conceptual framework for assessing the response of delta channel networks to holocene sea level rise abstract recent research has identified two fundamental unit processes that build delta distributary channels the first is mouth bar deposition at the shoreline and subsequent channel bifurcation which is driven by progradation of the shoreline the second is avulsion to a new channel a result of aggradation of the delta topset the former creates relatively small branching networks such as wax lake delta the latter generates relatively few long distributaries such as the mississippi and atchafalaya channels on the mississippi delta the relative rate of progradation to aggradation and hence the creation of accommodation space emerges as a controlling parameter on channel network form field and experimental research has identified sea level as the dominant control on holocene delta growth worldwide and has empirically linked channel network changes to changes in the rate of sea level rise here i outline a simple modeling framework for distributary network evolution and use this to explore large scale changes in holocene channel pattern that have been observed in deltas such as the rhine meuse and mississippi rapid early to mid holocene sea level rise forced many deltas into an aggradational mode where i hypothesize that avulsion and the generation of large scale branches should dominate slowing of sea level rise in the last 6000 yr allowed partitioning of sediment into progradation facilitating the growth of smaller scale distributary trees at the shorelines of some deltas and a reduction in the number of large scale branches significant antecedent topography modulates delta response the filling of large incised valleys for example caused many deltas to bypass the aggradational phase human effects on deltas can be cast in terms of geologic controls affecting accommodation constriction of channels forces rapid local progradation and mouth bar bifurcation while accelerated sea level rise increases aggradation and induces more frequent channel avulsion

quaternary blind thrusting in the southwestern sacramento valley california patterns of microearthquakes and quaternary surface deformation suggest that the tectonic setting of the sw sacramento valley is similar to areas of the western san joaquin valley known to be underlain by seismogenic blind thrust faults on the basis of previous work and analysis of geologic and seismic reflection data the following late cenozoic tectonic features and processes are identified 1 uplift of the northern coast ranges beginning approximately 3 4 ma and eastward propagation of uplift into the southwestern sacramento valley by 1 0 ma 2 uplift and homoclinal flexure of plio pleistocene strata at the eastern coast ranges mountain front 3 uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern sacramento valley similar associations of structures and processes have been observed in thrust belts in pakistan the peruvian andes and the canadian cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone by using other thrust belts as analogs the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern coast ranges mountain front seismic reflection profiles reveal that 1 the triangle zone consists primarily of east vergent blind thrusts and 2 west vergent backthrusts exposed in the the eastern coast ranges and southwestern sacramento valley are rooted in the east vergent thrusts transfer of slip from the east vergent blind thrusts to the west vergent backthrusts occurs locally beneath the southwestern sacramento valley fault bend folding in the hanging walls of the backthrusts has created a north northwest striking chain of low hills approximately 15 km east of the mountain front the folds deform 3 4 1 0 ma fluvial sediments and thus are middle pleistocene in age or younger local variations in strike suggest that the fold chain is segmented like the new idria coalinga kettleman hills segmented fold chain in the southwestern san joaquin valley stein and ekstrom 1989 these data have implications for seismic hazard assessment anecdotal accounts indicate that two m 6 0 events of the 1892 winters vacaville earthquake sequence probably occurred beneath the eastern coast ranges dale 1977 toppozada et al 1981 ground cracking was observed following the main shocks along the mountain front in the southwestern sacramento valley we propose that the earthquakes were generated by slip on a blind thrust beneath the coast ranges and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone the 1892 earthquake sequence suggests that blind thrusts beneath the southwestern sacramento valley are active and capable of generating moderate to large magnitude earthquakes

redistribution of dynamic stress during coseismic ruptures evidence for fault interaction and earthquake triggering we investigate the spatiotemporal evolution of dynamic stress outside a rupturing extended fault the dynamic stress variations caused by a coseismic rupture in a half space are computed by using the discrete wavenumber and reflectivity methods after a transient phase the stress time history evolves to the final static stress value we compare the static stress changes resulting from this model with those computed from a static dislocation model we have applied this method to study the interactions between the first two normal faults which ruptured during the 1980 ms 6 9 irpinia earthquake these two subevents are separated in time by nearly 20 s while the third and last subevent occurred 40 s after the rupture onset we compute the dynamic stress changes caused by the rupture of the first subevent our modeling results show that the dynamic stress peak on the second subevent fault plane is reached between 7 s and 8 s after the rupture initiation on the main fault on the average the static stress level on the second subevent 20 s fault plane is reached nearly after 14 s the dynamic rupture did not jump from a rupturing segment to the adjacent one immediately but the triggering of the 20 s subevent is delayed by roughly 10 s with respect to the instant of occurrence of the dynamic stress peak induced by the 0 s event the dynamic stress pulse propagates along the strike direction of the second subevent fault plane at an average velocity of nearly 3 4 km s the delayed triggering of the second subevent can be interpreted in terms of the frictional properties of the faults in particular rate and state dependent frictional law can explain a delayed instability after a sudden change in stress using the estimated values of the subevent triggering delay and the shear stress change we attempt to constrain the parameter a on the 20 s fault the values here inferred agree well with those resulting from previous studies

interpreting avulsion process from ancient alluvial sequences guadalope matarranya system northern spain and wasatch formation western colorado alluvial deposits of the guadalope matarranya system oligocene ebro basin spain and the wasatch formation eocene western colorado provide time integrated records of the process of river channel avulsion these sequences consist of isolated channel belt sandstones incised into and abruptly overlain by flood plain siltstones indicating deposition by avulsive river systems the geometry and distribution of channel incisions suggest that avulsion was not controlled by tectonics climate or base level changes but formed by autocyclic processes measurements from 221 channel fills in the guadalope matarranya system and 38 from the wasatch formation allow us to statistically characterize channel geometries we infer to be associated with establishment and abandonment of individual river avulsions paleoflow depths in both systems average 1 4 to 1 6 m aggradation height superelevation of channel margin levees are on average 0 6 and 1 1 times paleoflow depth in the guadalope matarranya and wasatch systems respectively these results are consistent with values from recently avulsed modern rivers and suggest that 1 flow depth is the appropriate parameter against which to scale the critical superelevation necessary for channel avulsion and 2 the increase in potential energy due to channel perching drives the lateral instability that is needed for avulsion to be successful numerous stacked channel fills indicate repeated reoccupation of the same site by avulsing channels these reoccupation channels indicate that inherited flood plain topography here abandoned channel forms was an important control on the arrival site of newly avulsed channels comparison of our results to others suggests two end member types of avulsion can take place incisional avulsion seen here is characterized by an early incision phase followed by infilling by migrating bar forms aggradational avulsion begins with aggradation followed in time by stream integration into a single downcutting channel we suggest that the type of avulsion is strongly influenced by whether or not the adjacent flood plain is well or poorly drained in both cases subsequent aggradation and channel perching increase the chances that some triggering event will lead to avulsion

three dimensional dynamic simulation of the 1992 landers earthquake the 1992 landers california earthquake magnitude 7 3 was modeled as the propagation of a spontaneous rupture controlled by a realistic prestress distribution with the use of a three dimensional finite difference method the dynamic rupture reproduces the general slip pattern used to compute the initial stress distribution and generates near fault ground motions at the surface similar to observations the simulated rupture propagates on the fault along a complex path with highly variable speed and rise time changing the magnitude and pattern of the stress significantly the method provides the framework to estimate earthquake rupture parameters from recorded seismic and geodetic data

discriminating stacked distributary channel from palaeovalley fill sand bodies in foreland basin settings abstract stacked fluvial distributary channel deposits and palaeovalley fills can form major multi storey sand bodies with similar thicknesses and with lateral extents often greater than a single exposure consequently they can be difficult to tell apart from one another using outcrop data this study addresses this problem by quantitatively analysing the architecture of five stacked fluvial distributary channel deposits and two palaeovalley fills from the pennsylvanian pikeville and hyden formations of the central appalachian basin usa the a priori interpretation of the sand bodies as stacked distributary channels and palaeovalley fills is possible because a robust in place coal seam correlation framework allows for the recognition of different basin scale architectures for each type aspect ratios 1000 and a regional basinward facies shift at the bases of palaeovalley fills sand body thickness storey thickness position and length of storey contacts within the sand body are similar in both types however they can be distinguished by different up system to down system changes in their respective architectures stacked distributary channel sand bodies thin down system display a decrease in storey thickness an increase in the mean position of storey heights in the sand body and a decrease in the length of storey contacts these trends are the result of down system decrease in channel size and confinement associated with radially distributive fluvial systems palaeovalley fill sand bodies thicken down system display an increase in storey thickness a decrease in the mean position of storey heights and a decrease in the length of storey contacts the increase in sand body and storey thickness are the result of down system increases in original channel size consistent with trunk axial fluvial systems fed by tributaries that predominate during valley formation the down system increase in amalgamation reflects a down system decrease in accommodation from the higher subsidence rate active margin of the basin and is therefore not necessarily characteristic of palaeovalley fill architectures in all basin settings this study emphasises the requirement for detailed correlation work and quantitative analysis of external and internal architectures before the interpretation of sand bodies as stacked distributary channels or palaeovalley fills

ophiolitic basement to the great valley forearc basin california from seismic and gravity data implications for crustal growth at the north american continental margin the nature of the great valley basement whether oceanic or continental has long been a source of controversy a velocity model derived from a 200 km long east west reflection refraction profile collected south of the mendocino triple junction northern california in 1993 further constrained by density and magnetic models reveals an ophiolite underlying the great valley great valley ophiolite which in turn is underlain by a westward extension of lower density continental crust sierran affinity material we used an integrated modeling philosophy first modeling the seismic refraction data to obtain a final velocity model and then modeling the long wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model the crustal section of great valley ophiolite is 7 8 km thick and the great valley ophiolite relict oceanic moho is at 11 16 km depth the great valley ophiolite does not extend west beneath the coast ranges but only as far as the western margin of the great valley where the 5 7 km thick great valley ophiolite mantle section dips west into the present day mantle there are 16 18 km of lower density sierran affinity material beneath the great valley ophiolite mantle section such that a second deeper present day continental moho is at about 34 km depth at mid crustal depths the boundary between the eastern extent of the great valley ophiolite and the western extent of sierran affinity material is a near vertical velocity and density discontinuity about 80 km east of the western margin of the great valley our model has important implications for crustal growth at the north american continental margin we suggest that a thick ophiolite sequence was obducted onto continental material probably during the jurassic nevadan orogeny so that the great valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle

river channel adjustment to meander cutoffs on the river bollin and river dane northwest england morphological adjustment of a channel after cutoff should be expected but rarely has the mode and timescale of adjustment in natural channels been measured four natural cutoffs which have occurred since 1980 on two rivers in northwest england have been monitored from pre cutoff stage two of the cutoffs were neck cutoffs of large loops two were chute cutoffs in which significant bend curvature remained all the cutoffs were the terminal result of progressive bank erosion and or scour of the floodplain rather than overflow and all the cutoffs took place in peak flow events though two of these were of relatively low magnitude high frequency rapid widening and sedimentation took place within the straight channels produced by neck cutoffs immediately after breaching adjustment was mainly by formation of multiple riffles and bars producing a variable morphology in the first 2 4 years subsequently the morphology became more regular and in one case had stabilised within about eight years progressive steepening and acceleration of bank erosion appears to have been propagated upstream in one case but in the other cases erosion was very localised rates of vertical accretion at the entrances to the old channel were much higher than most quoted in the literature rates of change in all morphological sedimentary and biotic components exhibit an exponential decline with time but with slightly differing timescales on these active gravel bed streams major adjustment within the new channels may be completed within 6 12 years though lakes persist in the abandoned channels for much longer

contribution of radar interferometry to a two step inversion of the kinematic process of the 1992 landers earthquake we study the rupture process of the 1992 landers earthquake to limit the trade off between slip amplitude and rupture time that affects solutions using only seismological data we adopt a two step approach we first constrain the slip distribution and its uncertainty by independent geodetic data to recover in the second step the temporal details of the rupture propagation the first step consists of an inversion of interferometric data and global positioning system measurements both independently and together to constrain slip distribution on a three segment fault model along both strike and dip direction we use a genetic algorithm to test the uniqueness of the solution and a least squares formulation to find the model which best fits the data we conclude from the results of these geodetic inversions that interferometric data are rich enough to access the slip distribution in the case of the landers earthquake since the surface deformations are more sensitive to shallow slip in our configuration the slip amplitude is better resolved near the surface than at depth the resulting slip distribution is in agreement with geological observations at the surface and confirms the heterogeneous nature of the landers earthquake most of the slip occurs at shallow depths on the homestead valley fault second segment with a maximum value of around 7 m another high slip zone is observed on the johnson valley fault first segment at 8 km depth in the second step we invert strong motion data with the a priori final slip amplitude and its uncertainty deduced from geodetic data to constrain the time history of the rupture process this second step emphasizes a strong variation of the temporal development of the earthquake fast rupture front velocities appear within high slip zones and the rupture slows when it encounters a resistance along the fault on average the rupture front propagates with velocities close to the s wave velocity and terminates about 20 s after initiation the large variations in both slip amplitude and rupture velocity suggest that the rupture process is better described by successively breaking asperities than by a pulse propagating with constant velocity

chute channel dynamics in large sand bed meandering rivers meander bends of many large sand bed meandering rivers are partitioned by chute channels that convey permanent flow and co exist with the mainstem for decades as a first step toward understanding the dynamics and morphodynamic implications of these bifurcate meander bends this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics regression models developed for the strickland river papua new guinea the lower paraguay river paraguay argentina and the beni river bolivia revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate the rate at which a bend elongates in a direction perpendicular to the valley axis trend image analyses of all rivers and field observations from the strickland suggest that the majority of chute channels form during scroll slough development rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank resulting in widely spaced scrolls with intervening sloughs that are positively aligned with primary over bar flow the rivers plot in order of increasing chute activity on an empirical meandering braided pattern continuum defined by potential specific stream power pv and bedload calibre d50 increasing stream power is considered to result in higher bend extension rates with implications for chute initiation in addition chute stability is shown to depend on river sediment load relative to flow discharge qs q such that while the beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power the formation of stable chute channels is suppressed by the high sediment load this tendency is consistent with previous experimental studies and results in a planform that is transitional between single thread meandering and braided copyright 2011 john wiley sons ltd

quantifying channel development and sediment transfer following chute cutoff in a wandering gravel bed river abstract three dimensional morphological adjustment in a chute cutoff breach alluvial channel is quantified using digital elevation model dem analysis for a ca 0 7 km reach of the river coquet northumberland uk following cutoff in january 1999 channel and bar topography was surveyed using a total station on five occasions between february 1999 and december 2000 analysis of planform change coupled with dem differencing elucidates channel and barform development following cutoff and enables quantification of sediment transfers associated with morphological adjustment within the reach this exercise indicates an initial phase of bed scour followed by a period characterised by extensive bank erosion and lateral channel migration where erosion including bed scour totalled some 15 000 m3 of sediment the channel in the post cutoff disequilibrium state is highly sensitive to relatively low magnitude floods and provision of accommodation space by bank erosion encouraged extensive lateral bar development bar development was further facilitated by infilling of channels abandoned by repeated within reach avulsion and large scale aggradation of sediment lobes deposited by higher magnitude floods calculations indicate that at least 6600 m3 of sediment was deposited on emerging bars within the reach over the survey period and 2300 m3 deposited within the channel sediment losses from the reach may have exceeded 6500 m3

shoreline autoretreat substantiated in flume experiments abstract under conditions of constant sediment supply s to the basin and a steady rate of rise a in relative sea level the autoretreat theory predicts that shorelines can retreat landwards at a relatively early stage of delta growth and attain an autobreak state after which the existing subaqueous slope begins to be starved of sediment and thereby lose its clear delta front configuration a numerical model based on this theory further suggests that shoreline migration depends on the inclination of basin s basement slope and subaqueous depositional slope and to a lesser degree on the inclination of subaerial depositional slope two dimensional flume experiments in which a miniature delta is subject to steady forcing a const 0 s const 0 were conducted to test the expectations of the autoretreat theory the experiments consisted of two different series of runs one was of modulation associated with a narrow range of the ratio of water discharge to sediment discharge q s the other was of q s modulation with kept nearly constant the q s ratio was mostly responsible for changes in but also affected to some extent all experimental runs substantiated the landward turnaround feature of shoreline accretion i e autoretreat and the geometrical break of the delta front shape during its retreat phase i e autobreak the obtained shoreline trajectories hold their geometrical patterns even after they were nondimensionalized in terms of the s a ratio a key hypothesis confirmed in the experiments is that a and s can never balance each other both of the experimental series brought some systematic change of shoreline trajectories but the variations of shoreline trajectory due to modulation were much more striking than those related to q s even these contrasting results are consistent with the numerical predictions the present experimental results support the applicability of the autoretreat concept to natural environments and help clarify the processes governing the regression and transgression of a shoreline on a constructional margin

seismicity and geometry of a 110 km long blind thrust fault 1 the 1985 kettleman hills california earthquake the august 4 1985 kettleman hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110 km long fold chain bounding the eastern california coast ranges the 1982 mw 5 4 new idria 1983 mw 6 5 coalinga and 1985 mw 6 1 kettleman hills events define a southward progression of seismic activity beneath the fold we use teleseismic waveforms geodetic modeling hypocenters relocated in a three dimensional velocity model and subsurface structural data to investigate the kettleman hills earthquake the main shock results from motion on a shallowly dipping thrust fault buried at 10 km depth aftershocks and coseismic fault slip extend 20 km along the fold axis nearly the full extent of the kettleman hills north dome anticline aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline the main shock and several foreshocks occurred at a 2 km right step in the quaternary fold axis which also corresponds to the southern end of the 1983 coalinga and northern end of the 1985 kettleman hills aftershock zones from this we infer that the step in the fold is caused by an offset or tear in the underlying fault the scalar seismic moment is 1 6 1018 n m consistent with the geodetic deformation and the duration of rupture is 16 s 3 4 times greater than for the average earthquake with this scalar moment the slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event the peak of the geodetic uplift is located 5 km perpendicular to the quaternary fold axis we argue that the fault is propagating northeast into the undeformed san joaquin valley sediments and that the overlying fold is growing at about 0 5 mm yr

seismic activity of the san francisco bay region moment magnitude m with objective confidence level uncertainties are estimated for felt san francisco bay region earthquakes using bakun and went worth s 1997 analysis strategy for seismic intensity observations the frequency magnitude distribution is well described for m 5 5 events since 1850 by a gutenberg richter relation with a b value of 0 90 the seismic moment rate m yr since 1836 is 2 68 1018 n m yr 95 confidence range 1 29 1018 n m yr to 4 07 1018 n m yr the seismic moment rate since 1850 is nearly the same m yr in the 56 years before 1906 is about 10 times that in the 70 years after 1906 in contrast m yr since 1977 is about equal that in the 56 years before 1906 80 1 14 of the plate motion moment accumulation rate is available for release in earthquakes the historical m yr and the portion of the plate motion moment accumulation rate available for release in earthquakes are used in a seismic cycle model to estimate the rate of seismic activity in the twenty first century high and low rates of future seismic activity are both permissible given the range of possible seismic cycle recurrence times t and the uncertainties in the historical m and in the percentage of plate motion available for release in earthquakes if the historical seismic moment rate is not greater than the estimated 2 68 1018 n m yr and the percentage of the plate motion moment accumulation available for release in earthquakes is not less than the estimated 80 then for all t the rate of seismic moment release from now until the next 1906 sized shock will be comparable to the rate from 1836 to 1905 when m 6 1 2 shocks occurred every 15 to 20 years

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

sediment storage partitioning in alluvial stratigraphy the influence of discharge variability abstract numerical models of formation of alluvial stratigraphy often specify either explicitly or implicitly the proportion of channel and overbank sediments that are deposited during a given interval of time however little is known about the factors that affect the partitioning of sediment between channels and the overbank environment over long time intervals and the fidelity with which that partition is preserved in the stratigraphic record here we use physical experiments to investigate the role that discharge variability plays in this partitioning in fluvial stratigraphy we find that channels formed under constant flow conditions have low lateral mobility and act mostly as conduits for sediments to reach the shoreline the bulk of the aggradation in this case is derived from sediment laden flow that escapes the main channels by contrast including floods increases channel lateral mobility and this change is recorded in stratigraphy as an increased proportion of channel deposits relative to overbank deposits when variable flow is included as an input condition a large volume of in channel deposition occurs rendering the channels substantial contributors to stratigraphic volume on their own the increase in channel deposit volume is driven mainly by a threefold increase in the average time that a location is subject to in channel aggradation other factors include a slight increase of in channel aggradation rates and an increase in erosion of the overbank environment that results from energetic overbank flows our study shows that the character of a river s hydrograph exerts a significant influence on the proportion of channel to overbank sediment bodies in alluvial successions which is an unexamined source of uncertainty in common stratigraphic models

autogenic attainment of large scale alluvial grade with steady sea level fall an analog tank flume experiment a graded river conveys its sediment load without net deposition or erosion the graded state is thought to represent the long term response of alluvial rivers to steady external forcing we show here that alluvial rivers building deltas can be in grade as an autogenic response to steady sea level fall consider an antecedent graded river profile the upstream end of which consists of an alluvial bedrock transition and the downstream end of which is a fixed overfall where constant sea level is maintained the antecedent graded profile is then drowned by a jump in sea level after which sea level drops the result is a new river profile ending in a prograding delta that deposits on top of the antecedent profile if the rate of sea level fall is constant and the length of the antecedent reach is sufficient the new profile eventually becomes parallel or quasi parallel to the antecedent profile maintaining grade as it progrades in the experiments reported here series of graded river profiles with prograding deltas are created by stacking fluviodeltaic systems each graded profile and its associated delta is stacked on its immediate predecessor for each fluviodeltaic system a graded alluvial profile is attained with any constant rate of sea level fall provided that the antecedent profile is of sufficient length experiments suggest that this autogenic approach to grade is more rapid for higher rates of sea level fall lower rates of sediment supply and higher water discharges

seismicity and geometry of a 110 km long blind thrust fault 2 synthesis of the 1982 1985 california earthquake sequence during 1982 1985 three 5 4 mw 6 5 earthquakes migrated 65 km along the north half of a string of quaternary folds at the east front of the california coast ranges there is no surface fault associated with these earthquakes but the fold chain oriented parallel to the san andreas fault 30 km to the west takes up contraction normal to the san andreas fault identification of active blind thrust faults is hindered by the absence of a fault trace but may be revealed by the presence of growing folds at the surface and by earthquakes at depth because of excellent seismic geodetic and geologic data this earthquake sequence illuminates the geometry of the blind faults and enables us to probe their mechanics we relocate seismicity and compute focal mechanisms in a three dimensional velocity space and model the vertical deformation associated with the 1983 m 6 5 coalinga earthquake the aftershock zones abut at echelon offsets in the fold axes and the mainshocks display reverse slip perpendicular to the axes suggesting that the folds conceal a contiguous segmented thrust fault background seismicity concentrates at bends and breaks in the fold chain sites that may correspond to tears and ramps in the thrust fault at depth seismic reflection profiles reveal thrust and reverse faults dipping toward the san andreas fault at depths of 5 10 km with several kilometers of cumulative slip and high angle reverse faults in the anticlines with several hundred meters cumulative slip coseismic fold uplift accompanied the coalinga and kettleman hills north dome earthquakes suggesting that fold growth is episodic and coupled closely to repeated earthquakes on the underlying thrusts the north half of the fold chain has been the site of several 6 m 6 5 earthquakes since 1885 thus kettleman hills middle dome the next fold segment to the south has an elevated seismic potential the aftershock zones of the three main shocks are diffuse and occupy a region much larger than the site of seismic slip aftershocks occur 5 7 km from the fault regions where on the basis of a boundary element model the shear strain increase caused by the fault slip exceeds 20 ppm equivalent to about 0 7 mpa we argue that the broad aftershock zone is a product of high sustained off fault stress caused by repeated displacement on faults that do not cut the earth s surface the fault tip stresses lead to the formation of secondary faults which can become sites of aftershocks and postseismic creep

3d simulations of multi segment thrust fault rupture thrust faults such as those that underlie the los angeles basin are typically segmented by tear faults that offset the thrust fault segments we perform 3d finite difference simulations of earthquake rupture to evaluate the effectiveness of these offsets in retarding rupture the simulations include orthogonal intersecting faults to model the interaction of the thrust fault segments with the tear fault for reasonable assumptions for fault segment length strength and stress drop rupture can jump offsets of up to 2 km if a tear fault is present consistent with observations of well studied thrust earthquakes absent a tear fault the maximum offset that can be breached is an order of magnitude smaller

accelerated river avulsion frequency on lowland deltas due to sea level rise sea level rise subsidence and reduced fluvial sediment supply are causing river deltas to drown worldwide affecting ecosystems and billions of people abrupt changes in river course called avulsions naturally nourish sinking land with sediment however they also create catastrophic flood hazards existing observations and models conflict on whether the occurrence of avulsions will change due to relative sea level rise hampering the ability to forecast delta response to global climate change here we combined theory numerical modeling and field observations to develop a mechanistic framework to predict avulsion frequency on deltas with multiple self formed lobes that scale with backwater hydrodynamics results show that avulsion frequency is controlled by the competition between relative sea level rise and sediment supply that drives lobe progradation we find that most large deltas are experiencing sufficiently low progradation rates such that relative sea level rise enhances aggradation rates accelerating avulsion frequency and associated hazards compared to preindustrial conditions some deltas may face even greater risk if relative sea level rise significantly outpaces sediment supply then avulsion frequency is maximized delta plains drown and avulsion locations shift inland posing new hazards to upstream communities results indicate that managed deltas can support more frequent engineered avulsions to recover sinking land however there is a threshold beyond which coastal land will be lost and mitigation efforts should shift upstream

source characterization and ground motion modeling of the 1892 vacaville winters earthquake sequence california a sequence of several earthquakes in april 1892 produced significant damage in the towns of winters dixon allendale and vacaville along the boundary between the southwestern sacramento valley and northern coast ranges of california the largest event occurred on 19 april 1892 with a maximum modified mercalli intensity of ix and was assigned a moment magnitude m of 6 5 based on felt area these earthquakes occurred within a zone of active crustal shortening accommodated by postulated blind thrust faults seismotectonic and structural analyses are used to evaluate the depth geometry and segmentation of thrust faults that were the probable sources of the 1892 earthquake sequence synthetic ground motion modeling demonstrates that rupture of a 17 km long segment of the thrust fault system can produce the magnitude and distribution of intensities documented from anecdotal accounts of the 19 april 1892 earthquake including probable directivity effects east of the range front integrated structural and seismotectonic analyses also are used to interpret the role of inferred geometric segment boundaries in arresting the 19 april 1892 earthquake rupture and the subsequent occurrence of the 21 april 1892 aftershock

conditions for branching in depositional rivers it is often taken for granted that rivers organize transport into a single active channel in some net depositional environments however flow of water and sediment is distributed in several stable channels such branching rivers may be confined in valleys anabranching or anastomosed or unconfined on deltas distributaries and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion i e the rapid abandonment of a channel in favor of a new path at lower elevation the requisite conditions for avulsion have been the subject of ongoing debate here we resolve the conditions leading to channel avulsion and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion a compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth from the relative rates of bank erosion and channel sedimentation we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur results may be directly applied to modeling landscape evolution over human and geologic time scales and for inverting formative environmental conditions from channel deposits on earth and other planetary surfaces

nonlinear sediment response during the 1994 northridge earthquake observations and finite source simulations we have addressed the long standing question regarding nonlinear sediment response in the los angeles region by testing whether sediment amplification was similar between the northridge earthquake and its aftershocks comparing the weak and strong motion site response at 15 sediment sites we find that amplification factors were significantly less for the main shock implying systematic nonlinearity the difference is largest between 2 and 4 hz a factor of 2 and is significant at the 99 confidence level between 0 8 and 5 5 hz the inference of nonlinearity is robust with respect to the removal of possibly anomalous sediment sites and how the reference site motion is defined furthermore theoretical ground motion simulations show no evidence of any bias from finite source effects during the main shock nonlinearity is also suggested by the fact that the four sediment sites that contain a clear fundamental resonance for the weak motion exhibit a conspicuous absence of the peak in the strong motion although we have taken the first step of establishing the presence of nonlinearity it remains to define the physics of nonlinear response and to test the methodologies presently applied routinely in engineering practice the inference of nonlinearity implies that care must be exercised in using sediment site data to study large earthquakes or predict strong ground motion

predicting delta avulsions implications for coastal wetland restoration river deltas create new wetlands through a continuous cycle of delta lobe extension avulsion and abandonment but the mechanics and timing of this cycle are poorly understood here we use physical experiments to quantitatively define one type of cycle for river dominated deltas the cycle begins as a distributary channel and its river mouth bar prograde basinward eventually the mouth bar reaches a critical size and stops prograding the stagnated mouth bar triggers a wave of bed aggradation that moves upstream and increases overbank flows and bed shear stresses on the levees an avulsion occurs as a time dependent failure of the levee where the largest average bed shear stress has been applied for the longest time r 2 0 93 these results provide a guide for predicting the growth of intradelta lobes which can be used to engineer the creation of new wetlands within the delta channel network and improve stratigraphic models of deltas

reply to the comment by j e t channell and j s stoner on magnetostratigraphy of the hettangian langmos section adnet austria evidence for time delayed phases of magnetization in our paper we have considered that the early origin of haematite in the reddish wackestones from the langmoos section was questionable for several reasons 1 no clear reversal test antipodal directions is available in this lithology 2 we have evidence for remagnetization at least in parts of the sequence and 3 only one site was studied the comparison i e a fold test between the characteristic component obtained from the reddish samples and the ones obtained from the adnet formation channell et al 1990 1992 does not provide a clear answer the fold test is negative because the bedding attitude is rather flat in this area therefore no direct comparison can be made between the results from the langmoos section which does not belong to the adnet formation and those obtained from the sites near by adnet considering this point we have only proposed that the magnetization in our samples was acquired before the large rotation of the nca

structural transect of the western transverse ranges california implications for lithospheric kinematics and seismic risk evaluation a retrodeformable cross section that integrates surface and subsurface data across the western transverse ranges california illustrates an actively developing fold and thrust belt that began forming at 2 3 ma high level thrusts are interpreted to root in a mid crustal detachment at 12 15 km depth which coincides with the maximum depth of earthquakes the cross section documents 53 km of convergence above the mid crustal detachment dividing this by the time since onset of deformation yields convergence rates of 17 6 26 5 mm yr the high density lithospheric anomaly beneath the transverse ranges is related to subduction of lower crust and lithosphere below the mid crustal detachment to balance the shallow crustal shortening thrust ramps coincide with zones of high seismicity in the transverse ranges this suggests that ramp regions have the highest potential for compressive earthquake events e g the recent whittier narrows earthquake of october 1 1987 m sub l 5 9 occurred along the eastern part of a ramp zone

a revised chronology for mississippi river subdeltas radiocarbon measurements by accelerator mass spectrometry relating to three of the four late holocene mississippi river subdeltas yielded consistent results and were found to differ by up to 2000 carbon 14 years from previously inferred ages these geological dataare in agreement with archaeological carbon 14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework which has implications for prehistoric human settlement patterns coastal evolution and wetland loss and sequence stratigraphic interpretations

drowning of the mississippi delta due to insufficient sediment supply and global sea level rise global sea level rise reduced sediment supply and subsidence threaten the stability of the mississippi delta calculations of riverine sediment load and storage indicate that 10 000 13 500 km2 of the delta could be submerged by ad 2100

active faulting and deformation of the coalinga anticline as interpreted from three dimensional velocity structure and seismicity this work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake interpretation of hypocenters and fault plane solutions from the 1983 coalinga coast range california earthquake sequence in combination with the three dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher velocity material above the main listric blind thrust there is a conjugate fault steeply northeast dipping that provides the western limit of the aftershocks within the coalinga anticline and that corresponds in location and spatial extent with the adjacent pleasant valley syncline the character of the seismicity varies with the degree of previous deformation on each section of the anticline where the previous uplift was largest the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure diffuse seismicity characterizes the area with the least previous deformation the mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features the upward extent of the mainshock rupture ended at the approximate boundary between franciscan and great valley sequence rocks above that depth the main thrust appears to splay into a steeper segment and a near horizontal segment thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material with the three dimensional velocity model each individual hypocenter moved slightly 0 2 km in accord with the details of the surrounding velocity structure so that secondary features in the seismicity pattern are more detailed than with a local one dimensional model and station corrections the overall character of the fault plane solutions was not altered by the three dimensional model but the more accurate ray paths did result in distinct changes in particular the mainshock has a fault plane dipping 30 southwest instead of the 23 obtained with the one dimensional model

robust inversion of iasp91 travel time residuals for mantle p and s velocity structure earthquake mislocations and station corrections using both p and s arrival time information 41 108 events in the international seismological centre isc catalog for the years 1964 to 1987 are relocated relative to the iasp91 velocity model the mean absolute horizontal relocation is 7 7 km and the mean absolute depth relocation is 9 1 km the mean absolute origin time shift is 1 2 s the relocation procedure increased the p residual standard deviation slightly from 2 3 s to 2 5 s while decreasing the s residual standard deviation from 6 8 s to 6 1 s when plotted as bottoming point averages the resulting iasp91 p and s arrival time residuals show coherent patterns as a function of geographic location an iterative lp residual norm minimization algorithm is used to estimate the set of p and s velocity variations as well as the earthquake relocation and seismographic station parameters which best explain the travel time residuals the procedure is robust in that extremely large travel time residuals which are common in the isc data do not unduly influence the velocity estimates both the p and s models of lateral heterogeneity contain prominent circum pacific low velocities 1 to 2 perturbations underlying the back arc basins between 35 and 200 km depth this ring of negative deviations continues into the depth interval 200 400 km the continental cratons are underlain by high velocity anomalies with maximum amplitudes of 2 iceland and the azores are underlain by low velocity mantle material that extends down to at least 400 km the benioff zones are only intermittently imaged as 1 2 high velocity regions in the uppermost 400 km they are best resolved in the p velocity variations both the p and s velocity models contain a circum pacific ring beneath the benioff zones of 1 2 positive velocity deviations in the depth range 660 870 km coherent high velocity features are seen below south america borneo tonga fiji the marianas and the northern kuriles the anomalies beneath south america and borneo extend into the 870 1070 km depth range below depths of 1270 km for p variations and 1070 km for s variations the amplitude of the heterogeneity has decreased significantly it is only in the lowermost mantle 2670 km to the core mantle boundary that the level of p heterogeneity rises significantly above the estimated noise level in this depth range a partial ring around the pacific basin is observed although this

meander cutoffs nonlocally accelerate upstream and downstream migration and channel widening nonlocal influence of cutoffs the hydrologic and sediment dynamics within and near cutoffs have long been studied establishing them as effective agents of rapid local geomorphic change however the morphodynamic impact of individual cutoffs at the reachwide scale remains unknown mainly due to insufficient observations of channel adjustments over large areal extents and at high temporal frequency here we show via annually resolved landsat derived channel masks of the dynamic meandering ucayali river in peru that cutoffs act as perturbations that nonlocally accelerate river migration and drive channel widening both up and downstream of the cutoff locations by tracking planform changes of individual meander bends near cutoffs we find that the downstream distance of cutoff influence scales linearly with the length of the removed reach the discovery of nonlocal cutoff influence supports the hypothesis of avalanche type behavior in meander cutoff dynamics and presents new challenges in modeling and prediction of rivers self adjusting responses to perturbations

a physical model for strain accumulation in the san francisco bay region stress evolution since 1838 1 understanding of the behavior of plate boundary zones has progressed to the point where reasonably comprehensive physical models can predict their evolution the san andreas fault system in the san francisco bay region sfbr is dominated by a few major faults whose behavior over about one earthquake cycle is fairly well understood by combining the past history of large ruptures on sfbr faults with a recently proposed physical model of strain accumulation in the sfbr we derive the evolution of regional stress from 1838 until the present this effort depends on 1 an existing compilation of the source properties of historic and contemporary sfbr earthquakes based on documented shaking geodetic data and seismic data bakun 1999 and 2 a few key parameters of a simple regional viscoelastic coupling model constrained by recent gps data pollitz and nyst 2004 although uncertainties abound in the location magnitude and fault geometries of historic ruptures and the physical model relies on gross simplifications the resulting stress evolution model is sufficiently detailed to provide a useful window into the past stress history in the framework of coulomb failure stress we find that virtually all m 5 8 earthquakes prior to 1906 and m 5 5 earthquakes after 1906 are consistent with stress triggering from previous earthquakes these events systematically lie in zones of predicted stress concentration elevated 5 10 bars above the regional average the sfbr is predicted to have emerged from the 1906 shadow in about 1980 consistent with the acceleration in regional seismicity at that time the stress evolution model may be a reliable indicator of the most likely areas to experience m 5 5 shocks in the future index terms 1206 geodesy and gravity crustal movements interplate 8155 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1243 geodesy and gravity space geodetic surveys keywords crustal deformation plate boundary zones viscoelastic relaxation san francisco bay region

three dimensional p wave velocity model for the san francisco bay region california 1 a new three dimensional p wave velocity model for the greater san francisco bay region has been derived using the double difference seismic tomography method using data from about 5 500 chemical explosions or air gun blasts and approximately 6 000 earthquakes the model region covers 140 km ne sw by 240 km nw se extending from 20 km south of monterey to santa rosa and reaching from the pacific coast to the edge of the great valley our model provides the first regional view of a number of basement highs that are imaged in the uppermost few kilometers of the model and images a number of velocity anomaly lows associated with known mesozoic and cenozoic basins in the study area high velocity vp 6 5 km s features at 15 km depth beneath part of the edge of the great valley and along the san francisco peninsula are interpreted as ophiolite bodies the relocated earthquakes provide a clear picture of the geometry of the major faults in the region illuminating fault dips that are generally consistent with previous studies ninety five percent of the earthquakes have depths between 2 3 and 15 2 km and the corresponding seismic velocities at the hypocenters range from 4 8 km s presumably corresponding to franciscan basement or mesozoic sedimentary rocks of the great valley sequence to 6 8 km s the top of the seismogenic zone is thus largely controlled by basement depth but the base of the seismogenic zone is not restricted to seismic velocities of 6 3 km s in this region as had been previously proposed

the lowermost mississippi river a mixed bedrock alluvial channel in this study the distribution of channel bed sediment facies in the lowermost mississippi river is analysed using multibeam data complemented by sidescan sonar and compressed high intensity radar pulse seismic data as well as grab and core samples of bed material the channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls the consolidated substratum is made up of latest pleistocene and early holocene fluvio deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends the exposed substratum commonly displays a suite of erosional features including flutes that are quantitatively similar in form to those produced under known laboratory conditions a total of five bed facies are mapped three of which include modern alluvial deposits and two facies that are associated with the relict substratum a radius of curvature analysis applied to the mississippi river centreline demonstrates that the reach scale distribution of channel bed facies is related to river planform from a broader perspective the distribution of channel bed facies is related to channel sinuosity higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment for example the ratio of alluvial cover to substratum is ca 1 5 1 for a 45 km segment of the river that has a sinuosity of 1 76 and this ratio increases to ca 3 1 for a 120 km segment of the river that has a sinuosity of 1 21 the exposed substratum is interpreted as bedrock and given the relative coverage of alluvial sediment in the channel the lowermost mississippi river can be classified as a mixed bedrock alluvial channel the analyses demonstrate that a mixed bedrock alluvial channel boundary can be associated with low gradient and sand bed rivers near their marine outlet

a new asymptotic method for the modeling of near field accelerograms we study high frequency radiation from a dislocation model of rupture propagation at the earthquake source we demonstrate that in this case all the radiation emanates from the rupture front and by a change of variables that at any instant of time the high frequency waves reaching an observer come from a line on the fault plane that we call isochrone an asymptotic approximation to near source velocity and acceleration is obtained that involves a simple integration along the isochrones for every time step it is shown that wave front discontinuities critical or stopping phases are radiated every time an isochrone becomes tangent to a barrier this leads to what we call the critical ray approximation which is given in a closed form the previous results are compared with discrete wavenumber synthetics obtained by bouchon 1982 for the gilroy 6 recording of the coyote lake earthquake of 1980 the fit between the asymptotic and full numerical method is extremely good the critical ray approximation permits the identification of different phases in bouchon s synthetics and the prediction of the behavior of the signal in the vicinity of their arrival time

meander hydromorphology of ephemeral streams similarities and differences with perennial rivers abstract the geomorphology hydrology and processes of ephemeral streams are poorly known and studies on the geomorphic characteristics of ephemeral meandering streams ems are even less investigated we collected geomorphic data namely channel width sinuosity wavelength and curvature radius from 107 ems reaches in different drylands of the world from googleearth pro satellite images geomorphology features processes such as neck cutoffs point scrolls and chute cutoffs were also assessed the main aim of this paper is to investigate similarities and differences between ems and perennial counterparts of more humid areas on which almost all hydrogeomorphic models are based for the purpose of explaining why meandering rivers form in environments where the lack of vegetation sporadic flows and a high sediment supply should favour the development of braided rivers ems showed both geomorphological similarities and differences with perennial rivers point bar chute cutoffs and point bar scrolls are the most common geomorphic features of ems suggesting a certain stability of meander behaviour though within a continuous process of changes towards the most probable form similarly to perennial streams the bimodal distribution of curvature ratio values departs substantially from that of perennial rivers that are commonly in the 2 3 range data from the rivers in this study indicate that their channel pattern is well described by the same sine generated function used for perennial rivers wavelength to channel width ratio and bankfull discharge to streambed gradient of ems unexpectedly plot in different areas of classical diagrams developed for perennial rivers we interpret such differences in terms of excess energy expenditure for high sediment load transport we also propose a new function to discriminate the meandering from the braided pattern of ephemeral streams and present a conceptual model of meander formation in ephemerals streams based on an autogenic process of bank collapse and bar deposition

the style of late cenozoic deformation at the eastern front of the california coast ranges the 1983 coalinga earthquake occurred at the eastern boundary of the california coast ranges in response to northeast directed thrusting such movements over the past 2 ma have produced coalinga anticline by folding above the blind eastern tip of the coalinga thrust zone the 600 km length of the coast ranges boundary shares a common structural setting that involves westward upturn of cenozoic and cretaceous strata at the eastern front of the coast ranges and a major southwest facing step in the basement surface beneath the western great valley like coalinga anticline pliocene and quaternary folding and faulting along the rest of the boundary also result from northeast southwest compression acting nearly perpendicular to the strike of the san andreas fault we suggest that much of this deformation is related to active thrusts beneath the eastern coast ranges the step in the basement surface beneath the great valley seems to have controlled the distribution of this deformation and the shape of the coast ranges boundary

punctuated sand transport in the lowermost mississippi river 1 measurements of sand flux and water flow in the mississippi river are presented for a portion of the system 35 50 km upstream from the head of its subaerial delta these data are used to provide insight into how nonuniform flow conditions present in the lower reaches of large alluvial rivers affect the timing and magnitude of sand transport near the river outlet field surveys during both low and high water discharge include 1 sequential digital bathymetric maps defining mobile river bottom topography which were used to estimate bed material flux 2 multiple water velocity profiles and 3 multiple suspended sediment profiles collected using a point integrated sampler these data show that total sand transport increases by two orders of magnitude over the measured range in water discharge 11 300 to 38 400 m3 s 1 during low water discharge no sand is measured in suspension and sand discharge via bed form migration is minimal during high water discharge 54 of the sand discharge is measured in suspension while 46 of the sand discharge is part of bed form migration the component of boundary shear stress associated with moving this sediment is estimated using a set of established sediment transport algorithms and values for the total boundary shear stress are predicted by fitting logarithmic velocity functions to the measured profiles the estimates of boundary shear stress using measurements of suspended sand transport bed form transport and downstream oriented velocity profiles are internally consistent moreover the analyses show that boundary shear stress increases by nearly 10 fold over the measured water discharge range we show how this increase in shear stress is consistent with backwater flow arising where the river approaches its outlet the hydrodynamic properties of backwater flow affect the timing and magnitude of sand flux and produce punctuated sand transport through the lowermost mississippi river our field data are used to evaluate the influence of this sand transport style on development of the mixed bedrock alluvial channel for the lowermost mississippi river

three dimensional model of alluvial stratigraphy theory and applications abstract a three dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution proportion and connectedness of coarse grained channel belt deposits in alluvial strata as a function of channel belt width floodplain width bankfull channel depth channel belt and overbank sedimentation rates avulsion location and period compaction and tectonism tilting and faulting in this model a floodplain surface of variable width and length is occupied by a single channel belt changes in floodplain topography are produced by spatial and temporal variation of channel belt and floodplain deposition rates and by compaction and local or regional tectonism the location and timing of avulsions are determined by local changes in floodplain slope relative to channel b lt slope and by flood magnitude and frequency the diverted channel belt follows the locus of maximum floodplain slope at the end of each simulation architectural parameters are calculated including channel deposit proportion and connectedness and the dimensions of channel belt sandstone bodies three dimensional perspective diagrams mesh surfaces and two dimensional stratigraphic sections can be plotted to illustrate depositional surfaces time planes and the location and geometry of coarse grained channel belt deposits within finer grained overbank deposits the model predicts that channel belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross section orientation upstream from avulsion points sandstone bodies have low width thickness ratios because of aggradation in a fixed channel belt immediately downstream from avulsion points channel belts tend to be connected resulting in sandstone bodies with high width thickness ratios avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period such sequences may produce successions in which channel belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years down valley increases in aggradation rate or down valley decreases in floodplain slope for example associated with a rise in base level may result in an increase in channel belt proportion and connectedness because of high avulsion frequencies in down valley regions of the floodplain down valley decreases in aggradation rate as in alluvial fans foreland basins and during base level fall may result in high avulsion frequencies in up valley parts of the floodplain tectonic tilting and faulting locally increase avulsion probabilities and channel belts generally shift toward areas of maximum subsidence under certain conditions however depositional topography may cause channels to shift away from areas of maximum subsidence channel deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain but distribution clustering of channel belts may not be a reliable indicator of fault geometry or displacement models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified the three dimensional model presented here predicts many of the features of channel behavior observed in modern rivers but there is a pressing need for better models and adequate natural data to test them

progressive arcward contraction of a mesozoic tertiary fore arc basin southwestern sacramento valley california the modern sacramento valley intermontane basin is the successor to a deformed mesozoic tertiary fore arc basin based on analysis of seismic reflection profiles drill hole data and surface mapping we propose a model for progressive arcward contraction of the fore arc basin in the rumsey hills region southwestern sacramento valley beginning in cretaceous and continuing episodically through cenozoic time we present our interpretation in the form of a kinematically restorable forward model the major tectonic and depositional events include 1 mid cretaceous or older shortening of fore arc basin strata along imbricate west dipping thrust faults 2 erosion of the deformed fore arc strata 3 onlap of cenomanian and younger fore arc strata onto deformed older strata and crystalline sierran basement 4 renewed shortening in latest cretaceous to early tertiary accommodated by delamination of upper cretaceous strata from the mid cretaceous angular unconformity tectonic wedging and west directed backthrusting 5 erosion of the deformed upper cretaceous strata and deposition of the eocene capay formation 6 uplift and east directed shortening from eocene to late neogene with local erosion of the capay formation over the ancestral rumsey hills 7 uplift of the northern coast ranges beginning ca 3 4 ma and deposition of the pliocene pleistocene tehama formation in the western sacramento valley as a syntectonic molasse and 8 fault propagation folding of the tehama formation beginning ca 0 5 1 0 ma with the exception of quaternary shortening progressive arcward contraction of the western basin margin occurred during east dipping subduction and plate convergence beneath western california the deformation of the ancestral sacramento valley fore arc basin is analogous to arcward contraction of the tobago trough in the lesser antilles arc trench system and suggests that tectonic wedging may be a relatively common process in the evolution of fore arc regions late quaternary shortening has occurred in a transpressional setting and suggests a continuity in structural style despite a change in plate boundary kinematics from convergence to transpression during the past 3 5 m y

detailed river stage mapping and head gradient analysis during meander cutoff in a laboratory river analytical models of river evolution predict meander narrowing and elongation which creates sinuosity driven hyporheic exchange across the meander neck by decreasing flow distance and increasing head loss we used a laboratory river table and close range photogrammetry to map and analyze sinuosity as a driver of head gradients and hyporheic exchange during cutoff the river valley had relatively high slopes 1 8 and moderately cohesive sediment 10 talc 90 sand to facilitate cutoff and ratios of horizontal to vertical scaling were distorted to achieve dynamic similitude re 3200 incipient to cutoff the head gradient across the neck increased due to a narrowing neck upstream aggradation and downstream degradation longitudinal and transverse river surface slopes around the meander bend increased as the meander approached cutoff the steep head gradient across the moderately cohesive meander neck generated seepage erosion and scour that formed a low sinuosity avulsion sediment rich flow in the avulsed channel aggraded the downstream bed and separated the active channel and oxbow lake the limitation in geometric and dynamic similitude in the river table limits extrapolation to natural rivers yet river evolution may involve aggradation and degradation induced channel head loss and turnover hyporheic exchange as well as seepage induced meander neck erosion our submillimeter maps of meander morphology and water stage provide data to parameterize river evolution and hyporheic exchange models and may inform analysis and mapping of field sites

crustal structure of the ancestral northwestern california forearc region from seismic reflection imaging implications for convergent margin tectonics abstract analysis of a suite of 2 d seismic reflection profiles reveals that the northwestern sacramento valley and eastern coast range foothills northern california are underlain by a system of blind west dipping thrust faults homoclinally east dipping and folded mesozoic marine forearc strata exposed along the western valley margin define the forelimbs of northeast vergent fault propagation folds developed in the hanging walls of the thrusts exhumed coherent blueschists of the accretionary complex and attenuated remnants of the ophiolitic forearc basement presently exposed in the eastern coast ranges are in the hanging wall of the blind thrust system and have been displaced from their roots in the footwall deep east dipping magnetic reflectors in the footwall of the thrust system may be fragments of sheared serpentinized and attenuated ophiolitic basement restoration of slip on the thrusts suggests that the coast range fault which is the exposed structural contact between the coherent blueschists and attenuated ophiolite originally dipped east and is associated with the east dipping magnetic reflectors in the footwall this interpretation of the reflection data is consistent with previous inferences about the deep structure in this region and supports a two stage model for blueschist exposure in the eastern coast ranges 1 blueschist exhumation relative to the forearc basin by attenuation of the ophiolitic basement along the east dipping coast range fault system in late cretaceous 2 blueschists attenuated ophiolite and forearc strata all were subsequently uplifted and folded in the hanging wall of the blind thrust system beginning in latest cretaceous early tertiary the blind thrust system probably rooted in and was antithetic to the east dipping subduction zone beneath the forearc region active transpressional plate motion in western california is locally accommodated in part by reactivation of blind thrust faults that originally developed during the convergent regime

historical analysis indicates seepage control on initiation of meandering in analytical and numerical models of river meandering initiation of meandering typically occurs uniformly along the streamwise coordinate in the channel based on a historical analysis of the nierskanaal here we show how and under which circumstances meandering has initiated in isolated sections of a channel the nierskanaal was constructed by the end of the 18th century as a straight channel between the river niers and the river meuse the purpose of this measure was to reduce flood risk in the downstream reaches of the river niers the banks on the dutch part of the channel were left unprotected and developed into a morphodynamically active channel featuring a meandering planform and valley incision the planform development and incision process is analysed using topographic maps and airborne lidar data meandering initiated in three sections of the channel where the channel sinuosity developed asynchronously sedimentary successions in the study area show layers of iron oxide indicating groundwater seepage from aeolian river dunes and river deposits located nearby only at the spots where meandering has initiated iron oxide is found close to the surface level this provides a clue that seepage triggered bank erosion by increasing moisture content of the banks the isolated meandering sections expanded in the longitudinal direction valley incision has developed in the first decades after the construction of the channel and diminished after a gravel layer was reached gravel was deposited in the downstream half of the channel bed acting as an armouring layer the spatial variation in meandering behaviour as observed in the nierskanaal justifies efforts to implement the influence of floodplain heterogeneity and the effect of seepage on bank erosion in meander models

chute cutoff as a morphological response to stream reconstruction the possible role of backwater stream restoration efforts often aim at creating new unconstrained meandering channels without weirs and bank revetments in reconstructed streams the initial morphological response of the new streams is often rapid until a dynamic equilibrium is reached here we report on a chute cutoff that occurred within 3 months after realization of a stream restoration project caused by a plug bar that formed in response to a backwater effect the temporal evolution of the morphology of both the new and the old channels was monitored over a period of nearly 8 months including precutoff conditions the observations can be separated into three stages stage 1 is the initial period leading to cutoff vulnerability stage 2 is the actual cutoff and stage 3 is the morphological adjustment in response to the cutoff in stage 1 a plug bar was deposited in one of the channel bends hydrodynamic model results show the location of the plug bar coincides with a region where bed shear stress decreased in downstream direction due to backwater longitudinal channel bed profiles show that the channel slope decreased soon after channel reconstruction hence sediment from upstream was available to form the plug bar after the plug bar was deposited an embayment formed in the floodplain at a location where the former channel was located stage 2 the former channel was filled with sediment prior to channel construction it is likely that the sediment at this location was less consolidated and therefore prone to erosion the chute channel continued to incise and widen into the floodplain and after 6 months acted as the main channel conveying the discharge during the majority of time stage 3 the cutoff channel gradually continued to fill with sediment from the moment the plug bar formed until the chute channel incised into the floodplain sedimentary successions of the deposited material show upward fining which is in agreement with observations of chute cutoffs in rivers although the artificial setting limits the degree in which the observed processes can be projected on natural rivers the observations prompt to investigate the role of backwater effects in natural chute cutoff initiation

a physical model for strain accumulation in the san francisco bay region summary strain accumulation in tectonically active regions is generally a superposition of the effects of background tectonic loading steady state dislocation processes such as creep and transient deformation in the san francisco bay region sfbr the most uncertain of these processes is transient deformation which arises primarily in association with large earthquakes as such it depends upon the history of faulting and the rheology of the crust and mantle which together determine the pattern of longer term decade scale post seismic response to earthquakes we utilize a set of 102 gps velocity vectors in the sfbr in order to characterize the strain rate field and construct a physical model of its present deformation we first perform an inversion for the continuous velocity gradient field from the discrete gps velocity field from which both tensor strain rate and rotation rate may be extracted the present strain rate pattern is well described as a nearly uniform shear strain rate oriented approximately n34 w 140 nanostrain yr 1 plus a n56 e uniaxial compression rate averaging 20 nanostrain yr 1 across the shear zone we fit the velocity and strain rate fields to a model of time dependent deformation within a 135 km wide arcuate shear zone bounded by strong pacific plate and sierra nevada block lithosphere to the sw and ne respectively driving forces are purely lateral consisting of shear zone deformation imposed by the relative motions between the thick pacific plate and sierra nevada block lithospheres assuming a depth dependent viscoelastic structure within the shear zone we account for the effects of steady creep on faults and viscoelastic relaxation following the 1906 san francisco and 1989 loma prieta earthquakes subject to constant velocity boundary conditions on the edges of the shear zone fault creep is realized by evaluating dislocations on the creeping portions of faults in the fluid limit of the viscoelastic model a priori plate boundary pb parallel motion is set to 38 mm yr 1 ag rid search based on fitting the observed strain rate pattern yields a mantle viscosity of 1 2 10 19 p as and a pb perpendicular convergence rate of 3m m yr 1 most of this convergence appears to be uniformly distributed in the pacific sierra nevada plate boundary zone

avulsion cycles and their stratigraphic signature on an experimental backwater controlled delta river deltas grow in large part through repeated cycles of lobe construction and channel avulsion understanding avulsion cycles is important for coastal restoration and ecology land management and flood hazard mitigation emerging theories suggest that river avulsions on lowland deltas are controlled by backwater hydrodynamics however our knowledge of backwater controlled avulsion cycles is limited here we present results from an experimental delta that evolved under persistent backwater hydrodynamics achieved through variable flood discharges shallow bed slopes and subcritical flows the experimental avulsion cycles consisted of an initial phase of avulsion setup an avulsion trigger selection of a new flow path and abandonment of the parent channel avulsions were triggered during the largest floods 78 of avulsions after the channel was filled by a fraction 0 3 0 13 of its characteristic flow depth at the avulsion site which occurred in the upstream part of the backwater zone the new flow path following avulsion was consistently one of the shortest paths to the shoreline and channel abandonment occurred through temporal decline in water flow and sediment delivery to the parent channel experimental synthetic stratigraphy indicates that bed thicknesses were maximum at the avulsion sites consistent with our morphologic measurements of avulsion setup and the idea that there is a record of avulsion locations and thresholds in sedimentary rocks finally we discuss the implications of our findings within the context of sustainable management of deltas their stratigraphic record and predicting avulsions on deltas

contemporary seismicity and tectonics of the northern and central coast ranges sierran block boundary zone california abstract beneath the physiographic boundary between the coast ranges and the great valley a fundamental tectonic boundary exists between the coast ranges province and the sierran block recent geophysical studies have revealed the coast ranges sierran block crsb boundary zone to be a complex region of compressional tectonics wherein wedges of comparatively ductile franciscan complex of the coast ranges have overridden the competent sierran block basement and peeled up the overlying sediments of the great valley sequence the boundary zone was the probable source of the two 1892 winters earthquakes ml 6 7 and the 1983 coalinga earthquake ml 6 7 we evaluated the seismicity within the northern and central portions of the zone from red bluff south to san luis reservoir for the period 1969 1985 seismicity was spatially diffuse most of the earthquakes occurred in somewhat isolated concentrations within the upper crust a low activity level temporal clustering and episodic behavior also characterized this seismicity the largest earthquake in the boundary zone was a ml 4 2 event near madison 20 events exceeded ml 3 0 in a few cases the seismogenic sources appear to be eastward dipping high angle reverse faults although some right slip north trending faults also appear to be seismogenic in comparison the coalinga and 1985 kettleman hills main shocks in the southern portion of the zone appeared to involve gently westward dipping thrust faults although aftershocks also occurred on moderately to steeply dipping reverse faults fault plane solutions along the zone exhibited a tectonic stress field of ne to e trending compression normal to the boundary such compression may be a consequence of one or a combination of several influences 1 the westward movement of the sierran block against the coast ranges due to late cenozoic crustal extension in the great basin 2 slightly convergent motion between the pacific and north american plates and 3 fault normal crustal compression due to the low shear strength of the san andreas fault the tectonic deformation manifested by the coalinga and recent moderate sized earthquakes in the southern zone appears to be occurring along the full extent of the boundary albeit at varying rates for different segments thus the potential for large earthquakes may exist along segments of the crsb boundary and the 1892 main shock may represent a characteristic earthquake for that portion of the zone

mesozoic paleogeography of the northern calcareous alps evidence from paleomagnetism and facies analysis the northern calcareous alps are generally thought to have been part of the same southern tethyan continental margin as the southern alps during late jurassic and early cretaceous time contrasting liassic paleomagnetic data from the northern calcareous alps and the southern alps can be explained by large scale clockwise rotation during late cretaceous and tertiary tectonic emplacement of the northern calcareous alps onto the european continental margin alternatively the data can be reconciled by postulating that the northern calcareous alps did not undergo the african mesozoic rotational history seen in the southern alps and were separated from the southern alps by a short lived branch of jurassic ocean facies analysis in the karwendel thiersee syncline of the northern calcareous alps indicates that the jurassic basin sediments in this area were deposited in an east west oriented sinistral strike slip fault system in the southern alps and central austroalpine complex coeval basin sedimentation was controlled by north south oriented normal faults this contrasting basin evolution and evidence for a source of ophiolitic detritus south of the northern calcareous alps argue for a short lived ocean basin south of the northern calcareous alps that was closed by early cretaceous time

scaling relationships and evolution of distributary networks on wave influenced deltas 1 no genetic model can explain the variability in distributary network pattern on modern deltas here we derive scaling relationships for two processes known to create distributary channels and with these laws construct a simple model for distributary network evolution the first process is mouth bar deposition at the shoreline and subsequent channel bifurcation the second is avulsion the wholesale abandonment of a channel in favor of a new path the former creates relatively small networks with power law distributions of channel length the latter generates relatively few backwater scale distributaries frequency magnitude plots of channel length on natural deltas agree with theoretical predictions and show a clear separation in scale that reflects these processes mouth bar distributary lengths scale with the width of the parent channel and avulsive distributary lengths scale with the backwater length intermediate channel lengths are relatively rare wave energy controls network topology by suppressing mouth bar development thereby preferentially eliminating smaller scale distributaries

fluvial and marine controls on combined subaerial and subaqueous delta progradation morphodynamic modeling of compound clinoform development 1 fluviodeltaic systems commonly display a compound clinoform geometry that consists of a subaerial subaqueous delta couplet the extent of subaqueous delta development varies significantly and in modern systems is a function of fluvial input and basin hydrodynamics we present a model of fluviodeltaic progradation in which the repeated occurrence of characteristic terrestrial floods and large coastal storms drives fluvial and shallow marine morphodynamics respectively we couple fluvial and shallow marine sediment dynamics via the surf zone which we collapse to a shock condition and treat as a moving boundary with steady sediment supply and sea level and simple basin geometry our model naturally develops prograding deltas with compound clinoform geometries the subaerial delta grows via fluvial aggradation and shoreface progradation whereas the subaqueous delta expands through foreset progradation with only minor topset aggradation the interplay of fluvial input with the wave current field controls the basic partitioning of sediment between subaerial and subaqueous deltas and by extension the compound clinoform geometry increasing the frequency or magnitude of coastal storms decreasing flood frequency or discharge and reducing grain size all increase the fraction of sediment delivered to the shallow marine environment and the extent of subaqueous delta progradation relative to subaerial delta development our model which emphasizes the intrinsic coupling of fluvial and shallow marine sediment dynamics and downplays the importance of allogenic fluctuations can explain many of the first order morphologic features of natural delta systems including significant lateral separation of the shoreline and clinoform rollover and differing rates of subaerial and subaqueous delta progradation

post eocene movement on the coast range thrust northern sacramento valley california subsurface structure mapping with more than 600 wells and 200 miles of seismic data in a portion of the northern sacramento basin and surface geologic mapping in the rumsey hills area to the west indicates that upper cretaceous strata along the western edge of the basin are doubled in thickness along thrust faults these east dipping detachments are part of the coast range thrust fault system eocene strata crop out in the fault zone and indicate that considerable post eocene movement occurred cretaceous movement on these faults can be surmised but not proven from reconstructions similarly analysis from five subsurface structure maps to the east shows that deformation there also is post eocene only minor upper cretaceous deformation can be discerned underthrusting of franciscan accretionary rocks best accounts for the development of these faults and a western high along the basin margin

filtering spatial error from dems implications for morphological change estimation abstract scour and fill estimation from digital elevation model dem subtraction or differencing is an increasingly common technique in morphological and sediment transport investigations the technique is commonly used to estimate scour and fill volumes and to produce scour and fill maps that provide process based information to geomorphologists accounting for sources of uncertainty within the dem is of critical importance dem error is spatially variable and has a tendency to be greater at breaks of slope such as bar and bank edges in the past however this has been achieved using a uniform error metric across the dem resulting in over conservative estimates of error in turn this has led to over conservative scour and fill volumes and incorrect process interpretation this paper applies a new approach that permits assessment of spatially distributed error across a dem the method is tested on a sequence of field surveys of the gravel bed river nent cumbria uk the results demonstrate some dramatic differences application of conventional techniques that account for mean error across a dem led to a 15 and 31 underestimation in scour and fill volumes respectively between july and october 1998 whilst for the october 1998 june 1999 subtraction 31 and 13 of scour and fill were underestimated respectively use of a uniform error across a surface captures less change in comparison to a spatially distributed approach furthermore the changes captured using a uniform error are biased toward areas of the channel that have more local topographic variability such as bar and bank edges in contrast the use of a spatially distributed approach provides information on change from flatter surfaces such as bar tops that would otherwise be missed this study also demonstrates that estimates of morphological change can be misleading in the absence of an error filter where the raw survey data is available it is recommended that sediment budgeting studies take account of the spatial variability of error in each dem involved in the subtraction

a synthetic stratigraphic model of foreland basin development the processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy we present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry topography and facies patterns in the model steady crustal shortening occurs according to a wedgethickening model erosion and deposition follow a diffusive process and the lithosphere is compensated elastically erosion and deposition are controlled by the transport coefficients r of the diffusion equation for a range of thrust velocities and lithospheric rigidities transport coefficients are of order 102 103 m yr in the mountain belt these values are much higher than those derived from the study of scarp degradation in the sedimentary basin transport coefficients of order 104 m2 yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition rapid thrusting results in a narrow underfilled basin while slow thrusting results in a wide overfilled basin in addition by varying the erosional and depositional transport coefficients while holding other parameters constant we generate both overfilled and underfilled basins these results suggest that changes in the rate of thrust loading the climate or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere clearly to understand either the long term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata the manner in which a basin was filled must be considered we apply the model to the evolution of the modern sub andean foreland and find that an erosional transport coefficient of 3000 m2 yr and a depositional transport coefficient of 20 000 m2 yr successfully predict the observed basin geometry

paleomagnetism and paleogeography of the northern calcareous alps austria prefolding magnetization components have been resolved at 38 sites of liassic adnet limestones in the central northern calcareous alps nca and at one site in the drauzug the site mean directions are northeasterly and down and are closer to coeval european directions than to southern alpine african directions the western end of both the nca and of the drauzug yield different site mean directions and negative fold tests indicating remagnetization the sampling localities have been rotated relative to one another during thrusting however comparison of paleomagnetic poles with the european apparent polar wander path implies an average clockwise rotation of about 30 for the nca relative to europe larger scale clockwise rotations in excess of 80 relative to the southern alps are implied by comparison with the southern alpine african polar wander path facies analysis of jurassic and lower cretaceous basinal sediments in the central part of the nca suggests that liassic subsidence was related to presently e w sinistral strike slip faults whereas malm berriasian subsidence was controlled by presently e w south dipping normal faults both paleomagnetic and facies data tend to indicate that the nca did not undergo the african rotation seen in southern alpine paleomagnetic data this implies the existence of a short lived tract of triassic jurassic ocean floor between the nca and the southern alps which linked the pindos ocean to the ligurian trough and allowed adria to rotate independently from the nca during the late jurassic eo alpine deformation and metamorphism are associated with the early cretaceous closure of this hallstatt trough

a regional view of urban sedimentary basins in northern california based on oil industry compressional wave velocity and density logs compressional wave sonic and density logs from 119 oil test wells provide knowledge of the physical properties and impedance contrasts within urban sedimentary basins in northern california which is needed to better understand basin amplification these wire line logs provide estimates of sonic velocities and densities for primarily upper cretaceous to pliocene clastic rocks between 0 1 and 5 6 km depth to an average depth of 1 8 km regional differences in the sonic velocities and densities in these basins largely 1reflect variations in the lithology depth of burial porosity and grain size of the strata but not necessarily formation age for example miocene basin filling strata west of the calaveras fault exhibit higher sonic velocities and densities than older but finer grained and or higher porosity rocks of the upper cretaceous great valley sequence as another example hard eocene sandstones west of the san andreas fault have much higher impedances than eocene strata mainly higher porosity sandstones and shales located to the east of this fault and approach those expected for franciscan complex basement rocks basement penetrations define large impedence contrasts at the sediment basement contact along the margins of several basins where quaternary pliocene and even miocene deposits directly overlie franciscan or salinian basement rocks at depths as much as 1 7 km in contrast in the deepest geographic centers of the basins such logs exhibit only a modest impedance contrast at the sediment basement contact at depths exceeding 2 km prominent up to 1 km sec and thick up to several hundred meters velocity and density reversals in the logs refute the common assumption that velocities and densities increase monotonically with depth online material borehole log regression parameters and metadata

morphodynamics of subaqueous levee formation insights into river mouth morphologies arising from experiments 1 levee formation by deposition from sediment laden flows debouching into open waters has been conceptually and analytically linked to jet hydrodynamics since the middle of the 20th century despite this long standing association between jets and levees the morphodynamics controlling subaqueous levee development remain largely unquantified and poorly understood here we report the results of physical experiments on subaqueous levee development modeled on floodplain tie channel processes in the laboratory we created subaqueous levees from a sediment laden jet entering a basin of still water levee formation occurred where the local shear velocity of the jet declined below the critical shear velocity for entrainment of sediment into suspension the rate of levee deposition depended on the rate of lateral sediment transfer to the jet margins and the settling velocity of suspended sediment in our friction dominated flows lateral sediment flux to the levees was primarily due to dispersive lateral transport driven by turbulence high rates of sediment transfer from the jet core to the margins led to levee growth that outpaced deposition along the flow centerline under hypopycnal conditions we failed to produce levees we observed distinct differences in levee morphology due to changes in the size and density of suspended sediment under identical hydrodynamic conditions our experimental results and review of published data on hypopycnal river mouths suggest that suspended sediment characteristics relative magnitude of bed friction and channel outlet aspect ratios may have a more significant influence on river mouth morphology than the buoyancy of discharging waters as envisioned in existing conceptual models

experimental study on the process of neck cutoff and channel adjustment in a highly sinuous meander under constant discharges abstract neck cutoff is an essential process limiting evolution of meandering rivers in particular the highly sinuous ones yet this process is extremely difficult to replicate in laboratory flumes here we reproduced this process in a laboratory flume by reducing at the 1 2500 scale the current planform of the qigongling bend centerline length 13 km channel width 1 2 km and neck width 0 55 km in the middle yangtze river with geometric similarity in five runs with different constant input discharges hydraulic parameters water depth surface velocity and slope bank line changes and riverbed topography were measured by flow meter and point gauges and bank line migration and a neck cutoff process were captured by six overhead cameras mounted atop the flume by analyzing the neck cutoff process development of the cutoff channel and adjustment of the old channel to cutoff we found that i bank erosion around the upstream and downstream channel segments of the neck reduced its distance subsequently increased water head gradient on both sides of the neck inducing the occurrence of neck cutoff in a short time period ii the width of the new cutoff channel increased quickly after neck cutoff because the increased local slope generated a higher unit stream power in the cutoff channel and iii neck cutoff significantly increased bank erosion and channel widening in upstream and downstream channels as it is a gradual process compared with chute cutoff these results suggest that bank revetment around the qigongling bend might be necessary to prevent the abrupt occurrence of natural neck cutoff in the future

a record of large earthquakes during the past two millennia on the southern green valley fault california we document evidence for surface rupturing earthquakes events at two trench sites on the southern green valley fault california sgvf the 75 80 km long dextral sgvf creeps 1 4 mm yr we identify stratigraphic horizons disrupted by upward flowering shears and infilled fissures unlikely to have formed from creep alone the mason rd site exhibits four events from 1013 ce to the present the lopes ranch site lr 12 km to the south exhibits three events from 18 bce to present including the most recent event mre 1610 52 yr ce 1 and a two event interval 18 bce 238 ce isolated by a millennium of low deposition using oxcal to model the timing of the four event earthquake sequence from radiocarbon data and the lr mre yields a mean recurrence interval ri or of 199 82 yr 1 and 35 yr standard error of the mean the first based on geologic data the time since the most recent earthquake open window since mre is 402 yr 52 yr well past 200 yr the shape of the probability density function pdf of the average ri from oxcal resembles a brownian passage time bpt pdf i e rather than normal that permits rarer longer ruptures potentially involving the berryessa and hunting creek sections of the northernmost gvf the model coefficient of variation cv is 0 41 but a larger value cv 0 6 fits better when using bpt a bpt pdf with of 250 and cv of 0 6 yields 30 yr rupture probabilities of 20 25 versus a poisson probability of 11 17 online material oxcal model for ages of paleoearthquakes lopes site 1998 logs and description of a model for estimation of rupture frequency

alluvial cutoffs as indicators of former channel conditions although alluvial cutoffs record accurately the geometry bedforms and bed material of the channel when last active few attempts have been made to use cutoffs in studies of channel changes a detailed record of historical channel changes on the lower hunter river in southeastern australia has shown that this channel responds to naturally alternating periods of high and low flood activity called flood and drought dominated regimes respectively sinuosity decreased from 3 84 in 1870 to 2 66 in 1893 and to 1 38 in 1970 through the development of eight cutoffs the channel also aggraded with medium sand burying the former bed material of mixed mud coarse sand and gravel channel straightening was a response to increased flood frequencies during the flood dominated regimes of the late 19th and 20th centuries combined with localized river engineering works and increased sand load detailed stratigraphic studies were carried out on three neck cutoffs and one chute cutoff which were abandoned in 1890 1950 1952 and 1956 a comparison of former and present bed elevations and bed material size showed similar trends to those determined by the historical record confirming the reliability of cutoffs as indicators of former channel conditions the sedimentary infills of the cutoffs are not uniformly fine grained as recorded previously in the literature relatively thin fine grained fills were deposited during the drought dominated regime of the first half of this century but thick coarser grained fills were deposited after 1949 during the flood dominated regime all fills fine upwards cutoff infills provide a record of changing flood activity and sediment loads

backwater controls of avulsion location on deltas river delta complexes are built in part through repeated river channel avulsions which often occur about a persistent spatial node creating delta lobes that form a fan like morphology predicting the location of avulsions is poorly understood but it is essential for wetland restoration hazard mitigation reservoir characterization and delta morphodynamics following previous work we show that the upstream distance from the river mouth where avulsions occur is coincident with the backwater length i e the upstream extent of river flow that is affected by hydrodynamic processes in the receiving basin to explain this observation we formulate a fluvial morphodynamic model that is coupled to an offshore spreading river plume and subject it to a range of river discharges results show that avulsion is less likely in the downstream portion of the backwater zone because during high flow events the water surface is drawn down near the river mouth to match that of the offshore plume resulting in river bed scour and a reduced likelihood of overbank flow furthermore during low discharge events flow deceleration near the upstream extent of backwater causes enhanced deposition locally and a reduced channel fill timescale there both mechanisms favor preferential avulsion in the upstream part of the backwater zone these dynamics are fundamentally due to variable river discharges and a coupled offshore river plume with implications for predicting delta response to climate and sea level change and fluvio deltaic stratigraphy

dynamic changes and processes in the mississippi river delta research in the modern delta of the mississippi river has revealed short term changes and processes that are of significant magnitude deltaic lobes each lobe covering an area of 30 000 sq km and having an average thickness of 35 km switch sites of deposition on an average of every 1 500 yr through short periods of geologic time this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas within a single delta lobe and operating on an even higher frequency are bay fills and overbank splays bay fills having areas of 250 sq km and thickness of 15 m require only 150 yr to accumulate four major events have taken place in the modern balize delta since 1838 overbank splays are much smaller covering areas of less than 2 sq km and having thicknesses of 3 m but are associated with high floods on the river at the river mouth continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long 8 km wide and a thickness of 80 m in a period of only 200 yr differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr on the subaqueous delta platform sediment instabilities operate nearly continuously mass moving large quantities of shallow water deposits to deeper water environments via arcuate rotational slides and mudflow gullies and depositional lobes all of these changes and processes operate at differing spatial and temporal scales but all result in deposition of large volumes of sediment over extremely short periods of time

flow structure and channel morphodynamics of meander bend chute cutoffs a case study of the wabash river usa received 25 october 2012 revised 23 september 2013 accepted 25 september 2013 published 12 december 2013 1 this paper documents the three dimensional structure of flow and bed morphology of two developing chute cutoffs on a single meander bend on the lower wabash river usa and relates the flow structure to patterns of morphologic change in the evolving cutoff channels the upstream end of the cutoff channels is characterized by 1 a zone of flow velocity reduction stagnation and bar development in the main channel across from the cutoff entrance 2 flow separation and bar development along the inner left bank of the cutoff channel immediately downstream from the cutoff entrance and 3 helical motion and outward advection of flow momentum entering the cutoff channel leading to erosion of the outer right bank of the cutoff channel at the downstream end of the cutoff channels the majorhydrodynamic and morphologic features are 1 flow stagnation along the bank of the main channel immediately upstream of the cutoff channel mouth 2 convergence of flows from the cutoff and main channels 3 helical motion of flow from the cutoff 4 a zone of reduced velocity along the bank of the main channel immediately downstream from the cutoff channel mouth and 5 development of a prominent bar complex that penetrates into the main channel and extends from the stagnation zone upstream to downstream of the cutoff mouth these results provide the basis for a conceptual model of chute cutoff dynamics in which the upstream and downstream ends of a cutoff channel are treated as a bifurcation and confluence respectively

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

experimental study of avulsion frequency and rate of deposition in existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate however if avulsion is driven by superelevation of a river bed above its surrounding flood plain one might expect avulsion rate to increase with sedimentation rate we have carried out a series of experiments with laboratory scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition in the regime of increasing avulsion frequency the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate although our experimental results cannot simply be scaled up to natural rivers they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture these models should be applied with caution until avulsion mechanics are better understood

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

evaluation of recurrence intervals characteristic earthquakes and slip rates associated with thrusting along the coast range central valley geomorphic boundary california abstract a seismically active blind fold and thrust belt underlies the coast range central valley geomorphic boundary crcv california earthquakes associated with this fault system include the 1983 m w 6 5 coalinga and 1985 m w 6 1 kettleman hills events published estimates of the crcv slip rate derived from published studies range from 1 to 10 mm yr and estimated recurrence intervals of coalinga type events range from 200 to 2000 yr relative consistency of geomorphic expression and regional geologic relationships suggests that deformation rates are similar along the 500 km length of the crcv we evaluated historical seismicity to further constrain recurrence interval and slip rate estimates based on structural geology geomorphology and historical seismicity the crcv may comprise 18 to 25 segments from 12 to 57 km in length with most segments between 20 and 30 km the characteristic earthquake for the average length segment may be m w 6 3 to 6 4 or slightly smaller than the coalinga earthquake and the longest segments may yield earthquakes as large as m w 6 8 to 7 1 during the approximately 150 yr period of historical record 11 earthquakes of m w 5 8 to 6 8 of which eight may be characteristic events have taken place on the crcv the characteristic earthquakes appear to have ruptured single segments using the estimates of co seismic rupture length for the 1983 coalinga and 1985 kettleman hills earthquakes to calibrate earlier events the historical earthquake record corresponds to rupture of 32 to 39 of the length of the crcv in 142 yr a recurrence interval of 360 to 440 yr for the average segment combining the recurrence estimates with estimates for co seismic displacement yields slip rates of 1 8 to 8 0 mm yr because recent vlbi studies suggest that the total convergence rate normal to the pacific north american plate boundary at the latitude of the crcv is 3 mm yr the lower end of the slip rate range 1 8 to 3 mm yr is favored for the crcv if these estimates are correct the crcv accommodates most of the plate convergence between the north american and pacific plates indicating that the historical earthquakes efficiently released strain accumulated on the crcv and suggesting that aseismic deformation along the crcv is minor

replication of apparent nonlinear seismic response with linear wave propagation models it is necessary to understand ground motion amplification by sediment defined as the ratio of ground motions at sediment sites to those at rock sites to predict seismic loadings for earthquake engineering at sediment sites observed weak motion amplifications from magnitude 3 to 4 aftershocks of the 1994 northridge earthquake were twice as large as magnitude 6 7 mainshock amplifications amplitude dependent nonlinear amplification by sediment is one explanation however earthquake simulations with empirical impulse responses and elastic finite difference calculations with weakly heterogeneous random three dimensional 3d crustal velocity variations show that linear wave propagation can explain observed apparently nonlinear sediment responses random 3d velocity variations also reproduce the observed log normal dispersion of peak ground motions deterministic wave propagation models are not adequate to quantify the scaling and dispersion of near source ground motions

unravelling the conundrum of river response to rising sea level from laboratory to field part i laboratory experiments the most recent deglaciation resulted in a global sea level rise of some 120 m over approximately 12 000 years in this part i of two parts a moving boundary numerical model is developed to predict the response of rivers to this rise the model was motivated by experiments at small scale which have identified two modes describing the transgression of a river mouth autoretreat without abandonment of the river delta no sediment starvation at the topset foreset break and sediment starved autoretreat with abandonment of the delta in the latter case transgression is far more rapid and its effects are felt much further upstream of the river mouth the moving boundary numerical model is checked against experiments the generally favourable results of the check motivate adaptation of the model to describe the response of the much larger fly strickland river system papua new guinea to holocene sea level rise this is done in the companion paper part ii

distributary channels and their impact on sediment dispersal a global analysis of world deltas with details from a natural and an anthropogenic influenced delta demonstrates how distributary channels control the flux of sediment into the coastal ocean the study addresses the range in the number of distributary channels across world deltas using remote sensing techniques a power law relationship is found between the number of distributary channels and the length of river and separately the delta gradient these relationships hold for all types of deltas whether controlled or strongly influenced by waves river discharge tides or ice permafrost or sea ice nature controlled deltas such as the klinaklini delta have distributary channels that act as overflow conduits that become active during flooding events anthropogenic controlled deltas like the po delta have distributary channels that are controlled for flood mitigation or low flow maintenance anthropogenic influences greatly impact the natural rate of delta progradation through changes in sediment supply controlling the position of distributary channels and impacting subsidence from gas and groundwater extraction even with flood controls the po delta traps 16 of the sediment load in its channels that are becoming super elevated at rates of 4 to 10 cm yr with respect to the delta plain a new model is formulated and shown to predict accurately the sediment flux through each channel along with their hydraulic properties deltas with high numbers of distributary channels produce hypopycnal plumes with reduced transport capacity as a result sediment diffuses out of the multi channel deltas as a buoyantly driven plume rather than as a momentum driven jet