petroleum systems of the oldest gas field in china neoproterozoic gas pools in the weiyuan gas field sichuan basin abstract weiyuan the oldest gas field in china is a rare example of the giant neoproterozoic gas pools this field is located in southwestern sichuan basin with a proven in place gas reserve around 40 billion cubic meters bcm or 1 41 trillion cubic feet tcf in reservoirs of the sinian dengying formation this field is almost depleted now with a cumulative gas production of 14 4 bcm and a recovery factor of around 36 the neoproterozoic gas pools are also geographically overlapped by gas pools in the lower permian maokou formation with an additional proven in place gas reserve about 0 861 bcm or 30 2 bcf gases in the sinian reservoirs are relatively rich in nitrogen co 2 h 2 s helium and argon with gaseous alkanes showing l 3 c 1 13 c 2 and l3 c 1

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

formation of rhythmic sorted bed forms on the continental shelf an idealised model an idealised model is presented to study the formation of sorted bed forms generated by a wind driven along shore current the study employs a linear stability analysis to describe the time development of perturbations of both bottom composition and bed elevation superimposed on a flat bed composed of a sediment mixture homogeneously distributed in space the model considers both bed and suspended loads and takes into account the averaged influence of waves on the flow field and the transport of sediment the results show that the positive coupling between waves along shore current and the erodible heterogeneous bed leads to the amplification of two modes which exhibit distinct characteristics a first mode is found to be dominant when moderate hydrodynamic conditions are considered and is primarily amplified by the convergence of sediment transport induced by the changes in the bed elevation this mode has wavelengths of the order of hundred metres and has coarse fine sediments in its troughs crests by increasing the height of the waves and or the strength of the steady current the second mode can become dominant this mode is characterised by shorter wavelengths and results from the interaction between the convergence of sediment transport related to changes in the bottom composition and that induced by perturbations of the bed elevation these bed features can have an up current or a down current shift between the centre of the coarse grained bands and the trough of the bottom wave typical growth times of the amplified features are of the order of hundreds of days and the migration rates in the direction of the along shore current range between 0 1 and 10 m per day a qualitative comparison of the model results with field observations indicates that the generation of two distinct modes provides a possible explanation for the broad range of characteristics of the natural bed features

revisiting the linear theory of sand dune formation a linear stability theory for dune and antidune formation is presented that implements a rotational two dimensional flow model as in previous linear theories the phase lag between sediment transport and bed elevation remains the main mechanism driving instability however it is shown that this phase lag varies significantly in a neighbourhood of the bed moreover since the layer in which sediments are moving has a finite though small thickness it is assumed that the perturbations of the fluid stress driving bedload transport should be evaluated at the top of the layer itself it is shown that such an apparently minor modification of the classical approach alters remarkably the balance between stabilizing and destabilizing effects that drives the instability process this also allows a clarification of the debated role of bedslope on the formation of dunes and antidunes following the above ideas antidunes are shown to form in the absence of suspended sediment load and without any effect associated with sediment inertia the present analysis ultimately allows a successful unification of the theories of dune and antidune formation

nonlinear modelling of shoreface connected ridges impact of grain sorting and interventions abstract the evolution of shoreface connected sand ridges on micro tidal inner shelves and the variations in the mean grain size over these ridges are investigated with process based models a review of previous studies is presented as well as new results that concern the influence of grain sorting on the finite amplitude behaviour of the ridges the application of the model to la barrosa beach and the role of wave topography feedbacks the ridges initially form due to morphodynamic self organisation in which the presence of waves and a storm driven current are crucial predicted growth time scales migration speeds topography and spatial pattern of the mean grain size agree with field data collected on micro tidal shelves in the case that both bedload and suspended load sediment transport are accounted for together with spatially non uniform wave orbital motion the model can not successfully explain the presence of large scale ridges observed on la barrosa inner shelf because strong and complex behaving tidal currents occur in that area nonlinear model simulations show that on the long term the height of the ridges evolves towards a finite constant value whilst their migration speed hardly changes during the evolution in the saturated stage the ridges have asymmetrical profiles with steep slopes on the downstream sides the maximum variation in mean grain size also tends to a constant value and during the evolution the spatial lag between the patterns in the mean grain size and topography decreases the processes that cause these changes are identified and explained model results can be obtained for transverse bottom slopes up to 50 of their observed values on micro tidal shelves extrapolation of results to realistic values of the inner shelf slope yields in case of long island shelf a final height that agrees with observed ridge heights but the modelled variation in mean grain size is small compared to field data finally the response of ridges to large scale interventions is considered experiments reveal that extraction of sand on the inner shelf causes a decrease of the sand volume stored in the surf zone

monitoring aseismic forcing in fault zones using earthquake time series the dynamics of earthquake occurrences is controlled both by fault in teraction processes and by long term tectonic loading of the faults in addition tran sient loading can be caused by aseismic deformation episodes for example during crustal fluid migration or slow slip events these forcing transients are best revealed by geodetic measurements however this type of instrumentation is not always avail able or is not always sensitive enough to detect significant anomalies in such cases one is better off exploiting the seismicity signature of these transients in order to char acterize them we here explore different ways to do so interearthquake time statistics are found to be prone to damping out fluctuations in forcing rate a more accurate method is developed by comparing the data with a triggering model that accounts for earthquake interactions the changes in fault loading rates are then well recovered both in duration and in intensity

crustal deformation in the india eurasia collision zone from 25 years of gps measurements the india eurasia collision zone is the largest deforming region on the planet direct measurements of present day deformation from global positioning system gps have the potential to discriminate between competing models of continental tectonics but the increasing spatial resolution and accuracy of observations have only led to increasingly complex realizations of competing models here we present the most complete accurate and up to date velocity field for india eurasia available comprising 2576 velocities measured during 1991 2015 the core of our velocity field is from the crustal movement observation network of china i ii 27 continuous stations observed since 1999 56 campaign stations observed annually during 1998 2007 1000 campaign stations observed in 1999 2001 2004 and 2007 260 continuous stations operating since late 2010 and 2000 campaign stations observed in 2009 2011 2013 and 2015 we process these data and combine the solutions in a consistent reference frame with stations from the global strain rate model compilation then invert for continuous velocity and strain rate fields we update geodetic slip rates for the major faults some vary along strike and find that those along the major tibetan strike slip faults are in good agreement with recent geological estimates the velocity field shows several large undeforming areas strain focused around some major faults areas of diffuse strain and dilation of the high plateau we suggest that a new generation of dynamic models incorporating strength variations and strain weakening mechanisms is required to explain the key observations seismic hazard in much of the region is elevated not just near the major faults

sorted bed forms as self organized patterns 1 model development 1 here we present a modified version of the exploratory numerical model originally presented by murray and thieler 2004 and use it to further investigate the development of sorted bed forms on the inner continental shelf the new version of the model is based on widely accepted parameterizations for hydrodynamics and sediment transport that are more detailed and empirically tested than the parameterizations used in the original model the shallow water wave equations are replaced by the general equations and changes in water depth related to morphological development are accounted for this model reproduces the main results of the previous one supporting the hypothesis that sorted bed forms are a self organized pattern and develop as a result of a coupling between bed composition and sediment flux the sensitivity of the model predictions evaluated in terms of the geometry of the sorted bed forms and their growth rate with respect to external forcing variables and internal model parameterizations shows increasing decreasing sorted bed form spacing height with increasing wave height and period the opposite occurs for increasing water depths more complicated behavior is observed when the ratio fine to coarse sediment diameter or the percentage of fine and coarse material is varied the shape of the suspended sediment and current profiles and the formulation used to predict the geometry of wave generated ripples can significantly affect or even inhibit growth rates of sorted bed forms results from a sensitivity analysis show that there are deficiencies in the description of small scale processes that limit the ability to quantitatively predict sorted bed form evolution highlighting gaps in our knowledge of sediment transport over mixed grain beds that need to be filled

pseudotachylite on impact marks of block surfaces in block and ash flows at merapi volcano central java indonesia surfaces of meter sized blocks in the 1998 block and ash flow deposits of merapi volcano are partially covered by centimeter sized randomly orientated impact marks which consist of an outer glassy pseudotachylite underlain by a cataclastic layer whole rock pseudotachylite melt inclusion in plagioclase and host rock groundmass compositions indicate that the pseudotachylite was generated by remelting of bulk rock on block impact the occurrence and distribution of this new type of collision related pseudotachylite on volcanic block surfaces demonstrate that blocks were transported by chaotic rotation saltation and tumbling the random orientation of impact marks suggests grain flow as the dominant process rather than any other currently discussed pyroclastic flow mechanism in addition the chaotic orientation of striations is interpreted as reflecting momentum transfer having been dominated by short lived intergranular collisions the blocks have apparently been transported in the collisional regime of grain flow

earthquakes induced by water injection at 3 km depth within the rongchang gas field chongqing china 1 unwanted water amounting more than 1 million m 3 has been injected intermittently at a pumping pressure of 2 1 2 9 mpa over hydrostatic at 2 6 2 9 km depth within the rongchang gas field western chongqing china since july 1988 the injections have induced more than 32 000 surface recorded earthquakes including 2 of m l 5 14 of m l 4 and more than 100 of m l 3 up until the end of 2006 we examined the epidemic type aftershock sequence etas model and the temporal evolution of several statistics of the earthquake sequence the random component in the etas model can be considered as an indicator of fluid driven activity while the rate of omori law type aftershocks indicates stress triggering of the preceding earthquakes we observed three distinct seismic activity phases that were coincident with different injection periods phase i corresponds to initial injections with low injection rate long time interval and shows a vast majority of omori type events and only 8 externally forced activity phase ii corresponds to a period of high injection rates and shows intense earthquake activity characterized by a high percentage 45 of fluid driven earthquakes phase iii demonstrates decreasing seismicity as a result of a lowering in the injection rate more than 70 of earthquakes occurred randomly over time indicating the dominance of external triggering the physical mechanism that induced the rongchang earthquake sequence was a change in coulomb s failure stress caused by pore pressure diffusion and transformed from seismic slip of preceding earthquakes a pressure increase of 2 1 2 9 mpa at the injection well is more than sufficient to induce failure within the local formations

a detailed view of the injection induced seismicity in a natural gas reservoir in zigong southwestern sichuan basin china 1 seismicity at a gas reservoir located in the relatively stable sichuan basin china mirrors the injection pressure of unwanted water suggesting that the seismicity is injection induced injection under high pressure on a routine basis began on 9 january 2009 and continued to july 2011 during the injection period over 120 000 m3 of water was pumped under a wellhead pressure of up to 6 2 mpa into the limestone formation of permian 2 45 to 2 55 km beneath the surface the injection induced more than 7000 surface recorded earthquakes including 2 m4 the largest one was ml4 4 20 m3 and more than 100 m2 events data observed by a nearby local seismic network and five temporal stations provide a detailed view of the spatiotemporal distribution of the induced earthquakes most events were limited to depths ranging from 2 5 to 4 km which is consistent with the limestone formation of permian in a map view hypocenters are concentrated in a nnw extended ellipsoidal zone approximately 6 km long and approximately 2 km wide centered approximately at the injection well multisources of evidence such as the shear mechanism pattern of hypocenter distribution and small elevated pore pressure as compared with the least principal stress in the region show that the induced earthquakes occurred as a result of lowering of the effective normal stress on known or unknown preexisting blind faults which are critically loaded under the regional stress field epidemic type aftershock sequence modeling results indicate that injection inducing and earthquake triggering are both important during earlier periods of injection while later periods are dominated by forced injection induced seismicity

horizontal and vertical sediment sorting in tidal sand waves modeling the finite amplitude stage the bed of coastal seas displays a large number of rhythmic bed features of which sand waves are relevant to study from an engineering perspective sediments tend to be well sorted over these bed forms which is so far poorly understood in terms of modeling of finite amplitude sand waves using delft3d we employ bed stratigraphy and consider four different grain size classes which are normally distributed on the phi scale the standard deviation sortedness is then varied whereas the geometric mean grain size is kept constant the results show that typically the crests of sand waves are coarser than the troughs residual flow causes net sedimentation on the lee side of the crest and consequently the general sorting pattern is distorted since larger grains experience a larger settling velocity they are deposited on the upper lee slope whereas the smaller grains are found on the lower lee slope due to sand wave migration also the internal structure of the sand wave is revealed which follows the same sedimentation pattern as the lee slope surface these results qualitatively agree with sorting patterns observed offshore the sorting processes lead to longer wavelengths and lower wave heights as a function of standard deviation this relates to the dampening effect of suspended sediment transport for fine grains finally it appears that the modeled wave heights fall in the same range as observations in the north sea these results are valuable for e g predicting the morphological response after engineering activities and determining suitable aggregates for sand extraction

morphodynamics of a field of crescent shaped rippled scour depressions northern monterey bay ca abstract despite the prevalence of rippled scour depression rsd on the world s continental shelves and their importance as nursery habitats for many commercially important species the processes responsible for their formation and geomorphic evolution are still not well understood most studies that focused on rsd evolution have been based on data acquired over multiple years to decades and often during calmer summer months here multiple geophysical seafloor mapping surveys of a field of rsd s off santa cruz california usa were conducted over the course of two winters with distinctly different oceanographic forcing these results were compared to time series oceanographic measurements made within the survey area and put into the context of long term trends from survey data collected during previous decades although the migration of these features across the seafloor occurs on decadal time scales significant change was detected within a given year depending on the wave climate and fluvial sediment input rsds shrank up to 16 or grew up to 28 in a span of a few months such change was predominantly the result of redistribution of fine grained sediment input to the system by local rivers migration of the seafloor features appears to be the result of accumulation of fine grained sediment at the boundary between the rsd and the surrounding fine grained cap

global review of human induced earthquakes the human induced earthquake database hiquake is a comprehensive record of earthquake sequences postulated to be induced by anthropogenic activity it contains over 700 cases spanning the period 1868 2016 activities that have been proposed to induce earthquakes include the impoundment of water reservoirs erecting tall buildings coastal engineering quarrying extraction of groundwater coal minerals gas oil and geothermal fluids excavation of tunnels and adding material to the subsurface by allowing abandoned mines to flood and injecting fluid for waste disposal enhanced oil recovery hydrofracturing gas storage and carbon sequestration nuclear explosions induce earthquakes but evidence for chemical explosions doing so is weak because it is currently impossible to determine with 100 certainty which earthquakes are induced and which not hiquake includes all earthquake sequences proposed on scientific grounds to have been human induced regardless of credibility challenges to constructing hiquake include under reporting which is 30 of m 4 events 60 of m 3 events and 90 of m 2 events the amount of stress released in an induced earthquake is not necessarily the same as the anthropogenic stress added because pre existing tectonic stress may also be released thus earthquakes disproportionately large compared with the associated industrial activity may be induced knowledge of the magnitude of the largest earthquake that might be induced by a project mmax is important for hazard reduction observed mmax correlates positively with the scale of associated industrial projects fluid injection pressure and rate and the yield of nuclear devices it correlates negatively with calculated inducing stress change likely because the latter correlates inversely with project scale the largest earthquake reported to date to be induced by fluid injection is the 2016 m 5 8 pawnee oklahoma earthquake by water reservoir impoundment the 2008 m 8 wenchuan people s republic of china earthquake and by mass removal the 1976 m 7 3 gazli uzbekistan earthquake the minimum amount of anthropogenic stress needed to induce an earthquake is an unsound concept since earthquakes occur in the absence of industrial activity the minimum amount of stress observed to modulate earthquake activity is a few hundredths of a megapascal and possibly as little as a few thousandths equivalent to a few tens of centimeters of water table depth faults near to failure are pervasive in the continental crust and induced earthquakes may thus occur essentially anywhere in intraplate regions neither infrastructure nor populations may be prepared for earthquakes human induced earthquakes that cause nuisance are rare but in some cases may be a significant problem e g in the hydrocarbon producing areas of oklahoma usa as the size of projects and density of populations increase the potential nuisance of induced earthquakes is also increasing and effective management strategies are needed

modern sedimentation on the shoreface and inner continental shelf at wrightsville beach north carolina u s a abstract the geologic framework and surficial morphology of the shoreface and inner continental shelf off the wrightsville beach north carolina barrier island were mapped using high resolution sidescan sonar bathymetric and seismic reflection surveying techniques a suite of over 200 diver vibracores and extensive seafloor observations by divers the inner shelf is a sediment starved active surface of marine erosion modern sediments where present form a patchy veneer over tertiary and quaternary units the lithology of the underlying units exerts a primary control on the distribution texture and composition of surficial sediments as well as inner shelf bathymetry the shoreface is dominated by a linear cross shore morphology of rippled scour depressions rsds extending from just seaward of the surf zone onto the inner shelf on the upper shoreface the rsds are incised up to 1 m below surrounding areas of fine sand and have an asymmetric cross section that is steeper sided to the north on the inner shelf the rsds have a similar but more subdued cross sectional profile the depressions are floored primarily by shell hash and quartz gravel vibracore data show a thick up to 1 5 m sequence of rsd sediments that unconformably overlies ancient coastal lithosomes in this sediment starved inner shelf setting rippled scour depressions probably form initially on preexisting coarse sediment substrates such as modern lag deposits of paleofluvial channel lithosomes or ancient tidal inlet thalwegs interannual observations of seafloor morphologic change and the longer term record contained in vibracores suggest that the present seafloor morphology is either relatively stable or represents a recurring preferential morphologic state to which the seafloor returns after storm induced perturbations the apparent stability is interpreted to be the result of interactions at several scales that contribute to a repeating self reinforcing pattern of forcing and sedimentary response which ultimately causes the rsds to be maintained as sediment starved bedforms responding to both along shore and across shore flows sediment accumulation from over 30 years of extensive beach nourishment at wrightsville beach appears to have exceeded the local shoreface accommodation space resulting in the leaking of beach and shoreface sediment to the inner shelf a macroscopically identifiable beach nourishment sediment on the shoreface and inner shelf was used to identify the decadal scale pattern of sediment dispersal the nourishment sediment is present in a seaward thinning wedge that extends from the beach over a kilometer onto the inner shelf to waters depths of 14 m this wedge is best developed offshore of the shoreline segment that has received the greatest volume of beach nourishment

sorting and bed waves in unidirectional shallow water flows the stability of a uniform flow above an erodible bed composed of a bimodal mixture of sediments is investigated by means of linear analysis results show that for any given set of the flow and sediment parameters two distinct modes of instability arise each one characterized by its own wave speed growth rate and longitudinal wavelength each one involving spatial variations of both grain size density and bed elevation although at a linear level no information on the amplitude of the perturbations is gathered the analysis of the eigenvectors associated with the two modes of instability allows for an easy classification in terms of the relative amplitudes of the perturbations of bed elevation and size density one eigenvalue is shown to be associated with the modifications of bed forms induced by the presence of the heterogeneous mixture such as the local accumulation of finer and coarser material along the unit wavelength the other with the formation of the low amplitude sorting waves known as bedload sheets in the present unidirectional shallow water framework only the sorting wave is found to be unstable since dunes and antidunes the relevant bed forms for this case require a more refined rotational flow model in order to become unstable on the other hand the simple flow model adopted allows for the formulation of an algebraic eigenvalue problem that can be solved analytically allowing for a deep insight into the mechanisms that drive both instabilities

dome collapse mechanisms and block and ash flow emplacement dynamics inferred from deposit and impact mark analysis mono craters ca abstract characteristics of the panum block and ash flow baf deposit mono craters ca were analyzed to determine the mechanisms of collapse of the parent dome and dynamics of emplacement of the baf granulometry componentry and obsidian water content data were used to define distinct facies of the panum baf deposit these suggest a sequential three stage collapse model for the ancestral dome of the panum vent with destabilization first of its cold brittle outer margins and then of its hot ductile interior to an estimated depth of 260 m below the free air surface impact marks on clast faces that resulted from clast to clast interactions recorded details of the impacting mechanism we analyzed 15 000 marks and report for the first time their size shape and sorting to infer the flow regime of the baf during emplacement to a distance of 2 6 km from the vent clast to clast interactions occurred over a broad spectrum from purely collisional normal to clast faces to purely frictional clasts shearing in a unidirectional flow and recorded a transition to a more frictional flow regime in distal reaches as the sliding head of the baf decelerated and halted finally we briefly differentiate a similar but previously undescribed older deposit that predates the 1325 1350 a d north mono eruptive episode

bottom topography and roughness variations as triggering mechanisms to the formation of sorted bedforms 1 a new model is presented that addresses the occurrence of sorted bedforms on inner shelves of coastal seas these features consist of alternating bands of coarse and fine grains with small topographic undulations we show that inherent feedbacks between alongshore currents sea waves bottom topography and roughness variations result in the emergence of morphological patterns similar to the observed bed features we find that two modes are amplified which display different characteristics yielding a possible explanation to the diversity exhibited by sorted bedforms in the field the results illustrate that both roughness variations and bottom topography influence the generation of sorted bedforms

the 17 july 1999 block and ash flow baf at colima volcano new insights on volcanic granular flows from textural analysis abstract on july 17 1999 a strong explosion occurred at colima volcano mexico that produced a 10 km high eruptive column the partial column collapse originated a block and ash flow baf that flowed to the south along the san antonio and montegrande ravines travelling 3 3 km from the volcano summit the flow filled the ravines with a volume estimated at 7 9 105 m3 the erosion of these deposits occurred between 1999 and 2002 time of sampling providing excellent longitudinal outcrops that allowed their detailed textural study the study was carried out by means of quantitative textural analysis 1 rosiwal intersections for carrying out vertical granulometric profiles 2 total grain size analysis from 11 to 9 and 3 fourier and fractal analysis of the particle morphology grain size and morphometric parameters obtained with these methods were used to identify vertical and longitudinal variation patterns in the baf deposit the grain size variations allowed to infer the main particle segregation mechanisms that acted during transport and deposition of the studied bafs the two methods used for studying the particle shape morphologies yielded results with different accuracy and reliability in particular fractal analyses have been found to be the most effective in describing the particle support mechanisms that acted during transport and deposition of the studied bafs the results highlight the importance of the information obtained by means of these techniques and provide new insights in transportation and deposition mechanisms of bafs

detailed spatiotemporal evolution of microseismicity and repeating earthquakes following the 2012 mw 7 6 nicoya earthquake we apply a waveform matching technique to obtain a detailed earthquake catalog around the rupture zone of the 5 september 2012 moment magnitude 7 6 nicoya earthquake with emphasis on its aftershock sequence starting from a preliminary catalog we relocate 7900 events using tomodd to better quantify their spatiotemporal behavior relocated aftershocks are mostly clustered in two groups the first is immediately above the major coseismic slip patch partially overlapping with shallow afterslip the second one is 50 km se to the main shock nucleation point and near the terminus of coseismic rupture in a zone that exhibited little resolvable afterslip using the relocated events as templates we scan through the continuous recording from 29 june 2012 to 30 december 2012 detecting approximately 17 times more than template events we find 190 aftershocks in the first half hour following the main shock mostly along the plate interface later events become more scattered in location showing moderate expansion in both along trench and downdip directions from the detected catalog we identify 53 repeating aftershock clusters with mean cross correlation values larger than 0 9 and indistinguishably intracluster event locations suggesting slip on the same fault patch most repeating clusters occurred within the first major aftershock group very few repeating clusters were found in the aftershock grouping along the southern edge of the peninsula which is not associated with substantial afterslip our observations suggest that loading from nearby afterslip along the plate interface drives spatiotemporal evolution of aftershocks just above the main shock rupture patch while aftershocks in the se group are to the se of the observed updip afterslip and poorly constrained

rippled scour depressions on the inner continental shelf off central california abstract side scan sonar records taken during the recent coastal ocean dynamics experiment code show elongate shore normal rippled depressions of low relief on the inner continental shelf off central california between bodega bay and point arena these features extend up to 2 km from the coast into water depths of up to 65 m the proposed mechanism for their generation is storm generated bottom currents associated with coastal downwelling during the late fall and winter which scour the surficial fine sand sediment and expose the coarser sand substrate in the depressions the zones of most intense erosion and the irregular spacing of the features may be controlled by submerged rock ledges and other prominent coastal features the large straight crested ripples within the depressions heights to 40 cm wavelengths to 1 7 m are probably formed by large amplitude long period surface waves generated by winter storms

sorted bed forms as self organized patterns 2 complex forcing scenarios 1 we employ a numerical model to study the development of sorted bed forms under a variety of hydrodynamic and sedimentary conditions results indicate that increased variability in wave height decreases the growth rate of the features and can potentially give rise to complicated a priori unpredictable behavior this happens because the system responds to a change in wave characteristics by attempting to self organize into a patterned seabed of different geometry and spacing the new wavelength might not have enough time to emerge before a new change in wave characteristics occurs leading to less regular seabed configurations the new seabed configuration is also highly dependent on the preexisting morphology which further limits the possibility of predicting future behavior for the same reasons variability in the mean current magnitude and direction slows down the growth of features and causes patterns to develop that differ from classical sorted bed forms spatial variability in grain size distribution and different types of net sediment aggradation degradation can also result in the development of sorted bed forms characterized by a less regular shape numerical simulations qualitatively agree with observed geometry spacing and height of sorted bed forms also in agreement with observations is that at shallower depths sorted bed forms are more likely to be affected by changes in the forcing conditions which might also explain why in shallow waters sorted bed forms are described as ephemeral features finally simulations indicate that the different sorted bed form shapes and patterns observed in the field might not necessarily be related to diverse physical mechanisms instead variations in sorted bed form characteristics may result from variations in local hydrodynamic and or sedimentary conditions

decay rate of fluid induced seismicity after termination of reservoir stimulations we present a model describing the seismicity rate of fluid injection induced seismicity we put the focus on seismicity induced after termination of fluid injections here our primary objective is the identification of parameters controlling the decay rate of seismicity the particular importance of a theoretical model for postinjection seismicity is underlined by observations after stimulations of geothermal reservoirs at different locations for instance the postinjection phase is relevant for a seismic risk which up to now has been difficult to control because processes leading to postinjection events are not well understood based on the assumption of pore pressure diffusion as the governing mechanism leading to the triggering of seismic events we develop a method to calculate the seismicity rate during and after fluid injections we find that the decay rate of seismicity after termination of injection is very similar to the omori law which describes the decay rate of aftershock activity after t

grain sorting effects on the formation of tidal sand waves a model is developed to investigate the process which leads to the formation of sand waves in shallow tidal seas characterized by a heterogeneous sea bed composition the main goal of the analysis is the evaluation of the effects that a graded sediment has on the formation of the bottom forms and the investigation of the sorting process induced by the growth of the bottom forms the analysis is based on the study of the stability of the flat bed configuration i e small amplitude perturbations are added to the flat bottom and a linear analysis of their time development is made for an oscillatory tidal current dominated by one tidal constituent the results show that the graded sediment can stabilize or destabilize the flat bottom configuration with respect to the uniform sediment case depending on the standard deviation sigma of the grain size distribution and on the ratio r over cap between the horizontal tidal excursion and the water depth for moderate values of r over cap i e values just larger than the critical value for which the sediment is moved and sand waves appear the presence of a sand mixture stabilizes the flat bed on the other hand for large values or r over cap the mixture has a destabilizing effect in both cases the effect that a sand mixture has on the stability of the flat bed configuration is relatively small moreover for moderate values of r over cap the fine fraction of the mixture tends to pile up at the crests of the bottom forms while the coarse fraction moves towards the troughs for large values or r over cap the grain size distribution depends on the value of sigma the results are physically interpreted and provide a possible explanation of the apparently conflicting field observations of the grain size distribution along the sand wave profile carried out in the north sea

migration of early aftershocks following the 2004 parkfield earthquake a significant number of the early aftershocks following large shallow earthquakes are missing in existing earthquake catalogues an analysis of the waveforms of 3 647 relocated earthquakes along the parkfield section of the san andreas fault reveals 11 times more aftershocks within three days of the 2004 parkfield earthquake than listed in a standard catalogue large shallow earthquakes are immediately followed by numerous aftershocks a significant portion of these events is missing in existing earthquake catalogues mainly because seismicity after the mainshock can be masked by overlapping arrivals of waves from the mainshock and aftershocks1 2 3 4 however recovery of the missing early aftershocks is important for understanding the physical mechanisms of earthquake triggering2 3 4 and for tracking postseismic deformation around the rupture zone associated with the mainshock5 6 7 here we use the waveforms of 3 647 relocated earthquakes8 along the parkfield section of the san andreas fault as templates9 10 to detect missing aftershocks within three days of the 2004 magnitude 6 0 parkfield earthquake we identify 11 times more aftershocks than listed in the standard catalogue of the northern california seismic network we find that the newly detected aftershocks migrate in both along strike and down dip directions with logarithmic time since the mainshock consistent with numerical simulations of the expansion of aftershocks caused by propagating afterslip11 12 the cumulative number of early aftershocks increases linearly with postseismic deformation in the first two days supporting the view that aftershocks are driven primarily by afterslip13 14

holocene sedimentation in the shallow nearshore zone off nauset inlet cape cod massachusetts abstract present conditions and sedimentary evolution of the shallow offshore region near nauset inlet on cape cod massachusetts were clarified using high resolution seismic reflection profiles sidescan sonar records surface grab samples and current meter measurements the study area contains three provinces 1 a nearshore province shallower than 18 m with a relatively steep slope 0 6 and a cover of medium sand 2 a northern offshore province covered with coarse sand gravel and boulders interpreted to be glacial drift and 3 a southern offshore province with a gentle seaward dipping slope 0 3 and a surface sediment of coarse sand the glacial drift exposed in the northern offshore province can be traced southward under the coarse sand province the overlying fill is comprised of either outwash sediment derived from the pleistocene south channel ice lobe to the east or holocene aged marine sediments eroded from seacliffs to the north latest holocene sediment appears to be limited to the zone shoreward of 18 m where the medium sand occurs near bottom mean flows measured over two winter months in 10 m water depth average 6 cm sec 1 to the south mean flows exceeded 20 cm sec 1 approx 23 of the time ninety percent of the flows exceeding 20 cm sec 1 were directed to the south reflecting the dominant atmospheric forcing during these winter months waves had an average variance of 650 cm 2 with variance exceeding 5000 cm 2 3 of the time indicating moderate wave activity present processes are actively reshaping the nearshore province which is characterized by many east to northeast trending shore oblique channels that do not extend seaward of the 18 m contour coarse sand in the floors of these channels suggests they may be erosional features and the presence of megaripples oriented perpendicular to the channel axes indicates active transport in these channels megaripple orientation and the current and wave regime of the study area support a rip current origin for these channels

effect of grain size sorting on the formation of shoreface connected sand ridges 1 field data of shoreface connected ridges show persistent spatial variations of mean grain size over the bed forms in the shore normal direction the profiles of bottom topography and mean grain size are approximately 90 out of phase to investigate the mechanisms responsible for the observed grain size distribution and the influence of sediment sorting on the temporal and spatial characteristics of shoreface connected ridges a model is developed and analyzed a linear stability analysis of an alongshore uniform basic state describing a storm driven flow on a microtidal inner shelf with respect to small bottom perturbations is carried out the transport of nonuniform sediment is described by formulations for both bed load and suspended load both of which account for dynamic hiding effects a one layer model for the bed evolution and a bottom friction term which depends on the grain size are used the initial formation of the ridges is studied for a bimodal sediment mixture the results of the model indicate that the phase shift between bed topography and mean grain size for shoreface connected ridges is due to the selective transport via suspended load of grains with different sizes a net stabilizing effect on the growth of bed forms and enhanced migration are predicted caused by the bimodal character of the sediment the wavelengths of the bed forms are only slightly affected including a tidal current or a grain size dependent formulation for the bottom friction has no effect on the results a physical explanation for the model results is also given

nearly pure sorting waves and formation of bedload sheets bedload sheets are coherent migrating patterns of bed material recently observed both in flume studies and in field streams with beds of coarse sand and fine gravel this newly recognized feature is inherently associated with the heterogeneous character of the sediment and consists of sorting waves with distinct coarse fronts only one or two coarse grains high the question of the formation of bedload sheets poses an interesting and peculiar stability problem for the grain size distribution sorting waves are essentially two dimensional migrating perturbations associated with variations of this distribution we show that their growth is strictly associated with grain sorting in fact the latter gives rise to perturbations of bedload transport which drive small perturbations of bottom elevation the amplitude of which scales with grain size the sorting wave also induces spatial variations of bottom roughness and consequently alters the fluid motion which conversely exerts a spatially varying stress on the bed the feature of bedload sheets which allows them to be distinguished from dunes over beds with coarse sand or fine gravel is then the fact that sorting is the dominant effect controlling their growth rather than being a relatively small perturbation of the mechanism which gives rise to dunes in the case of uniform sediment the requirement that perturbations should not alter the sediment budget leads to an integral condition which gives rise to an integro differential mathematical problem with the help of recently developed bedload relationships suitable for mixtures as well as appropriate modelling of turbulent channel flow over a bed with spatially periodic perturbations of bottom elevation and roughness we are able to derive a general dispersion relation which can be readily solved in terms of undisturbed size densities in the form of sums of dirac distributions perturbations are found to be unstable within a range of wavenumbers depending on the relative roughness and froude number we show that when the effects of perturbations of bottom elevation are neglected the unstable region corresponds to the range of conditions where the bottom stress leads bottom roughness a range distinct from that which characterizes the formation of dunes this result is given a physical explanation which depends crucially on the deviation from equal mobility of different grain sizes in the surface layer the effect of perturbations of bottom elevation is however not negligible when the bottom roughness is fairly large compared to depth in the latter case perturbations of bottom elevation and of bottom roughness are equally important and gravel sheets are not easily distinguished from small amplitude dunes comparison with the field observations of whiting et al 1985 1988 is satisfactory insofar as the bedload sheet mode is unstable under the conditions of the experiments and the predicted wavelengths fall within the experimental range the laboratory observations of kuhnle southard 1988 on the other hand appear to fall within a range of bottom roughness where the observed bedforms do not exhibit features unambiguously distinct from those of small amplitude dunes

stochastic prediction of sediment transport in sand gravel bed rivers classical deterministic bedload transport predictors are applied to sand gravel bed rivers the turbulent bed shear stress is modeled according to a probability distribution to obtain realistic bedload transport rates at incipient motion in extending the predictors to stochastic predictors for nonuniform sediment many parameters that represent near bed turbulence and the particle size distribution must be chosen the parameters that give realistic results are chosen by analyzing the results of a new experimental flume dataset with relatively large water depths choosing other combinations of parameters may give equal total bedload transport rates but at the cost of large errors in fractional transport rates attention is given to the hiding exposure phenomenon and a hindrance effect related to nonuni form sediment validation based on two independent field datasets shows that successful predictions of particle sizes near the threshold for motion are feasible using the stochastic approach while the deterministic approach gives successful predictions well above incipient motion

friction marks on blocks from pyroclastic flows at the soufriere hills volcano montserrat implications for flow mechanisms friction marks on surfaces of blocks 1 to 10 m in pyroclastic flow deposits generated by lava dome collapse in the 1995 1999 eruption of the soufriere hills volcano montserrat consist of a thin 5 to 10 mm layer of cataclasite with a polished slickensided surface in some examples thin surface layers and veins of pseudotachylyte are found the friction marks occur in several orientations on all block surfaces they are typically 0 5 to 10 cm wide and up to 1 m long the marks are similar to tectonic structures on faults they indicate that the blocks were tumbling and sliding within the flows large transient stresses result when the mass of a large sliding block is applied to a small contact area on a neighboring block or the ground compositional data indicate that the pseudotachylytes involved melting of the flow matrix these observations support models of pyroclastic flows as rapid granular avalanches

high fluid pressure and triggered earthquakes in the enhanced geothermal system in basel switzerland 1 we analyzed 118 well constrained focal mechanisms to estimate the pore fluid pressure field of the stimulated region during the fluid injection experiment in basel switzerland this technique termed focal mechanism tomography fmt uses the orientations of slip planes within the prevailing regional stress field as an indicator of the fluid pressure along the plane at the time of slip the maximum value and temporal change of excess pore fluid pressures are consistent with the known history of the wellhead pressure applied at the borehole elevated pore fluid pressures were concentrated within 500 m of the open hole section which are consistent with the spatiotemporal evolution of the induced microseismicity our results demonstrate that fmt is a robust approach being validated at the meso scale of the basel stimulation experiment we found average earthquake triggering excess pore fluid pressures of about 10 mpa above hydrostatic overpressured fluids induced many small events m 3 along faults unfavorably oriented relative to the tectonic stress pattern while the larger events tended to occur along optimally oriented faults this suggests that small scale hydraulic networks developed from the high pressure stimulation interact to load hydraulically isolated high strength bridges that produce the larger events the triggering pore fluid pressures are substantially higher than that predicted from a linear pressure diffusion process from the source boundary and shows that the system is highly permeable along flow paths that allow fast pressure diffusion to the boundaries of the stimulated region