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

lower mantle inhomogeneities inferred from pcp precursors a search of phases arriving between p and pcp the reflection from the core mantle boundary cmb was performed in the bulletins of the international seismological center bisc the distribution of the turning reflection points of anomalous waves in the lowermost mantle indicates the areas which are possibly anomalous regions the regions exhibiting these anomalies cover only a small fraction of the lowermost mantle sampled the areas which can be associated with anomalous arrivals are the lowermost mantle under the north pole region northern siberia central siberia western mongolia afghanistan northeastern china korea and the north atlantic ridge these are the regions which should be studied in more detail by analyzing the seismograms i e the amplitude and waveform information at the corresponding stations in addition to the travel time information reported to the isc

investigating ultra low velocity zones in the southern hemisphere using an antarctic dataset abstract given limited seismic coverage of the lowermost mantle less than one fifth of the core mantle boundary cmb has been surveyed for the presence of ultra low velocity zones ulvzs investigations that sample the cmb with new geometries are therefore important to further our understanding of ulvz origins and their potential connection to other deep earth processes using core reflected scp waves recorded by the recently deployed transantarctic mountains northern network in antarctica the current study aims to expand ulvz investigations in the southern hemisphere our dataset samples the cmb in the vicinity of new zealand providing coverage between an area to the northeast where ulvz structure has been previously identified and another region to the south where prior evidence for a ulvz was inconclusive this area is of particular interest because the data sample across the boundary of the pacific large low shear velocity province llsvp the weddell sea region near antarctica is also well sampled providing new information on a region that has not been previously studied a correlative scheme between a large database of 1 d synthetic seismograms and the observed scp data demonstrates that ulvzs are required in both study regions modeling uncertainties limit our ability to definitively define ulvz characteristics but also likely indicate more complex 3 d structure given that ulvzs are detected within along the edge of and far from the pacific llsvp our results support the hypothesis that ulvzs are compositionally distinct from the surrounding mantle and are not solely related to partial melt ulvzs may be ubiquitous along the cmb however they may be thinner in many regions than can be resolved by current methods mantle convection currents may sweep the ulvzs into thicker piles in some areas pushing these anomalies toward the boundaries of llsvps

double beam imaging mapping lower mantle heterogeneities using combinations of source and receiver arrays we present a new technique for imaging spatially distributed heterogeneities using information from combinations of source and receiver arrays the method is based on the single scattering assumption and is closely related to the double beam method of kruger et al 1993 1995 1996 in that it exploits amplitude delay time slowness and azimuth information in two arrays simultaneously a crucial step in the method is the application of static time corrections for a chosen reference phase here pcp one type of image is obtained as a spatial likelihood distribution for prescribed source and receiver array slowness and azimuth values and delay times with respect to a reference phase this allows the determination of the spatial origin of coherent phases in the p coda in a complementary approach we perform a double beam stack migration according to the theoretical slowness and azimuth values for candidate scatterers distributed over a three dimensional 3 d grid the scattering strength is expressed as the resulting beam power beam amplitude or semblance in a time window determined from the theoretical delay time with respect to the reference phase while likelihood mapping provides an image reflecting the probability of a region to explain observed kinematic phase properties by single scattering double beam migration provides information about the scattering strength the method is applied to nuclear explosion sources in eastern kazakhstan recorded at the yellowknife array in northern canada we identify several scattering volumes within the lower mantle below the arctic producing individual anomalous lower mantle phases in the p coda these anomalies range in depth from the core mantle boundary cmb up to 500 km into the lowermost mantle and are mainly located under the eurasian side of the arctic the joint interpretation of these results with the results of previous studies suggests a connection of these anomalies to the cenozoic and mesozoic subduction of the pacific plate and the kula plate the determination of the nature of such 3 d anomalies necessitates further array studies with better azimuthal coverage

probing two low velocity regions with pkp b caustic amplitudes and scattering summary we probe low velocity regions of the lower mantle with pkp waves using south pacific earthquakes recorded at stations from the temporary ethiopia afar geoscientific lithospheric experiment array in ethiopia and the german regional seismic network grsn germany the ethiopian stations show strong scattering of pkpab waves which arrive as precursors to pkpdf they also show anomalously large amplitude diffractions from the pkp b caustic in contrast the events recorded at the grsn show scattering of pkpab as precursors to pkpdf but more normal amplitudes in the pkp b caustic diffractions to understand the cause for the observed amplitudes we test a large set of 1 d models with anomalies in p and s wave velocities and density in the lowermost mantle and outer core structures that can cause the observed large pkp b caustic amplitudes are low velocity regions just above the core mantle boundary of the order of 50 km thick for the south pacific to ethiopia path with velocity reductions of around 5 per cent for p and 25 per cent for the s wave velocity alternatively a 30 km thick lamella starting at 50 km above the core mantle boundary with 10 per cent p wave velocity reduction and 25 per cent s wave velocity reduction also produces a good fit to the data we find that the low velocity regions act as waveguides enhancing the amplitude of the pkp b caustic diffracted waves and causing superposition of pkp b diffractions with pkpdf the low velocity regions cannot be smooth layers but must show a certain degree of small scale heterogeneity to produce the high frequency scattering observed in the data the path to germany needs small scatterers in the lowermost mantle but the low velocity region is less strong compared with the south pacific ethiopia path a plausible explanation for these low velocity regions is the presence of melt inclusions perhaps entrained in small scale convective systems that act as high frequency wavefield scatterers in regions where these inclusions or convective systems are abundant they appear as a large scale low velocity zone to lower frequency waves these observations support ideas that ultra low velocity zones at the base of the mantle may be heterogeneous regions of melt rich material

absence of short period ulvz precursors to pcp and scp from two regions of the cmb we present results from a study of the core mantleboundary cmb underneathmexico centralamer ica and the northeast pacic using core reflected phases pcpand scp which provide a direct means of sampling the deep mantle precursor arrivals to these phases pro duced by reflection from the top of an ultra low velocity layer ulvz at the base of themantle would contain infor mation about the thickness of such a layer and the velocity reductions within it stacks of these phases from 17 earth quakes recorded by the northern and southern california seismographic networks do not exhibit coherent or inco herent precursor arrivals from a ulvz in precursor time windows ratios of stack amplitudes to those of the cmb reflected phases fall below predicted values from published ulvz models we conclude that ulvzs if present in these regions havevp andvs reductions less than 10 are less than ve km thick or have transitions greater thanve km wide at their upper boundaries

a water retention model for deformable soils the paper presents an experimental study on water retention behaviour of two reconstituted soils a silty clay speswhite kaolin and a sandy clayey silt barcelona silt and two compacted soils a silty clay speswhite kaolin and a silty sandy gravel adige river embankment these soils were used with the aim of encompassing a wide range of soil types and soil fabrics specimens were subjected to drying and wetting hydraulic paths by removing or adding water under zero total stress and to mechanical wetting paths by compressing specimens at constant water content to investigate a broad range of void ratios the reconstituted sandy clayey silt was initially reconstituted from slurry by applying different consolidation vertical stresses whereas the compacted speswhite kaolin and adige river embankment were compacted to a broad range of water contents and compaction energy on the basis of the experimental data a modification to the van genuchten s model is proposed to account for the effect of voi

suction controlled multistage triaxial testing on clayey silty soil abstract multistage triaxial testing offers a potentially reliable and time saving alternative method of determining the shear strength of soils under suction controlled conditions while addressing the issues related to soil variability and lack of replicates however evidence of its suitability for testing geomaterials in unsaturated conditions is rather limited in this work a series of suction controlled multistage triaxial tests were performed on compacted specimens of clayey silty soil wherein each test was performed on the same soil specimen at a constant suction under drained condition but at varying net confining pressures the point of termination of the shearing stage was defined through a selection process involving either the stiffness or the volume change in monotonically sheared specimen based on whichever prevents premature failure of the specimen this was the first attempt where all the stages of a multistage triaxial test were compared against that from single stage triaxial tests for validation of the approach moreover the influence of errors introduced by multistage triaxial testing on the shear strength predictions of unsaturated soil was considered by constructing a three dimensional failure envelope on p s space using a well known relation and comparing them against those obtained from single stage triaxial tests it was also noted that for determining the shear strength parameters of soil the multistage testing reduced the testing times to half of that required by single stage triaxial testing which would translate into significant reduction of cost of testing thereby making unsaturated soil testing much more appealing to the practitioners

a doubling of the post perovskite phase boundary and structure of the earth s lowermost mantle a new model of the deep earth allows geophysicists to measure variations in earth s temperature near the boundary between the rocky mantle and core using a recently discovered phase change in the lowest part of the earth s mantle where atoms rearrange themselves in crystals under extreme pressure and temperature the model also explains odd features of this part of the mantle such as patchy regions that reflect seismic waves generated by large earthquakes and inferences of dense partial melting combined with observations the results imply very large variations in temperature providing further evidence for circulation of material throughout the earth s mantle the thermal structure of the earth s lowermost mantle the d layer spanning depths of 2 600 2 900 kilometres1 is key to understanding the dynamical state and history of our planet earth s temperature profile the geotherm is mostly constrained by phase transitions such as freezing at the inner core boundary or changes in crystal structure within the solid mantle that are detected as discontinuities in seismic wave speed and for which the pressure and temperature conditions can be constrained by experiment and theory a recently discovered phase transition at pressures of the d layer2 3 4 is ideally situated to reveal the thermal structure of the lowermost mantle where no phase transitions were previously known to exist here we show that a pair of seismic discontinuities observed in some regions of d can be explained by the same phase transition as the result of a double crossing of the phase boundary by the geotherm at two different depths this simple model can also explain why a seismic discontinuity is not observed in some other regions and provides new constraints for the magnitude of temperature variations within d

a pollen record of holocene climatic changes from the dunde ice cap qinghai tibetan plateau pollen preserved in ice cores from the dunde ice cap provides a sensitive record of holocene climatic changes and vegetational response in the northern qinghai tibetan plateau at time scales ranging from millennia to centuries and decades pollen analysis of the annually resolvable ice layers for a 30 yr period 1957 1986 suggests that total pollen concentration is correlated positively with summer precipitation and negatively with summer temperature thus it is a sensitive indicator of moisture availability and vegetation density in the steppe and desert regions around dunde high pollen concentrations between 10 000 and 4800 yr b p suggest that the summer monsoon probably extended beyond its present limit to reach dunde and westernmost tibet in response to orbital forcing the summer monsoon retreated time transgressively across the qinghai tibetan plateau during the middle holocene relatively humid periods occurred at 2700 2200 1500 800 and 600 80 yr b p probably as a result of neoglacial cooling prominent pollen changes during the medieval warm period 790 620 yr b p and the little ice age 330 80 yr b p suggest that the vegetation in the qinghai tibetan plateau region is sensitive to abrupt century scale climatic changes such as those anticipated in scenarios of greenhouse warming

ice core pollen record of climatic changes in the central andes during the last 400 yr abstract this paper presents a high resolution ice core pollen record from the sajama ice cap bolivia that spans the last 400 yr the pollen record corroborates the oxygen isotopic and ice accumulation records from the quelccaya ice cap and supports the scenario that the little ice age lia consisted of two distinct phases a wet period from ad 1500 to 1700 and a dry period from ad 1700 to 1880 during the dry period xerophytic shrubs expanded to replace puna grasses on the altiplano as suggested by a dramatic drop in the poaceae asteraceae p a pollen ratio the environment around sajama was probably similar to the desert like shrublands of the southern bolivian highlands and western andean slopes today the striking similarity between the sajama and quelccaya proxy records suggests that climatic changes during the little ice age occurred synchronously across the altiplano

compaction behaviour of clay this paper presents an experimental study of the compaction behaviour of non active clay one dimensional static compaction tests were carried out at high and medium water content with matric suction monitoring using trento high capacity tensiometers at lower water contents a transistor psychrometer was used to measure post compaction suction samples were compacted on the dry side of optimum to cover a wide range of compaction water contents and vertical stresses three water content regions were identified in the compaction plane depending on whether post compaction suction increased decreased or remained constant as the degree of saturation was increased at constant water content hydraulic paths of specimens subjected to loading unloading cycles at constant water content have clearly shown that post compaction suction may increase as the degree of saturation increases this non intuitive behaviour was demonstrated to be associated with the coupling between mechanical and water retention behaviour to this end a coupled mechanical water retention model was formulated irreversible one dimensional mechanical paths were modelled by a boundary surface in the space average skeleton vertical stress modified suction and void ratio irreversible hydraulic wetting paths were modelled by a boundary surface in the space suction degree of saturation and void ratio this study was completed by investigating the pore size distribution of compacted samples through mip tests

a quantitative comparison of microfossil extraction methods from ice cores microfossil records from ice archives allow vegetation fire and land use activity reconstructions on broad spatial scales samples typically contain low microfossil concentrations therefore large ice volumes are often needed for palynology hence it is crucial to extract maximum microfossil numbers through appropriate physical chemical treatments we compare six methods covering the main water reduction procedures evaporation filtration and centrifugation with snow samples adding a known number of lycopodium marker spores prior to sample treatment and a second marker eucalyptus after laboratory processing allows a quantitative microfossil loss assessment during pollen extraction we applied the best performing method average loss of 22 to high alpine firn cores from colle gnifetti glacier for validation with a natural archive containing extremely low microfossil concentrations we conclude that samples processed with different microfossil extraction protocols may give different results for pollen concentrations percentages and ratios between different pollen types especially if vesiculate conifer pollen is an important pollen assemblage component we recommend a new evaporation based method which delivers the smallest and least variable losses among the tested approaches since microfossil losses are inevitable during laboratory procedure adding markers prior to sample processing is mandatory to achieve reliable microfossil concentration and influx estimates

comments on brugger and others 2018 a quantitative comparison of microfossil extraction methods from ice cores in our comments we re evaluate brugger and others 2018 lycopodium eucalyptus double marker approach based on the fact that previous evidence already demonstrated that the batch of eucalyptus tablets used by brugger and others 2018 is not suitable for quantitative comparisons as they are characterized by inconsistent pollen concentration we present clear evidence that the eucalyptus tablets do feature inaccurate pollen concentrations and are therefore improper for all quantitative comparisons of microfossil extraction methods consequently the results of the quantitative and qualitative assessment of different pollen extraction methods from ice samples compiled by brugger and others 2018 are highly questionable due to the use of faulty marker tablets

new insights into the p and s wave velocity structure of the d discontinuity beneath the cocos plate summary broad band p and s waves from earthquakes in south america recorded at californian network stations are analysed to image lateral variations of the d discontinuity beneath the cocos plate we apply two array processing methods to the data set a simplified migration method to the p wave data set and a double array method to both the p and s wave data sets allowing us to compare results from the two methods the double array method images a dipping reflector at a depth range from 2650 to 2700 km in the southern part of the study area we observe a step like topography of 100 km to a shallower reflector at about 2600 km depth to the north as well as evidence for a second deeper reflector at a depth range from 2700 to 2750 km in the north results from the simplified migration agree well with those from the double array method similarly locating a large step in reflector depth in a similar location about 2650 km depth in the south and about 2550 km in the north as well as the additional deeper reflector at the depth of about 2750 km in the north waveform modelling of the reflected waves from both methods suggests a positive velocity contrast for s waves but a negative velocity contrast for p waves for the upper reflector in agreement with predictions from mineral physical calculations for a post perovskite phase transition the data also show some evidence for the existence of another deeper reflector that could indicate a double intersection of the geotherm with the post perovskite stability field that is the back transformation of post perovskite to perovskite close to the core mantle boundary

probing the core mantle boundary beneath europe and western eurasia a detailed study using pcp summary we use pcp the core reflected p phase recordings of deep earthquakes and nuclear explosions from the grafenberg germany and norsar norway arrays to investigate the core mantle boundary region beneath europe and western eurasia we find evidence for a previously unknown ultra low velocity zone 600 km south east of moscow located at the edge of a middle size low shear velocity region imaged in seismic tomography that is located beneath the volga river region the observed amplitude variations of pcp can be modelled by velocity reductions of p and s waves of 5 and 15 respectively with a density increase of 15 travel time delays of pre and postcursors are indicating a thickness of about 13 km for this ultra low velocity region ulvz however our modelling also reveals highly ambiguous amplitude variations of pcp and a reflection off the top of the anomaly for various ulvzs and topography models accordingly large velocity contrasts of up to 10 in vp and 20 in vs cannot be excluded in general the whole volga river region shows a complex pattern of pcp amplitudes caused most likely by cmb undulations further pcp probes beneath paris kiev and northern italy indicate likely normal cmb conditions whereas the samples below finland and the hungary slovakia border yield strongly amplified pcp signals suggesting strong cmb topography effects we evaluate the amplitude behaviour of pcp as a function of distance and several ulvz models using the 1d reflectivity and the 2d gauss beam method the influence of the velocity and density perturbations is analysed as well as the anomaly thickness the dominant period of the source wavelet and interface topographies strong variation of the pcp amplitude are obtained as a function of distance and of the impedance contrast we also consider two types of topographies undulations atop the cmb in the presence of flat ulvzs and vice versa where a broad range of cmb topography dimensions lead to large pcp amplitude variations only large ulvz undulations generate significant amplitude scattering consequently this indicates that topography effects of anomalies may mask the true medium parameters as well as the ulvz thickness moreover there might be a possibility of misinterpreting the precursor as pcp in particular for thin ulvzs

small scale changes of core mantle boundary reflectivity studied using core reflected pcp abstract short period pcp phases from earthquakes in the western aleutians recorded at the canadian yellowknife array yka sample the core mantle boundary cmb beneath the alaskan kenai peninsula and the cook inlet the pcp to p amplitude ratios for these events show large variability with some amplitude ratios more than an order of magnitude larger than predicted by radial earth models such as iasp91 or prem the amplitude ratios vary laterally with pcp core mantle boundary reflection points showing a 1 region with exceptionally large amplitude ratios it is likely that the variability in relative pcp amplitudes is due in large part to cmb topography other mechanisms such as radiation pattern mantle attenuation focusing and defocusing and core mantle boundary velocity and density perturbations could cause variations on scale lengths of less than a degree as found in this study but are unable to produce variations as large as those we observe

lateral variations in d thickness from long period shear wave data we explore global variations in d shear wave structure by examining 12 years of long period global digital seismograph network gdsn data to identify reflected phases from the d layer in the lowermost mantle we restrict our search to epicentral distances between 63 and 74 where a precritical d reflection will lie between the s and scs arrivals with gdsn long period records a d signal at these ranges is generally obscured by the stronger neighboring phases so to isolate the d reflections we develop a technique for stripping away the interfering s and scs waveforms we interpret travel time variations in the observed sds phases in terms of variations in d thickness and generate maps of inferred d thickness for areas of good seismic coverage the regions of observed d reflections are beneath australasia north central asia the arctic alaska and central america we find considerable variation in d structure in these regions a result consistent with previous studies which have found evidence for lateral heterogeneity in d on average the estimated d thickness is 260 km but there is a fairly uniform distribution of thicknesses between 150 km and 350 km a possible correlation is observed between d thickness and regions of predicted strong horizontal mantle flow

subducted banded iron formations as a source of ultralow velocity zones at the core mantle boundary ultralow velocity zones ulvzs are regions of the earth s core mantle boundary about 1 10 kilometres thick exhibiting seismic velocities that are lower than radial earth reference models by about 10 20 per cent for compressional waves and 10 30 per cent for shear waves it is also thought that such regions have an increased density of about 0 20 per cent ref 1 a number of origins for ulvzs have been proposed such as ponding of dense silicate melt2 core mantle reaction zones3 or underside sedimentation from the core4 here we suggest that ulvzs might instead be relics of banded iron formations subducted to the core mantle boundary between 2 8 and 1 8 billion years ago consisting mainly of interbedded iron oxides and silica such banded iron formations were deposited in the world s oceans during the late archaean and early proterozoic eras we argue that these layers as part of the ocean floor would be recycled into the earth s interior by subduction5 sink to the bottom of the mantle and may explain all of the observed features of ulvzs

axial force displacement behaviour of a buried pipeline in saturated and unsaturated sand several pipeline projects have been proposed in the world in recent years owing to the rapid growth of the petroleum industry onshore pipeline systems are usually buried above the natural groundwa

scp a probe of ultralow velocity zones at the base of the mantle the core reflected phase scp is a sensitive probe of the properties of an ultralow velocity zone ulvz at the base of the mantle in synthetic experiments a ulvz with large depressions in p and s velocities results in scp having two diagnostic precursors and one later arrival whose timing depends on ulvz vp vs and thickness amplitudes of the additional arrivals can be large if ulvz shear velocity has extreme reductions such as 30 and greater some data examples are presented for southwest pacific events recorded at short period and broadband arrays in hawaii which show evidence for a ulvz resolving all three additional arrivals will help in addressing the strength and ratio of vs and vp reductions in ulvz and thus the issue of partial melt

summer temperature reconstruction on the central tibetan plateau during 1860 2002 derived from annually resolved ice core pollen 1 we present a 140 year ice core pollen record with annual time resolution from the puruogangri ice field in the central tibetan plateau a linear regression model between the sum of the steppe and meadow pollen taxa and summer temperature at naqu accounts for 66 of the summer temperature variance for the period 1955 2002 on the basis of this model we reconstructed summer temperature history for the central tibetan plateau from 1860 to 2002 three marked warm periods during the 1870s 1890s and 1990s and five cold periods during the 1860s 1880s 1900s 1950s and 1960 1970s were identified within the last 100 years late 20th century warming is distinctive and may have had a major role in the present retreat of most glaciers on the tibetan plateau the pollen record is also significantly correlated with average annual temperatures over the whole tibetan plateau suggesting that these proxy data have the potential for reflecting large scale regional temperature change the statistically significant similarity between the puruogangri ice core pollen record and the 18o time series indicates that both are reliable indicators of temperature change at annual to multiyear timescales in this region

measurements of drying and wetting permeability functions using a new stress controllable soil column direct measurement of a permeability function k where suction of an unsaturated soil is often tedious and time consuming thus researchers have proposed various semiempirical predictive methods to determine k indirectly from a soil water characteristic curve swcc or a particle size distribution also stress effects on k are generally not considered to explore any stress effect on k a new 1 m high stress controllable soil column is developed to measure stress dependent swcc sdswcc and k of soils directly the objectives of this paper are to investigate the effects of 1 two stress state variables matric suction and net normal stress and 2 a drying wetting cycle on both sdswcc and k of a compacted decomposed silty clay each compacted soil column is subjected to an evaporation ponding cycle under various vertical net normal stresses the variations of pore water pressure and volumetric water content vwc w profiles with time are measured instantaneously the k

deep mantle seismic modeling and imaging detailed seismic modeling and imaging of earth s deep interior is providing key information about lower mantle structures and processes including heat flow across the core mantle boundary the configuration of mantle upwellings and downwellings phase equilibria and transport properties of deep mantle materials and mechanisms of core mantle coupling multichannel seismic wave analysis methods that provide the highest resolution deep mantle structural information include network waveform modeling and stacking array processing and 3d migrations of p and s wave seismograms these methods detect and identify weak signals from structures that cannot be resolved by global seismic tomography some methods are adapted from oil exploration seismology but all are constrained by the source and receiver distributions long travel paths and strong attenuation experienced by seismic waves that penetrate to the deep mantle large and small scale structures with velocity variations ranging from a fraction of a percent to tens of percent have been detected and are guiding geophysicists to new perspectives of thermochemical mantle convection and evolution

the core mantle boundary under the gulf of alaska no ulvz for shear waves the earth s core mantle boundary cmb marks the boundary between the hot molten iron core and the silicate mantle and is a thermal chemical and flow boundary previous observations of very slow compressional wavespeeds suggest that thin ultra low velocity zones ulvzs possibly composed of a mixture of molten iron and silicates exist at the base of the mantle a molten or partially molten layer would cause a large shear wavespeed decrease however this velocity drop has not been observed here we use core reflected scp phases to investigate the shear properties of ulvzs these phases reveal at least two distinct regions one region under central america which is distinctly average prem and iasp91 like and a second region under the entire gulf of alaska that produces large scp reflections with amplitudes of up to 30 times larger than calculated with average earth models the large amplitudes suggest a combination of focusing by cmb topography high shear wavespeeds at the bottom of the mantle and low attenuation high q along the scp path high shear wavespeeds and low attenuation are opposite from what would be expected from a shear wave ulvz a ulvz in compressional wavespeeds ultra low vp has previously been observed in this region however in addition to the large amplitudes the short period scp waveforms show no complexity that can be related to a ulvz thus either there is not a ulvz under the gulf of alaska or if it does exist it is restricted to compressional wavespeed changes precluding interpretation as partial melt but rather suggesting a chemical origin fl 2000 published by elsevier science b v all rights reserved

scp constraints on ultralow velocity zone density and gradient thickness beneath the pacific short period teleseismic recordings of scp from events in the tonga fiji tf and marianas mar subduction complexes made by the u s geological survey hawaiian volcano observatory seismograph network are examined for evidence of pre and post scp phases such arrivals would be caused by interaction of scp with ultralow velocity zone ulvz structures at the base of the mantle and can therefore be used to constrain the properties and occurrence of ulvzs the relative timings and amplitudes of scp two pre scp phases and one post scp phase potentially provide very good constraints on ulvz compressional and shear velocities density and thickness for example the amplitude of the topside s to p conversion sdp is very strongly density contrast dependent while the amplitude of the last arriving scsp a core reflection that converts to p upon emergence from the ulvz is very strongly dependent on the shear velocity contrast previous data largely consisting of diffracted spdiffks arrivals and reflected pcp precursors have been unable to provide good constraints on these two properties in our data set though we find very little evidence for pre and post scp arrivals even for tf paths where abundant evidence for the presence of ulvz exists for the tf data distance range 43 48 precursor amplitudes are at or below the data noise level if the density contrast ln is 10 km thick which lower scsp amplitudes below detectable levels taken together these properties favor ulvz material composed dominantly of partial melt but a contribution from core mantle reaction products cannot be ruled out for the mar distance range 55 58 pre and post scp arrival amplitudes are predicted to be higher although such arrivals are absent from the mar recordings however the quality of our data is not sufficiently good to rule out the presence of thinner and or more subdued ulvz in that region

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

localized double array stacking analysis of pcp d and ulvz structure beneath the cocos plate mexico central pacific and north pacific abstract a large high quality p wave data set comprising short period and broadband signals sampling four separate regions in the lowermost mantle beneath the cocos plate mexico the central pacific and the north pacific is analyzed using regional one dimensional double array stacking and modelling with reflectivity synthetics a data screening criterion retains only events with stable pcp energy in the final data stacks used for modelling and interpretation this significantly improves the signal stacks relative to including unscreened observations allows confident alignment on the pcp arrival and allows tight bounds to be placed on p wave velocity structure above the core mantle boundary cmb the pcp reflections under the cocos plate are well modelled without any ultra low velocity zone from 5 to 20 n at latitudes from 15 to 20 n we find evidence for two p wave velocity discontinuities in the d region the first is 182 km above the cmb with a ln vp of 1 5 near the same depth as a weaker discontinuity

the influence of soil structure and stress history on the soil water characteristics of a compacted till the soil water characteristic defines the relationship between the soil suction and gravimetric water content w or the volumetric water content or the degree of saturation s theoretical and

aspects of the behaviour of compacted clayey soils on drying and wetting paths a relatively large number of drying and wetting tests have been performed on clayey soils compacted at the standard or modified proctor optimum water content and maximum density and compared with tests on normally consolidated or overconsolidated soils the results show that drying and wetting paths on compacted soils are fairly linear and reversible in the void ratio or water content versus negative pore water pressure planes on the wet side of the optimum the wetting paths are independent of the compaction water content and can be approached by compaction tests with measurement of the negative pore water pressure correlations have been established between the liquid limit of the soils and such properties as the optimum water content and negative pore water pressure the maximum dry density and the swelling or drying index although based on a limited number of tests these correlations provide a fairly good basis to model the drying wetting paths when all the necessary data are not available key wor

performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration a major infrastructure project the south to north water transfer project has been proposed to carry potable water from the yangtze river region in the south to many arid and semi arid areas in the northern regions of china including beijing the proposed 1200 km middle route of the project is likely to be an open channel with a trapezoidal cross section at least 180 km of the proposed excavated canal will pass through areas of unsaturated expansive soils an 11 m high cut slope in a typical expansive clay with medium plasticity in zaoyang close to the middle route in hubei was selected for a comprehensive and well instrumented field study of the effects of rainfall infiltration two artificial rainfall events were created during a month of field investigation and monitoring prior to the rainfall events high soil suction was measured within the top 1 m of soil this high initial soil suction created an upward flux of water and moisture the abundant cracks and fissures near the ground surface p

microstructure and soil water retention behavior of compacted and intact silt loess abstract there are an increasing number of mega engineering projects of removing the top of hills to fill in the valley for new land creation in the loess plateau of china which significantly changes the hydrological and geological equilibrium state of the site the formation of a new hydrological equilibrium state is associated with the water movement in the compacted and intact loess layers most disasters occurring in the loess area are related to water the soil water retention curve swrc is the key to the implementation of unsaturated soil mechanics in engineering practice and is required to numerically model the movement of water in the soil in this study the swrcs and microstructure of intact and compacted specimens from lanzhou and yan an two typical cities that have new land creation projects are investigated the swrc is obtained using the filter paper method the mercury intrusion porosimetry mip and scanning electron microscope sem techniques are used to explore the microstructure the results highlight that the intact loess has a higher air occlusion value aov and a similar slope of swrc in the transition zone compared with the remolded specimens that compacted at the natural water content the normalized swrc i e degree of saturation versus suction avo of the intact specimen is approximately the same as the remolded specimens compacted as the natural water content the difference in the microstructure between intact and compacted specimens originates from different existing states of the clay particles and further contributes to the variation of the swrc

shear strength and dilative characteristics of an unsaturated compacted completely decomposed granite soil shear strength and dilative characteristics of a re compacted completely decomposed granite cdg soil are studied by performing a series of single stage consolidated drained direct shear tests under different matric suctions and net normal stresses the axis translation technique is applied to control the pore water and pore air pressures a soil water retention curve swrc is obtained for the cdg soil from the equilibrium water content corresponding to each applied matric suction value for zero net normal stress using a modified direct shear apparatus shear strength increases with matric suction and net normal stress and the failure envelope is observed to be linear the apparent angle of internal friction and cohesion intercept increase with matric suction a greater dilation angle is found at higher suctions with lower net normal stresses while lower or zero dilation angles are observed under higher net normal stresses with lower suctions also at a saturated condition experimental shear strength

water retention and volumetric characteristics of intact and re compacted loess a laboratory testing program was conducted to investigate the effects of microstructure on the water retention curve wrc and wetting drying induced volume change in loess the axis translation an

seismic detection of rigid zones at the top of the core data from earthquakes in the tonga fiji region recorded at a seismic array in northern australia show evidence for rigid zones at the top of the outer core the scp waveforms can be modeled by thin 0 12 to 0 18 kilometer zones of molten iron mixed with solid material with a small but positive s wave velocity 0 6 to 0 8 kilometer per second that enables the propagation of s waves in the outermost core the zones may be topographic highs of the core mantle boundary filled by light core sediments and might be important for variation of earth s nutation and for convection of the outer core

intense seismic scattering near the earth s core mantle boundary beneath the comoros hotspot i report strong seismic pkp precursors recorded in the seismic stations in tanzania for earthquakes occurred in the fiji subduction zone the observed pkp precursors show clear onsets and rapid variations of amplitude for different events and stations i develop a technique to use the precursor onset times to determine the location of seismic scatterers and apply forward modeling of precursor amplitudes to place constraints on the magnitude of seismic anomalies the travel time analysis indicates that these recorded pkp precursors are caused by the seismic scattering in the lowermost 360 km of the mantle beneath the comoros hotspot the amplitudes of the observed pkp precursors suggest a rather complex seismic structure with a p velocity variation of at least 8 likely associated with partial melt

influence of compaction conditions on pore size distribution and saturated hydraulic conductivity of a glacial till the paper examines the hydraulic conductivity of a nonplastic till from northern quebec it is shown that the hydraulic conductivity is strongly influenced by the compaction degree of saturation and the variation of hydraulic conductivity with void ratio is influenced by compaction conditions determination of pore size distributions and microphotographs provide evidence that changes in hydraulic conductivity are related to the fabric of the compacted specimens and macroporosity developing when the soil is compacted at degrees of saturation less than that at the optimum key words till hydraulic conductivity microfabric

on the evaluation of unsaturated flow in a natural slope in rio de janeiro brazil abstract geotechnical engineers and geologists are frequently concerned with the complex phenomenon of rainstorm induced landslides in mountainous tropical regions this paper analyses the hydrological conditions that contributed to a huge deep seated slide of a slope in rio de janeiro brazil the slope failure occurred after a short period of intense rainfall buried cars and caused structural damages to an adjacent building on the following day the failure surface was completely saturated despite the fact that no groundwater was observed within the slope understanding a rain induced landslide is a considerable challenge in the present case there was an extra question to be answered why were no slope movements observed a few months earlier when a more intense rainfall reached the slope geological and geotechnical surveys were carried out aiming at determining complete soil profile characterization piezometric levels and hydraulic parameters field investigations indicated a variable thickness unsaturated residual soil layer overlying a gneissic rock the transition between the saprolitic soil and the sound rock consisted of a highly fractured and weathered rock 3d flow modelling has shown that the amount of rain infiltration was insufficient to produce positive pore water pressures it was likely that development of preferential infiltration paths could explain the hydrological conditions of the slope during landslide water infiltration through the fractured soil rock transition induced a rapid development of positive pore water within the soil mass this favourable scenario might have been created by the lack of maintenance of the crest drainage channel adjacent to the slide area it is a common engineering practice to try to associate rainfall amounts to the risk of slope failure the role that bedrock plays in generating pore pressure is seldom considered the numerical analyses presented in this paper pointed out at the complexity of flow modeling of rain infiltration through unsaturated residual soil slopes it essentially revealed that flow mechanisms other than rain infiltration over the slope surface might play a major role on hydrological pattern of slopes

localized boundary layer below the mid pacific velocity anomaly identified from a pcp precursor dense record sections from deep earthquakes in fiji and argentina recorded on hundreds of short period stations in california at distances of 81 to 85 are used to investigate the detailed p wave velocity structure above the core mantle boundary cmb in the fiji data a secondary phase arriving 2 to 4 s after the direct p is identified as a precursor to pcp this phase provides good evidence for a reflection off the top of a thin low velocity layer above the cmb comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5 10 compared to the overlying mantle a record section from an argentina event does not show the pcp precursor indicating that the low velocity layer is not a global feature this thin low velocity layer is in the same place as a much larger s wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the cmb under the mid pacific

extraction of weak pcp phases using the slant stacklet transform i method and examples summary in order to study fine scale structure of the earth s deep interior it is necessary to extract generally weak body wave phases from seismograms that interact with various discontinuities and heterogeneities the recent deployment of large scale dense arrays providing high quality data in combination with efficient seismic data processing techniques may provide important and accurate observations over large portions of the globe poorly sampled until now major challenges are low signal to noise ratios snr and interference with unwanted neighbouring phases we address these problems by introducing scale dependent slowness filters that preservetime spaceresolution wecombinecomplexwaveletandslant stacktransformstoobtain the slant stacklet transform this is a redundant high resolution directional wavelet transform with a direction here slowness resolution that can be adapted to the signal requirements to illustrate this approach we use this expansion to design coherence driven filters that allow us to obtain clean pcp observations a weak phase often hidden in the coda of the p wave for events with magnitude mw 5 4 and distances up to 80 in this context we then minimize a linear misfit between p and pcp waveforms to improve the quality of pcp p traveltime measurements as compared to a standard cross correlation method this significantly increases both the quantity and the quality of pcp p differential traveltime measurements available for the modelling of structure near the core mantle boundary the accuracy of our measurements is limited mainly by the highest frequencies of the signals used and the level of noise we apply this methodology to two examples of high quality data from dense arrays located in north america while focusing here on body wave separation the tools we propose are more general and may contribute to enhancing seismic signal observations in global seismology in situations of low snr and high signal interference

seismological constraints on the ultralow velocity zones in the lowermost mantle from core reflected waves abstract analyses of reflected seismic phases pcp and scp from the core mantle boundary cmb enable us to constrain the physical properties of the ultralow velocity zones ulvzs at the base of the mantle the cmb region beneath the western and southern pacific is investigated in detail by using a large amount of waveform data recorded by the short period array stations clear arrivals of postcursor to scp scsp are detected from several event receiver pairs which sample the cmb regions beneath philippine kalimantan and east of australia indicating the existence of ulvzs under the regions no scp precursors sdp and spcp are found in our data set except for a few events with a large epicentral distance over 45 no additional arrivals related to pcp are found in the region where scsp is observed in the scp wavelet elastic properties of the ulvz are precisely constrained by waveform modeling of both pcp and scp which sample the cmb region under philippine kalimantan different ulvz structures are found between the northeastern and southwestern parts of the philippine kalimantan the ratio of shear to compressional velocity reductions is 2 0 2 5 and the ratio of density to shear velocity changes is 0 5 to 0 33 in the northeastern part while minimal changes in compressional velocity and large shear velocity decrease in the southwestern part are found a certain degree of density increases and shear velocity decreases are also observed in the ulvz beneath east of australia increase in density and decrease in shear velocity throughout our observation imply that the iron incorporation into the lowermost mantle minerals may generate the ulvz anomaly however other mechanisms such as partial melting can explain the structural difference between the northeastern and southwestern parts of the ulvz beneath the philippine kalimantan region

on the absence of an ultralow velocity zone in the north pacific 1 using an unusually large earthquake near the big island of hawaii we study the core mantle boundary cmb beneath the north northeastern pacific between hawaii and north america a dense sampling of the cmb is achieved using the core reflected phase pcp recorded at a large number of high quality stations in north america including networks in california oregon washington and alaska as well as at earthscope s usarray stations we apply an adaptive stacking technique for optimal record alignment on specific phases namely p and pcp and subsequently stack seismograms to obtain summation traces possessing relatively high signal to noise ratios of pcp and p anomalous ultralow velocity zone ulvz layering at the cmb has been noted to exist in various parts of the pacific beneath the lowermost mantle large low shear velocity province imaged by tomography ulvz structure produces anomalous pcp waveform variations in the form of precursors to pcp these pcp data however lack precursory energy indicating either that 1 ulvz layering is lacking or 2 that a ulvz is present and thinner than our detection threshold i e less than a few kilometers thick we use synthetic waveform modeling to establish the sensitivity and utility of investigating the time window ahead of pcp for precursors generated from fine scale cmb layering these results combined with evidence for ulvz structure in other parts of the pacific suggest that ulvzs are intermittent and possibly only detectable in regions where mantle currents collect ulvz material whether or not partially molten presumably beneath or near upwellings or plumes

hydrological effects of live poles on transient seepage in an unsaturated soil slope centrifuge and numerical study abstractlive poles have been used to stabilize shallow slopes most of the existing work has focused on pole reinforcement the effects of pole transpiration on slope stability are unclear this study aims to investigate the effects of pole transpiration on rainfall induced slope hydrology through centrifuge model tests a novel live pole modeling technique using real branch cuttings was adopted to simulate the hydrological effects of the transpiration induced suction in the centrifuge suction responses during rainfall were recorded and back analyzed by seepage analysis higher suction was preserved after rainfall when pole transpiration induced higher suction before rainfall this is because inducing higher suction would lower the soil water storage capacity and hydraulic conductivity after a rainfall event with a return period of 20 years up to 10 kpa of suction was preserved in the pole supported slope to provide stabilization effects

cracking and vertical preferential flow through landfill clay liners abstract cracking is a major concern in the long term performance of landfill clay liners cracking of clay liners is affected by the roots of the vegetation planted to control erosion while vegetation roots may restrict cracking they may introduce preferential flow through the liners the impact of vegetation roots on the hydraulic properties of clay liners remains unclear this study investigates the effect of vegetation roots on crack formation and preferential flow through clay liners three experimental soil columns were used to observe crack formation by exposure to the weather and to measure the flow through them caused by artificial rainfall the clay in each column was compacted to 90 relative compaction the first column was not vegetated and no crack developed in it the second column was vegetated and underwent drying wetting cycles outdoor the third column did not have vegetation but underwent drying wetting cycles outdoor cracks developed in the second and third columns due to the exposure to the weather image analysis in the experimental soil columns revealed that vegetation roots restricted crack formation especially in the early stages of drying wetting cycles infiltration tests performed on the experimental soil columns showed that the unsaturated hydraulic conductivity of the vegetated soil in the second column was respectively about two orders and one order of magnitude larger than those of the soils in the first and third columns the experimental results imply that the effect of vegetation roots on the preferential flow through landfill clay liners should be considered in practical designs

a sharp and flat section of the core mantle boundary the transition zone between the earth s core and mantle plays an important role as a boundary layer for mantle and core convection1 this zone conducts a large amount of heat from the core to the mantle and contains at least one thermal boundary layer2 3 the proximity of reactive silicates and molten iron leads to the possibility of zones of intermediate composition4 here we investigate one region of the core mantle boundary using seismic waves that are converted from shear to compressional waves by reflection at the boundary the use of this phase known as scp the large number of receiving stations and the large aperture of our array all provide higher resolution than has previously been possible5 7 for the 350 km long section of the core mantle boundary under the northeast pacific sampled by the reflections the local boundary topography has an amplitude of less than 500 m no sharp radial gradients exist in the 400 km above the boundary and the mantle to core transition occurs over less than 1 km the simplicity of the structure near and above the core mantle boundary argues against chemical heterogeneity at the base of the mantle in this location

a scattering region near the core mantle boundary under the north atlantic summary we analysed pcp precursors in seven events from central america recorded at the uk array of short period stations to determine the most likely source of this signal combinations of observations of slowness traveltime and backazimuth were used to investigate the properties of the phase with the result that its source is identified as p to p scattering from an anomalous region in the lower mantle within d using a joint likelihood method the best estimate of a point source for scattering is at 44 0 n 71 5 w under nova scotia and 2720 km depth analysis of the spectrum of the scattered signal indicates that the region contains small scale heterogeneities on the order of 0 6 to 0 7 km and is 700 km in size the velocity contrast between the scattering bodies and ambient mantle is as high as 0 4 to 0 7 per cent vp and 1 4 to 6 6 per cent vs on account of their small size positive velocity anomaly and association with the reconstructed position of ancient subducted material we conclude that they represent chemical heterogeneity in the lower mantle

making unsaturated soil mechanics accessible for engineers preliminary hydraulic mechanical characterisation and stability assessment the paper presents an accessible approach for preliminary hydraulic and mechanical characterisation of unsaturated soils and stability analysis of geo structures under unsaturated conditions the approach is accessible in the sense that the laboratory investigation and geotechnical analyses can be completed in a relatively short time and at a reasonable cost the overall philosophy behind this approach is that laboratory testing should not be eliminated but kept to a minimum and that any simplification introduced should be conservative the paper illustrates that shear strength can be characterised conservatively by performing constant water content tests with no facilities to control monitor suction and that water retention behaviour can be predicted successfully by performing a single water retention measurement the paper also demonstrates that stability analyses can be carried out within the classical framework of limit analysis we show that a preliminary analysis of the influence of infiltrating rainwater on suction can be carried out using a simple and conservative method that makes use of solutions available in saturated geotechnical textbooks such an approach may be used to assess when it is worthwhile or necessary to conduct further time consuming and costly unsaturated experimental testing field monitoring and numerical analyses when considering the stability of geostructures

small scatterers in the lower mantle observed at german broadband arrays seismograms of earthquakes from the south pacific recorded at a german broadband array and network show precursors to pkpdf these precursors mainly originate from off path scattering of pkpab or a nearby pkpbc to p for receiver side scattering or from scattering of p to pkpab or pkpbc on the pkpdf path for source side scattering standard array processing techniques based on plane wave approximations such as vespagram or frequency wavenumber analysis are inadequate for investigating these precursors since scattered waves cannot be approximated as plane waves for arrays and networks larger than 300 300 km for short period waves we therefore develop a migration method to estimate the location of scatterers in the mantle at the core mantle boundary and at the top of the outer core with our method we are able to find isolated scatterers at the source side and the receiver side although the depth of the scatterer is not well constrained however from looking at the first possible arrival time of precursors at different depth and the region where scattering can take place scattering volume we believe that the location of the scatterers is in the lowermost mantle since we have detected scatterers in regions where ultralow velocity zones have been discovered recently we think that the precursor energy possibly originates from scattering at partial melt at the base of the mantle comparing results from broadband and band pass filtered data the detection of small scale structure of the ultralow velocity zones becomes possible

why loss matters reply to the comments of festi and others on a quantitative comparison of microfossil extraction methods from ice cores by brugger and others 2018

modelling the applied vertical stress and settlement relationship of shallow foundations in saturated and unsaturated sands the bearing capacity and settlement of foundations are determined experimentally or modelled numerically based on conventional soil mechanics for saturated soils in both methods bearing capacity

a very slow basal layer underlying large scale low velocity anomalies in the lower mantle beneath the pacific evidence from core phases a multi phase analysis using long period world wide standardized seismograph network and canadian network data has been conducted using core phases for deep focus events from the southwest pacific these include sks s2ks svd iff and sp dks the last phase emerges from sks near 106 and is associated with a p wave diffracting along the bottom of the mantle patterns in s2ks sks differential travel times t s2ks sks correlate with those in sp dks sks t spdks sks t s2ks sks values strongly depend on variations in v s structure in the lower third of the mantle whereas t spdks sks values mainly depend on v p structure and variations in a thin zone 100 km or less at the very base of the mantle anomalously large t s2ks sks and t spdks sks values relative to the preliminary reference earth model prem are present for fiji tonga and kermadec events recorded in north and south america along with anomalously large sv diff amplitudes well into the core s shadow more northerly paths beneath the pacific to north america for indonesian and solomon events display both prem like and anomalous times a model compatible with the observations is presented and contains a thin very low velocity layer at the base of the mantle that underlies the large volumetric lower mantle low velocity regions in the southwest pacific a low velocity layer of 20 100 km thickness with reductions of up to 5 10 relative to prem can reproduce t spdks sks as well as sv diff amplitudes large scale more than 1000 km lower mantle v s heterogeneity 2 4 can explain long wavelength trends in t s2ks sks the exact thickness and velocity reduction in the basal layer is uncertain owing to difficulties in resolving whether anomalous structure occurs on the source and or receiver side of wavepaths at the cmb

constraining the short scale core mantle boundary topography beneath kenai peninsula alaska with amplitudes of core reflected pcp wave previous analysis of short period pcp with seismic array reported rapid variation of pcp amplitude and suggested small scale variation of core mantle boundary cmb topography beneath kenai peninsula alaska in order to constrain the length scale l and topographic height variation h in the region we quantitatively compute synthetic pcp seismograms using the seismic representation theorem for a core mantle boundary cmb with topography bump h 0 or depression h 0 we model pcp amplitudes for l in the range of 60 500 km h in the range of 6 km depression to 5 km bump it is observed that cmb depressions do produce amplification and cmb bumps generate de amplification for short period pcp amplitudes after computation for various combinations of l and h amplification in the range of 2 4 5 is observed in the above mentioned parameter ranges the topography model of a cmb depression h 6 km l 6 predicts 1 hz pcp amplification consistent with observed pcp amplitude variation beneath kenai peninsula however pcp amplitude can also be affected by heterogeneities in the crust and mantle the estimated topography could be less pronounced

short period p wave constraints on d reflectivity we present evidence of a p wave reflector in d beneath the central and eastern equatorial pacific obtained from nearly 7000 short period seismic recordings of earthquakes in the tonga fiji and south america subduction complexes both regions overlap previous observations of an s wave triplication at similar depths double array waveform stacking places the reflector 190 20 km above the core mantle boundary and allows for a velocity transition zone no thicker than 50 km the data admit a wide range of compressional v p and shear wave velocities v s the 2 3 increase in v s suggested by earlier long period s wave studies however necessitates only a 0 5 0 6 increase in v p the data stacks represent spatial averages over large areas of the lower mantle such that reflector intermittence and topography decrease stack amplitude and lower the apparent v p jump our results constrain these processes to act over length scales of 10 100 km

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

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

integrated approaches for predicting soil water characteristic curve and resilient modulus of compacted fine grained subgrade soils this paper combines a series of approaches for predicting the soil water characteristic curve swcc and the variation of the resilient modulus mr of compacted fine grained subgrade soils with moisture content which is the key information required in mechanistic pavement design methods the presented approaches for the swcc and mr are integrated as i they are developed following the same philosophy ii they require only the measurements of the suction and moisture content or mr at saturated and optimum moisture content conditions for prediction and iii the predicted swcc is used for predicting the mr moisture content relationship experimental studies have been performed on five fine grained subgrade soils that were collected from different regions in ontario canada to determine their mr at various external stress levels and post compaction moisture contents as well as their swccs after the mr tests experimental measurements are predicted using the integrated approaches and the empirical

high resolution cmb imaging from migration of short period core reflected phases abstract the fine scale structure of the core mantle boundary cmb region beneath southern alaska is studied using core reflected p waves pcp recorded at the canadian yellowknife array seismic studies of the lowermost mantle have detected a multitude of structures at this contact zone between iron alloy liquid of the core and the silicates of the mantle these structures offer key constraints for the dynamics of mantle and core high quality waveform information and migrated array data have been used to infer the existence of thin layering at the cmb in the form of ultra low velocity zones the data clearly show the absence of an ultra low velocity zone thicker than about 7 km in this region while there is evidence for a weak p wave reflector about 110 km above the cmb beneath the kenai peninsula these observations are in good agreement with the hypothesis that ultra low velocity zones are related to areas with lower than average seismic velocities in tomographic images and absent in regions of fast seismic velocities likely related to subduction processes such as beneath alaska

an insight into the water retention properties of compacted clayey soils experimental data from different testing methodologies on different compacted clayey soils with dominant bimodal pore size distribution are presented and analysed to provide a comprehensive picture of the evolution of the aggregated fabric along hydraulic and mechanical paths fabric changes are analysed both from the porous network viewpoint by means of careful mercury intrusion porosimetry investigation and from the soil skeleton viewpoint by quantifying swelling and shrinkage of the aggregates in an environmental scanning electron microscopy study the consequences of the aggregated fabric evolution on the water retention properties of compacted soils are analysed and discussed a new model for water retention domain is proposed which introduces a dependence of the intra aggregate pore volume on water content the model succeeds in tracking correctly the evolution of the hydraulic state of the different soils investigated along generalised hydromechanical paths the proposed approach brings to l

experimental investigations of the soil water characteristics of a volcanic soil rain induced landslides are common around the world to analyse transient seepage and to predict pore water pressure distribution in unsaturated slopes subjected to rainfall infiltration it is essential to study soil water char acteristics and water permeability functions the soil water characteristic curve swcc is a relationship between suc tion and water content or degree of saturation conventionally only the drying soil water characteristic curve of soil specimens is determined in a pressure plate extractor without the application of any external stress in this paper the influences of initial dry density and initial water content history of drying and wetting soil structure and the stress state upon the desorption and adsorption soil water characteristics of a completely decomposed volcanic soil in hong kong are examined and discussed the experimental results presented are obtained by using a conventional volumetric pressure plate extractor and a newly modified one dimensional stress controllable pressure plate extractor with deforma tion measurements the swcc of a recompacted specimen is very different from that of a natural specimen with the same initial soil density and initial water content the swcc of the recompacted specimen is highly dependent on the history of drying and wetting the rates of desorption and adsorption are substantially higher at the first drying and wetting cycle than at the second drying and wetting cycle the size of the hysteresis loop of the recompacted specimen is considerably larger than that of the natural specimens the swcc of soil is stress state dependent for recompacted specimens subjected to different stress states the higher the applied stresses the lower the rate of desorption and the smaller the size of the hysteresis loops however for natural specimens the size of the hysteresis loops seems to be independent of the stress state under a higher applied stress natural specimens exhibit lower rates of desorption and adsorption

a search for anomalies in the lowermost mantle using seismic bulletins abstract for deep events and epicentral distances around 75 the bulletins of the international seismological centre bisc contain many secondary arrivals between p and pcp the reflection from the core mantle boundary cmb which cannot be explained by current spherical symmetric earth models assuming that these arrivals in the p coda are reflections from p velocity anomalies in the lower mantle we calculate the associated reflection points geographical coordinates and distance from the cmb and map both strength relative density of reflection points in bins in the lower mantle and distance of the reflectors from the cmb on a global scale the regions with these anomalous bins cover 3 of the 50 of the lowermost mantle that can be analysed with this data set the most significant regions with anomalous bins values in parentheses give a preliminary estimate of the distance of the reflector from the cmb are under the north atlantic ridge 250 km northern central siberia 230 km japan 180 km the central mid atlantic ridge 310 and 260 km and off the coast of ecuador 340 km the presence of reflectors in one of these regions has already been confirmed by detailed seismic studies using waveform data from arrays and networks a slight 2 3 p velocity contrast is sufficient to produce measurable reflection amplitudes in the distance range of 70 80 ten percent of the lower mantle sampled by this study is covered by bins with no or very few secondary arrivals in the p coda the most significant regions with such a normal lower mantle are under central america the caroline and marshall islands in the eastern pacific the area of hawaii and the easter islands and the northern atlantic identification of the regions with anomalous bins serves to locate the areas of the lowermost mantle which should be analysed further by using the full information content of seismograms the study of the travel time the amplitude and the form of the wavefield recorded preferably at seismic arrays dense networks will allow the determination of the anomalous velocity structure of the core mantle transition zone in more detail the most promising areas for such a study are given here but further anomalies are likely to exist

small scale ultralow velocity zone structure imaged by scp 1 short period teleseismic scp phases from events in the tonga fiji region recorded at the short period small aperture warramunga array in australia are used to study structure at the core mantle boundary cmb we examine the data for evidence of precursors and postcursors to scp such additional arrivals are generated by the interaction of scp with structure at the reflection and conversion point at or near the cmb we show precursors and postcursors to scp indicating an ultralow velocity zone ulvz at the cmb in the region between tonga fiji and australia one dimensional forward modeling of the short period data leads to models of the ulvz with thickness varying between 5 and 20 km velocity reductions of 0 10 and 10 30 for p velocity and s velocity respectively and 0 50 density increase many events however show simple waveforms consistent with a locally simple cmb models for proximal events can vary greatly indicating very complex three dimensional structure with scale lengths of a few tens of kilometers these results point to complex chemical and physical processes in the lowermost mantle

relationship between soil structure and mechanical behavior for an expansive unsaturated clay to investigate the soil structure and mechanical behavior of an unsaturated expansive clay soil triaxial and mercury intrusion tests have been performed and are presented in this paper also an optical test system was developed for studying the internal structure evolution of the unsaturated expansive soil during shear loading tests the research results show a linear relationship between the cohesion intercept and the degree of saturation of the unsaturated clay for the test data for the tested soil the pore size distribution curves were found to be bimodal the measurement results from mercury intrusion tests confirm that the micropores of guangxi expansive soils of this study remain constant and that the macropores vary with the dry density of soil samples comparing the volumetric strain curves with the soil particle area curves some correlation and dependence on the macropore variation of soil samples during shear loading tests can be shown to exist key words soil structure mechanical behavior

detection of an ultralow velocity zone at the core mantle boundary using diffracted pkkpab waves 1 seismic phases diffracted around earth s core contain information about lowermost mantle wave speeds by measuring the slowness of incident diffracted energy from array recordings seismic velocity along the diffracted path can be estimated here we apply this principle to diffraction of the major arc seismic phase pkkpab recorded at the canadian yellowknife array to estimate p wave velocity variations along the core mantle boundary we observe pkkpabdiff about 7 5 past the ray theoretical cutoff distance for pkkpab we utilize 330 western pacific rim earthquakes that allow us to probe the core mantle boundary beneath the north atlantic and the south pacific oceans using pkkpabdiff slowness and back azimuth are measured by frequency wave number analysis mapping pkkpabdiff slowness variations suggest 4 19 p wave velocity reductions relative to prem in good agreement with the magnitude of velocity reductions previously mapped in ultralow velocity zones the pkkpabdiff slowness and back azimuth variations combined with results from previous ulvz studies using spdiffks imply that the lowered velocities occur at the base of the mantle beneath the north atlantic ocean along the receiver side of raypaths pkkpabdiff array measurements thus hold important potential for mapping ultralow velocity zone structure in so far unprobed regions of the lower mantle as well as for providing additional and independent information about lower mantle structure

wetting front advancing column test for measuring unsaturated hydraulic conductivity unsaturated hydraulic conductivity is the primary soil parameter required when performing seepage analyses for unsaturated saturated soil systems unsaturated hydraulic conductivity is also one of

a new probe of ulvz s wave velocity structure array stacking of scs waveforms 1 s wave velocity reductions in thin layers above the core mantle boundary cmb are resolved by modeling precursors to the tangential components of core reflected scs phases unlike most phases used to study ultra low velocity zone ulvz structure scs provides direct sensitivity to shear velocity structure vs recordings of deep earthquakes in the tonga fiji and south american subduction zones at broadband seismic networks in western north america are used to map small scale ulvz variability a two layer low vs structure exists under the central pacific an abrupt 0 8 2 0 decrease 60 86 km above the cmb is underlain by a 24 30 km thick ulvz layer having reductions of 3 3 7 4 with respect to prem no corresponding velocity reductions are found under the cocos plate the structure above the ulvz may be associated with reverse transformation of post perovskite to perovskite in a steep thermal gradient in a boundary layer above the cmb

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt