explain the influence of river training on channel changes in mountain rivers also considered are the causes of failure of different training schemes the research was conducted on the regulated mszanka and porebianka rivers belonging to the raba river drainage basin in the polish flysh carpathian mountains channel mapping carried out in 2004 drew attention to the contemporary morphology of the channels and the development of their dynamic typology general changes in channel morphometry and land cover were identified by comparing cartographic sources from various years archive material from cracow s regional water management authority rzgw was used to analyse the detailed channel changes caused by each regulation structure the material consisted of technical designs of individual training works as well as plans longitudinal profiles and cross sections of trained channel reaches a series of minimum annual water stages at the mszana dolna gauging station was used to determine the tendency of channel bed degradation over 53 years during the first half of the 20th century the middle and lower courses of the mszanka and porebianka rivers had braided patterns the slopes mostly covered with crops were an important source of sediment delivery to the river channels today both channels are single threaded narrow and sinuous downcutting is the leading process transforming the channels they cut down to bedrock along about 60 of their lengths the main type of channel is an erosion channel which occurs also in the middle and lower courses of the rivers the channel sediment deficit is an important cause for river incision sediment supply to the channels was reduced after a replacement of crops on the slopes by meadows or forests gravel mining has also caused channel downcutting the rapid channel changes began after 1959 as systematic training was introduced channel regulation seems therefore to be a major factor determining channel adjustment debris dams and groynes were built before 1980 and these caused the greatest change of channel pattern increase of channel gradient and magnitude of river incision after that date the measures mostly involved drop structures from then on the rate of downcutting decreased considerably but has not ceased the rivers continued to incise until bedrock was exposed or training structures were destroyed after that a tendency to lateral migration and local braiding were observed in the deepened channel the channels displayed a tendency to return to their morphology and dynamic from before the training the results demonstrate that river training distorts the equilibrium of channel systems a channel becomes divided into artificial reaches which later follow different evolutionary patterns most training schemes on mountain channels are ineffective in the long term as river managers seem to consider a channel at a reach scale only individual channel reaches however are not independent but rather form a system that must be managed at the entire channel scale

absorption band q model for the earth body wave surface wave and normal mode data are used to place constraints on the frequency dependence of q in the mantle with a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range the quality factor qs is proportional to in the band and to and 1 at higher and lower frequencies respectively as appropriate for a relaxation mechanism with a spectrum of relaxation times the parameters of the band are q min 80 0 15 and width 5 decades the center of the band varies from 101 seconds in the upper mantle to 1 6 103 seconds in the lower mantle the shift of the band with depth is consistent with the expected effects of temperature pressure and stress high qs regions of the mantle are attributed to a shift of the absorption band to longer periods to satisfy the gravest fundamental spheroidal modes and the scs data the absorption band must shift back into the short period seismic band at the base of the mantle this may be due to a high temperature gradient or high shear stresses a preliminary attempt is also made to specify bulk dissipation in the mantle and core specific features of the absorption band model are low q in the body wave band at both the top and the base of the mantle low q for long period body waves in the outer core an inner core qs that increases with period and low qp qs at short periods in the middle mantle the short period qs increases rapidly at 400 km and is relatively constant from this depth to 2400 km the deformational q of the earth at a period of 14 months is predicted to be 463

compositional vs thermal buoyancy and the evolution of subducted lithosphere we formulate 2 d cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity increase at the boundary between the upper and lower mantle subducted lithosphere is represented as a cold stiff layered composite of denser eclogite underlain by more buoyant harzburgite slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thicken and fold strongly as they penetrate the lower mantle approximately a factor of two thickening occurs via pure shear just above the discontinuity with additional enhancement due to folding by over a factor of two no separation of the individual slab components occurs at the discontinuity and direct comparison with models in which compositional buoyancy is explicitly ignored indicates that slab evolution is largely controlled by the thermal buoyancy these results are at odds with hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab megaliths or a compositionally layered upper mantle

lower mantle scattering profiles and fabric below pacific subduction zones abstract the earth s mantle is compositionally evolving principally through the process of mid ocean ridge basalt production 95 by lithospheric area but also by way of arc magmatism 5 and continental growth reymer a schubert g 1984 phanerozoic addition rate to the continental crust and crustal growth tectonics 3 63 77 rowley d b 2002 rate of plate creation and destruction 180 ma to present geol soc am bull 114 927 933 convection eventually must mix these products back into the mantle but the details of the mixing process are unknown we report here new observational data documenting the location of differentiated material in the present day lower mantle and scattering power profiles with depth small scale 10 km objects which efficiently scatter short period s waves are present in the mantle lying below the pacific ocean rim at depths between 1100 and 1800 km we also observe a reduction in scattering power with depth indicating that the objects represent bodies richer in differentiated basalt than the surrounding mantle the scattering objects appear to be deformed to various degrees and usually form clusters separated from each other by a distance of 100 to 200 km the clusters probably represent folds of the order of 100 km probably corresponding to early stage of mixing process because the scattering power does not increase with depth neither does a permanent barrier to flow exist in the mid mantle nor does the intrinsic density of basalt prevent re entrainment into mantle flow from the bottom of the mantle

the measurement of river bank erosion and lateral channel change a review a detailed review and chronological survey is presented of the various techniques which have been used for the measurement of river bank erosion and channel change the techniques are classified according to the time scales involved long intermediate and short and each is discussed with respect to accuracy and repeatability the methods covered include sedimentological evidence botanical evidence historical sources planimetric resurvey repeated cross profiling erosion pins and terrestrial photogrammetry prospects for future developments are also discussed

the relationship between bank erosion local aggradation and sediment transport in a small carpathian stream abstract river bank erosion is the main process in small mountainous basins especially those where the main channel is built of poorly resistant sedimentary rock or alluvium exemplified by the slomka basin the main goal of the study based on the field survey work and the erosion pin method was to define the relation between river bank erosion accumulation and sediment transport the investigations were carried out in the slomka stream basin watercourse length of 25 2 km basin area of 69 9 km2 on an 1 0 km long stream reach strongly anthropogenically transformed between watercourse kilometres 5 0 and 6 0 we found that with an increase in flow the retreat bank rate increased nonlinearly an average migration rate of the undercut banks ranged from 0 10 m for the flow rate of 11 5 m3 s 1 to 0 92 m for the flow rate of 35 5 m3 s 1 the maximum observed rate exceeded 0 8 m for the flow rate of 11 5 m3 s 1 and 9 0 m for the flow rate of 24 8 m3 s 1 with respect to the channel vertical morphology over the whole measurement section we found that bed gradeline was raised on average by 0 04 m the trend is contrary to the results of field surveys performed earlier on the carpathian watercourses the phenomenon is related to the abundant supply of rock rubble into the stream channel high flow stages in the slomka stream which caused the stream bed material to move had a predominant impact on stream channel morphology processes such as destructive activity of rain drops aeolian erosion sandblasting soil expansion and shrinkage during the rainy and dry seasons were of minor significance

high temperature anomalies oceanward of subducting slabs at the 410 km discontinuity abstract our p wave whole mantle tomography revealed a low velocity region oceanward of the northern honshu slab of the pacific plate at depths around the 410 km seismic discontinuity resolution tests and scrutiny of the traveltime residuals for the ray paths passing through the low velocity region indicate that this anomaly is a resolvable feature and not an artifact due to the strong slab anomalies the existence of the slow anomalies is also supported by the analysis of the p wave records from the j array a large aperture seismic array in japan for a bonin earthquake the p arrivals to northern honshu at epicentral distances of 13 20 are strongly triplicated because of the 410 km discontinuity the later arrivals along the retrograde branch where ray paths pass through the low velocity region are anomalously slow comparison of the observed and synthetic waveforms indicates not only slow anomalies but also depression of the 410 km discontinuity this depression represents the direct evidence for the low velocity zone of primarily thermal origin an excess temperature of 200 k and the associated fractional melt of less than 1 can explain both the results of the tomographic and waveform analyses