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

sub basin scale dust source geomorphology detected using modis 1 the spatial and temporal variability of dust emissions from different surfaces in the lake eyre basin australia is determined using modis data for 2003 6 the sources of 529 dust plumes were classified overall 37 of plumes originated in areas of aeolian deposits 30 from alluvial deposits and floodplains and 29 from ephemeral lakes or playas at this sub basin scale the relative importance of different dust source geomorphologies varied primarily in response to sediment supply and availability and was not related to aeolian transport capacity suggesting the lake eyre basin is a supply limited system

earthquake scaling laws for creeping and non creeping faults the dependence of the summed moment release on the fault dimension differs between non creeping faults and creeping faults or volcanic regions this can be attributed to different earthquake scaling laws the number n of events with fault length l scales for non creeping faults as n l l 2 and for creeping faults as n l l 3 this difference offers a means for mapping the distribution of creep in the seismogenic crust

sedimentary characteristics of the gravel sand transition in the river rhine abstract a gravel bed river often changes abruptly into a sand bed river the causes of such so called gravel sand transitions have been elucidated in recent years but detailed observations of gravel sand transitions in large rivers are scarce the present study fills this knowledge gap by investigating the sedimentary characteristics of the gravel sand transition in the river rhine data on bed composition and flow characteristics are used in combination with a novel method for the evaluation of the bed structure the analysis shows that in contrast to common belief the bed structure in the gravel sand transition zone does not change from fully clast supported to fully matrix supported instead there is a strong cross stream variation in bed structure especially upstream of the transition zone the analysis also shows that the downstream decrease in gravel mobility in the gravel sand transition zone which is usually attributed to the change in bed structure and the so called patchiness effect is at least partly caused by the larger scale bend sorting process which concentrates gravel in outer bends where it is only infrequently mobile dune sorting which has been suggested to have an effect on the gravel mobility similar to that of bend sorting was found to be relatively unimportant in the rhine partly because it is strongly counteracted by armoring processes that affect an uncommonly thick sediment layer the gravel sand transition in the rhine with its length of 50 km is much less abrupt than the gravel sand transitions of smaller rivers reported on in the literature although the dependence of the transition length on river size intuitively is correct the exact causes remain unclear

decoding downstream trends in stratigraphic grain size as a function of tectonic subsidence and sediment supply downstream grain size fining in stratigraphy is driven primarily by selective deposition of sediment and the long term efficiency of this process is determined by 1 the magnitude and characteristics of the input sediment supply 2 the spatial distribution of subsidence rate which creates accommodation for sediment preservation and 3 the dynamics of sediment transport and deposition a key challenge is to determine how these first two factors control the caliber and spatial distribution of deposits over time scales of 10 4 10 6 yr without incorporating sediment transport details that are largely unknowable for time averaged stratigraphy in the geological past we address this using grain size data collected from fluvial conglomerates in the eocene pobla basin spanish pyrenees a synorogenic basin where the timing of sediment deposition is well constrained the sediment budget is closed and good exposure enables time lines within stratigraphy to be picked out unambiguously for successive stratigraphic horizons downstream trends in grain size and composition are derived for basin filling sediment routing systems with length scales of 6 and 40 km respectively our data show that the rate of grain size fining varies over time and with system length and can be linked to changes in source area these results are contrasted with grain size data from the antist group a 60 km long oligocene system that mantles the southern pyrenees where very slow rates of grain size fining on the wedge top of this fold and thrust belt are observed we apply a self similarity based selective deposition model to quantify the competing controls of tectonic subsidence and sediment supply on derived grain size trends and model results are compared with independent constraints on the eocene oligocene evolution of the pyrenees our results suggest that it is now possible to invert time averaged grain size trends in stratigraphy to gain quantitative information on the geological boundary conditions governing the evolution of sedimentary basins

advance retreat and halt of abrupt gravel sand transitions in alluvial rivers downstream fining of bed sediment in alluvial rivers is usually gradual but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm i e a gravel sand transition gst or gravel front here we present an analytical model of gst migration that explicitly accounts for gravel and sand transport and deposition in the gravel reach sea level change subsidence and delta progradation the model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel wedge and predicts the circumstances required for the gravel front to advance retreat and halt predicted modern gst migration rates agree well with measured data at allt dubhaig and the fraser river and the model qualitatively captures the behavior of other documented gravel fronts the analysis shows that sea level change subsidence and delta progradation have a significant impact on the gst position in lowland rivers

experimental study of incipient motion in mixed size sediment transport rates of five sediments were measured in a laboratory flume three of these sediments had the same mean size the same size distribution shape and different values of grain size distribution standard deviation the critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate the sorting of the sediment mixture had little effect on the critical shear stress of individual fractions once the median size d50 of the mixture and a fraction s relative size di d50 are accounted for our data combined with previously published data show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction s relative grain size despite a broad variation in the available data of mixture sorting grain size distribution shape mean grain size and grain shape all fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions this result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment recirculating systems as well as for systems where the fractional transport rates are imposed on the system feed systems this equivalence in initial motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination

stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

systematic survey of high resolution b value imaging along californian faults inference on asperities understanding and forecasting earthquake occurrences is presumably linked to understanding the stress distribution in the earth s crust this cannot be measured instrumentally with useful coverage however the size distribution of earthquakes quantified by the gutenberg richter b value is possibly a proxy to differential stress conditions and could therewith act as a crude stress meter wherever seismicity is observed in this study we improve the methodology of b value imaging for application to a high resolution 3 d analysis of a complex fault network in particular we develop a distance dependent sampling algorithm and introduce a linearity measure to restrict our output to those regions where the magnitude distribution strictly follows a power law we assess the catalog completeness along the fault traces using the bayesian magnitude of completeness method and systematically image b values for 243 major fault segments in california we identify and report b value structures revisiting previously published features e g the parkfield asperity and documenting additional anomalies e g along the san andreas and northridge faults combining local b values with local earthquake productivity rates we derive probability maps for the annual potential of one or more m6 events as indicated by the microseismicity of the last three decades we present a physical concept of how different stressing conditions along a fault surface may lead to b value variation and explain nonlinear frequency magnitude distributions detailed spatial b value information and its physical interpretation can advance our understanding of earthquake occurrence and ideally lead to improved forecasting ability

mechanism of secondary currents in open channel flows 1 this paper describes the conditions for initiation and maintenance of secondary currents in open channel flows by analyzing the reynolds equation in the wall normal and wall tangent directions this study reveals that like other types of vortices the secondary currents are originated in the near boundary region and the magnitude or strength of secondary flow is proportional to the lateral gradient of near wall velocity the near wall secondary flow always moves from the region with lower velocity or lower boundary shear stress to the location with higher velocity or higher boundary shear stress subsequently the near boundary secondary flow creeps into the main flow and drives circulation within a region enclosed by lines of zero total shear stress leading to anisotropy of turbulence in the main flow region this paper also discusses typical secondary currents in open channel flows and presents the relationship between sediment transport and secondary currents the formation of sand ridges widely observed on the earth surface is explained in the light of the proposed relationship

long term monitoring of creep rate along the hayward fault and evidence for a lasting creep response to 1989 loma prieta earthquake we present results from over 30 yr of precise surveys of creep along the hayward fault along most of the fault spatial variability in long term creep rates is well determined by these data and can help constrain 3d models of the depth of the creeping zone however creep at the south end of the fault stopped completely for more than 6 years after the m7 1989 loma prieta earthquake lpeq perhaps delayed by stress drop imposed by this event with a decade of detailed data before lpeq and a decade after it we report that creep response to that event does indeed indicate the expected deficit in creep

sampling techniques for gravel sized sediments the methods commonly used for sampling the coarser surface layers of gravel bed streams are reviewed it is found that while an areal sample is biased toward the coarser grains compared with a volumetric sample of the same material the conversion formula suggested by a voidless cube model overcompensates for this effect a modified version of the cube model that accounts for porosity is proposed the modified cube model and tests conducted with areal sampling using wax indicate that area by weight analyses can be converted successfully to the equivalent bulk sieve analyses the average depth of the wax samples ranged from d65 d91 increasing with median grain size the equivalence of the grid by number and bulk sieve analyses is reaffirmed

distribution of aseismic slip rate on the hayward fault inferred from seismic and geodetic data 1 we solve for the slip rate distribution on the hayward fault by performing a least squares inversion of geodetic and seismic data sets our analysis focuses on the northern 60 km of the fault interferometric synthetic aperture radar insar data from 13 independent ers interferograms are stacked to obtain range change rates from 1992 to 2000 horizontal surface displacement rates at 141 bench marks are measured using gps from 1994 to 2003 surface creep observations and estimates of deep slip rates determined from characteristic repeating earthquake sequences are also incorporated in the inversion the fault is discretized into 283 triangular dislocation elements that approximate the nonplanar attributes of the fault surface south of the city of hayward a steeply east dipping fault geometry accommodates the divergence of the surface trace and the microseismicity at depth the inferred slip rate distribution is consistent with a fault that creeps aseismically at a rate of 5 mm yr to a depth of 4 6 km the interferometric synthetic aperture radar insar data require an aseismic slip rate that approaches the geologic slip rate on the northernmost fault segment beneath point pinole although the insar data might be complicated by a small dip slip component at this location a low slip rate patch of 1 mm yr is inferred beneath san leandro consistent with the source location of the 1868 earthquake we calculate that the entire fault is accumulating a slip rate deficit equivalent to a mw 6 77 0 05 per century however this estimate of potential coseismic moment represents an upper bound because we do not know how much of the accumulated strain will be released through aseismic processes such as afterslip

morphology and controls on the position of a gravel sand transition fraser river british columbia alluvial river channels often exhibit a relatively abrupt transition from gravel to sand bedded conditions the phenomenon is well documented but few prior studies have analyzed the spatial variability through reaches where transitions occur the downstream fining pattern observed in the fraser river is cited as a classic example of an abrupt gravel sand transition in a large alluvial channel however important questions regarding the exact location of the transition its sedimentology and morphology and what controls its location remain unanswered here we present observations of the downstream change in bed material grain size river bed topography and channel hydraulics through the reach within which the transition occurs these observations indicate that the gravel sand transition is characterized by a terminating gravel wedge but there are patches of gravel downstream of the wedge forming a diffuse extension we show that there is a dramatic decrease in shear stress at the downstream end of the wedge and a consequent cessation of general gravel mobility we argue that the patches of gravel observed beyond the wedge are the result of enhanced mobility of fine gravel over a sand bed we also find that sand in suspension declines rapidly at the downstream end of the wedge suggesting that sand is delivered to the bed completing the sedimentary conditions for a gravel sand transition we propose that the break in river slope associated with the transition is a consequential feature of the transition

an experimental study of flow bedload transport and bed topography under conditions of erosion and deposition and comparison with theoretical models the nature of flow sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size density sediment under equilibrium and non equilibrium aggradational degradational conditions and compared with theoretical models during each experiment water depth bed and water surface elevation flow velocity bed shear stress bedload transport and bed state were continuously monitored equilibrium uniform flow was established with a discharge of about 0 05 m3 s 1 a flow depth of about 0 01 m a flow velocity of about 0 81 0 88 m s 1 a spatially averaged bed shear stress of about 1 7 2 2 pa and a sediment transport rate of about 0 005 0 013 kg m 1 s 1 i e close to the threshold of sediment transport such equilibrium flow conditions were established prior to and at the end of each aggradation or degradation experiment pebble clusters bedload sheets and low lying bars were ubiquitous in the experiments heavy minerals were relatively immobile and occurred locally in high concentrations on the bed surface as lag deposits aggradation was induced by 1 increasing the downstream flow depth flume tilting and 2 sediment overloading tilt induced aggradation resulted in rapid deposition in the downstream half of the flume of a cross stratified deposit with downstream dipping pebbles pseudo imbricated and caused a slight decrease in the equilibrium mean water surface slope and total bedload transport rate these differences between pre and post aggradation equilibrium flow conditions are due to a decrease in the local grain roughness of the bed sediment overloading produced a downstream fining and thinning wedge of sediment with upstream dipping pebbles imbricated whereas the equilibrium flow and sediment transport conditions remained relatively unchanged degradation was induced by 1 decreasing the downstream flow depth flume tilting and 2 cutting off the sediment feed tilt induced degradation produced rapid downstream erosion and upstream deposition due to flow convergence with little change to the equilibrium flow and sediment transport conditions the cessation of sediment feed produced degradation and armour development a reduction in the mean water surface slope and flow velocity an increase in flow depth and an exponential decrease in bedload transport rate as erosion proceeded a bedload transport model predicted total and fractional transport rates extremely well when the coarse grained or bedform trough areas of the bed are used to define the sediment available to be transported a sediment routing model midas also reproduced the equilibrium and non equilibrium flow conditions total and fractional bedload transport rates and changes in bed topography and texture very well

geodetic and seismic constraints on some seismogenic zone processes in costa rica new seismic and geodetic data from costa rica provide insight into seismogenic zone processes in central america where the cocos and caribbean plates converge seismic data are from combined land and ocean bottom deployments in the nicoya peninsula in northern costa rica and near the osa peninsula in southern costa rica in nicoya inversion of gps data suggests two locked patches centered at 14 2 and 39 6 km depth interplate microseismicity is concentrated in the more freely slipping intermediate zone suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone the rupture zone for future large earthquakes at least over the short 1 year observation period we also estimate northwest motion of a coastal sliver block at 8 3 mm yr probably related to oblique convergence in the osa region to the south convergence is orthogonal to the trench cocos caribbean relative motion is partitioned here with 8 cm yr on the cocos panama block boundary including a component of permanent shortening across the fila costena fold and thrust belt and 1 cm yr on the panama block caribbean boundary the gps data suggest that the cocos plate panama block boundary is completely locked from 10 50 km depth this large locked zone as well as associated forearc and back arc deformation may be related to subduction of the shallow cocos ridge and or younger lithosphere compared to nicoya with consequent higher coupling and compressive stress in the direction of plate convergence

influence of sand on hydraulics and gravel transport in a braided gravel bed river measurements in a braided reach of the proglacial white river washington are used to compare the bed load capacity and competence of flow over the predominant gravel bed and over a narrow ribbon of mobile sand which extended alongside the talweg analysis of 144 velocity profiles and 222 bed load samples shows that shear stress and roughness height were significantly lower over sand than gravel but maximum bed load diameter and total gravel transport rate significantly higher as were transport rates of given size fractions at similar shear stresses these differences in competence and capacity are explained by relative size effects on the threshold shear stress for entrainment the influence on gravel transport of local variation in bed material size may explain the self sorting of bed load sheets described by other workers however in channels like that described here the continuous longitudinal ribbon of sand allows the flow to develop a lower shear stress over the smoother bed which partly offsets the relative size effects on competence and capacity

repeating earthquakes as low stress drop events at a border between locked and creeping fault patches the source of repeating earthquakes on creeping faults is modeled as a weak asperity at a border between much larger locked and creeping patches on the fault plane the x 1 2 decrease in stress concentration with distance x from the boundary is shown to lead directly to the observed scaling t m 1 6 between the average repeat time and average scalar moment for a repeating sequence the stress drop in such small events at the border depends on the size of the large locked patch for a circular patch of radius r and representative fault parameters 7 6 m r 3 5 mpa which yields stress drops between 0 08 and 0 5 mpa 0 8 5 bars for r between 2 km and 100 m these low stress drops are consistent with estimates of stress drop for small earthquakes based on their seismic spectra however they are orders of magnitude smaller than stress drops calculated under the assumption that repeating sources are isolated stuck asperities on an otherwise creeping fault plane whose seismic slips keep pace with the surrounding creep rate linear streaks of microearthquakes observed on creeping fault planes are trivially explained by the present model as alignments on the boundaries between locked and creeping patches

new constraints on the nubia sinai dead sea fault crustal motion abstract the subduction between nubia and eurasia and activities along the red sea gulf of suez and gulf of aqaba may control the surface deformation in the north eastern corner of the african continent using very few gps stations in the egyptian region previous studies indicate northward motion of northern nubia with respect to eurasia of about 5 mm yr mcclusky et al 2000 reilinger et al 2006 mahmoud et al 2005 in order to constrain better the movement rate of northern nubia and interaction between nubia eurasia and arabia plates we are using for the first time 16 permanent gps stations in combination with 47 non permanent stations covering egypt for the period 2006 2012 this paper extends the previous study saleh and becker 2013 by processing in addition to the egyptian stations 86 permanent stations belonging to three different tectonic plates this led to a considerably better coverage of the sinai dead sea region this work intends to be the first comprehensive analysis of the permanent stations in combination with the various campaign data in selected regions of special interest in egypt the gps sites show a relative motion between nubia and eurasia of about 6 5 1 mm yr which increased toward the hellenic trench 8 2 0 8 mm yr in sinai peninsula 14 2 1 4 mm yr in the north of arabian plate and 22 3 0 7 mm yr in eastern and central turkey the strain computation indicates high strain rates where the deformation is strongly localized along the dead sea fault and low to moderate strain rate in the gulf of suez and southern sinai

the role of unbalanced mesoscale circulations in dust storms 1 in this study two dust storms in northwestern nevada february 2002 and april 2004 are investigated through the use of weather research and forecasting wrf model simulations the focus of the study is twofold 1 examination of dynamic processes on the meso scale for both cases and 2 analysis of extreme upper air cooling prior to storm formation and the development of a nearly discontinuous gust front in the 2002 case that could not be validated in an earlier synoptic scale study results of the simulations suggest that the driving mechanism for dust storm dynamics derives from the breakdown and subsequent balance between the advection of geostrophic wind and total wind in the exit region of the polar jet in this process the deviation from quasi geostrophic q g balance creates a plume of ascent along and to the right of the jet s exit region the cold pool generation in the mid lower troposphere in consequence of this adjustment sets up the kinetic energy in the planetary boundary layer and creates a forward leaning slope from north to south cold front under the jet exit region surface heating is coupled with this frontal structure and rapid surface pressure falls rises occur initially later in response to diabatic adiabatic processes the adjustments occur at fast time scales scales that are radically different from those in studies that followed the q g tenets of the danielsen paradigm the results of this study indicate that meso scale features associated with subgeostrophy in the exit region of the curved jet aloft and associated thermal wind imbalance 700 500 hpa lead to significant velocity divergence aloft mass momentum adjustments and the associated cooling strengthen the baroclinic zone aloft the restoration to thermal wind balance accompanying this cooling resulted in a narrow zone of surface pressure rise and strong low level isallobaric winds the turbulent momentum for dust ablation comes from this sequence of processes

variations in rupture process with recurrence interval in a repeated small earthquake in theory and in laboratory experiments friction on sliding surfaces such as rock glass and metal increases with time since the previous episode of slip1 this time dependence is a central pillar of the friction laws widely used to model earthquake phenomena2 3 on natural faults other properties such as rupture velocity4 5 porosity and fluid pressure6 11 may also vary with the recurrence interval eighteen repetitions of the same small earthquake separated by intervals ranging from a few days to several years allow us to test these laboratory predictions in situ the events with the longest time since the previous earthquake tend to have about 15 larger seismic moment than those with the shortest intervals although this trend is weak in addition the rupture durations of the events with the longest recurrence intervals are more than a factor of two shorter than for the events with the shortest intervals both decreased duration and increased friction are consistent with progressive fault healing during the time of stationary contact

postearthquake relaxation and aftershock accumulation linearly related after the 2003 m 6 5 chengkung taiwan and the 2004 m 6 0 parkfield california earthquakes we treat both the number of earthquakes and the deformation following a mainshock as the superposition of a steady background accumulation and the postearthquake process the preseismic displacement and seismicity rates r u and r e are used as estimates of the background rates let t be the time after the mainshock u t u 0 the postseismic displacement less the background accumulation r u t and n t the observed cumulative number of postseismic earthquakes less the background accumulation r e t for the first 160 days duration limited by the occurrence of another nearby earthquake following the chengkung m 6 5 10 december 2003 eastern taiwan and the first 560 days following the parkfield m 6 0 28 september 2004 central california earthquakes u t u 0 is a linear function of n t the aftershock accumulation n t for both earthquakes is described by the modified omori law d n dt 1 t p with p 0 96 and 0 03 days although the chengkung earthquake involved sinistral reverse slip on a moderately dipping fault and the parkfield earthquake right lateral slip on a near vertical fault the earthquakes share an unusual feature both occurred on faults exhibiting interseismic fault creep at the surface the source of the observed postseismic deformation appears to be afterslip on the coseismic rupture the linear relation between u t u 0 and n t suggests that this afterslip also generates the aftershocks the linear relation between u t u 0 and n t obtains after neither the 1999 m 7 1 hector mine southern california nor the 1999 m 7 6 chi chi central taiwan earthquakes neither of which occurred on fault segments exhibiting fault creep

unequal mobility of gravel and sand in weakly bimodal river sediments bed load was trapped during flood events over a 20 month period at the lower end of the allt dubhaig a small river in scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile the channel bed near the trap is predominantly gravel with a secondary sand mode total transport in each event depended mainly on peak shear stress rather than duration over a threshold bed load was mainly sand in smaller events bimodal in intermediate events and mainly gravel in the biggest floods mean and maximum grain diameter both increased with peak shear stress but in different ways analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility the slight selectivity in gravel transport is consistent with previous field studies of near equilibrium unimodal beds and supports assumptions made in the numerical model of hoey and ferguson 1994 which successfully simulates the observed amount of downstream fining over the 2 5 km upstream of the bed load trap

the gravel sand transition sediment dynamics in a diffuse extension as gravel bedded rivers fine in the downstream direction they characteristically exhibit an abrupt transition from gravel to sand bedded conditions the prevailing theory for why abrupt gravel sand transitions emerge is based on bed load sorting of a bimodal sediment the abruptness is thought to be a consequence of sand overwhelming the gravel sand mixture once it reaches a critical coverage on the bed the role suspension plays in the development of gravel sand transitions has not been fully appreciated the fraser river british columbia is an archetypical abrupt gravel sand transition with a diffuse extension composed of a sand bed with some patches of gravel we examine flow shear stress and suspended sediment flux in the diffuse extension to better understand sediment dynamics where the sand bed emerges sand is carried in suspension upstream of the primary abrupt gravel sand transition but in the diffuse extension sand is moved as both bed load and suspended load we do not observe downstream gradients in shear stress or suspended sand flux through the diffuse extension that would suggest a gradual rain out of sand moving downstream which raises the question how is the sand bed formed sediment advection length scales indicate that with the exception of very fine sand that moves as wash load in the diffuse extension fractions coarser than the median sand size cannot be carried in suspension for more than one channel width this suggests that sand is deposited en masse at the beginning of the diffuse extension forming a sediment slug at low flood flows that is smeared downstream at high flood flows to form the sand reach

earthquake potential along the northern hayward fault california the hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace dislocation models of the surface deformation adjacent to the hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at 7 millimeters per year to the bottom of the seismogenic zone along a 20 kilometer long northern fault segment microearthquakes with the same waveform repeatedly occur at 4 to 10 kilometer depths and indicate deep creep at 5 to 7 millimeters per year the difference between current creep rates and the long term slip rate of 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern hayward fault adjacent to the locked 1868 earthquake rupture which broke the southern 40 to 50 kilometers of the fault the potential for a major independent earthquake of the northern hayward fault might be less than previously thought

measuring and defining bimodal sediments problems and implications gravel bed rivers close to the transition to a sand bed often have bimodal grain size distributions these can be quantified in several ways giving wide differences in derived statistics and calculations problems associated with measuring summarizing and defining bimodality are examined using evidence from flume experiments and a simple numerical simulation of sand deposition no single index of bimodality serves all purposes alternatives are suggested measured near bed hydraulic properties relate more closely to grain size statistics calculated by area than by volume this has implications for the abrupt downstream transition often found from gravel to sand bed

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

dust storm over the black rock desert larger scale dynamic signatures 1 a dust storm that originated over the black rock desert brd of northwestern nevada is investigated our primary goal is to more clearly understand the sequence of dynamical processes that generate surface winds responsible for entraining dust from this desert in addition to reliance on conventional surface and upper air observations we make use of reanalysis data sets ncar ncep and narr blends of primitive equation model forecasts and observations from these data sets we obtain the evolution of vertical motion patterns and ageostrophic motions associated with the event in contrast to earlier studies that have emphasized the importance of indirect transverse circulations about an upper level jet streak our results indicate that in this case the transition from an indirect to a direct circulation pattern across the exit region of upper level jet streak is central to creation of low level winds that ablate dust from the desert it is further argued that the transition of vertical circulation patterns is in response to adjustments to geostrophic imbalance an adjustment time scale of 6 9 h although unproven we suggest that antecedent rainfall over the alkali desert 2 weeks prior to the event was instrumental in lowering the bulk density of sediments and thereby improved the chances for dust ablation by the atmospheric disturbance we comprehensively compare contrast our results with those of earlier investigators and we present an alternative view of key dynamical signatures in atmospheric flow that portend the likelihood of dust storms over the western united states

influences on the location of repeating earthquakes determined from a and b value imaging to explore where earthquakes tend to recur we statistically investigated repeating earthquake catalogs and background seismicity from different regions parkfield hayward calaveras and chihshang faults we show that the location of repeating earthquakes can be mapped using the spatial distribution of the seismic a and b values obtained from the background seismicity molchan s error diagram statistically confirmed that repeating earthquakes occur within areas with high a values 2 8 3 8 and high b values 0 9 1 1 on both strike slip and thrust fault segments however no significant association held true for fault segments with more complicated geometry or for wider areas with a complex fault network the productivity of small earthquakes responsible for high a and b values may thus be the most important factor controlling the location of repeating earthquakes we inferred that the location of high creep rate in planar listric fault structures might be indicated by a values of 3 and b values of 1

numerical modeling of strike slip creeping faults and implications for the hayward fault california abstract the seismic potential of creeping faults such as the hayward fault san francisco bay area ca depends on the rate at which moment slip deficit accumulates on the fault plane thus it is important to evaluate how the creep rate observed at the surface is related to the slip on the fault plane the surface creep rate scr depends on the geometry of locked and free portions of the fault and on the interaction between the fault zone and the surrounding lithosphere using a viscoelastic finite element model we investigate how fault zone geometries and physical characteristics such as frictionless or locked patches affect the observed surface creep when the system is driven by far field plate motions these results have been applied to creep observations of the hayward fault this analysis differs from most previous fault creeping models in that the fault in our model is loaded by a distributed viscous flow induced by far field velocity boundary conditions instead of imposed slip beneath the major faults of the region the far field velocity boundary conditions simulate the relative motion of the stable pacific plate respect to the rigid sierra nevada block leaving the rheology fault geometry and mechanics locked or free to creep patches to determinate the patterns of fault creep our model results show that the fault geometry e g length and depth of creeping and the local rheology influence the surface creep rate scr and the slip on the fault plane in particular we show that the viscoelastic layer beneath the elastic seismogenic zone plays a fundamental role in loading the fault additionally the coupling with the surrounding lithosphere results in a smooth transition from regions free to creep to locked patches

controls on gravel termination in seven distributary channels of the selenga river delta baikal rift basin russia the selenga river delta lake baikal russia is 600 km2 in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge the delta is positioned along the deep water 1600 m margin of lake baikal a half graben styled rift basin qualifying it as a modern analogue of a shelf edge delta system this study provides a detailed field survey of channel bed sediment composition channel geometry and water discharge the data and analyses presented here indicate that the selenga delta exhibits downstream sediment fining over tens of kilometers ranging from predominantly gravel coarse pebble and sand near its apex to silt and sand at the delta lake interface we developed an analytical framework to evaluate the downstream elimination of gravel within the multiple distributary channels the findings include the following 1 the selenga river delta consists of at least eight orders of distributary channels 2 with increasing channel order downstream channel cross sectional area width depth ratio water discharge boundary shear stress and sediment flux systematically decrease 3 the downstream elimination of gravel in distributary channels is caused by declining boundary shear stress as a result of water discharge partitioning among the bifurcating channels 4 over longer time scales gravel is contained on the delta topset due to frequent and discrete seismic events that produce subsidence and accommodation so that coarse sediment cannot be transported to the axis of the baikal rift basin the distribution of sediment grain size in deltaic channels as related to hydrodynamics and sediment transport plays a critical role in influencing stratigraphy because the sustained tectonism leads to high preservation potential of the delta topset sedimentary deposits therefore the selenga river delta provides an opportunity to explore the interactions between modern deltaic sedimentation processes and tectonics that affect the production of basin stratigraphy

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

subtropical polar jet interactions in southern plains dust storms 1 the origin of two separate southern high plains dust storms which occurred over a 2 day period in february 2007 is traced to an interaction between the subtropical jet stj and the polar jet pj a large scale thermal wind imbalance resulting from the confluence of these two jets led to a series of mesoscale circulations that ultimately produced the dust storms understanding the connectivity between the dust storms with differing geometries is central to the present investigation the study rests on the interpretation of analyses from upper air and surface observations complemented by imagery from satellites the 32 km gridded data set from the north american regional reanalysis and a fine resolution 6 km grid simulation from the weather research and forecasting model principal assertions from the present study are 1 scale interaction is fundamental to the creation of an environment conducive to dust storm development 2 low to middle tropospheric mass adjustment is the primary response to a large scale imbalance 3 the mesoscale mass adjustment is associated with circulations about a highly accelerative jet streak resulting from the merger of the pj and stj 4 the structure of the jet streak resulting from this merger governs the evolution of the geometry of the dust plumes with plumes that initially had a straight line orientation developing a semicircular geometry and 5 it is concluded that improvements in dust storm prediction will depend on an augmentation to the upper air network in concert with a flow dependent data assimilation strategy

downstream variation in bed sediment size along the east carpathian rivers evidence of the role of sediment sources taking as an example six main rivers that drain the western flank of the eastern carpathians a conceptual model has been developed according to which fluvial bed sediment bimodality can be explained by the overlapping of two grain size distribution curves of different origins thus for carpathian tributaries of the siret coarse gravel joins an unimodal distribution presenting a right skewness with enhanced downstream fining the source of the coarse material distributions is autohtonous by abrasion and hydraulic sorting mechanisms a second distribution with a sandy mode is in general skewed to the left the source of the second distribution is allohtonous the quantity of sand that reaches the river bed through the erosion of the hillslope basin terrains the intersection of the two distributions occurs in the area of the 0 5 8 mm fractions where in fact the right skewness for gravel and left skewness for sand histogram tails meet this also explains the lack of particles in the 0 5 8 mm interval for rivers where fine sediment sources are low the 0 5 8 mm fractions have a higher proportion than the fractions under 1 mm for the siret river itself bed sediment bimodality is greatly enhanced due to the fact that the second mode is more than 25 of the full sample as opposed to its tributaries the source of the first mode of gravel is allohtonous to the siret river generated by the massive input of coarse sediment through the carpathian tributaries while the second mode of the sands is local in this case we can also observe that the two distributions of particles of different origins overlap in the 0 5 8 mm fraction domain creating the illusion of particle lack in the fluvial bed sediments copyright 2007 john wiley sons ltd

episodic creep events on the san andreas fault caused by pore pressure variations recent seismic and geodetic observations indicate that interseismic creep rate varies in both time and space the spatial extent of creep pinpoints locked asperities while its temporary accelerations known as slow slip events may trigger earthquakes although the conditions promoting fault creep are well studied the mechanisms for initiating episodic slow slip events are enigmatic here we investigate surface deformation measured by radar interferometry along the central san andreas fault between 2003 and 2010 to constrain the temporal evolution of creep we show that slow slip events are ensembles of localized creep bursts that aseismically rupture isolated fault compartments using a rate and state friction model we show that effective normal stress is temporally variable on the fault and support this using seismic observations we propose that compaction driven elevated pore fluid pressure in hydraulically isolated fault zone and subsequent frictional dilation cause the observed slow slip episodes we further suggest that the 2004 mw6 parkfield earthquake might have been triggered by a slow slip event which increased the coulomb failure stress by up to 0 45 bar per year this implies that while creeping segments are suggested to act as seismic rupture barriers slow slip events on these zones might promote seismicity on adjacent locked segments

channel sediment variability along a river a case study of the siret river romania the siret river has the largest drainage basin 42 274 km2 in romania it gathers all the rivers from the eastern part of the eastern carpathians a fact that causes marked asymmetry of the basin this study is principally concerned with changes in the form of the longitudinal profile and the grain size variability introduced by the carpathian tributaries channel sediment analyses considered the petrography granulometry and morphometry of the pebbles relating these to the river bed and floodplain geometry and to some properties of the drainage basin the following conclusions arise the siret river undergoes an intense regrading of its longitudinal profile with marked aggradation between transects 24 and 26 see figures 1 and 2 this reflects selective accumulation of coarse material due to the massive contribution of the carpathian tributaries this phenomenon has been continuous throughout the holocene resulting in the gravel sheet formation of the pericarpathian piedmont

re evaluation of the 1836 hayward fault and the 1838 san andreas fault earthquakes abstract current seismic hazard models include two major earthquakes m 7 in the san francisco bay area that are close in space and time an 1836 event on the northern hayward fault and an 1838 event on the peninsula section of the san andreas fault analysis and interpretation of the available historical accounts indicate that the 1836 event occurred east of monterey bay far from the hayward fault and was of m 6 also the 1838 event was not confined to the 60 km peninsula san andreas as current models indicate instead faulting probably extended from san francisco to san juan bautista 140 km indicating a significantly larger earthquake m 7 than previously thought damaging effects of the 1836 earthquake were reported only from santa clara to carmel and no contemporary effects were reported to the north of santa clara or near the hayward fault the illusion of an 1836 hayward earthquake evolved from a newspaper reminiscence published following the 1868 hayward earthquake stating that the 1868 effects in the east bay were similar to those of an 1836 event the article describes various strong effects in the east bay that differ completely from the effects recorded for the 1836 earthquake but are very similar to those documented for the major 1838 san andreas earthquake that caused extensive damage on both sides of san francisco bay based on this and other evidence we conclude that the reminiscence describes the destructive june 1838 effects but it erroneously indicates the date as june 1836 there is no evidence for any major historical earthquakes in the san francisco bay area before the 1838 earthquake back to the founding of mission san francisco dolores in 1776 during the 1838 san andreas fault earthquake the shaking intensity in monterey was as strong as or stronger than during the great 1906 san andreas fault earthquake this suggests that the 1838 san andreas fault rupture may have extended to san juan bautista as it did in 1906 numerous probable aftershocks were felt in the area south of san juan bautista these damaged carmel and santa cruz in 1840 and alisal 16 km west of the san andreas fault in 1841 the northern end of the 1838 faulting was previously assumed to be 25 km south of san francisco however mission san francisco dolores was damaged in 1838 but not in 1906 suggesting that the 1838 faulting extended to san francisco also the 1838 aftershocks were felt in oakland as frequently and violently as those following the major 1868 hayward earthquake suggesting that the 1838 faulting on the san andreas extended to the latitude of oakland the 1838 fault segment ruptured again 68 years later as part of the overlapping 1906 san andreas fault rupture this and similar evidence from southern california indicates that m 7 san andreas fault earthquakes can recur at intervals of 68 years or less when they are followed by m 8 earthquakes on overlapping segments of the fault

a silent mw 4 7 slip event of october 2006 on the superstition hills fault southern california 1 during october 2006 the 20 km long superstition hills fault shf in the salton trough southern california slipped aseismically producing a maximum offset of 27 mm as recorded by a creepmeter we investigate this creep event as well as the spatial and temporal variations in slip history since 1992 using ers 1 2 and envisat satellite data during a 15 year period steady creep is punctuated by at least three events the first two events were dynamically triggered by the 1992 landers and 1999 hector mine earthquakes in contrast there is no obvious triggering mechanism for the october 2006 event field measurements of fault offset after the 1999 and 2006 events are in good agreement with the interferometric synthetic aperture radar data indicating that creep occurred along the 20 km long fault above 4 km depth with most of the slip occurring at the surface the moment released during this event is equivalent to a mw 4 7 earthquake this event produced no detectable aftershocks and was not recorded by the continuous gps stations that were 9 km away modeling of the long term creep from 1992 to 2007 creep using stacked ers 1 2 interferograms also shows a maximum creep depth of 2 4 km with slip tapering with depth considering that the sediment thickness varies between 3 km and 5 km along the shf our results are consistent with previous studies suggesting that shallow creep is controlled by sediment depth perhaps due to high pore pressures in the unconsolidated sediments

mechanical implications of creep and partial coupling on the world s fastest slipping low angle normal fault in southeastern papua new guinea we use densely spaced campaign gps observations and laboratory friction experiments on fault rocks from one of the world s most rapidly slipping low angle normal faults the mai iu faul

critical shear stress of natural sediments the critical shear stress of individual fractions ci in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture for strongly bimodal sediments ci increases with grain size an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes ci to deviate from size independence in the direction of unisize shields values a quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of ci is observed and to predict the variation of ci with grain size in bimodal sediments because properties of sediment grain size distributions other than bimodality appear to have little influence on ci the trends presented here for 14 unimodal and bimodal sediments may be quite general

modelling earthquake rupture rates in fault systems for seismic hazard assessment the eastern betics shear zone abstract earthquake surface fault ruptures can show very complex geometries and involve different faults simultaneously consequently modern fault based probabilistic seismic hazard assessments psha need to account for such complexities in order to achieve more realistic modellings that treat fault systems as a whole and consider the occurrence of earthquake ruptures as aleatory uncertainties we use sherifs a recent approach of modelling annual rates of complex multi fault ruptures to obtain system level magnitude frequency distributions mfds for the eastern betics shear zone ebsz spain considering four fault rupture hypotheses we then analyze the consistency of each scenario based on data from the earthquake catalogue and paleoseismic studies the definition of the different rupture hypotheses was discussed within the frame of fault2sha esc working group and critical fault input data is extracted from previous published studies the four rupture hypotheses are defined as incremental scenarios based on fault geometry and kinematics with lengths varying from minimal fault sections to a rupture of nearly the whole system the results suggest that multi fault ruptures involving lengths up to single to several whole faults are consistent with the annual rates from both the instrumental catalogue and paleoseismic record the method does not allow to completely discard any hypothesis but it allows to weight the different models in a logic tree for seismic hazard assessment the approach is revealed as a practical tool for obtaining fault system mfds and as a useful tool for highlighting limitations and uncertainties in geological and paleoseismic data to be assessed this study aims to constitute a step forward in the consideration of complex multi fault ruptures for future seismic hazard assessments in the region

turbulence and secondary flow over sediment stripes in weakly bimodal bed material longitudinal stripes are a common bed form in heterogeneous alluvial sediments and consist of periodic spanwise variations in bed texture and elevation that are aligned parallel to the mean flow direction this paper quantifies mean and turbulent flow structures over self formed sediment stripes in a weakly bimodal sand and gravel mixture turbulence anisotropy generates two secondary circulation cells across the channel half width which produce a cross stream perturbation in boundary shear stress the interaction between this flow structure and the selective transport of bed material generates spanwise sediment sorting that is symmetrical about the centerline finer sediments are entrained from regions of high shear stress transported laterally by the secondary flow and deposited in regions of lower shear stress lateral changes in bed texture further enhance the near bed secondary flow which provides a positive feedback mechanism for stripe growth in bimodal sediments at shear stresses just above the entrainment threshold stripes may replace lower stage plane beds at higher shear stresses the coarser sediment becomes more mobile and the stripes are replaced by flow transverse bed forms read more http ascelibrary org doi abs 10 1061 asce 0733 9429 1999 125 5 463

the gravel sand transition along river channels abstract this paper discusses a hitherto neglected aspect of fluvial downstream fining the transition from a gravel bed to a sand bed from the few known examples and by additional study of new sites some general features of such transitions are given these transitions are generally rapid and involve a change in bed texture from unimodal gravels through bimodal sediments to exclusively sand material and are often associated with a break in slope we propose three possible causes local base level control an excess of sand supply and abrasion breakdown of fine gravel we further argue that such transitions occur in a variety of fluvial environments and are geographically more widespread than previously supposed and on this basis we suggest they may also aid in interpreting ancient sediment showing bimodality and rapid proximal to distal changes in grain size

the temporal variability of centimeter scale surface roughness in a playa dust source synthetic aperture radar investigation of playa surface dynamics abstract emission of mineral dust aerosols is highly dynamic in part due to variability in surface erodibility investigation of the variations of surfaces within dust source regions has the potential to elucidate the processes that control erodibility and to improve model representations of dust emission in this study we investigate surface temporal variability in the black rock playa nevada usa using synthetic aperture radar sar satellite data from between 2004 and 2010 the sar backscatter 0 observations of the playa surface are compared to weather station observations and temporally resolved measurements of water inundation derived from multi spectral modis satellite data the data illustrate that centimeter scale surface roughness on the playa is surprisingly heterogeneous over multi meter length scales and evolves dynamically on annual time scales interannual changes in surface roughness on the playa are quite large compared to the variability of surface roughness during the summer months july september suggesting that summer anthropogenic and aeolian processes are substantially less important than water related processes for controlling the evolution of the playa surface playa median 0 is relatively low in years with high annual precipitation 140 mm suggesting that surface water controls centimeter scale surface roughness and potentially erodibility spatially there is a relationship between 0 and the time of year at which a surface dries areas drying in july have significantly higher 0 than surfaces that dry earlier in the year suggesting that spatial heterogeneity in 0 is controlled by cycles of wetting and drying at the edge of the playa lake the implications in the black rock playa and potentially more broadly are that water is a critical factor controlling playa surface evolution and the annual time scale is crucial for investigations of playa erodibility

surface based fractional transport rates mobilization thresholds and partial transport of a sand gravel sediment twenty eight coupled observations of flow transport and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted bimodal grain size distribution these observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface the key to our observations is the use of a sediment in which each size fraction has been painted a different color which permits reliable repeatable and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface at a given flow the fractional transport rates may be divided into two parts a finer grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate and a coarser grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run we define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion of that fraction zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude a result strongly in contrast to many earlier observations but consistent with our observations of incipient motion in sediments with bimodal grain size distributions

evidence for surface rupture in 1868 on the hayward fault in north oakland and major rupturing in prehistoric earthquakes wgcep90 estimated the hayward fault to have a high probability 0 45 in 30 yr of producing a future m7 bay area earthquake this was based on a generic recurrence time and an unverified segmentation model because there were few direct observations for the southern fault and none for the northern hayward fault to better constrain recurrence and segmentation of the northern hayward fault we trenched in north oakland unexpectedly we observed evidence of surface rupture probably from the m7 1868 earthquake this extends the limit of that surface rupture 13 km north of the segmentation boundary used in the wgcep90 model and forces serious re evaluation of the current two segment paradigm although we found that major prehistoric ruptures have occurred here we could not radiocarbon date them however the last major prehistoric event appears correlative with a recently recognized event 13 km to the north dated ad 1640 1776

inferences drawn from two decades of alinement array measurements of creep on faults in the san francisco bay region we summarize over 20 years of monitoring surface creep on faults of the san andreas system in the san francisco bay region using alinement arrays the san andreas fault is fully locked at five sites northwest from san juan bautista the southern end of the 1906 earthquake rupture that is no creep 1 mm yr is observed likewise the san gregorio rodgers creek and west napa faults show no creep the measured creep rate on the calaveras paicines fault from hollister southward is either 6 or 10 mm yr depending on whether the arrays cross all of the creeping traces northward of hollister the central calaveras creep rate reaches 14 2 mm yr but drops to 2 mm yr near calaveras reservoir where slip transfers to the southern hayward fault at a maximum creep rate of 9 mm yr at its south end however the hayward fault averages only 4 6 mm yr over most of its length the northern calaveras fault now creeping at 3 4 mm yr steps right to the concord fault which has a similar rate 2 5 3 5 mm yr which is slightly slower than the 4 4 mm yr rate on its northward continuation the green valley fault the maacama fault creeps at 4 4 mm yr near ukiah and 6 5 mm yr in willits the central and southern segments of the calaveras fault are predominantly creeping whereas the hayward northern calaveras and maacama faults are partly locked and along with the rodgers creek and san andreas have high potential for major earthquakes manuscript received 6 november 2002

geodetic constraints on san francisco bay area fault slip rates and potential seismogenic asperities on the partially creeping hayward fault 1 the hayward fault in the san francisco bay area sfba is sometimes considered unusual among continental faults for exhibiting significant aseismic creep during the interseismic phase of the seismic cycle while also generating sufficient elastic strain to produce major earthquakes imaging the spatial variation in interseismic fault creep on the hayward fault is complicated because of the interseismic strain accumulation associated with nearby faults in the sfba where the relative motion between the pacific plate and the sierra block is partitioned across closely spaced subparallel faults to estimate spatially variable creep on the hayward fault we interpret geodetic observations with a three dimensional kinematically consistent block model of the sfba fault system resolution tests reveal that creep rate variations with a length scale of 15 km are poorly resolved below 7 km depth in addition creep at depth may be sensitive to assumptions about the kinematic consistency of fault slip rate models differential microplate motions result in a slip rate of 6 7 0 8 mm yr on the hayward fault and we image along strike variations in slip deficit rate at 15 km length scales shallower than 7 km depth similar to previous studies we identify a strongly coupled asperity with a slip deficit rate of up to 4 mm yr on the central hayward fault that is spatially correlated with the mapped surface trace of the 1868 mw 6 9 7 0 hayward earthquake and adjacent to gabbroic fault surfaces

selective sorting and abrasion of river gravel i theory most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fining is due to some combination of selective sorting by which finer grains are preferentially transported downstream and abrasion by which individual particles are reduced in size here a framework for the simultaneous treatment of both phenomena is developed the analysis is restricted to bed load transport of gravel only abrasion due to binary collisions between moving bed load and stationary bed particles is considered the formulation includes the possibility of differential basin uplift and subsidence due to tectonism and mixtures of rock types with differing coefficients of abrasion the formulation of selective sorting is based on the concept of hiding as applied to a surface based bed load transport relation abrasion is first treated in terms of a lagrangian formulation for individual grains and t

bedload sheets in heterogeneous sediment field observations in streams with beds of coarse sand and fine gravel have revealed that bedload moves primarily as thin migrating accumulations of sediment and coarse grains cluster at their leading edge these accumulations are one or two coarse grains high and are much longer 0 2 0 6 m long in sand 0 5 2 0 m in fine gravel than their height the authors propose the term bedload sheet for these features and the authors argue that they result from an instability inherent to bedload movement of moderately and poorly sorted sediment in essence coarse particles in the bedload slow or stop each other trap finer particles in their interstices and thus cause the coarse particles to become mobile again bedload sheets develop on the stoss side of dunes causing the dune to advance incrementally with the arrival of each sheet successive deposition of coarse sediment from the leading edge followed by fine sediment may generate the grain size sorting that distinguishes cross bedding available flume experiments and field observations indicate that bedload sheets are a common but generally unrecognized feature of heterogeneous sediment transport

seismic controls on contemporary sediment export in the siret river catchment romania abstract while differences in catchment sediment yield sy t km 2 y 1 are generally attributed to topography lithology climate and land use recent studies have highlighted that also seismic activity may have an important impact on sy nonetheless relatively little is known about the importance of this factor and the processes and mechanisms explaining its influence therefore this study explores the role of seismic activity in explaining spatial and temporal variation in sediment export within the siret basin romania 45 000 km 2 a catchment with a large variability in seismic activity based on previously unpublished long term 30 years sy measurements for 38 subcatchments of the siret we analyze the correlation between average sy seismic activity and various other catchment characteristics our results showed that spatial variation in average sy was indeed strongly correlated with the degree of seismic activity in each catchment r 2 0 74 also catchment lithology explained an important part of the differences in sy r 2 0 67 the combination of these two factors accounted for about 80 of the observed variation in sy while other factors e g topography land use climate and runoff did not significantly contribute to the explained variance in average sy to explore the impact of a specific earthquake event on sediment export we analyzed daily variations in suspended sediment concentrations of 10 subcatchments five years before and after an earthquake of mw 7 4 that affected the vrancea region in 1977 and triggered a substantial number of landslides only one catchment showed a clear 3 fold increase in sediment concentrations at unit discharge for the other nine catchments no consistent increase could be observed this indicates that the impact of seismic activity on average sy is mainly indirect and not associated with sudden pulses of sediments caused by earthquake triggered landslides potential mechanisms that could explain such indirect responses are discussed

model for calculating bed load transport of sediment a bed load transport model based on the mechanics of sediment moving by saltation yields predicted values of bed load flux as a function of boundary shear stress grain diameter and density the parameters required to calculate bed load transport particle velocity bed load sediment concentration and the height of the bed load layer can all be determined from our model which computes sequences of trajectories of individual saltating grains as well as the concentration of moving grains that the flow can support the latter is related to the momentum the accelerating grains extract from the flow predicted curves of bed load transport vs boundary shear stress agree well with transport data collected by gilbert meyer peter iet al n and the waterways experimental station measured shear stress was corrected for pressure drag when bed forms were present comparison of predicted bed load transport with common bed load equations reveals considerable similarity among the relationships the best agreement with the data is produced by relationships in which the transport rate vanishes as the shear stress approaches the critical shields number as in our model and yalin s equation

slope induced changes in channel character along a gravel bed stream the allt dubhaig scotland changes in channel character along a small river in the scottish highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0 02 to o ooo15 because of local baselevel control this decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed the channel pattern alters progressively rather than abruptly in the sequence 1 near braided 2 meandering with active point bar chutes 3 meandering with active outer bank talweg 4 stable equiwidth sinuous the changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical cdncepts than by purely empirical discriminant or trend equations measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload it is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions are typical of many streams in mountainous areas and can be understood in terms of slope induced changes in hydraulic properties

active fault creep variations at chihshang taiwan revealed by creep meter monitoring 1998 2001 1 the daily creep meter data recorded at chihshang in 1998 2001 are presented the chihshang creep meter experiment was set up across the chihshang thrust fault the most active segment of the longitudinal valley fault which is the present day plate suture between the eurasian and the philippine sea plates in eastern taiwan near continuous data recording at two sites revealed different surface fault motions yet similar annual shortening rates 16 2 mm at the tapo site comprising two connected creep meters and 15 0 mm at the chinyuan site three creep meters straddling parallel fault branches four of the five creep meters showed a seasonal variation with the fault moving steadily during the rainy season from april to october and remaining quiescent during the rest of the year the only exception was recorded by the creep meter located on a melange composed hillslope where local gravitational landsliding played an additional role other than tectonic faulting through comparison with daily precipitation data we inferred that moderate rainfall suffices to trigger or facilitate slippage on the surface fault during the transition period of the dry wet season during the observation period from 1998 to 2001 the subsurface seismicity exhibited clusters of microearthquakes on the chihshang fault at depths of 10 25 km recurrent earthquakes occurred regardless of whether the season was wet or dry indicating that the stress relaxation associated with seismicity in the seismogenic zone did not transfer immediately up to the surface the accumulated strain on the chihshang fault at shallow surface levels was released through creep during the wet season in addition to these short term seasonal variations an apparent decrease in the annual slipping rate on the chihshang fault during the last few years deserves further investigation in order to mitigate against seismic hazard

historic creep rate and potential for seismic slip along the hayward fault california the hayward fault is considered the most likely source of one or more major earthquakes in the san francisco bay area in the next few decades historically at least one and probably two major earthquakes about m 6 8 occurred along the hayward fault one in 1836 and another in 1868 little is known about the 1836 event but the 1868 earthquake was accompanied by a surface rupture that extended as much as 41 km along the southern part of the fault although the amount of surface slip in 1868 is uncertain right slip including afterslip reached at least several centimeters and possibly several decimeters in places this paper documents the spatial variation of creep rate along the hayward fault since the 1868 earthquake creep aseismic fault slip occurs over at least 66 km and may extend over the fault s entire 82 km length of which about 13 km lies underwater creep rate seems nearly constant over decades but short term variations occur we derive creep rate mainly from our own systematic surveying of offset cultural features curbs fences and buildings on each feature we solve directly for accumulated creep by using multiple linear regression creep rate mostly falls in the range of 3 5 6 5 mm yr but systematic variation occurs along strike fault segments with distinctly higher and lower rates generally correspond to parts of the fault most salient from the overall average alinement of the fault most distinctive is a 4 km long section near the south end of the fault that creeps at about 9 mm yr such a high rate has occurred there at least since the 1920s and probably since the 1868 earthquake as indicated by an offset railroad track built in 1869 we suggest that this 9 mm yr slip rate may approach the long term or deep slip rate that controls average recurrence interval between major earthquakes if so assuming an elastic rebound model the potential for slip in large earthquakes below the surficial creeping zone is now 1 1 m in the southern 1868 segment of the fault and 1 4 m in the northern 1836 segment subtracting surface creep rates from a long term slip rate of 9 mm yr gives present potential for surface slip in large earthquakes of up to 0 8 m with an average of 0 6 m in the northern segment and 0 4 m in the southern segment we present a simple hypothesis for rupture potential that is compatible with historic creep rate microseismicity distribution and geodetic data if seismic rupture occurs on segments 41 km long by 10 km deep 7 km fully locked 3 km creeping today s potential for seismic moment release is 1 4 1019 and 1 1 1019 n m for both 1836 and 1868 segments respectively and 2 5 1019 n m for both segments jointly converting moment to magnitude gives ml 6 8 in the northern segment ml 6 7 in the southern segment and ml 7 0 for simultaneous rupture of both

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

microseismicity and creeping faults hints from modeling the hayward fault california usa abstract creeping segments of strike slip faults are often characterized by high rates of microseismicity on or near the fault this microseismicity releases only a small fraction of the slip occurring on the fault and the majority of the accumulating elastic strain is released either through aseismic creep or in rare large events distinguishing between creeping or non creeping patches on faults and determining the resulting accumulated slip deficit is important in assessing the seismic hazard associated with a fault unfortunately surface creep data alone are insufficient to constrain the creep at depth on the fault here we analyze the possibility of using microseismicity as a further constraint an analysis of the accumulation of coulomb stress associated with the fault creep indicates that the transition from creeping regions to locked patches has the potential to affect the local seismicity pattern precise relative relocations of the microseismicity of the hayward fault 1 f waldhauser w l ellsworth fault structure and mechanics of the hayward fault california from double difference earthquake locations j geophys res 107 3 doi 10 1029 2000jb000084 2002 indicate that a fraction of the events repeat indicating recurrent ruptures of the same small patch a comparison of the creeping pattern resulting from a finite element deformation model with this precisely relocated microseismicity indicates that the non repeating earthquakes mainly occur in the transitional zones from creeping to locked patches while recurrent repeating earthquakes cluster in high creep rate regions building from this observation we have developed an analysis approach to better define patterns of creep and thus the slip deficit on the hayward fault additionally this creep rate and its spatial pattern on the fault vary as a function of time after the system is loaded by earthquakes on the locked patches

effect on flow structure of sand deposition on a gravel bed results from a two dimensional flume experiment 1 particle imaging velocimetry piv was used to quantify the entire two dimensional flow field over a series of fixed test beds in a laboratory flume the test beds used in the experiments provide a two dimensional representation of gravel beds with contrasting roughness the two dimensional form of the beds combined with the nonintrusive nature of piv allowed the velocity field to be quantified right down to the bed surface the three beds used were designed to simulate the progressive effect of sand deposition on a gravel bed with median grain size d50 of 25 mm such a situation is common in bimodal beds but as compared with gravel and sand cases little work has been done to investigate the interaction between the variable roughness of a bimodal bed and the flow structure above it the results demonstrate that as the effective roughness decreases the reverse flow often found in the lee of gravel particles is eliminated furthermore near bed velocities increase while shear stress and turbulent kinetic energy decrease turbulent properties are also diminished higher up in the profile although this is not the case for the mean downstream velocity which remains unaffected except at the bed quadrant analysis reveals that q2 and q4 events become less frequent around high points as effective roughness decreases all these responses to the changing bed conditions are most pronounced in areas where the effective roughness height h approaches 10 mm or less h d50 0 4 such a situation is often found immediately upstream of gravel sand transitions in natural rivers it is hypothesized that the hydraulic response outlined here provides a detailed physically based explanation for the gravel sand transition the combination of reduced bed shear stress and q2 q4 events around gravel particles will result in greater mobility of the sand fraction relative to gravel these experiments thus appear to demonstrate that there is a fundamental threshold between gravel bed and sand bed states that has not previously been quantified

modelling microseismicity of a producing reservoir from coupled fluid flow and geomechanical simulation in this paper we investigate production induced microseismicity based on modelling material failure from coupled fluid flow and geomechanical simulation the model is a graben style reservoir characterized by two normal faults subdividing a sandstone reservoir into three compartments the results are analysed in terms of spatial and temporal variations in distribution of material failure we observe that material failure and hence potentially microseismicity is sensitive to not only fault movement but also fluid movement across faults for sealing faults failure is confined to the volume in and around the well compartment with shear failure localized along the boundaries of the compartment and shear enhanced compaction failure widespread throughout the reservoir compartment for non sealing faults failure is observed within and surrounding all three reservoir compartments as well as a significant distribution located near the surface of the overburden all shear enhanced compaction failures are localized within the reservoir compartments fault movement leads to an increase in shear enhanced compaction events within the reservoir as well as shear events located within the side burden adjacent to the fault we also evaluate the associated moment tensor mechanisms to estimate the pseudo scalar seismic moment of failure based on the assumption that failure is not aseismic the shear enhanced compaction events display a relatively normal and tight pseudo scalar seismic moment distribution centred about 106 pa whereas the shear events have pseudo scalar seismic moments that vary over three orders of magnitude overall the results from the study indicate that it may be possible to identify compartment boundaries based on the results of microseismic monitoring

fault structure and mechanics of the hayward fault california from double difference earthquake locations 1 the relationship between small magnitude seismicity and large scale crustal faulting along the hayward fault california is investigated using a double difference dd earthquake location algorithm we used the dd method to determine high resolution hypocenter locations of the seismicity that occurred between 1967 and 1998 the dd technique incorporates catalog travel time data and relative p and s wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events the relocated seismicity reveals a narrow near vertical fault zone at most locations this zone follows the hayward fault along its northern half and then diverges from it to the east near san leandro forming the mission trend the relocated seismicity is consistent with the idea that slip from the calaveras fault is transferred over the mission trend onto the northern hayward fault the mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the calaveras fault at calaveras reservoir in some locations discrete structures adjacent to the main trace are seen features that were previously hidden in the uncertainty of the network locations the fine structure of the seismicity suggests that the fault surface on the northern hayward fault is curved or that the events occur on several substructures near san leandro where the more westerly striking trend of the mission seismicity intersects with the surface trace of the aseismic southern hayward fault the seismicity remains diffuse after relocation with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation the seismicity is highly organized in space especially on the northern hayward fault where it forms horizontal slip parallel streaks of hypocenters of only a few tens of meters width bounded by areas almost absent of seismic activity during the interval from 1984 to 1998 when digital waveforms are available we find that fewer than 6 5 of the earthquakes can be classified as repeating earthquakes events that rupture the same fault patch more than one time these most commonly are located in the shallow creeping part of the fault or within the streaks at greater depth the slow repeat rate of 2 3 times within the 15 year observation period for events with magnitudes around m 1 5 is indicative of a low slip rate or a high stress drop the absence of microearthquakes over large contiguous areas of the northern hayward fault plane in the depth interval from 5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large magnitude earthquakes

slip in the 1868 hayward earthquake from the analysis of historical triangulation data despite the importance of the hayward fault for earthquake hazards in the san francisco bay area neither the rupture length nor the average coseismic slip in the most recent major earthquake which occurred in 1868 is well known we estimate slip in the 1868 earthquake by analyzing triangulation data collected between 1853 1860 and 1876 1891 we apply a denuisancing procedure that projects out the dependence of that data on station coordinates allowing their displacements to be estimated directly the best fit to the data is obtained with a 52 km long by 10 km deep rupture that extends from warm springs to near berkeley considerably longer than the 30 km of reported surface breakage the estimated slip for an earthquake of this length is 1 9 0 4 m and the seismic moment is m0 3 0 0 7 1019 n m corresponding to mw 7 0 the data are inconsistent with regionally high strain rate prior to the 1906 earthquake as had been previously suggested the triangulation data record northeastward displacement of station loma prieta presumably as a result of the 1865 earthquake the fault normal displacement is not easily explained by slip on either the san andreas fault or the 1989 loma prieta rupture surface this observation is however consistent with an mw 6 3 4 thrust event northeast of the san andreas fault in the southern santa cruz mountains thrust belt

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

selective sorting and abrasion of river gravel ii applications most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fin

the step mountain coordinate model description and performance for cases of alpine lee cyclogenesis and for a case of an appalachian redevelopment abstract the problem of the pressure gradient force error in the case of the terrain following sigma coordinate does not appear to have a solution the problem is not one of truncation error in the calculation of space derivatives involved thus with temperature profiles resulting in large errors an increase in vertical resolution may not reduce and is even likely to increase the error therefore an approach abandoning the sigma system has been proposed it involves the use of step mountains with coordinate surfaces prescribed to remain at fixed elevations at places where they touch and define or intersect the ground surface thus the coordinate surfaces are quasi horizontal and the sigma system problem is not present at the same time the simplicity of the sigma system is maintained in this paper design of the model silhouette averaged mountains properties of the wall boundary condition and the scheme for calculation of the potential to kinetic energy conversion are presented for an

effect of lateral bed roughness variation on particle suspension in open channels in this paper the effect of variation of bed roughness along lateral direction on suspension concentration distribution in open channel turbulent flows was investigated starting from the mass and momentum conservation equations this study demonstrates that both the reynolds shear stress overline u v and sediment diffusivity depends on bed roughness from the theoretical analysis it is found that both the reynolds shear stress and the sediment diffusivity increase over smooth bed surfaces and decrease over rough bed surfaces at the junction of smooth and rough bed surface the effect of bed roughness on the reynolds shear stress and sediment diffusion is almost negligible including this effect suspension concentration distribution is also studied and from the hunt s diffusion equation an analytical model for predicting suspension concentration is proposed apart from this effect the effects of moveable bed roughness and stratification are also considered in the model it is observed that the rouse equation is obtained from the proposed model as a special case when the flow is considered as single phase and there is no effect of secondary current stratification and bed roughness variation on the basis of experimental data available in literature the proposed model is validated and also compared with the rouse equation to get a quantitative idea about the goodness of fit weighted relative error is calculated the comparison results and calculated errors indicate that the present model is capable of describing the suspension concentration distribution more accurately than rouse model throughout the flow depth in open channel flow

variations in creep rate along the hayward fault california interpreted as changes in depth of creep variations in surface creep rate along the hayward fault are modeled as changes in locking depth using 3d boundary elements model creep is driven by screw dislocations at 12 km depth under the hayward and other regional faults inferred depth to locking varies along strike from 4 12 km 12 km implies no locking our models require locked patches under the central hayward fault consistent with a m6 8 earthquake in 1868 but the geometry and extent of locking under the north and south ends depend critically on assumptions regarding continuity and creep behavior of the fault at its ends for the northern onshore part of the fault our models contain 1 4 1 7 times more stored moment than the model of burgmann et al 2000 45 57 of this stored moment resides in creeping areas it is important for seismic hazard estimation to know how much of this moment is released coseismically or as aseismic afterslip

bedload transport of fine gravel observed by motion picture photography motion pictures taken at duck creek a clear stream 6 5 m wide and 35 cm deep near pinedale wyoming provide detailed quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them bed shear stress was approximately 6 pa 60 dynes cm 2 which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material and transport was almost entirely as bedload the displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones and rarely by brief sliding of large angular ones entrainment was principally by rollover of the larger particles and liftoff of the smaller ones and infrequently by ejection caused by impacts whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones the displacement times averaged about 0 2 0 4 s and generally were much shorter than the intervening repose times the collective motions of the particles were characterized by frequent brief localized random sweep transport events of very high rates of entrainment and transport which in the aggregate transported approximately 70 of the total load moved these events occurred 9 of the time at any particular point of the bed lasted 1 2 s affected areas typically 20 50 cm long by 10 20 cm wide and involved bedload concentrations approximately 10 times greater than background the distances travelled during displacements averaged about 15 times the particle diameter despite the differences in their dominant modes of movement the 8 16 mm particles typically travelled only about 30 slower during displacement than the 2 4 mm ones whose speeds averaged 21 cm s 1 particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely with diffusivities of 4 6 and 0 26 cm 2 s 1 respectively the bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler the 2 4 mm particles were entrained 6 times faster on finer areas of the bed where 8 16 mm particles covered 6 of the surface area than on coarser ones where they covered 12 even though 2 4 and 4 8 mm particles covered practically the same percentage areas in both cases the 4 8 and 8 16 mm particles in contrast were entrained at the same rates in both cases to within the statistical uncertainty the rates of distrainment balanced the rates of entrainment for all three sizes and were approximately proportional to the corresponding concentrations of bedload

downstream fining in large sand bed rivers abstract downstream fining of bed sediments is a well known phenomenon in gravel bed rivers but also occurs in large sand bed rivers the underlying processes however are not necessarily the same due to the difference in sediment mobility grain size distribution width and geographical setting downstream fining studies focusing on large sand bed rivers are scarce nevertheless in this paper the processes that affect downstream fining in gravel bed rivers are reviewed and it is evaluated to what extent they are relevant for large sand bed rivers furthermore several new downstream fining processes are discussed ending in a conceptual model of downstream fining in large sand bed rivers in deriving the conceptual model downstream fining processes are divided into three categories abrasion selective transport and sediment addition extraction abrasion rates in large sand bed rivers are small due to the durable lithologies the small grain size the high degree of rounding the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment only in the upstream part of large sand bed rivers abrasion rates may be somewhat higher selective transport rates in large sand bed rivers everywhere overrun abrasion rates but are small compared to selective transport rates in gravel bed rivers because the unimodal sediments in sand bed rivers exhibit stronger hiding exposure effects during threshold of motion conditions but especially because many sand bed rivers though not all are in state of fully mobilised transport making differences in threshold of motion between coarse and fine grains relatively unimportant selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting which are all processes that act on a relatively large scale within sand bed rivers the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency there are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand bed rivers note that in contrast to abrasion selective transport only produces a stable downstream fining pattern if the river profile is concave generally a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand bed rivers overbank sedimentation for instance may significantly decrease the downstream fining rate in large sand bed rivers because especially fine grains are removed from the bed dredging can also strongly affect the downstream fining trend depending on the volume and the grain size of the sediment involved meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances increasing the downstream fining rate river bifurcations can cause a discontinuity in the downstream fining pattern because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch all these addition and extraction processes not only have a direct effect on the grain size composition of the river bed but also indirectly influence downstream fining by their effect on the longitudinal bed profile other sediment addition and extraction processes such as tributary confluences non alluvial sediment sources and dumping are less common in large sand bed rivers but can locally have a great effect a full determination of the relative importance of all downstream fining processes in large sand bed rivers requires extensive field measurements the construction of detailed fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above

the gravel sand transition flume study of channel response to reduced slope this paper reports flume experiments with a bimodal gravel sand bed and sediment feed designed to investigate river channel response to declining slope in successive runs an initial near equilibrium channel was subjected to progressively bigger reductions in flume slope this simulated a series of sites along a concave profile approaching a local base level and undergoing a transition from gravel to sand bed it also models the effects of a rise in base level on a fixed site slope reduction led to immediate increase in depth and reduction in velocity and shear stress hence progressive aggradation deposition was size selective and led to the following sequence of bed texture change with increasing severity of slope reduction 1 sand in lee of gravel clusters 2 elongated sandy patches and thin streaks 3 wider sand ribbons covering gravel completely and developing ripples there was feedback from the gradual bed surface changes to hydraulic conditions near bed velocity increased while depth shear stress and roughness height decreased these feedbacks were more pronounced with a bigger reduction in slope these results support the view that the gravel sand transition along river channels with a local base level control is assisted by positive feedback between changes in bed surface texture and near bed flow

fault creep and microseismicity on the hayward fault california implications for asperity size 1 the hayward fault is documented to undergo significant creep with some patches accommodating 50 or more of the long term fault displacement in spite of this the fault has also experienced moderate to large earthquakes by comparing the patterns of microseismicity observed on the fault with models of fault zone creep we can investigate the long term displacement deformation history of the fault in terms of the relative roles of aseismic creep fault slip accommodated through microseismicity and strain accumulation slip deficit we find that microseismicity on the hayward fault produces a negligible percentage of the seismic moment dissipated on the fault combining seismicity with our fault creep models allows us to calculate the size of asperities on the creeping fault for small asperities associated with repeating earthquakes on the hayward fault the rupture areas of these asperities range from 20 to 60 m2

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

spatiotemporal evolution of surface creep in the parkfield region of the san andreas fault 1993 2004 from synthetic aperture radar the parkfield section of the san andreas fault saf is defined as a transitional portion of the fault between slip release behavior types in the creeping section of the saf to the northwest and the apparently locked section to the southeast the parkfield section is characterized by complex frictional fault behavior because it represents a transition zone from aseismic creep to stick slip regime at least six historic earthquakes of mw 6 have occurred in this area in 1881 1901 1922 1934 1966 and 2004 it was observed in the 2004 mw 6 0 parkfield earthquake that 70 of the total coseismic and postseismic moment release occurred aseismically to understand the saf behavior in this area it is of particular interest to measure and analyze not only the spatial evolution of the surface displacement in this area but also its evolution over time using radar data acquired by the european space agency s european remote sensing ers1 2 satellites we constructed descending interferograms and retrieved time series of surface displacements along the central saf for the decade preceding the 2004 parkfield earthquake we focus on characterizing the space and time evolution of surface creep in the parkfield and cholame sections the spatial pattern of the interseismic displacement rate indicates that tectonic strain was not uniformly distributed along the strike of the fault between 1993 and 2004 our data indicate not only a decrease in the creep rate from the parkfield section to south of highway 46 from 1 4 0 3 cm y to 0 6 0 3 cm y but also a small but significant creep rate increase in the cholame section to 0 2 0 1 cm y the evidence for episodic creep in the cholame section of the saf south east of parkfield is in contrast with previously published interpretations of gps and trilateration data the cholame section of the saf merits close monitoring because it was likely the nucleation site of the 1857 fort tejon earthquake and because it has shown recent evidence of deep slow slip as revealed by deep tremors

post 1906 stress recovery of the san andreas fault system calculated from three dimensional finite element analysis 1 the m 7 8 1906 san francisco earthquake cast a stress shadow across the san andreas fault system inhibiting other large earthquakes for at least 75 years the duration of the stress shadow is a key question in san francisco bay area seismic hazard assessment this study presents a three dimensional 3 d finite element simulation of post 1906 stress recovery the model reproduces observed geologic slip rates on major strike slip faults and produces surface velocity vectors comparable to geodetic measurements fault stressing rates calculated with the finite element model are evaluated against numbers calculated using deep dislocation slip in the finite element model tectonic stressing is distributed throughout the crust and upper mantle whereas tectonic stressing calculated with dislocations is focused mostly on faults in addition the finite element model incorporates postseismic effects such as deep afterslip and viscoelastic relaxation in the upper mantle more distributed stressing and postseismic effects in the finite element model lead to lower calculated tectonic stressing rates and longer stress shadow durations 17 74 years compared with 7 54 years all models considered indicate that the 1906 stress shadow was completely erased by tectonic loading no later than 1980 however the stress shadow still affects present day earthquake probability use of stressing rate parameters calculated with the finite element model yields a 7 12 reduction in 30 year probability caused by the 1906 stress shadow as compared with calculations not incorporating interactions the aggregate interaction based probability on selected segments not including the ruptured san andreas fault is 53 70 versus the noninteraction range of 65 77

a review of the structure and dynamics of upper level frontal zones abstract this article presents a review of upper level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves the discussion begins by tracing present day concepts concerning the structure of upper level frontal systems which are based on composite analyses of radiosonde and aircraft data from their origins in the pioneering analyses of upper air data in the 1930s perspectives from scales both smaller and larger than upper level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper level frontogenesis the dynamics of upper level fronts are shown to comprise the interactions between the primary geostrophic and secondary ageostrophic cir

inferred depth of creep on the hayward fault central california a relation between creep rate at the surface trace of a fault the depth to the bottom of the creeping zone and the rate of stress accumulation on the fault is derived from weertman s 1964 friction model of slip on a fault a 5 1 km depth for the creeping zone on the hayward fault is estimated from the measured creep rate 5 mm yr at the fault trace and the rate of stress increase on the upper segment of the fault trace inferred from geodetic measurements across the san francisco bay area although fault creep partially accommodates the secular slip rate on the hayward fault a slip deficit is accumulating equivalent to a magnitude 6 6 earthquake on each 40 km segment of the fault each century thus the current behavior of the fault is consistent with its seismic history which includes two moderate earthquakes in the mid 1800 s