the grain size gap and abrupt gravel sand transitions in rivers due to suspension fallout median grain sizes on riverbeds range from boulders in uplands to silt in lowlands however rivers with 1 5 mm diameter bed sediment are rare this grain size gap also marks an abrupt transition between gravel and sand bedded reaches that is unlike any other part of the fluvial network abrupt gravel sand transitions have been attributed to rapid breakdown or rapid transport of fine gravel or a bimodal sediment supply but supporting evidence is lacking here we demonstrate that rivers dramatically lose the ability to transport sand as wash load where bed shear velocity drops below 0 1 m s forcing an abrupt transition in bed material grain size using thresholds for wash load and initial motion we show that the gap emerges only for median bed material grain sizes of 1 5 mm due to reynolds number dependencies in suspension transport the grain size gap therefore is sensitive to material properties and gravity with coarser gaps predicted on mars and titan

effects of large historical earthquakes viscous relaxation and tectonic loading on the 2008 wenchuan earthquake 1 the 2008 wenchuan earthquake m7 9 occurred on the longmenshan fault in sichuan china both the seismicity and slip rate of the fault are lower than those of other faults in the region by incorporating large earthquakes in the region during the past 222 years into our investigation the effects of historical earthquakes viscous relaxation and tectonic loading on the wenchuan earthquake were studied using a quasi static finite element model in this model we first simulated an initial stress field constrained by fault slip rates obtained by gps and geologic observations in the region we then simulated earthquakes by fault softening the results show that tectonic loading has the most significant effect on the longmenshan fault whereas viscous relaxation of the lower crust and upper mantle has the least effect the effects of the historical earthquakes are variable some promote the earthquake potential and others retard the earthquake potential

the 1999 mw 7 1 hector mine california earthquake sequence complex conjugate strike slip faulting the 1999 m w 7 1 hector mine mainshock showed right lateral strike slip faulting with an initial strike of n6 w and vertical dip the mainshock was preceded within 20 hours by 18 recorded foreshocks of 1 5 m 3 8 within a few kilometers distance of the mainshock hypocenter the aftershocks delineate how the hector mine earthquake ruptured with strike n6 w to the south for a distance of 15 km and possibly to the north for a distance of several kilometers the two largest aftershocks of m 5 9 and m 5 7 occurred near the north and south ends of the first mainshock rupture segment the second segment of rupture starting 15 km to the south away from the mainshock hypocenter delineated by strike slip and thrust faulting aftershocks extends 10 km farther away with a strike of s140 e along the bullion fault the aftershocks also outline an unusual third rupture segment extending from about 5 km south of the hypocenter with a strike of n30 w to n35 w for a distance of 20 km approximately 10 to 25 km farther to the north and west of the mainshock epicenter several clusters form a complex aftershock distribution three dimensional vp and vp vs models of the region exhibit only small regional changes as is typical for the mojave region nonetheless the mainshock rupture started within a region of rapidly varying vp and at least three regions of low vp vs are imaged within the aftershock zone the rate of decay for the hector mine earthquake sequence has been slightly above the mean for both p values and b values in southern california the focal mechanisms of the aftershocks and the state of stress are consistent with strike slip faulting including a component of normal faulting most prominent to the north the orientation of the regional maximum horizontal stress the variation in orientation of the mainshock fault segments by 30 and scattered distribution of aftershocks suggest that the mainshock and aftershock deformation field exhibit volumetric shear deformation accommodated by complex conjugate sets of strike slip faults

δ objects as a gauge for stress sensitivity of strain rate in mylonites abstract our understanding of the flow properties of deforming rocks in the earth s lithosphere is mainly based on theoretical work and on the extrapolation of high strain rate experimental data to the low strain rates of rock deformation in nature the geometry of structures in naturally deformed rocks can be an additional source of information on the rheology of the lithosphere flow experiments show that the geometry of a mantle of recrystallised material around a rigid object can be used to distinguish between a linear or power law relation of differential stress and strain rate in strongly deformed rocks such as mylonites this means that it is possible to use geometrical patterns in deformed rocks to constrain aspects of the rheology and to check the applicability of theoretical models and the extrapolation of laboratory data on creep in rocks to nature

predicting boundary shear stress and sediment transport over bed forms to estimate bed load sediment transport rates in flows over bed forms such as ripples and dunes spatially averaged velocity profiles are frequently used to predict mean boundary shear stress however such averaging obscures the complex nonlinear interaction of wake decay boundary layer development and topographically induced acceleration downstream of flow separation and often leads to inaccurate estimates of boundary stress particularly skin friction which is critically important in predicting skin friction over 2d bed forms the approach is based on combining the equations describing the mechanics of the internal boundary layer with semi empirical structure functions to predict the velocity at the crest of a bed form where the flow is most similar to a uniform boundary layer significantly the methodology is directed toward making specific predictions only at the bed form crest and as a result it avoids the difficulty and questionable validity of spatial averaging the model provides an accurate estimate of the skin friction at the crest where transport rates are highest simple geometric constraints can be used to derive the mean transport rates as long as bedload is dominant

sediment supply limited bedforms in sand gravel bed rivers the stability of bedforms in mixtures of gravel and sand is not well understood two bedform types are characteristic flow parallel sand ribbons and flow transverse barchans flume experi ments and field data presented here show that gradual transitions exist from sand ribbons to barchans and from barchans to fully de veloped dunes barchans and sand ribbons occur when not enough transportable sediment is available for the formation of fully devel oped ripples or dunes the reason is that a part of the bed sediment is immobile e g with an armor layer which limits the sediment sup ply and thus the volume of sediment available for the formation of bedforms bedform stability diagrams are shown to be extendable to sedi ment supply limited bedforms in sand gravel sediment if the par ticle parameters of the diagrams are derived from the transported sediment instead of the bed sediment barchans and forms transi tional to fully developed dunes plot in the dune stability fields sand ribbons on the other hand plot in the ripple lower plane bed and dune fields in the case of sediment supply limitation bedforms are partly or completely related to the characteristics of the sediment supply from upstream the sediment underlying the bedforms may be a stable armor and the exchange of sediment between this armor and the bedforms may be small or non existent consequently bedform char acteristics in sand gravel mixtures in supply limited conditions often are not predictable from the local hydraulics and sediment charac teristics

velocity distribution and bed roughness in high gradient streams a model for the velocity field over a poorly sorted bed is derived herein and applied to flow in steep streams with coarse gravel beds in such streams large clasts often are comparable in size to stream depth and act as obstacles to the flow the effect of these obstacles on a flow is estimated by partitioning total stress proportional to the depth slope product into a purely fluid component and a form drag component associated with flow around the obstacles an eddy viscosity closure is used with a length scale dependent on both distance from the boundary and wake dimension resulting velocity profiles agree well with profiles measured by marchand et al 1984 in nine mountain streams in colorado model runs for a range of conditions reveal that the velocity distribution depends primarily on d84z and flow depth h d84z is the length of the vertically oriented axis of clasts at the 84th percentile of the grain size distribution calculated values of mean velocity normalized by shear velocity are well represented by a simple log linear relationship in terms of relative roughness d84 h

the morphodynamics of fluvial sand dunes in the river rhine near mainz germany i sedimentology and morphology the dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply limited reach of the river rhine germany bed sediments dune geometry bedform migration rates and the internal structure of dunes are considered in this paper hydrodynamic and sediment transport data are considered in a companion paper the pebbles and cobbles d50 of 10 mm of the flat lag layer are rarely entrained dunes consist of well sorted medium to coarse sand d50 of 0 9 mm small pebbles move over the dunes by overpassing but there is a degree of size and shape selectivity populations of ripples in sand d50 0 6 mm and small and large dunes are separated by distinct breaks in the bedform length data in the regions of 0 7 1 m and 5 10 m ripples and small dunes may have sinuous crestlines but primarily exhibit two dimensional planforms in contrast large dunes are primarily three dimensional barchanoid forms ripples on the backs of small dunes rarely develop to maximum steepness small dunes may achieve an equilibrium geometry either on the gravel bed or as secondary dunes within the boundary layer on the stoss side of large dunes secondary dunes frequently develop a humpback profile as they migrate across the upper stoss slope of large dunes diminishing in height but increasing in length as they traverse the crestal region however secondary dunes more than 5 m in length are rare the dearth of equilibrium ripples and long secondary dunes is probably related to the limited excursion length available for bedform development on the parent bedforms large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry a depth limitation rather than a sediment supply limitation is the primary control on dune height dunes rarely exceed 1 m high in water depths of 4 m dune celerity increases as a function of the mean flow velocity squared but this general relationship obscures more subtle morphodynamics during rising river stage dunes tend to grow in height owing to crestal accumulation which slows downstream progression and steepens the dune form during steady or falling stage an extended crestal platform develops in association with a rapid downstream migration of the lee side and a reduction in dune height these diminishing dunes actually increase in unit volume by a process of increased leeside accumulation fed by secondary dunes moving past a stalled stoss toe a six stage model of dune growth and diminution is proposed to explain variations in observed morphology the model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload dominated dunes can result in dunes characterized by gentle lee sides with weak flow separation this finding is significant as other studies of dunes in large rivers have attributed this morphological response to a predominance of suspended load transport

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

equilibrium entrainment of fine sediment over a coarse immobile bed 1 the transported load in most fluvial systems including gravel bedded rivers includes fine grained sediment models for suspended sediment transport have focused on sand covered beds rendering incomplete the theoretical and empirical framework for predicting fine sediment transport and routing we conducted laboratory experiments involving sand transport over large immobile grains the experiments were scaled such that immobile particles were much larger than the mobile sediment but less than 10 of flow depth and that bed shear stresses scaled by the size of the mobile sediment were indicative of transport in suspension the experiments were conducted in equilibrium transport and included measurements of near bed sediment concentration and interstitial sand storage for a range of flow and transport rates partial filling of grain interstices occurred over a narrow range of flow and transport rates indicating a sharp threshold between no interstitial sand storage and a sand covered bed less sand coverage on the bed resulted in higher near bed sand concentrations per unit area of sand than runs with greater sand coverage as sand bed elevation decreased relative to the coarse grains turbulent wakes shed by the large grains appeared to enhance grain entrainment more than the corresponding decrease in bed area covered by sand resulted in decreased entrainment elevated concentrations were maintained until the bed was depleted of fine sediment these results are formalized in a proposed sand elevation correction function that scales the entrainment rate for a bed partially covered by sand to the entrainment rate that would be predicted for a sand covered bed

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

development of deflation lag surfaces a series of wind tunnel tests were carried out to investigate the development of deflation lags in relation to the non erodible roughness element concentration glass spheres 18 mm in diameter were placed along the complete length of the wind tunnel working section in regular staggered arrays using three different spacings d 18 30 and 60 mm and completely covered with a 0 27 mm erodible sand a pre selected free stream velocity above threshold 8m s 1 was established above the surface and the sediment transport measured at 2 s intervals using a wedge shaped trap in which an electronic balance is incorporated throughout each test the emerging lag surface was periodically photographed from above at two locations upwind of the trap the photographs were electronically scanned and analysed to calculate the lag element coverage and location as well as mean height and frontal area for each time period test results indicate that lag development has a profound effect on both the sediment flux and wind profile characteristics initially there is an increase in sediment flux above that for a rippled sand bed because of increased erosion around and reduced kinetic energy loss in highly elastic collisions with the emerging roughness elements with further emergence a dynamic threshold is reached whereupon the sediment flux decreases rapidly tending towards zero at this point the supply of grains to the air stream through fluid drag follows a reduction in aerodynamic roughness and therefore surface shearing stress at least as important is the lesser potential for grain ejection at impact because of reduced momentum imparted from the air stream during saltation although recent shear stress partitioning models indicate when particle movement may commence on varying surfaces our experimental results demonstrate that this partitioning has a further direct bearing upon the saltation flux ratio

quantifying the relative importance of flow regulation and grain size regulation of suspended sediment transport α and tracking changes in grain size of bed sediment β to predict changes in sediment transport it is essential to know whether transport is regulated mainly by changes in flow or by changes in grain size of sediment on the bed in flows where changes in suspended sediment transport are regulated purely by changes in flow grain size of bed sediment is constant increases in flow strength cause increases in both concentration and grain size of sediment in suspension because stronger flows are able to suspend more sediment and coarser grains under this constraint of constant grain size of bed sediment concentration and median diameter of suspended sediment are positively correlated in contrast where transport is regulated purely by changes in grain size of sediment on the bed concentration and median diameter of suspended sediment are negatively correlated because increasing the median diameter of the bed sediment causes the concentration to decrease while causing the median grain size in suspension to increase where both flow strength and grain size on the bed are free to vary the relation between concentration and grain size in suspension can be used to quantify the importance of grain size regulation relative to flow regulation of sediment transport a measure defined as to predict sediment transport in systems that are regulated dominantly by changes in grain size on the bed it is more useful to measure sediment input events or changes in grain size on the bed than to measure changes in flow more commonly grain size of bed sediment may be secondary to flow in regulating transport but may nevertheless be important the relative coarseness of bed sediment can be measured directly or like can be calculated from measurements of concentration and grain size of suspended sediment

bed material transport estimate in large gravel bed rivers using the virtual velocity approach this paper reports on a first attempt of using the virtual velocity approach to assess sediment mobility and transport in two wide and complex gravel bed rivers of northern italy displacement length and virtual velocity of spray painted tracers were measured in the field also the thickness of the sediment active layer during floods was measured using scour chains and post flood morphological changes as documented by repeated survey of channel cross sections the effects of eight and seven floods were studied on the tagliamento and brenta rivers where 259 and 277 spray painted areas were surveyed respectively in the tagliamento river 36 of the spray painted areas experienced partial transport whereas in the brenta river this accounted for 20 whereas full removal gravel deposition was observed on 37 and 26 of these areas on the tagliamento and brenta rivers respectively the mean displacement length of particles the thickness of the active layer and the extent of partial transport are well correlated with the dimensionless shear stress the virtual velocity approach allowed calculation of bed material transport over a wide range of flood magnitudes annual coarse sediment transport was calculated up to 150 for the tagliamento and 30 103 m3 yr 1 for the brenta the outcomes of this work highlight the relevance of partial transport condition as it could represent more than 70 of the total bed material transported during low magnitude floods and up to 40 for near bankfull events results confirm that bed material load tends to be overestimated by traditional formulas copyright 2016 john wiley sons ltd

mesoscopic structure of the punchbowl fault southern california and the geologic and geophysical structure of active strike slip faults we examine the distribution density and orientation of outcrop scale structures related to the punchbowl fault an exhumed ancient trace of the san andreas fault southern california in order to determine the structure of the fault zone the punchbowl fault has 44 km of right lateral slip and cuts the cretaceous pelona schist in the study area the mesoscopic structures examined include fractures small faults and veins they were inventoried using scan lines at closely spaced stations along three strike perpendicular traverses 200 250 m long across the fault the fault zone thickness is a function of the type of structure measured slip along narrow 2 m wide ultracataclasite cores of the faults results in foliation reorientation over a distance of 50 m from the cores fracture and fault densities appear to increase 50 80 m from the fault cores and vein densities are highly variable across the fault zone fractures and faults in the damaged zone have a variety of orientations but most are at high angles to the main fault zone when coupled with previous geochemical and microstructural data these data show that large displacement faults of the san andreas system are up to 200 250 m thick and enclose zones of mineralogic and geochemical alteration that are 20 30 m thick extreme slip localization occurs over zones 1 5 m thick when reconciled with geophysical imaging our data suggest that trapped headwaves travel in the damaged zone and that some aftershock events produce slip on faults and fractures which often have orientations very diaerent from the principal slip surfaces 7 2000 elsevier science ltd all rights reserved

when faults communicate viscoelastic coupling and earthquake clustering in a simple two fault system 1 3 d finite element models of a simplified northern and southern san andreas type fault system are presented with the goal of better understanding how great earthquakes m 7 5 on one major segment of a fault can affect the earthquake cycle on another colinear fault segment separated from the first by an asesimically creeping segment we find that the earthquake cycles of the two seismogenic fault segments become coupled as the lower crustal viscosity and or the fault separation distance are decreased further models with a 10 30 difference in relative fault breaking strengths exhibit a bi modal distribution of repeat times for each fault resulting in earthquakes that appear clustered in time

effects of sand addition on turbulent flow over an immobile gravel bed 1 the factors controlling the complex interaction of a coarse streambed with flow and sediment are difficult to measure however planning for reservoir flushing or dam removal requires knowledge of these interactions in both cases impounded sediments are introduced to channel beds that have had fine sediment particles removed without replacement the channel bed pore space interacts with the flow and provides storage for particles in order to address the need for information on such systems an adjustable slope recirculating laboratory flume was used to study the changes in flow and turbulence caused by sand added to an immobile gravel bed detailed measurements were made using an acoustic doppler velocimeter that collected three velocity components at a rate of 200 hz because of the rough nature of the bed individual velocity profiles varied significantly therefore in order to determine general trends the data were spatially averaged over six 10 20 cm planes parallel to the bed with the lowest plane about 2 cm below the maximum gravel elevation the increasing elevation of sand relative to the gravel layer resulted in decreased bed shear stress decreased reynolds stress increased relative turbulence intensity and a near bed shift toward sweep dominated turbulence

sand transport over an immobile gravel substrate abstractexperiments were conducted in a laboratory flume channel to evaluate the effects of increasing amounts of sand on its transport over and through an immobile coarse gravel bed detailed measurements of sand transport rate bed texture and bed topography were collected for four different discharges at approximately the same flow depth of 0 2 m for 11 different elevations of sand in the gravel bed sand transport was measured using both physical samples and a density cell for a given flow rate increases in the elevation of sand relative to gravel resulted in decreases of bed shear stress from 32 44 and increases in sand transport by three orders of magnitude for the highest two discharges the sand merged into a small number of long and low bed forms that translated through and over the gravel bed a collapse of the transport data was accomplished by relating the sand transport rate to the bed shear stress scaled by the cumulative probability distribution function of the gravel surface evaluated

viscosity of oceanic asthenosphere inferred from remote triggering of earthquakes a sequence of large interplate earthquakes from 1952 to 1965 along the aleutian arc and kurile kamchatka trench released accumulated stresses along nearly the entire northern portion of the pacific plate boundary the postseismic stress evolution across the northern pacific and arctic basins calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10 17 pascal seconds is consistent with triggering of oceanic intraplate earthquakes temporal patterns in seismicity at remote plate boundaries and space based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30 year period after the sequence

stress near geometrically complex strike slip faults application to the san andreas fault at cajon pass southern california slip on an undulatory strike slip fault induces predictable residual stresses in the adjacent crust elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries across fault parallel planes domains of reverse normal right lateral and left lateral residual stresses are induced near the bends in the fault these residual stresses increase in magnitude from one slip event to the next typically reaching failure level after several events a two dimensional analytic elastic plate model for slip on a sinusoidal fault provides general results on the scales and patterns of slip induced stresses and fourier synthesis allows for the solution with arbitrary but small fault distortions maximum fault normal residual stress is proportional to the square of the wavenumber of the sinusoidal trace and decays exponentially away from the fault reduced to 2 e times the maximum value at a distance equal to 1 wavenumber this fault parallel residual shear stress oscillates between dextral and sinistral along the fault with maximum magnitude adjacent to the maximum excursions in the fault trace fault parallel residual shear stress is maximum at a distance from the fault of 1 wavenumber where its magnitude is 1 e times the maximum normal stress on the fault two and three dimensional finite element analyses extend the analytic model they account for depth decaying fault perturbation amplitude and slip deficit and are valid for large fault undulations application of the model to the san andreas fault in the cajon pass region produces the complex distribution of fault parallel normal reverse right lateral and left lateral structures recognized near the main trace left lateral stresses on planes subparallel to the san andreas fault at the cajon pass well are predicted by this model

a statistical space time magnitude analysis on the aftershocks occurrence of the july 21th 2017 mw 6 5 bodrum kos turkey earthquake abstract in this study a detailed space time magnitude assessment on the aftershock sequence of july 21th 2017 mw 6 5 bodrum kos earthquake turkey is carried out by focusing on the analyses of aftershock parameters b value p value dc value and mamax b value is estimated as 0 90 0 05 with a completeness magnitude mc 1 6 and this relatively small b value may be resulted from the abundance of aftershocks with magnitude ml 4 0 however it is well represented by the gutenberg richter law with a typically b 1 0 p value is estimated as 1 04 0 02 with a c value 0 224 0 039 and is well characterized nearly close to the global p 1 0 this relatively high p value may be a result of the relative fast decay rate of the aftershock activity dc value is calculated as 1 74 0 09 and it means that aftershocks are homogeneously distributed temporal distribution of b value shows that small b values may be due to a stepwise increase in effective stress before the occurrence of larger aftershocks regional changes range from 0 5 to 1 2 in b value from 0 4 to 1 3 in p value and from 3 0 to 5 2 in mamax the smallest b values are found in and around mainshock including karaada bodrum akyarlar and turgutreis and these regions have high stress as well as coseismic deformation the largest p values are found in and around the mainshock epicenter including karaada and it is interpreted that this situation may be caused by coseismic deformation mamax values larger than 4 4 are observed in and around mainshock epicenter including bodrum and akyarlar and there is a clear correlation between spatial variations of b values and mamax these results show that stress changes and coseismic deformation seem highly effective on b and p value variations therefore a special interest needs to be given to these anomaly regions since aftershock hazard is highly associated with these parameters and may be developed considering their space time magnitude distribution

investigating the role of clasts on the movement of sand in gravel bed rivers the bed morphology of mountain rivers is characterized primarily by the presence of distinguishable isolated roughness elements such boulders or clasts the objective of this experimental study was to provide a unique insight into the role of an array of clasts in regulating sand movement over gravel beds for low relative submergence conditions h dc 1 and flow depth h to the diameter of the clast dc a process that has not been studied thoroughly to assess the role of clasts in controlling incoming sand movement detailed flume experiments were conducted by placing 40 equally spaced clasts atop a well packed glass bead bed for replicating the isolated roughness flow regime the experiments were performed for moderate 2 50 cr where cr is the critical dimensionless bed shear stress and high 5 50 cr applied bed shear stress conditions representative of gravel bed rivers for comparison purposes experiments were also repeated for nearly identical flow conditions but without the presence of

sediment entrainment and depletion from patches of fine material in a gravel bed river characteristics at shear stresses ranging from 5 to 60 n m 2 experiments demonstrate that patches of fine sediment control both the intensity and duration of bed load under hydraulic conditions characteristic of the early stages of floods the experiments allow quantifying bed load at the entrainment threshold providing the first empirical evidence that marginal bed load transport can be attributed to the mobilization of sediments from patches bed load transport was recorded consistently once shear stress exceeded 5n m 2 the experiments produced low bed load rates 6 g sm depletion of material in the patches occurred rapidly with bed load rates and particle sizes decreasing after only 5 minutes combining flume and helley smith data for the study reach a breakpoint in the relation between shear stress and bed load rate was calculated to be around 30 n m 2 this represents the transition between bed load transport phases below the breakpoint transport occurs at a low rate and is composed predominantly of fine sediment from patches but above it much higher rates occur from across the reach as a whole hydraulic conditions at the threshold are those which occur during small frequent floods 25 bankfull flow equaled or exceeded 15 of time this indicates that sediment entrainment from patches of fine material is a frequent process and the threshold change between bed load phases occurs regularly

calculating bed load transport in steep boulder bed channels 1 steep rough channels occupy a large fraction of the total channel length in mountainous regions most sediment mobilized on hillslopes must pass through these streams before reaching lower gradient channels steep channels have wide grain size distributions that are composed of finer more mobile sediment and large rarely mobile grains the large grains can bear a significant portion of the total shear stress and thereby reduce the stress available to move the finer sediment conventional bed load transport equations often overpredict the sediment flux in steep channels by several orders of magnitude we hypothesize that sediment transport equations overpredict the sediment flux because they do not 1 account for the stress borne by rarely mobile grains 2 differentiate between highly and rarely mobile sediment and 3 account for the limited availability of mobile sediment here we modify a conventional bed load transport equation to include these three effects we use measurements of the flow bed properties and sediment flux in a small steep flume to test this equation we supply gravel at a constant rate through fields of regularly spaced immobile spheres and measure the bed coverage by gravel and sphere protrusion the percent of the sphere that protrudes above the gravel deposit for a given sphere spacing the proportion of the bed covered by gravel increases and the sphere protrusion decreases with greater sediment supply thus bed coverage and immobile grain protrusion may serve as proxies for sediment availability in steep rough streams unlike most transport equations that we tested our modified bed load equation predicts sediment fluxes to within an order of magnitude of the measured values our results demonstrate that accurately predicting bed load transport in steep rough streams may require accounting for the effects of local sediment availability coverage by mobile sediment and drag due to rarely mobile particles

stress and strain estimates for newtonian and non newtonian materials in a rotational shear zone abstract stress and shear strain rate in a rotational shear zone between two concentric circular boundaries are estimated for both newtonian and non newtonian materials tangential shear stress in the shear zone r is proportional to 1 r2 where r is distance from the centre of rotation the principal stresses 1 and 3 at a point are oriented at 45 to the radius through the point from the centre of rotation the shear strain rate is proportional to 1 r2n where n is the stress exponent in the constitutive equation the distribution of strain ellipses systematically varies from place to place as a function of the total angle of rotation of the circular boundaries and of n deformation appears prominently concentrated in a narrow zone close to the interior boundary when n becomes large

postseismic stress diffusion in chile and south peru abstract by means of a spherical model of global postseismic deformation we perform an analysis of the postseismic stress diffusion process associated with the main seismic events which occurred in chile and south peru in the last century our aim is to ascertain if the viscoelastic relaxation of the mantle and asthenosphere following the earthquakes superimposed on the main mechanism of stress accumulation in the lithosphere that is to say the subduction process has in some way influenced the spatiotemporal pattern of seismic energy release in this region our investigations focus particularly on the areas where two important seismic gaps in an advanced mature state can be recognized we found that the evolution of the postseismic stress might give a satisfactory explanation for the anomalous duration of the seismic quiescence in the gap areas and also for the possible filling of one of these gaps

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

mixing in flow perturbations a model for development of mantled porphyroclasts in mylonites abstract the presence of a rigid object such as a porphyroclast in simple shear flow induces a flow perturbation with a relatively simple geometry domains of closed and open flow lines are bounded by a separatrix that surrounds the porphyroclast the concept of a separatrix to describe heterogeneous flow gives rise to some new insights in the development of porphyroclasts with a mantle of recrystallized material in mylonites the final shape of the recrystallized mantle depends on separatrix shape but also on the degree of intersection of the mantle and the separatrix and therefore on relative mantle width this model can explain the coexistence of several types of mantled porphyroclast in a single mylonite sample the occurrence of porphyroclasts that lack wings at high strain values and the presence of symmetric winged mantled porphyroclasts in ductile shear zones

estimating bottom stress in tidal boundary layer from acoustic doppler velocimeter data bed stresses in the bottom boundary layer of the york river estuary virginia were estimated from three dimensional near bottom velocities measured by acoustic doppler velocimeters advs and also by a profiling array of electromagnetic current meters by assuming the measurements were made in a constant stress layer four methods of stress estimation were evaluated using advs direct covariance cov measurement turbulent kinetic energy inertial dissipation utilizing the kolmogorov spectrum and log profile the four methods yielded similar estimates of frictional velocity u based on adv output from both 14 cm and 44 cm above bed all eight estimates of average u were consistent with the overall mean of 1 10 cm s to within the 95 confidence interval for individual burst estimates the cov method worked slightly better nearer the bed possibly because of the sensitivity of cov to the upper limit of the constant stress layer the inertial dissipation method performed marginally well at 14 cm above bed likely because of sediment induced stratification and insufficient separation of turbulent production and dissipation scales the log profile method was the most variable and appeared most sensitive to stratification and to the thickness of the constant stress layer the turbulent kinetic energy method was the most consistent at both heights and appears most promising for further development results encourage future use of the adv in estuarine environments but also favor the simultaneous use of several methods to estimate bottom stress

discrete particle modeling of entrainment from flat uniformly sized sediment beds the stability of randomly deposited sediment beds is examined using a discrete particle model in which individual grains are represented by spheres the results indicate that the threshold shear stress for flat beds consisting of cohesionless uniformly sized grains cannot be adequately described by a single valued parameter rather it is best represented by a distribution of values physically this result stems from the localized heterogeneity in the arrangement of surface grains for uniformly sized beds geometric similarity exists such that the critical entrainment shear stress distributions scale directly with grain size a shields parameter of 0 06 is commonly used to define threshold conditions and it was found that this corresponds to a point on the distributions where approximately 1 4 by weight of the surface is mobile furthermore the analysis includes a comparison of the contributions of sheltering to variation in critical entrainment shear stress it was found that remote sheltering ind

mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock aftershock interaction the diverse aftershock sequence of the 1989 loma prieta earthquake is inconsistent with conventional models of mainshock aftershock interaction because the aftershocks do not accommodate mainshock induced stress changes instead the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane this orientation implies that i stress drop in the mainshock was nearly complete ii mainshock induced decreases of fault strength helped were important in controlling the occurrence of after shocks and iii mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip

glacial erosion evolution of river long profiles and the organization of process domains in mountain drainage basins of coastal british columbia 1 in glaciated british columbia canada quaternary climate changes are responsible for profound spatial reorganization of earth surface processes these changes have left a landscape characterized by topographic anisotropy associated with a hierarchy of glacial troughs the evolution of glaciated landscapes is examined by analyzing the structure of geomorphic process domains and channel long profiles to identify process domains we use channel surveys and gis analysis to construct slope area transects of the channel network this analysis reveals generalized process form disequilibrium with a mismatch between topographic signatures and currently active geomorphic process domains at the landscape scale of source colluvial channels contributing area 1 km2 the glacial paraglacial signature commonly overrides that produced by contemporary debris flows along the axis of former ice flows relict glacial cirques introduce a hanging fluvial domain at contributing areas as small as 8 10 2 km2 and produce complex channel long profiles similar to those observed for rivers responding to tectonic forcing slope area relations typical of unglaciated equilibrium environments do not apply here the concept of process domains appears to hold however some major glacially forced modifications in the alluvial colluvial transition are observed and the definition of a depositional colluvial subdomain is proposed comparison between field and gis measured slopes reveals that gis associated error is not uniform between process domains and that gis based plots do not successfully discriminate field based process domains the combination of glacial and post glacial fingerprints and the effects of ongoing earth surface processes generate a complex landscape whose glacial signatures may persist until the onset of the next ice age

aftershocks due to property variations in the fault zone a mechanical model abstract studies of many large strike slip earthquakes have shown that the seismicity rate increases in quadrants in which the coulomb stress is increased by the earthquake rupture this mechanism explains the concentration of aftershocks near the tips of the fault rupture but cannot explain the aftershocks on the sides of the rupture zone where the aftershocks are commonly abundant we suggest that these aftershocks may be induced by variations of fault zone properties not included in the previous models in a finite element model that treats the rupture plane as a zone with heterogeneous physical properties as suggested by field observations and laboratory experiments the coulomb stress changes triggered by the mainshock no longer uniformly decrease on the sides of the rupture zone but show complex patterns of negative and positive changes these results may explain some of the aftershocks on the sides of the mainshock rupture

transport slopes sediment cover and bedrock channel incision in the henry mountains utah 1 field data from channels in the henry mountains of utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds the cover effect we compare several small channels that share tributary junctions and have incised into the same sedimentary bedrock unit navajo sandstone but contain differing amounts of coarse diorite clasts owing to the spatial distribution of localized sediment sources bedrock channels that contain abundant clasts diorite rich have steeper longitudinal slopes than tributaries of these channels with smaller drainage areas and less sediment diorite poor the diorite poor tributaries have incised more deeply to lower average slopes and have more reach scale slope variability which may reflect bedrock properties longitudinal sediment sorting and incision at lower sediment supply diorite rich channels have less bedrock exposed and smoother longitudinal profiles than diorite poor channels we find that 1 coarse sediment can mantle bedrock channel beds and reduce the efficiency of incision validating the hypothesized cover effect in fluvial incision models 2 the channel slope needed to transport the sediment load can be larger than that needed to erode bedrock suggesting that the slope of incising bedrock channels can become adjusted to the sediment load 3 when abundant sediment is available transport capacity rather than thresholds of motion can be dominant in setting bedrock channel slope and 4 cover effects can be important even when moderate amounts of bedrock are exposed in channel beds

aftershocks properties of the 2013 shonbe m w 6 3 earthquake central zagros iran abstract the spatial and temporal seismicity parameters stress state as well as the partitioning of the seismic energy of the shonbe earthquake sequence have been investigated the geodetic data and the stress tensor inversions of the focal mechanisms before and after the shonbe main shock indicate the same ne sw trend of the greatest principal stress this stress orientation demonstrates a non heterogeneous stress state in that region of the central zagros which can be confirmed by the fractal dimension and the estimated p value the aftershock focal mechanisms show a diversity which may be associated with the approximately uniaxial stress field acting nearly perpendicular to the main shock fault plane in which the co seismic stress release is nearly complete the spatial distributions of the b and p values have been investigated and compared the high p value regions e g far to the southeast south and southwest of the main shock are approximately consistent with the high b value regions and the low p value regions e g the limited areas at the north northeast of the main shock are nearly consistent with the low b value regions the limited areas surrounding the main shock and also at the east southeast of the main shock the p values are not in agreement with the b values which may be attributed to the local tectonic features in those regions the seismic moment ratio and the bath s law have been used to constrain the energy partitioning between the main shock and the aftershocks it is found that a high fraction of the energy is released by the shonbe main shock and nearly one third of the energy released by the aftershocks this may be associated with the high stress imparted by the blind thrust main shock to the overlying crust in which promotes failure in the surrounding area and is often relieved by secondary faults and typically produce a high number of aftershocks

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

the impact of coarse grain protrusion on near bed hydrodynamics in steep rivers sediment is often transported over immobile cobbles and boulders previous studies of such conditions have observed that the entrainment rate of the mobile sediment strongly depends on the level of protrusion of the immobile grains here experiments are conducted in a laboratory flume in order to quantify how different levels of protrusion of large aggregates above a fixed fine sediment bed modeled as a patch of hemispheres modify the local hydrodynamics near the fine sediment bed five protrusion levels defined by p k r 0 20 40 60 80 were investigated where k is the protrusion height and r the radius for small protrusion p 20 enhanced shear stress and turbulence intensity on the fine sediment bed is observed as the mixing layer generated at the hemisphere top impacts the fine sediment bed moreover sweep events generated near the top of the hemispheres reach the fine sediment bed for large protrusions p 60 the mixing layer generated near the top of the hemispheres does not reach the fine grains with the consequence that the shear stress drops the remaining turbulence near the fine sediment bed although enhanced by the wakes generated by the hemispherical caps is quasi isotropic the transition between these two distinct near bed flow regimes is found to be around p 40 corresponding to the protrusion levels observed by grams and wilcock 2014 https doi org 10 1002 2013jf002925 above which the erosion of fine sediment ceases

transport of fine sediment over a coarse immobile riverbed sediment transport in cobble boulder rivers consists mostly of fine sediment moving over a coarse immobile bed transport rate depends on several interrelated factors boundary shear stress the grain size and volume of fine sediment and the configuration of fine sediment into interstitial deposits and bed forms existing models do not incorporate all of these factors approaches that partition stress face a daunting challenge because most of the boundary shear is exerted on immobile grains we present an alternative approach that divides the bed into sand patches and interstitial deposits and is well constrained by two clear end member cases full sand cover and absence of sand entrainment from sand patches is a function of their aerial coverage entrainment from interstices among immobile grains is a function of sand elevation relative to the size of the immobile grains the bed sand coverage function is used to predict the ratio of the rate of entrainment from a partially covered bed to the rate of entrainment from a completely sand covered bed which is determined using a standard sand transport model we implement the bed sand coverage function in a morphodynamic routing model and test it against observations of sand bed elevation and suspended sand concentration for conditions of nonuniform fine sediment transport in a large flume with steady uniform flow over immobile hemispheres the results suggest that this approach may provide a simple and robust method for predicting the transport and migration of fine sediment through rivers with coarse immobile beds

can coseismic stress variability suppress seismicity shadows insights from a rate and state friction model 1 the paucity of detectable seismicity shadows in the days months following a main shock has raised the question as to whether dynamic rather than static triggering could be the main mechanism driving the seismicity at these timescales the lack of correlation between the stress unloading of the main fault and the typically vigourous aftershock production taking place on it however suggests that the spatial heterogeneity of the coseismic stress change could also be a factor in the apparent suppression of seismicity shadows at least on the main fault here we study whether this stress variability can indeed be an important aspect in the stress modeling of earthquake sequences a rate and state friction model of seismicity is used in conjunction with realistic levels of stress heterogeneity at the earthquake nucleation scale 1 10 m as predicted from these friction laws based on scale invariant models of coseismic slip distribution in this model a relatively weak on fault quiescence is delayed by months to years because of the high stress heterogeneity delayed quiescences due to slip heterogeneity are similarly predicted at distances of up to about half the rupture length away from the fault we also postulate that off fault seismicity can be significantly controlled by stress variability that originates from small scale crustal heterogeneity and the complexity in fault geometries rather than mapping stress changes by providing a single stress value at every grid point these results suggest that an estimate of the likely stress variability acting at the nucleation scale is also needed especially when trying to account for the seismicity occurring at relatively short timescales

experimental modelling of mantled porphyroclasts abstract mantled porphyroclasts are commonly used as sense of shear indicators in mylonites analogue experiments have been carried out to model the development of such objects rigid spheres mantled by a viscous plasticene putty mixture with power law behaviour were suspended in an optically transparent newtonian polymer matrix the samples were deformed between two cylinders one placed within the other and with coinciding axes this induces a non coaxial flow between the cylinders and allows the achievement of high finite strain the viscous mantle around the rigid sphere remained undeformed deformed into an ellipsoid or developed wings the final geometry depends mainly on the induced viscosity contrast between the mantle and the matrix which is a function of the flow induced stress on the surface of the mantle winged mantles only develop if the induced viscosity contrast between the mantle and the matrix polymer is sufficiently small if this critical induced viscosity contrast is exceeded the mantled sphere remains undeformed or obtains a stable ellipsoidal shape even at high finite strain shaped winged mantles only develop in simple shear flow if the mantle around a rigid sphere is thin

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

characterizing riverbed sediment using high frequency acoustics 2 scattering signatures of colorado river bed sediment in marble and grand canyons in this the second of a pair of papers on the statistical signatures of riverbed sediment in high frequency acoustic backscatter spatially explicit maps of the stochastic geometries length and amplitude scales of backscatter are related to patches of riverbed surfaces composed of known sediment types as determined by georeferenced underwater video observations statistics of backscatter magnitudes alone are found to be poor discriminators between sediment types however the variance of the power spectrum and the intercept and slope from a power law spectral form termed the spectral strength and exponent respectively successfully discriminate between sediment types a decision tree approach was able to classify spatially heterogeneous patches of homogeneous sands gravels and sand gravel mixtures and cobbles boulders with 95 88 and 91 accuracy respectively application to sites outside the calibration and surveys made at calibration sites at different times were plausible based on observations from underwater video analysis of decision trees built with different training data sets suggested that the spectral exponent was consistently the most important variable in the classification in the absence of theory concerning how spatially variable sediment surfaces scatter high frequency sound the primary advantage of this data driven approach to classify bed sediment over alternatives is that spectral methods have well understood properties and make no assumptions about the distributional form of the fluctuating component of backscatter over small spatial scales

the role of non uniform roughness in the formation of sand ribbons abstract sand ribbons typically are shallow water sedimentary features characterized as thin elongate bands of sand overlying a coarser lag sediment which is exposed between them the ribbons are oriented along the direction of the dominant current and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons a perturbation expansion coupled with flume experiments indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips the non uniform turbulence field arising from this roughness distribution drives a secondary helical flow the expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth 4 1 for unstratified channel flow conditions the secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress this serves to retard the movement of sand through such a system

an experimental study into the geometry of supply limited dunes the relationship between dune geometry and the volume of mobile sediment was studied in flume experiments in these flume experiments the volume of mobile sediment on top of an immobile coarse sediment layer was increased stepwise and the bedform characteristics were observed a strong relationship was found between the volume that is mobile and therefore available for bedform formation and the dune dimensions and regularity if the sediment supply is limited dunes are smaller and more regular series of experiments with a decreasing supply limitation were conducted for different flow velocities and water depths the relationship between the dune dimensions and the available volume is different for each series the observed relationships between dune dimensions and layer thickness collapse to one relationship for height and one for length if scaling parameters are introduced current models for bedform dimensions under alluvial conditions can be extended to partial transport conditions using this relationship

the morphodynamics of fluvial sand dunes in the river rhine near mainz germany ii hydrodynamics and sediment transport the dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply limited reach of the river rhine during daylong surveys suspended sediment concentration bedload transport rate water depth flow velocity turbulence intensity near bed shear stress and water temperature were recorded over individual isolated dunes this paper considers the hydrodynamic environment and sediment transport over the dunes a companion paper details the sedimentology and morphology of the dunes flow over the flat gravel lag upstream of large dunes is more uniform than that over dunes and gravels are rarely entrained by in bank discharges unsteady and non logarithmic velocity profiles are common within the boundary layer above the stoss side of large dunes and the near bed flow demonstrates evidence of large scale coherent low frequency flow structures these may reflect stacked sequences of separated boundary layers generated by secondary dunes however the low amplitude morphology of large dunes does not affect the statistical properties of turbulence production over the stoss sides bed roughness and near bed shear stress commonly increase steadily over the stoss of dunes but may decrease near the crest especially where a crestal platform exists that is devoid of secondary bedforms bed roughness scales with the physical size of bed roughness elements however variability in roughness lengths is large owing to the composite nature of bed roughness bedload transport over stoss slopes is spatially variable but often shows an increase with increasing bed shear stress over the stoss well formed wakes only develop downstream of lee slopes which are close to the angle of repose otherwise separation is weak and suspension and settling of fine sediments is of little consequence to dune evolution wake flow is characterized by turbulence production one order of magnitude greater than over the stoss side which may be related to vortex shedding from the dune however wake current speeds are extremely low and variable in direction reverse flow is not sustained and no retrogressive bedforms occur in the lee of large dunes near bed shear stress and bed roughness are usually low within the wake reflecting low current speeds weak wake flow reattachment occurs at a variable distance downstream up to several times the duneform height a two dimensional numerical model for flow over dune topography calibrated using average parameter values provided a reasonable description of the flow upstream and over the backs of large dunes as far as the crestal region wake flow could not be modelled however the temporal and spatial complexity of natural three dimensional flow over the dunes resulted in variance in parameter estimates this variance precluded modelling of flow and bedload transport consequently it was not possible to model dune evolution in a deterministic sense

prediction of sediment transport in step pool channels 1 in mountainous drainage networks sediment mobilized on hillslopes must first pass through steep streams before reaching lower gradient channels the bed of steep channels is typically composed of large relatively immobile boulders and finer more mobile gravel most sediment transport equations overpredict sediment flux in steep streams by several orders of magnitude because they do not account for the stress borne by immobile grains and the limited availability of the more mobile sediment we previously developed and tested in flume experiments a sediment transport equation that accounts for these two effects here we modify the parker 1990 bed load equation to include the resistance borne by steps and selective transport of the relatively mobile sediment using a range of hiding functions we test a number of resistance equations and hiding functions combined with our modified and the original parker equations against measured flow and sediment transport in three steep channels our modified sediment transport equation generally predicts the transported sediment volumes to within an order of magnitude of the measured values whereas the unmodified equations do not the most accurate sediment flux predictions were obtained from using our modified equation combined with a hiding function that calculates highly selective transport of the relatively mobile sediment furthermore this hiding function has a critical shields stress that is similar to those reported for lower gradient channels the effects of the immobile steps on flow and sediment transport are not adequately captured by simply increasing the critical shields stress to values reported in steep streams

study of the 1999 m 7 1 hector mine california earthquake fault plane by trapped waves we recorded fault zone trapped waves from aftershocks on portable seismometers in a tight linear array across the lavic lake fault which was one of several faults that ruptured in the m 7 1 hector mine california earthquake on 16 october 1999 trapped waves with large amplitudes and long duration at 4 to 7 hz produced by aftershocks occurring within the rupture zone were recorded at stations close to the fault trace however the s waves registered at stations farther from the rupture zone for the same events were much briefer trapped waves recorded at the hector mine rupture zone are similar to those observed in the landers rupture zone li et al 1994a b but show higher frequencies simulations of these trapped waves indicate a 75 to 100 m wide low velocity and low q waveguide along the hector mine rupture zone in which the s velocity is reduced by about 40 to 50 from wall rock velocities and q is 10 to 60 in the depth range from the surface to 10 km we interpret this low velocity waveguide as being a remnant of the process zone formed by inelastic deformation around the propagating crack tip during dynamic rupture in the 1999 hector mine earthquake the reductions of velocities and q within the hector mine rupture zone are similar to those within the landers rupture zone suggesting that the fault zone rock was damaged to the same degree in the two earthquakes the wave guide width 75 100 m on the hector mine rupture zone 40 km in the total length is half that 150 250 m of the landers rupture zone 80 km in the total length consistent with the scaling of process zone size to rupture length as predicted in some published dynamic rupture models locations of aftershocks for which we observed trapped waves show bifurcation of the northern hector mine rupture at depth although only the west rupture branch broke to the surface

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults