tidal wave propagation along the mekong deltaic coast a two dimensional barotropic numerical model was employed to investigate the dynamics of tidal wave propagation in the south china sea with a particular interest for its characteristics along the mekong deltaic coast the study indicates that tidal waves propagate from the pacific ocean into the south china sea mainly through the luzon strait ls where the k 1 diurnal tide dominates due to a quarter wavelength resonance in this semi enclosed basin and that the incoming tidal waves from the celebes open boundary play a more important role than those from the andaman and flores open boundaries previous studies have not explained why both adjacent seas including the south china sea and the gulf of thailand are dominated by a diurnal tide while a semidiurnal tide dominates along the eastern mekong deltaic coast by means of green s law continental shelf tidal resonance theory and standing wave theory this study clarifies that the large amplified m 2 semidiurnal amplitude leading to a prevailing mixed semidiurnal tide is caused not only by the shoaling effect and the continental shelf oscillation resonance phenomenon but also by the position on the standing wave anti node line moreover the finding of radial tidal currents occurring along the southern mekong estuarine coast has not been revealed in earlier studies based on a number of numerical geometrically schematised experiments we suggest that the interaction between the large amplified amplitude near the shoreline associated with the adjacent low amplitude band system causing convex hydraulic gradients of tidal amplitude due to basin geometry as well as sloping topography is the mechanism for developing these radial tidal current systems the results reveal that wind monsoon climate could cause either damped or amplified tidal amplitudes around the mekong deltaic coast of which approximately 2 3 cm is due to the changing atmospheric pressure the tangential stress of wind over the water surface and wind enhanced bottom friction also this study suggests that the tidal generating forces should be considered to achieve accurate model results depending on the geographical region of interest findings achieved from this study contribute to a deeper insight of tidal wave propagation from a deep ocean to a shallow flat basin similar to the south china sea and its mekong deltaic coast

characteristics of varved clays of the elk valley british columbia canada abstract the interrelationships among selected geotechnical characteristics and microstructure for late pleistocene glaciolacustrine varved clay from a site in the elk valley southeastern british columbia canada are presented oedometer test results revealed preconsolidation pressures greater than would be expected under the previous maximum overburden cover the presence of segmented failure envelopes i e possessing over as well as normally consolidated regions and moderately high liquidity indices suggest the presence of cementation bonding this conclusion is supported by the porous laminar matrix microstructure of the varved clay natural slope instability is influenced by horizontal seepage within highly permeable interbeds in the clay the main mode of failure being rotational slumping slopefailure appears governed by shear strength parameters that are intermediate between those yielded by the over and normally consolidated regions of the failure envelope

a survey of some geotechnical properties of the tees laminated clay of central middlesbrough north east england abstract the tees laminated clay was deposited in a proglacial lake which occupied the teesside area during late devensian times disappearing about 13 000 y ago the laminations consist of alternating thin layers of clay and silt and represent seasonal increments of sediment fissuring is common in the upper few metres of the laminated clays which reach a maximum thickness in parts of central middlesbrough of just over 9 m illite and kaolinite with lesser amounts of chlorite dominate the mineral composition quartz tends to account for less than 30 the notable carbonate content may be diagenetic in origin the birefringence ratio of the clay minerals indicates a medium degree of particle orientation which suggests a partially flocculated fabric these laminated clays have a high plasticity however the liquid and plastic limits of the silty layers differ significantly from those of the clay layers or the bulk samples the clay fraction has a greater influence on these limits than the silty fraction in addition the linear shrinkage liquidity indices and consistency indices of the silty layers differ appreciably from those of the clay layers and the bulk samples the clays are inactive the orientation of the laminations in relation to the direction of maximum principal stress influences the shear strength the lowest strengths being developed when these orientations are between 30 and 60 the stress paths anisotropic ratios and values of a f indicate that the tees laminated clay is normally consolidated or lightly overconsolidated the mclamore gray concept possibly can be applied to the anisotropic strength behaviour

integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the mekong delta abstract arsenic is contaminating the groundwater of holocene aquifers throughout south and southeast asia to examine the biogeochemical and hydrological processes influencing dissolved concentrations and transport of as within soils sediments in the mekong river delta a 50 km2 field site was established near phnom penh cambodia where aqueous as concentrations are dangerously high and where groundwater retrieval for irrigation is minimal dissolved as concentrations vary spatially ranging up to 1300 g l in aquifer groundwater and up to 600 g l in surficial clay pore water groundwaters with high as concentrations are reducing with negligible dissolved o2 and high concentrations of fe ii nh 4 and dissolved organic c within near surface environments these conditions are most pronounced in sediments underlying permanent wetlands often found within oxbow channels near the mekong river there labile c co deposited with as bearing fe hydr oxides under reducing conditions drives the reductive mobilization inclusive of fe and as reduction of as here conditions are described under which as is mobilized from these sediments and near surface as release is linked to aquifer contamination over long time periods 100s to 1000s of years site biogeochemistry is coupled with extensive hydrologic measurements and accordingly a comprehensive interpretation of spatial as release and transport within a calibrated hydraulic flow field is provided of an as contaminated aquifer that is representative of those found throughout south and southeast asia

geomorphic controls on groundwater arsenic distribution in the mekong river delta cambodia natural arsenic contamination of groundwater is a severe public health crisis affecting nearly 60 million people in south asia and southeast asia alone contamination is spatially heterogeneous and results from the coupled microbial decomposition of organic matter and reductive dissolution of arsenic bearing iron minerals here we demonstrate that elevated arsenic concentrations arise in regions of recent organic matter deposition and thus are controlled by fluvial geomorphic processes arsenic contamination is best expressed within recent geomorphic features such as docked islands scroll bars and avulsions within these features the deposition of rapidly buried reactive organic matter facilitates microbial iron reduction and arsenic release ultimately the organic matter supply is exhausted and the conditions necessary for soluble arsenic to persist diminish

a holocene pollen record of vegetation and coastal environmental changes in the coastal swamp forest at batulicin south kalimantan indonesia abstract pollen analysis of a coastal peat swamp core representing 9100 bp from batulicin south kalimantan indonesia shows that mangrove forest with rhizophora as its main element has been established since the early holocene vegetation development in general and particularly mangrove forest was influenced by holocene environmental changes the highest value of rhizophora at ca 8200 bp indicates an early holocene sea level drop and implies sea level at ca 9 m subsequently mangrove forest was severely disrupted by rapid sea level rise at ca 6400 bp prior to the holocene maximum however it quickly recovered following a lower rate of sea level rise or subsequent sea level drop at ca 6000 bp and flourished until ca 1000 bp from ca 6000 bp the environmental setting around the site seems to have gradually become more terrestrial and changed from mangrove forest to peat swamp forest due to higher precipitation and intensive progradation human influence is recognized from ca 1600 bp

rapid flooding of the southern vietnam shelf during the early to mid holocene a new sea level record derived from coastal deposits of the south vietnam shelf covers the deglacial sea level history between 13 0 and 9 0 ka bp this record reveals a relatively constant rate of sea level rise and matches well with model results based on the rses anu global ice sheet model however the sea level observations reveal a significant discrepancy with model results based on the ice 5g global ice sheet model which further questions the existence of meltwater pulse 1b the incised valley deposits of the south vietnam shelf also complete the deglacial coastal evolution of southern vietnam the regional sea level records of the south vietnam shelf the mekong delta and the cambodian lowlands reveal an offset of about 18 m between 9 0 and 8 2 ka bp the shoreline retreat of more than 200 km related to this offset was mainly caused by the differential hydro isostatic adjustment between the south vietnam shelf and the cambodian lowlands however the gia model results cannot fully explain this fast relative sea level rise due to the topographic changes and possibly alternative melt water sources not accounted for in the current models copyright c 2014 john wiley sons ltd

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

morphodynamic characteristics of the dextral diversion of the yangtze river mouth china tidal and the coriolis force controls this paper examines the morphological development of the yangtze river mouth which has been diverting southeasterly dextrally according to historical 150 years chart based digital evolution model and on site measured tidal flow data we reveal a significantly narrowing of the northern river mouth branch from formerly 30 km wide to presently 10 km wide due to rapid siltation net siltation there however decreases gradually which largely contrasts with the fact that the siltation has shifted to the southern river mouth area as shown by many newly emerged estuarine islands sandy shoals and bifurcated branches our data have further demonstrated that the ebb flow that dominates in the study area changes its direction gradually from east to southeast from the inner to outer river mouth area and its duration is much longer than the flood flow in the inner river mouth area but nearly equal at the river mouth area accordingly the sediment transport pathway has been diverted from east to southeast we examine whether the coriolis force could explain the dextral diversion of the ebb flow and the altered morphodynamical processes although too weak to strengthen the tidal flows the coriolis force can drag the ebb flow southeasterly and so influence sediment transport paths at the estuarine scale the coriolis force is limited in the inner river mouth but substantial at and in the outer river mouth area when gradually free of estuarine topographic constraints the coriolis force causes an offset in propagation of in out flow directions at the river mouth area to form a slack water setting prone to estuarine siltation using the present approach also enables explanation of the morphological development of the holocene yangtze delta coast that extends to the southeast copyright 2010 john wiley sons ltd

initiation of the mekong river delta at 8 ka evidence from the sedimentary succession in the cambodian lowland abstract modern deltas are understood to have initiated around 7 5 9 ka in response to the deceleration of sea level rise this episode of delta initiation is closely related to the last deglacial meltwater events and eustatic sea level rises the initial stage of the mekong river delta one of the world s largest deltas is well recorded in cambodian lowland sediments this paper integrates analyses of sedimentary facies diatom assemblages and radiocarbon dates for three drill cores from the lowland to demonstrate holocene sedimentary evolution in relation to sea level changes the cores are characterized by a tripartite succession 1 aggrading flood plain to natural levee and tidal fluvial channel during the postglacial sea level rise 10 8 4 ka 2 aggrading to prograding tidal flats and mangrove forests around and after the maximum flooding of the sea 8 4 6 3 ka and 3 a prograding fluvial system on the delta plain 6 3 ka to the present the maximum flooding of the sea occurred at 8 0 0 1 ka 2000 years before the mid holocene sea level highstand and tidal flats penetrated up to 20 50 km southeast of phnom penh after a period of abrupt 5 m sea level rise at 8 5 8 4 ka the delta progradation then initiated as a result of the sea level stillstand at around 8 7 5 ka another rapid sea level rise at 7 5 7 ka allowed thick mangrove peat to be widely deposited in the cambodian lowland and the peat accumulation endured until 6 3 ka since 6 3 ka a fluvial system has characterized the delta plain and the fluvial sediment discharge has contributed to rapid delta progradation the uppermost part of the sedimentary succession composed of flood plain to natural levee sediments reveals a sudden increase in sediment accumulation over the past 600 1000 years this increase might reflect an increase in the sediment yield due to human activities in the upper to middle reaches of the mekong as with other asian rivers

the palaeoenvironmental development of the northeastern vietnamese mekong river delta since the mid holocene three radiocarbon dated sediment cores from the northeastern vietnamese mekong river delta have been analysed with a multiproxy approach grain size pollen and spores macro charcoal carbon content to unravel the palaeoenvironmental history of the region since the mid holocene during the mid holocene sea level highstand a diverse zoned and widespread mangrove belt dominated by rhizophora covered the extended tidal flats the subsequent regression and coeval delta progradation led to the rapid development of a back mangrove community dominated by ceriops and bruguiera but also represented locally by e g kandelia excoecaria and phoenix along rivers this community seems to have endured even when the adjoining floodplain had already shifted to freshwater vegetation generally this freshwater vegetation has a strong swamp signature but locally arecaceae fabaceae moraceae urticaceae and myrsinaceae are important and mirror the geomorphological diversity of the delta plain the macro charcoal record

how did the 2013 lushan earthquake ms 7 0 trigger its aftershocks insights from static coulomb stress change calculations the lushan earthquake ms7 0 occurred at 08 02 beijing time 00 02 utc on 20 april 2013 located on the eastern margin of the tibetan plateau where the 2008 ms8 0 wenchuan earthquake took place the other strong shock following the wenchuan earthquake occurred on the longmen shan thrust fault a large number of aftershocks were recorded after the mainshock to investigate how the aftershocks were triggered we firstly relocated the 6 month aftershock sequence by the double difference algorithm next we calculated coulomb failure stress changes imparted by the lushan mainshock both on the optimally oriented plane and on the aftershock nodal planes then we examined the correspondence between the coulomb failure stress changes and the spatial distribution of the aftershocks the computed results show that most of the aftershock hypocenters did not occur on the region with the positive coulomb stress changes caused by the lushan mainshock moreover the majority of aftershock nodal planes were brought away from failure by coseismic coulomb stresses this implies that coseismic static coulomb stress changes may not be the governing process for aftershock genesis in contrast postseismic stress transfer by processes of afterslip and pore fluid flow are assumed to play an important part in aftershock triggering based upon the spatiotemporal distribution of the aftershocks

a strong correlation between induced peak dynamic coulomb stress change from the 1992 m7 3 landers california earthquake and the hypocenter of the 1999 m7 1 hector mine california earthquake the 1992 m7 3 landers earthquake may have played a role in triggering the 1999 m7 1 1 hector mine earthquake as suggested by their close spatial 20 km proximity current investigations of triggering by static stress changes produce differing conclusions when small variations in parameter values are employed here i test the hypothesis that large amplitude dynamic stress changes induced by the landers rupture acted to promote the hector mine earthquake i use a flat layer reflectivity method to model the landers earthquake displacement seismograms by requiring agreement between the model seismograms and data i can constrain the landers main shock parameters and velocity model a similar reflectivity method is used to compute the evolution of stress changes i find a strong positive correlation between the hector mine hypocenter and regions of large 4 mpa dynamic coulomb stress changes peak f t induced by the landers main shock a positive correlation is also found with large dynamic normal and shear stress changes uncertainties in peak f t 1 3 mpa are only 28 of the median value 4 6 mpa determined from an extensive set 160 of model parameters therefore the correlation with dynamic stresses is robust to a range of hector mine main shock parameters as well as to variations in the friction and skempton s coefficients used in the calculations these results imply dynamic stress changes may be an important part of earthquake trigging such that large amplitude stress changes alter the properties of an existing fault in a way that promotes fault failure

reasons for the low sedimentation and slow progradation in the pearl river delta southern china during the middle holocene abstract holocene sea level rise monsoonal discharge and human activity have been the main drivers for the evolutionary history of asian deltas in theory when sea level ceased to rise in around 7000 cal a bp deltas should have started to prograde seawards however previous studies revealed that the progradation of modern deltas was initially very slow and more importantly sediment accumulation within central deltaic basins was exceptionally low between 7000 and 4000 cal a bp in order to investigate the main reasons for such slow progradation and low sedimentation during the middle holocene this study reconstructed the sedimentary history of the head area of the pearl river delta the results show marine transgression from 9500 to 7500 cal a bp followed by marine regression starting from 7500 to 4000 cal a bp in response to sea level change during the latter period sedimentation was concentrated in the head area the amount of sediment entering the central basin was significantly reduced this limited amount of sediment was spread thinly across the wide deltaic basin a portion of it could have been transported out into the south china sea by the strong tidal currents resulting in an exceptionally low sedimentation in the central basin however in locations adjacent to former present islands locally supplied sediment helped sedimentation to continue from 7500 to 4000 cal a bp by about 4000 cal a bp the deltaic shoreline advanced into the central basin this period saw an increase of human activity that caused soil erosion and sediment supply leading to a marked increase in sedimentation rate active land reclamations also helped the acceleration of shoreline advance this sedimentary evolution model in the pearl river delta mirrors those of other deltas in the monsoonal asia

evidence for smoothly falling sea level relative to north queensland australia during the past 6 000 yr post glacial sea level curves are expected to differ between different parts of the world as a result of global isostatic changes1 or perhaps other forms of geoidal changes2 i report here results from the inner part of the northern great barrier reef which seem to show a pattern of smoothly falling sea level during the past 6 000 yr the results are consistent with stratigraphical results from prograded mangrove coasts on the nearby north queensland mainland the region is remote from the tectonically active margin of the south west pacific ocean and the results contribute to the global data set relevant to isostatic changes being assembled by the international sea level project igcp project no 61 they also provide a baseline relevant to holocene coral reef growth in the inner great barrier reef

a review on active tectonics and deep crustal processes of the western sichuan region eastern margin of the tibetan plateau abstract one of remarkable geological and geomorphological features in continental china is the presence of a north south trending tectonic zone which separates the seismically active tibetan plateau to the west from the tectonically stable south china and ordos blocks to the east the zone also owns a name of north south seismic belt as a large number of great historical earthquakes occur on it the most recent one the 2008 wenchuan earthquake attests its recent tectonic activity in this review i take the western sichuan region as an example to probe the structural styles and kinematic pattern as well as deep geological process associated with tectonic deformation of the north south trending zone through integrated studies on active faults gps crustal deformation and geophysical structure we show that deformation in the western sichuan is governed by interactions among three crustal blocks songpan chuandian and south china of distinctive rheological properties under the tectonic framework that eastward growth of the soft eastern tibet is blocked by the hard lithosphere of the south china block the left lateral xianshuihe fault continues to the north south trending fault system without crustal shortening to form a bounding fault to limit the northern extend of the magnificent clockwise rotation of crustal material around the eastern himalaya syntax upper crust of the three blocks is dominated by brittle deformation whereas the ductile flow of lower crust would drag the brittle upper crustal blocks to move with respect to each other the relative motions among the brittle upper crustal blocks cause strain accumulations among their bounding faults to generate large earthquakes deformation of the western sichuan region can thus be described in terms of combined model of rigid block movement and continuous deformation we suggest this combined model can be applied to entire continental china

stratigraphy and optically stimulated luminescence osl dating of a quaternary sequence along the dong nai river southern vietnam abstract the pre holocene cenozoic sequence outcrops in the terrestrial part of the eastern margin of the mekong basin however the stratigraphy of the sequence is still unclear its detailed stratigraphy and chronology were therefore studied along the dong nai river southern vietnam and the lithofacies and the relations among the formations were investigated from the outcrops the ages of the deposits were determined by using optically stimulated luminescence osl dating the ba mieu formation was deposited about 176 52 ka during marine isotope stage mis 7 6 the thu duc formation was deposited about 97 27 ka during mis 5 both the ba mieu and thu duc formations are composed of fluvial and tidally influenced coastal deposits the newly proposed nhon trach formation was originally an eolian blanket deposit but it has been partly reworked by fluvial processes the nhon trach formation was deposited about 10 9 4 7 ka in the last part of the pleistocene to the beginning of the holocene the osl ages for the ba mieu thu duc and nhon trach formations are younger than the ages previously assigned to these formations

late holocene evolution of the mekong subaqueous delta southern vietnam abstract as asia s third largest river with regard to sediment load the mekong river delivers approximately 160 million tons of sediment per year to the south china sea high resolution seismic profiling and coring during 2006 and 2007 cruises revealed a low gradient subaqueous delta system up to 20 m thick surrounding the modern mekong river delta mrd in the west of the south china sea based on clinoform structure grain size 210pb ams 14c and 13c results the subaqueous delta is divided into four zones defined by different sedimentary processes and depositional features over the past 3000 yr the evolution of the mrd has shown a morphological asymmetry indicated by a large down drift area and a rapid progradation around cape camau 200 km downstream from the river mouth this asymmetric feature is consistent with increased wave influence the strong southwestward coastal current strengthened by the strong ne monsoon plays an important role locally in longshore transport of resuspended sediments into the gulf of thailand a late holocene sediment budget for the mrd has been determined based on the area and thickness of deltaic sediment approximately 80 of mekong delivered sediment has been trapped within the delta area which together with a falling sea level resulted in a rapidly prograding mrd over the past 3000 yr

sedimentary processes on the mekong subaqueous delta clay mineral and geochemical analysis sedimentary processes on the inner mekong shelf were investigated by examining the characteristics of sediments sampled in gravity cores at 15 locations including grain size clay mineralogy sediment accumulation rates and the elemental and stable carbon isotopic composition of organic matter atomic c n ratios and delta c 13 deltaic deposits exhibit contrasting characteristics along different sides of the delta plain south china sea scs hereafter to the east and gulf of thailand got hereafter to the west as well as on and off the subaqueous deltaic system on one hand cores recovered from the subaqueous delta in the scs got are consisted of poorly well sorted sediments with similar different clay mineral assemblage with from mekong sediments excess pb 210 profiles supported by c 14 chronologies indicate either non steady scs side or rapid accumulation got side processes on the subaqueous delta the delta c 13 and c n ratio indicate a mixture of terrestrial and marine sourced organic matter in the deltaic sediment on the other hand cores recovered from areas with no deltaic deposits or seaward of the subaqueous delta show excess pb 210 profiles indicating steady state accumulation with a greater proportion of marine sourced organic matter core analysis s relevance with local depositional environment and previous acoustic profiling are discussed c 2012 elsevier ltd all rights reserved

past environmental and circulation changes in the south china sea input from the magnetic properties of deep sea sediments abstract the south china sea located at the transition between the pacific and the indian ocean receives every year mainly during the rain season enormous amounts of river sediments originating from the erosion weathering of rocks in the catchment basins at sea these sediments are carried by different water masses to their deposition site and they constitute a unique archive for past environmental studies in this region the magnetic fraction of deep sea sediments though forming a minority in volume provides incredibly valuable information for paleoceanographic reconstructions as long as its provenance and source to sink processes are well constrained after a brief description of the climatic sedimentological and oceanographic context of the south china sea scs a review of the information available so far in the literature about the magnetic properties of scs sediments is presented it shows a large variety of interpretations conclusions that finally results in a rather unclear picture because in such a context the characterization of the sediment at the source is critical the magnetic properties recently obtained from a set of samples from rivers and marine surface sediments are summarized to describe the present day situation they are then used to interpret paleorecords from a set of seven marine cores distributed from the southern to the northern basins at different water depths and all covering at least the last climatic cycle the results reported here for the first time suggest that the magnetic mineralogy remains rather stable in time on land and that its time and spatial distribution at sea is an interplay of changes in sea level and deep sea circulation during low sea level periods bottom deep sea circulation is weak and the deposited sediment originates from the proximal rivers on the contrary during high sea level the circulation is enhanced transporting more sediment most likely from taiwan and luzon to the northwestern part of the scs and also in smaller proportion to the southern basin where it mixes with the local river borne sediment by comparing the two longest records we observe that this pattern is repeated over the last 900 ka superimposed to the 100 kyr cyclicity we also observe a longer term evolution with a maximum in the bottom current strength around 500 ka coinciding with global changes in the deep ocean circulation and carbon cycle these new results based on a wide spectrum of magnetic properties of numerous marine sedimentary cores from the scs show that the magnetic fraction yields important insights into past changes of the sedimentary pathways in particular the dynamic of the deep sea circulation depending on the global climatic context

the role of sea level rise monsoonal discharge and the palaeo landscape in the early holocene evolution of the pearl river delta southern china abstract the early holocene history of the pearl river delta is reconstructed based on a series of sediment cores obtained from one of the main palaeo valleys in the basin sedimentary and microfossil diatom analyses combined with radiocarbon dating provide new evidence for the interactions between sea level rise antecedent topography and sedimentary discharge changes within the deltaic basin since the last glacial these new records show that river channels of last glacial age incised down to c 40 m into an older possibly mis5 age marine sequence which forms the floor of the deltaic basin and exists primarily at c 10 m 15 m below present mean sea level rapid postglacial sea level rise flooded the incised valleys by the beginning of the holocene and prior to c 9000 cal years bp marine inundation was largely confined within these incised valleys the confined available accommodation space of the incised valleys combined with strong monsoon driven freshwater high sediment discharge and a period of rapid rising sea level meant that sedimentation rates were exceptionally high towards c 8000 cal years bp as sea level rose to about 5 m marine inundation spilled out of the incised valleys and the sea flooded the whole deltaic basin as a result the mouth of the pearl river was forced to retreat to the apex of the deltaic basin water salinity within the basin increased markedly as the previously confined system dispersed across the basin and the sedimentation changed from fluvial dominated to tidal dominated sea level continued to rise albeit at a much reduced rate between 8000 and 7000 cal years bp and deltaic sedimentation was concentrated around the apex area of the basin during the last 7000 cal years bp the delta shoreline moved seawards and the sedimentary processes changed gradually from tidal dominated to fluvial dominated

ubiquity of wave dominated deltas in outer shelf growth faulted compartments abstract growth faults and deltaic coastlines are common features along basin margins with a high flux of sediment delivery it has been observed in previous studies that wave dominated deltas are the characteristic and long lived coastline type in such growth faulted extensional basins during deltaic progradation the structural and depositional systems migrate and adjust as sedimentation rates keep pace with fault displacement rates a numerical model indicates that increased subsidence along growth faults slows the progradation rate of deltas as new accommodation is being created the decrease in forward delta growth results in a longer period of wave reworking per unit of sediment due to the retention of large fetch and water depth since subsidence is maintained through time sediments tend to accumulate on delta topsets creating thick wave dominated delta front successions on the downthrown sides of faults the proposed model may guide subsurface interpretations where it is difficult to assess the shoreline type without cores it is proposed here to use values of expansion index derived from seismic reflection profiles or well log correlations as a proxy for the likelihood of wave dominance on the delta shoreline

thermoluminescence ages for a reworked coastal barrier southeastern vietnam a preliminary report thermoluminescence dating of quartz sand 90 125 m from the coastal barrier successions between phan thiet and tuy phong southeastern vietnam reveals that a substantial component was deposited during the last interglacial sensu lato oxygen isotope stage 5 between 108 and 85 ka the barrier successions have subsequently in places experienced multiple phases of aeolian reworking during the last glacial cycle and in particular between 62 and 19 ka possibly related to reduced vegetation cover and landscape instability caused by climatic change the difficulties of applying the thermoluminescence tl method in areas of intense tropical weathering are also examined 2002 elsevier science ltd all rights reserved author keywords coastal barrier deposition quaternary thermoluminescence dating vietnam index keywords coastal landform last interglacial paleoclimate paleoenvironment quaternary sand thermoluminescence viet nam

a km scale triaxial experiment reveals the extreme mechanical weakness and anisotropy of mica schists grandes rousses massif france abstract the development of andersonian faults is predicted according to theory and experiments for brittle frictional deformation occurring in a homogeneous medium in contrast in an anisotropic medium it is possible to observe fault nucleation and propagation that is non andersonian in geometry and kinematics here we consider post metamorphic brittle frictional deformation in the mechanically anisotropic mylonitic mica schists of the grandes rousse massif france the role of the mylonitic foliation and of any other source of mechanical anisotropy in brittle frictional deformation is a function of orientation and friction angle according to the relative orientation of principal stress axes and foliation a foliation characterized by a certain coefficient of friction will be utilized or not for the nucleation and propagation of brittle frictional fractures and faults if the foliation is not utilized the rock behaves as if it was isotropic and andersonian geometry and kinematics can be observed if the foliation is utilized the deviatoric stress magnitude is buffered and andersonian faults fractures cannot develop in a narrow transition regime both andersonian and non andersonian structures can be observed we apply stress inversion and slip tendency analysis to determine the critical angle for failure of the metamorphic foliation of the grandes rousses schists defined as the limit angle between the foliation and principal stress axes for which the foliation was brittlely reactivated this approach allows defining the ratio of the coefficient of internal friction for failure along the mylonitic foliation to the isotropic coefficient of friction thus the study area can be seen as a km scale triaxial experiment that allows measuring the degree of mechanical anisotropy of the mylonitic mica schists in this way we infer a coefficient of friction w e a k 0 14 for brittle frictional failure of the foliation or 20 of the isotropic coefficient of internal friction

quantifying early aftershock activity of the 2004 mid niigata prefecture earthquake mw6 6 1 we analyze the early aftershock activity of the 2004 mid niigata earthquake using both earthquake catalog data and continuous waveform recordings the frequency magnitude distribution analysis of the japan meteorological agency jma catalog shows that the magnitude of completeness of the aftershocks changes from values around 5 0 immediately after the main shock to about 1 8 12 hours later such a large incompleteness of early events can bias significantly the estimation of aftershock rates to better determine the temporal pattern of aftershocks in the first minutes after the niigata earthquake we analyze the continuous seismograms recorded at six high sensitivity seismograph network hi net stations located close to the aftershock distribution clear aftershocks can be seen from about 35 s after the main shock we estimate that the events we picked on the waveforms recorded at two seismic stations ngoh and ynth situated on opposite sides of the aftershock distribution are complete above a threshold magnitude of 3 4 the c value determined by taking these events into account is about 0 003 days 4 3 min statistical tests demonstrate that a small but nonzero c value is a reliable result we also analyze the decay with time of the moment release rates of the aftershocks in the jma catalog since these rates should be much less influenced by the missing small events the moment rates follow a power law time dependence from few minutes to months after the main shock we finally show that the rate and state dependent friction law or stress corrosion could explain well our findings

song hong red river delta evolution related to millennium scale holocene sea level changes abstract the song hong red river delta occurs on the northwest coast of the south china sea its evolution in response to holocene sea level changes was clarified on the basis of sedimentary facies and 14 radiocarbon dates from the 40 m long duy tien core from the delta plain and using previously reported geological geomorphological and archaeological data the delta prograded into the drowned valley as a result of early holocene inundation from 9 to 6 cal kyr bp as sea level rise decelerated the sea level highstand at 2 3 m from 6 to 4 cal kyr bp allowed widespread mangrove development on the delta plain and the formation of marine notches in the ha long bay and ninh binh areas during sea level lowering after 4 cal kyr bp the former delta plain emerged as a marine terrace and the delta changed into the present tide and wave influenced delta with accompanying beach ridges delta morphology depositional pattern and sedimentary facies are closely related to holocene sea level changes in particular falling sea level at 4 cal kyr bp had a major impact on the evolution of the song hong delta and is considered to be linked to climate changes

the holocene history and development of the tonle sap cambodia abstract the tonle sap the great lake of central cambodia is the central component of wetland ecosystems in the lower mekong river basin and is of enormous conservation value the lake s unusual hydraulic relationship with the mekong river and its consequent sensitivity to monsoon variability makes the tonle sap sensitive to climate change exploring the dynamics and development of this system under different climate regimes of the past offers a perspective on possible future impacts which is critical for sound management biostratigraphic and sedimentological data derived from cores of lake sediment indicate that during the period 7000 to ca 5500 14 c years before present the lake was less variable than present in terms of depth during the annual cycle of flood and may have been strongly influenced by saline tidal waters associated with higher than present seas levels as regional environments became drier and more seasonal in the late holocene more sediment was re suspended during the increasingly marked dry season lake level minimum lowering the effective sediment accumulation rate contrary to current interpretations of the history of the lake and associated wetland ecosystems the data presented here imply that regional hydraulic connections between the lake and the mekong river existed from at least the early holocene

luminescence dating of beach ridges for characterizing multi decadal to centennial deltaic shoreline changes during late holocene mekong river delta abstract empirical understanding of decadal to centennial scale deltaic shoreline changes in the past is essential for understanding the fate of coasts in the coming decades and centuries we tested the effectiveness of quartz optically stimulated luminescence osl dating of beach ridges to constrain shoreline changes of the mekong river delta over the last 3500 years forty seven osl ages have been analyzed from the tra vinh delta plain central mekong river delta the tra vinh beach ridges are recurved and branching showing the hierarchy of shoreline changes which include discontinuous shifts c 5 km seawards caused by the emergence and elongation of a delta front bar island and subsequent downdrift accretion of spits the spit accretion is interrupted by changes of shoreline orientation each of the discontinuous shifts resulted in a cluster of beach ridges which is referred to as ridgeset the beach ridge sediments were found to have excellent luminescence properties resulting in low age uncertainties of c 5 the osl chronology agrees well with shoreline changes over recent decades and with radiocarbon ages of tidal flat sediment underlying the beach ridges the chronology clearly illustrates the coastal progradation from 3500 years relative to ad 2010 onwards two main types of beach ridge are present recurved and trunk ridges osl ages of recurved ridges are consistently younger both downdrift and seawards documenting decadal to centennial scale shoreline migration especially over the last 1500 years trunk ridges in contrast have less systematic ages because a trunk ridge is formed where the sediment is likely to have been reworked by waves ages of updrift trunk ridges characteristically show relatively young ages in each ridgeset representing the extensive erosion and resedimentation of the updrift coast major changes in shoreline orientation occurred around 400 500 years suggesting strengthening of the northeasterly winter monsoon at the beginning of the little ice age the rate of progradation of the tra vinh delta plain is described in two ways frequency of the discontinuous shift and growth rate of the delta plain area both suggest the coast has expanded regularly over the last 1000 years a decrease in sand supply to the coast in the last few decades due to river dam construction and fluvial sand dredging is inferred possibly affecting the behavior of the modern and future shorelines which can be compared with the less human influenced centennial to decadal scale past shoreline changes reconstructed in this study

early holocene initiation of the mekong river delta vietnam and the response to holocene sea level changes detected from dt1 core analyses abstract the early holocene initiation of the mekong river delta vietnam was investigated based on sedimentary facies microfossil analyses and radiocarbon dating of dt1 core taken from the upper delta plain the dt1 core 105 38 51 e 10 17 02 n elevation 2 m is 51 5 m long and located in fill of a buried incised valley this incised valley of the paleo mekong river formed during the last glacial maximum which is 20 30 km wide has been traced spatially from the cambodian border to the south china sea coast by compilation of existing core data the dt1 core records the history of the estuarine channel and delta systems of the mekong river during the early to middle holocene the sedimentary facies determined particularly from sedimentary structures diatom and foraminifera assemblages grain size variation and the accumulation rate clearly show an upward deepening succession in the lower part of the core from marsh or intertidal flat to subtidal inner bay facies and an overlying upward shallowing succession from prodelta to intertidal flat floodplain the boundary between the two successions indicating the maximum flooding surface occurred at approximately 8 0 ka at the time that the mekong river delta was initiated the delta initiation and sediment facies succession are closely linked to holocene sea level changes particularly a standstill from 8 0 to 7 5 ka after a rapid rise of sea level from 8 8 to 8 2 ka during the further sea level rise from 7 5 to 7 0 ka sediments aggraded forming topset sedimentary deposits

early growth stage of a large delta transformation from estuarine platform to deltaic progradational conditions the northeastern mekong river delta vietnam abstract the initial growth stage of a river delta can be defined either as the beginning geomorphologic expansion of the river dominated coastal lowland or as the starting basinward directed progradation of the geological stratigraphic delta body differentiating between these two perspectives is necessary for understanding the sensitive reaction of a deltaic system to dynamic changes in sea level and sediment availability the holocene evolution of the northeastern part of the mekong river delta southern vietnam offers a great chance to depict these two different definitions and relate them genetically to each other since the transition between an old abrasion platform inner delta and the continental shelf basin outer delta is well developed in this marginal part of the delta the early stage of delta growth taking place during transgression related inundation between 8 cal ka bp maximum flooding and 5 7 6 0 4 8 cal ka bp sea level highstand was characterized by tide and saltwater influenced nearshore conditions allowing extensive mangrove and tidal flat deposits to aggradate on the wide abrasion platform with the onset of regression around 4 8 cal ka bp the depositional center shifted immediately beyond the seaward margin of this estuarine platform and thus true deltaic progradation could develop leading to rapid construction of a delta body and in addition to subsidence tilting of the inner delta zone directly at the transition of these two deposystems remains of shore related deposits indicate the sudden breakdown of the estuarine platform conditions around 4 6 cal ka bp

sedimentary facies diatom and foraminifer assemblages in a late pleistocene holocene incised valley sequence from the mekong river delta bentre province southern vietnam the bt2 core abstract a detailed description of sediment facies diatom mollusca and foraminifer assemblages and 14 c ages of the mekong river delta southern vietnam is presented in this paper a 71 m long core recovered in 1997 at the bentre province in the eastern part of the mekong river delta is divided into eight units and nine sediment facies on the basis of sedimentary properties diatom and foraminifer assemblages and 14 c ages these units show facies changes from transgression to regression in relation to the late pleistocene holocene sea level changes the postglacial transgression caused by a sea level rise led to infilling of the incised valley and formation of estuarine sediments and open bay muddy sediments in ascending order estuarine sediments consist of slightly oxidized yellowish grey silty sand and intercalated greenish grey and stiffly brownish grey sandy silt and silty clay the muddy facies 13 5 m thick is characterized by abundant marine planktonic diatoms and open sea foraminifers this facies indicates the maximum holocene marine influence was around 5300 cal yr bp at the bt2 site the regressive succession is composed of deltaic sediments from prodelta delta front sub to inter tidal flat and beach ridge in ascending order 14 c ages indicate that the delta front passed the core site at approximately 4000 3000 cal yr bp the coastal progradation rate was 17 18 m yr 1 during the 5300 3500 cal yr and decreased to 13 14 m yr 1 during the last 3500 cal yr

depositional model of a macrotidal estuary and floodplain south alligator river northern australia the south alligator river northern territory of australia has a macrotidal estuary tidal influence spring tidal range 5 6 m at the mouth extends 105 km up the channel it is dominated by freshwater in the wet season december april with a salt wedge near the mouth but is well mixed and becomes saline throughout the dry season the tidal channel can be divided into four different channel types an estuarine funnel a sinuous meandering segment a cuspate meandering segment in which the inside of bends are pointed and an upstream tidal channel the distribution of morphologically defined land classes and morphological units within each land class on the floodplain flanking the estuary differs from one channel type to another several stratigraphic and morphostratigraphic units have been recognized from drill holes on the coastal and deltaic estuarine plains and a model of development is proposed on the basis of extensive radiocarbon chronology and palynology the coastal plain has prograded with most rapid sedimentation between 5000 and 3000 yr bp a similar pattern of progradation is identified in the estuarine funnel in the sinuous segment of the estuary the channel has migrated laterally across the floodplain previous channel positions are indicated by palaeochannels and the meander tract is occupied by laminated channel sediments within the cuspate segment there are numerous sinuous palaeochannels on the plains in the upstream segment the channel and palaeochannels have long straight reaches with irregular bends and discontinuous levees and channel avulsion is indicated mangrove mud is a widespread stratigraphic unit throughout the plains the initial phase of development is a transgressive phase 8000 6800 yr bp when mangrove forests extended landwards into a pre existing valley as sea level rose as sea level stabilized the transgressive phase was followed by a widespread mangrove phase termed the big swamp 6800 5300 yr bp the mangrove forests disappeared from most of the plains as vertical accretion continued and were replaced by grass and sedge covered floodplains during the sinuous phase about 5300 2500 yr bp the channel migrated laterally and eroded the deltaicestuarine plain and deposited lateral accretion deposits laminated channel sediments part of the channel of the south alligator river has then progressed from sinuous to cuspate in form and erosion of river banks has occurred transgressive and big swamp phases occurred under rising and stabilizing sea level respectively later morphodynamic channel adjustments occurred under conditions of stable sea level the depositional model has direct application to other estuaries in northern australia and may be applied to other areas where sea level change has been similar

depositional facies and radiocarbon ages of a drill core from the mekong river lowland near phnom penh cambodia evidence for tidal sedimentation at the time of holocene maximum flooding abstract the depositional facies and radiocarbon ages of a long holocene sediment core ks from cambodia are reported here to clarify the sedimentary environments of the mekong river delta system at the time of holocene maximum flooding by the sea the 30 7 m long ks core from the upper mekong river lowland about 20 km southeast of phnom penh penetrated five depositional facies a to e in ascending order facies a is cross laminated fluvial sand facies b is laminated very fine sand with mud drapes facies c to e are salt marsh flood plain and natural levee deposits characterized by a succession of peat organic clay and reddish brown silt facies b dates to 9 0 7 5 ka and is interpreted as aggradational tidal deposits 16 5 m thick deposited during a rapid rise of sea level these deposits became overlain by an accumulation of salt marsh and flood plain deposits during the subsequent period of slowly rising and then falling sea level the maximum flooding surface lies in the facies b interval the presence of tidal deposits implies that the core site about 230 km inland from the present river mouth was near the shoreline and experienced strong tidal influences during the early holocene

pleistocene paleoceanography of the south china sea progress over the past 20 years article i nfo the south china sea scs has become one of the foci in paleoceanographic research and hydrocarbon explora tions over the last 20 years over the period at least 16 international cruises including odp ocean drilling pro gram leg 184 have investigated the scs and more than 2000 exploratory wells have been drilled the present paper attempts to synthesize progress in pleistocene paleoceanography because of the high depositional rates and favorable carbonate preservation the scs keeps valuable paleoceanographic archives for the low latitude pacific region which have helped to uncover potential ties between the low latitude oceanic biogeochemical evolution and step wise development of the boreal ice sheet it has been found that the 1 6 ma event marked a reorganization of the oceanic carbon reservoir associated with the restructuring of the southern and the tropical pacific ocean the mpt mid pleistocene transition and mbe mid brunhes event centered at 0 9 ma and 0 4 ma respectively were preluded by carbon isotope maxima 13 cmax iii and 13 cmax ii suggesting that major steps in the ice sheet development were preceded by significant changes in oceanic biogeochemistry the late pleistocene records have demonstrated the amplifying role of the scs in responding to glacial cycles as the amplitudes of sst and 18 ov ariations have been significantly amplified in the scs as compared with the open ocean at the same latitudes the amplification is ascribed to its semi enclosed morphology and the prevail ing monsoon climate the closure of the southern margin during glacial times led to stratification of the upper waters and specific faunal changes in the southern scs the seasonally reversing monsoon winds not only drive the surface circulation but also the biological production the n s gradient in oceanography and the sed iment transportdynamics such asthe asymmetric growth of the mekong river delta becauseof its distinguished position in the low latitudes between the largest continent and the largest ocean the scs provides important insight into the mechanism of environmental evolution with regards to ocean continent or high low latitude interactions

sediment distribution and provenance on the continental shelf off the mekong river se vietnam insights from heavy mineral analysis abstract the continental shelf off southern vietnam is relatively well studied regarding the dominant transport paths of bulk and fine grained sediments discharged by the mekong river thus this region is particularly suitable for testing the applicability of heavy mineral analysis in determining the extent and origin of the sandy fraction of continental shelf deposits supplied mainly by a large river and their distribution over the adjacent extensive shelf the fine sand fraction of 105 sediment samples from the mekong river delta and neighbouring continental shelf is investigated applying two approaches i considering only the transparent non micaceous mineral assemblage and ii considering all the identified heavy minerals the geological complexity of the source areas and local dominance of micaceous and non transparent heavy minerals require the inclusion of all heavy mineral components when interpreting sediment provenance and dominant transport paths the most diagnostic heavy minerals amphibole chlorite epidote limonite opaques tourmaline and zircon are used for this purpose the investigated continental shelf can be divided into two main regions characterised by various heavy mineral assemblages namely sw and ne off the mekong river delta the south western region is dominated by chlorite and limonite indicating prevailing southward sediment transport from the mekong river whereas the north eastern region is characterised by amphibole epidote opaques tourmaline and zircon indicating various sources from se vietnam and relict sediments partly covering the continental shelf between the two main regions a large mixing zone is found where sediments from all sources are present including mekong river sediments transported eastward

analysis of sediment distribution and transport for mitigation of sand deposition hazard in tam quan estuary vietnam tam quan estuary in the binh dinh province of vietnam provides shelter for about 2000 boats recently erosion and accretion associated with the mouth of the estuary have badly affected marine transportation and economic development an 850 m long jetty was constructed to mitigate the hazard however sand deposition still continues to be an ongoing problem this paper aimed to assess the reasons for sand deposition in the estuary based on sediment distributions and transport pathways following an investigation of topography geological and hydrodynamic conditions and sediment characteristics an analysis of sediment transport trends was undertaken results show that medium sand is distributed along the coast from the shoreline to 2 5 m water depth fine sands are found in deeper areas of 10 13 m where there are also medium sand deposits thought to be of ancient origin sand deposition has intensively occurred in the estuary due to a dominant north to south longshore sediment transport regime sediment from this regime is presently trapped by the jetty and deposited in the navigation channel as a solution it is suggested that a properly designed jetty stemming from the headland on the north side of the estuary could effectively control the patterns of sediment transport enabling the sand to bypass the entrance thereby avoiding entrapment inside its mouth

distribution of seismic wave speed changes associated with the 12 may 2008 mw 7 9 wenchuan earthquake 1 we used continuous recordings in sichuan china to track the temporal change of the seismic wave speed at a regional scale for 2 years including the wenchuan mw 7 9 earthquake the data are recorded by a temporary network of 156 broad band seismographs in a region that covers the southern 2 3 of the fault activated during the earthquake a doublet analysis applied on the codas of seismic noise cross correlation functions is used to detect temporal velocity changes we found clear evidence that the seismic velocity drops by up to 0 08 in the fault region just after the earthquake with fluctuations within 0 02 before the earthquake we compared the measurements in different sub arrays to get a spatial distribution of the velocity changes this distribution is consistent with the volumetric strain change during the wenchuan earthquake and shows that the co seismic velocity change is not controlled by the response of sediments

rupture of deep faults in the 2008 wenchuan earthquake and uplift of the longmen shan the longmen shan mountains rise up 6 000 m over a distance of just 100 km but the mechanisms driving formation of this striking topography are debated analyses of crustal movements during the 2008 wenchuan earthquake suggest that fault induced uplift plays a role in building the high topography

holocene evolution of the chan may coastal embayment central vietnam changing coastal dynamics associated with decreasing rates of progradation possibly forced by mid to late holocene sea level changes abstract southeast asian coastal environments are undergoing massive transformations with unprecedented population and infrastructure development these transformations are occurring on a backdrop of intense natural and anthropogenic environmental change which are increasing the risk to the burgeoning coastal population little is known about how central vietnamese coastal environments have changed naturally since the mid holocene sea level highstand and how recent anthropogenic change and sea level variation have affected the coastal system the chan may embayment in central vietnam allows us to examine how recent changes in both anthropogenic development and sea level change have affected the coastline the embayment preserves a series of prograding beach ridges and is subject to intense human pressures with the construction of a large economic and industrial park and expansion of tourist facilities using ground penetrating radar and quartz optical dating we identify a switch from 6000 years of prograding beach ridges to transgressive dunes within the past century resulting in a decreasing rate of beach ridge progradation possibly in the last 100 years the recent modes of sediment deposition through washovers and a transgressive dune indicates that coastal progradation has slowed and might have stopped

the changing mangrove shorelines in kuala kurau peninsular malaysia abstract a considerable extent of the shoreline along the west coast of peninsular malaysia is vegetated by mangrove forests typically one observes frequent change of position of the mangrove outer shore limit the kuala kurau coastline in northwest perak which provides a good example is selected for this study the geological evidence indicates that marine sediments prevail throughout the kuala kurau coastal plain greenish gray clay predominates with varying amounts of silt and sand underlying the basically flat topography the present shoreline has prograded from its initial position within a time span of about 5000 years similarly from 1914 to 1969 a prograding shoreline trend is indicated however from 1969 to 1986 there was widespread erosion although both accreting and retreating situations were indicated erosion in the mangrove fringes in the later years is related to sediment budget freshwater supply over exploitation and utilisation of mangrove forest and the effects indirect of the fishing industry

suspended sediment dynamics during the inter monsoon season in the subaqueous mekong delta and adjacent shelf southern vietnam land ocean interactions in the coastal zone are severely influenced by tidal processes in regions of high sediment discharge like the mekong river delta in southern vietnam these processes are even more significant three cruises in 2006 2007 and 2008 were carried out to investigate the sediment suspension and their spatial distribution additionally we investigated the influence of the tidal currents in relation to the suspended sediment therefore all cruises took place during the inter monsoon season between march and may where wave and wind influences are not dominant in contrast to the summer monsoon may to early october and winter monsoon season november to early march suspended sediment concentrations sscs in the particle size range between 2 5 and 500 iim were measured with an lisst instrument laser in situ scattering and transmissiometry current velocities and directions were recorded with an acoustic doppler current profiler adcp additionally data of different tidal gauge stations in the mekong river delta were correlated and compared to the mixed semidiurnal diurnal tidal cycle our results show significant areas of sscs greater than 25 mu l l in the mekong river branches and its subaqueous delta during the inter monsoon season 20 of all measured sscs in the subaqueous mekong delta exceed 100 mu l l highest concentrations occur close to the seabed sscs decrease at the transition to the open shelf the shelf region contains only low suspension loads especially on the south eastern shelf 99 of all samples 25 close to the seabed in water depths of 20 25 m two surveys lasting 25 h each were performed on mooring stations in 12 m mooring 1 and 26 m mooring 2 water depth and located 3 2 km apart on the subaqueous delta slope similar patterns of ssc over time show that concentrations of suspension load correlate with the tidal current velocities high tidal current velocities of up to 0 6 m s near the sea bottom generate increasing sscs of more than 25 gl in the water column additionally a significant trend of decreasing ssc from the near seabed to the upper part of the water column can be observed in terms of sediment transport the ebb phase dominates the tidal cycle by its higher tidal current velocities but the flood phase has the longer duration the switch of the tidal current direction from ebb to flood phase occurs rapidly against which the change from flood to ebb phase requires up to 3 h this leads to an asymmetry of the tidal ellipses and may cause a net sediment transport from the shelf into the subaqueous mekong delta in the subaqueous mekong delta and adjacent shelf seven transects show similar patterns of sscs dependent to the tidal phase a hypopycnal sediment plume from the subaqueous mekong delta into the shelf region was not observed our results imply that resuspension by tidal currents dominates the sediment transport in the subaqueous mekong delta and adjacent shelf regions during the inter monsoon season c 2012 elsevier ltd all rights reserved

late holocene sedimentary and environmental development of the northern mekong river delta vietnam abstract a sedimentological and palynological study of three sediment cores from the northern mekong river delta shows the regional sedimentary and environmental development since the mid holocene sea level highstand a sub to intertidal flat deposit of mid holocene age is recorded in the northernmost core shoreline deposits in all three cores show descending ages from n to s documenting 1 the early stages of the late holocene regression and 2 the subsequent delta progradation the delta plain successions vary from floodplain deposits with swamp like elements to natural levee sediments the uppermost sediments in all cores show human disturbance to varying degrees the most intense alteration is recorded in the northernmost core where the palynological signal together with a charcoal peak indicates the profound change of the environment during the modern land reclamation the sediments from at least one of the three presented cores do not show a true delta facies succession but rather estuary like features as also observed in records from southern cambodia this absence is probably due to lack of accommodation space during the initial phase of rapid delta progradation which impeded the development of true delta successions as shown in cores from the southern mekong river delta

infilling and flooding of the mekong river incised valley during deglacial sea level rise the abrupt transition from fluvial to marine deposition of incised valley fill sediments retrieved from the southeast vietnamese shelf accurately records the postglacial transgression after 14 ka before present bp valley filling sediments consist of fluvial mud whereas sedimentation after the transgression is characterized by shallow marine carbonate sands this change in sediment composition is accurately marked in high resolution x ray fluorescence xrf core scanning records rapid aggradation of fluvial sediments at the river mouth nearly completely filled the mekong incised valley prior to flooding however accumulation rates strongly reduced in the valley after the river mouth system flooded and stepped back this also affected the sediment supply to deeper parts of the southeast vietnamese shelf comparison of the mekong valley filling with the east asian sea level history of sub and inter tidal sediment records shows that the transgressive surface preserved in the incised valley fill records is a robust sea level indicator the valley was nearly completely filled with fluvial sediments between 13 0 and 9 5 ka bp when sea level rose rather constantly with approximately 10 mm yr as indicated by the east asian sea level record at shallower parts of the shelf significant sediment reworking and the establishment of estuarine conditions at the final stage of infilling complicates accurate dating of the transgressive surface nevertheless incised valley fill records and land based drill sites indicate a vast and rapid flooding of the shelf from the location of the modern vietnamese coastline to the cambodian lowlands between 9 5 ka and 8 5 ka bp fast flooding of this part of the shelf is related with the low shelf gradient and a strong acceleration of the east asian sea level rise from 34 to 9 meter below modern sea level mbsl corresponding to the sea level jump of melt water pulse mwp 1c c 2010 elsevier ltd all rights reserved

slip tendency analysis as a tool to constrain the mechanical properties of anisotropic rocks abstract the mechanical strength of foliated rocks is typically anisotropic because it varies with the orientation of the foliation relative to the applied principal stresses and commonly depends on phyllosicate content and phyllosicate physical interconnectivity we constrain the degree of mechanical anisotropy associated with pre existing planar discontinuities such as metamorphic foliations and inherited faults by combining paleostress analysis and meso and microscale characterization of brittle failure modes in different phyllosilicate bearing rocks outcropping in the sierras de cordoba sdc of central argentina the sdc show evidence of a long brittle deformation history from early triassic present with three distinct brittle deformational events each phase caused new strain increments accommodated by the formation of newly formed faults or by the reactivation of inherited discontinuities structural investigations reveal that gneisses and phyllites deformed by different failure modes during the different events therefore we were able to use a conceptual field scale triaxial experiment by applying a stress model based on normalised slip tendency analysis we constrained the friction coefficient for slip along the foliations s and along pre existing faults f to 0 2 to 0 3 and 0 4 respectively these values fit independent estimates for similar rocks confirming the potential of our approach for other case studies

surface ruptures on the transverse xiaoyudong fault a significant segment boundary breached during the 2008 wenchuan earthquake china abstract the 220 km long rupture of the 2008 mw 7 9 wenchuan earthquake breached several km scale geometric discontinuities along strike including the previously un mapped nw trending xiaoyudong fault connecting between the two major ne trending rupture planes on the beichuan and pengguan faults in this paper we present high resolution mapping of the 8 km long surface breaks and sinistral oblique thrusting coseismic slip on the xiaoyudong fault scarp height is the largest at the nw end reaching 3 5 m and decreases southward in steps to less than 0 2 m with an average slip gradient of 6 10 3 at a few tens of meters length scale but up to 50 10 3 locally left lateral offsets co vary with the vertical component the largest sinistral slip vector we observed is 2 2 m geological and geophysical evidence suggests that the xiaoyudong fault is likely a 30 sw dipping lateral ramp that soles into the pengguan fault and at its northwestern end intersects with the beichuan fault where the latter has a step in the fault plane kinematically the xiaoyudong fault functions as a tear and conjugate fault and coincides with significant coseismic slip rake rotations on both the beichuan and pengguan faults similar correlation of fault bends with sharp changes in faulting style occurs at other steps along the wenchuan rupture the xiaoyudong fault may have played a positive role in linking coseismic slip partitioning between parallel reverse fault planes facilitating the growth of a longer and more destructive rupture this highlights the role of tear faults in bridging ruptures between segments such that reverse type ruptures can breach steps wider than anticipated from strike slip fault examples transfer faults are common and perhaps poorly documented features in reverse fault systems and their roles in ruptures may increase the maximum potential earthquake magnitude for fold and thrust belts

step like migration of early aftershocks following the 2007 mw 6 7 noto hanto earthquake japan seismic data reveal that the aftershock zone expands with time after a main shock event here we examine the aftershock sequence recorded during the first 32 days following the 2007 noto hanto earthquake japan by applying a matched filter technique to the data we detected about 10 times more events than those listed in the routinely constructed earthquake catalog the aftershock area expanded along the fault strike as a logarithmic function of time beginning immediately after the main shock aftershock expansion toward the sw was especially extensive and developed more rapidly than toward the ne interestingly the aftershock area expanded in a step like manner by the activation of a series of spatially clustered seismic bursts spatial correlation between the aftershock extension and the afterslip distribution suggests that the expansion of the aftershock area could be mainly driven by aseismic afterslip the aftershock area continues to expand even though 7 years have now passed since the main shock

groundwater flow in an arsenic contaminated aquifer mekong delta cambodia abstract to advance understanding of hydrological influences on as concentrations within groundwaters of southeast asia the flow system of an as rich aquifer on the mekong delta in cambodia where flow patterns have not been disturbed by irrigation well pumping was examined monitoring of water levels in a network of installed wells extending over a 50 km2 area indicates that groundwater flow is dominated by seasonally variable gradients developed between the river and the inland wetland basins while the gradient inverts annually net groundwater flow is from the wetlands to the river hydraulic parameters of the aquifer k 10 4 ms 1 and overlying clay aquitard k 10 8 ms 1 were determined using grain size permeameter and slug test analyses when coupled with observed gradients they indicate a net groundwater flow velocity of 0 04 0 4 ma 1 downward through the clay and 1 13 ma 1 horizontally within the sand aquifer producing aquifer residence times on the order 100 1000 a the results of numerical modeling support this conceptual model of the flow system and when integrated with observed spatial trends in dissolved as concentrations reveal that the shallow sediments upper 2 10 m of fine grained material are an important source of as to the underlying aquifer

origin and evolution of interdistributary delta plains insights from mekong river delta the geometry of river deltas is considered to refl ect the interplay of coastal processes river discharge and sediment supply as 25 of the world s population lives on deltaic lowlands prediction of delta growth is critical knowledge of processes responsible for delta geometries is not well established although such knowledge is critical for the risk management of land use and settlements on deltas the mekong river delta of southern vietnam is one of the largest deltas in the world and offers a unique opportunity to understand the sedimentary evolution of mixed tide and wave dominated large river deltas we constrained the three dimensional sedimentary evolution of the delta plains based on optically stimulated luminescence dating of beach ridges coupled with radiocarbon dated sediment cores results show that the beach shoreline in the lower delta plain initiated ca 3 5 ka by aggradation on basement shoals the delta plain propagated laterally during the late holocene evolving from bars that resulted in asymmetric bifurcation of the river mouth asymmetry was caused by the southwestward longshore sediment drift enhanced by the winter monsoon bars were successively formed on the wider side of the bifurcated river mouths and subsequently accreted seaward being stabilized by tide effects to result in shore perpendicular elongate delta plains our work based on the mekong delta demonstrates for the fi rst time that bar emergence is a key process in the long term evolution of mixed tide and wave dominated deltas

changes in late pleistocene holocene sedimentary facies of the mekong river delta and the influence of sedimentary environment on geotechnical engineering properties abstract the aim of the study was to characterize a variety of sedimentary facies from the mekong river delta mrd and to determine their geotechnical properties the paper furthermore discusses the influences of sedimentary environment and conditions on geotechnical properties of the sedimentary facies a core at the vinhlong province mrd sufficiently presented the sedimentary facies of the area eight successive facies were identified based upon sedimentary properties characteristics of the unit showed development of sedimentary facies each sedimentary facies was formed under a different environment and revealed typical geotechnical properties estuarine channel estuarine marine delta front mouth bar and sub to inter tidal flat facies were formed under strong hydrodynamic conditions the sediments are from clay to coarse sand and even pebbles and the sedimentary structures are plentiful such as intercalated clay silt beddings and fine to coarse sand beddings with very different thicknesses strengths of these sedimentary facies vary significantly which can be observed by cone penetration test cptu results but not by standard penetration test spt the normalized values from cptu show saw tooth graphs with large variations especially delta front mouth bar and sub to inter tidal flat facies at shallower depth the soil behavior types determined from the normalized values are plentiful from cohesionless to cohesive soils as an example an estuarine channel facies experiencing a large consolidation pressure in relatively long time over 9920 yr bp has geotechnical properties with heavily overconsolidated conditions and high strength on the other hand marsh open bay pro delta and march flood plain facies which formed under relatively low hydrodynamic conditions have simple and homogenous sedimentary properties the sediments are commonly silt and clay and sedimentary structures are very faint interbedded clay and silt laminae and rarely very fine sand laminae strengths of these sedimentary facies increase linearly with depth the normalized values are all rather constant with depth and their soil behavior type is only normally consolidated clays from cptu results it could be said that the particular sedimentary conditions in the late pleistocene holocene deposits of mrd can be reasonably estimated by the cptu test

magnetic minerals in three asian rivers draining into the south china sea pearl red and mekong rivers the use of the marine sedimentary magnetic properties as tracers for changes in precipitation rate and in oceanic water masses transport and exchanges implies to identify and to characterize the different sources of the detrital fraction this is of particular importance in closed and or marginal seas such as the south china sea we report on the magnetic properties of sedimentary samples collected in three main asian rivers draining into the south china sea the pearl red and mekong rivers the geological formations as well as the present climatic conditions are different from one catchment to another the entire set of performed magnetic analyses low field magnetic susceptibility arm acquisition and decay irm acquisition and decay back field acquisition thermal demagnetization of three axes irm hysteresis parameters forc diagrams and low temperature magnetic measurements allow us to identify the magnetic mineralogy and the grain size distribution when magnetite is dominant some degree of variability is observed in each basin illustrating different parent rocks and degree of weathering on average it appears that the pearl river is rich in magnetite along the main stream while the mekong river is rich in hematite the red river is a mixture of the two compared to clay mineral assemblages and major element contents previously determined on the same samples these new findings indicate that the magnetic fraction brings complementary information of great interest for environmental reconstructions based on marine sediments from the south china sea

electrical resistivity tomography determines the spatial distribution of clay layer thickness and aquifer vulnerability kandal province cambodia despite being rich in water resources many areas of south east asia face difficulties in securing clean water supply this is particularly problematic in regions with a rapidly growing population in this study the spatial variability of the thickness of a clay layer controlling surface groundwater interactions that affect aquifer vulnerability was investigated using electrical resistivity tomography ert data were acquired along two transects showing significant differences in the imaged resistivities borehole samples were analyzed regarding particle density and composition and linked to their resistivity the obtained relationships were used to translate the field electrical resistivities into lithologies those revealed considerable variations in the thickness of the clay layer ranging from 0 m up to 25 m geochemical data highlighting zones of increased ingress of surface water into the groundwater confirmed areas of discontinuities in the clay layer which act as preferential flow paths the results may guide urban planning of the phnom penh city expansion in order to supply the growing population with safe water the presented approach of using geophysics to estimate groundwater availability accessibility and vulnerability is not only applicable to kandal province cambodia but also to many other areas of fast urbanization in south east asia and beyond

aftershock zones of large shallow earthquakes fault dimensions aftershock area expansion and scaling relations summary we determine the aftershock areas from relocated hypocentres for 64 dip slip and eight strike slip earthquakes in the period 1977 1996 together with those for three recent earthquakes the 1998 antarctic plate earthquake the 1999 izmit turkey earthquake and the 2000 wharton basin earthquake we also include the data for 27 strike slip earthquakes from pegler das 1996 we find that the location of the hypocentre is essentially random along strike for both strike slip and dip slip earthquakes subduction zone earthquakes appear to initiate more frequently towards the down dip edge of the fault whereas the non subduction zone dip slip earthquakes do not have any preferred depth of initiation the aftershock zones of subduction zone earthquakes often expand substantially along strike and up dip but far less in the down dip direction whereas those for non subduction zone earthquakes do not expand significantly in either the up or the down dip direction subduction zone thrust earthquakes have larger and more numerous aftershocks than earthquakes in all other tectonic settings for strikeslip earthquakes we find that slip increases at least linearly with length for dip slip earthquakes we find that the ratio of length to width increases systematically with length for lengths 40 km indicating that there is some restriction on fault width slip is found to be proportional to length over the moment range 10 17 nm m0 3r10 21 nm

global survey of aftershock area expansion patterns we developed an objective method to define the aftershock areas of large earthquakes as a function of time after the main shock the definition is based upon the amount of energy released by aftershocks the spatial distribution of the energy release is first determined and is contoured the 1 day aftershock area is defined by a contour line corresponding to the energy release level of 10 15 6 ergs 100 km 2 day the 10 day 100 day and 1 y aftershock areas are similarly defined by contour lines corresponding to 10 14 8 10 14 0 and 10 13 5 ergs 100 km 2 day respectively we also define the expansion ratios at time t by the ratio of the aftershock area at t to that at 1 day using this method we study the aftershock area expansion patterns of 44 large m s 7 5 and five moderate shallow earthquakes which occurred from 1963 to 1980 each aftershock sequence is examined at four different times i e 1 day 10 days 100 days and 1 y after the main event we define the aftershock area expansion ratios and e by s 100 s 1 and l 100 l 1 respectively here s t and l t are the area and the length of the aftershock area respectively at time t our study suggests that a distinct regional variation of aftershock area expansion patterns is present it is strongly correlated with the tectonic environment in general the subduction zones of the mariana type have large expansion ratios and those of the chilean type have small expansion ratios some earthquakes that occurred in the areas of complex bathymetry such as aseismic ridges tend to have large expansion ratios these results can be explained in terms of an asperity model of fault zones in which a fault plane is represented by a distribution of strong spots called the asperities and weak zones surrounding the asperities the rupture immediately after the main shock mostly involves asperities after the main rupture is completed the stress change caused by the main shock gradually propagates outward into the surrounding weak zones this stress propagation manifests itself as expansion of aftershock activity in this simple picture if the fault zone is represented by relatively large asperities separated by small weak zones chilean type then little expansion of aftershock activity would be expected on the other hand if relatively small asperities are sparsely distributed mariana type significant expansion occurs the actual distribution of asperities is likely to be more complex than the two cases described above however we would expect that the expansion ratio is in general proportional to the spatial ratio of the total asperity area to the fault area

monsoon influenced variations in morphology and sediment of a mesotidal beach on the mekong river delta coast abstract a mesotidal beach on the mekong river delta coast exhibits unique patterns of river sediment discharge and experiences reversals of ocean wave directions in response to the summer and winter monsoons we analyzed long term changes of the shoreline since 1936 from past topographic maps and satellite images and short term changes in geomorphology and sedimentology by undertaking repeated field surveys between november 2005 and february 2008 along six shore normal beach transects the shoreline of the mekong river delta coast has changed asymmetrically over the last 70 years in response to net southwestward sediment transport related to dry northeasterly winter monsoons during the summer rainy season large volumes of sediments are discharged from the river at this time wave direction is reversed in response to the relatively weak southwesterly summer monsoon mud and very fine sand in the surface sediments of the northeastern updrift beach transects tend to be removed during winter suggesting that the sediment supplied from the river during summer is temporarily deposited near the river mouth and later transported southwestward during the winter monsoon the relief of intertidal bars on the beach increased during winter in response to higher waves however previous studies have suggested that higher waves flatten bars we hypothesize that the increase in relief that we observed is related to a wide and shallow subtidal delta front platform that attenuates ocean waves at all times other than at high tide the beach profile varies longshore the accreting to stable beach dips seaward at a gradient of about 1 80 with up to three longshore intertidal bars classified here as either slip face bars or low amplitude ridges in contrast the eroding beach profile is much steeper 1 20 1 40 and linear without intertidal bars and is similar in form to a low tide terrace because there is little variation of wave size and grain size within the study area we consider that the spatial variation of the beach profile is related to the availability of sediment accumulated on the beach

dynamics of a fringe mangrove forest detected by landsat images in the mekong river delta vietnam mangrove forests dominate many tropical coastlines and are one of the most bio diverse and productive environments on earth however little is known of the large scale dynamics of mangrove canopies and how they colonize intertidal areas here we focus on a fringe mangrove forest located in the mekong river delta vietnam a fast prograding shoreline where mangroves are encroaching tidal flats the spatial and temporal evolution of the mangrove canopy is studied using a time series of landsat images spanning two decades as well as shuttle radar topography mission srtm elevation data our results show that fast mangrove expansion is followed by an increase in normalized difference vegetation index ndvi in the newly established canopy we observe three different dynamics of the mangrove fringe in the southwest part of the fringe near a deltaic distributary where the fringe boundary is linear the canopy expands uniformly on the tidal flats with a high colonization rate and high ndvi values in the northeast part of the fringe near another distributary the canopy expands at a much lower rate with low ndvi values in the fringe center far from the river mouths the fringe boundary is highly irregular and mangroves expansion in characterized by sparse vegetated patches displaying low ndvi values we ascribe these different dynamics to wave action and southwest longshore transport triggered by energetic northeasterly monsoons during winter we further link the large scale dynamics of the fringe to small scale physical disturbances waves erosion and deposition that might prevent the establishment of mangrove seedlings based on these results we include mangrove encroachment in an already published conceptual model of progradation of the mekong river delta we conclude that high ndvi values and a constantly linear vegetation water interface are indicative of stable mangrove canopies undergoing fast expansion probably triggered by sediment availability at the shore our results can be applied more generally to mangrove forests growing in minerogenic and high tidal range environments with high sediment inputs copyright 2016 john wiley sons ltd

sediment facies and late holocene progradation of the mekong river delta in bentre province southern vietnam an example of evolution from a tide dominated to a tide and wave dominated delta abstract the mekong river delta southern vietnam is a typical mixed tide and wave energy delta with a wide delta plain formed during the last 6 ka and is one of the largest deltas in the world three cores were taken from bentre province in the lower delta plain with the objective to describe the sediment facies and to clarify the changes from tide dominated to tide wave dominated delta of the mekong river delta during the late holocene three cores bt1 bt2 and bt3 were obtained in 1997 holocene sediments in bt1 and bt3 with thickness ranging from 10 to 20 m were mainly composed of deltaic sediments unconformably overlying the late pleistocene sediments the bt2 core however consisted of transgressive estuarine sediments covered by deltaic sediments infilling the paleo mekong river incised valley which is more than 70 m deep and was formed during the last glacial period the deltaic sediment facies and succession in the bt2 and bt3 cores taken from the outer delta plain with beach ridges were different from those of the bt1 core taken from the inland delta plain the bt2 and bt3 cores provide a good example of tide and wave dominated delta sediments and were characterized by a coarsening upward delta front facies covered by a fining upward subtidal to intertidal flat facies followed by a coarsening upward foreshore dune facies these changes from the inland delta plain to the beach ridge delta plain were accompanied by changes in progradation rates and subaqueous delta topography caused by increased wave influence

formation and fate of sedimentary depocentres on southeast asia s sunda shelf over the past sea level cycle and biogeographic implications abstract sea level variations are the major factor controlling sedimentation as well as the biogeographic patterns at continental margins over late quaternary times fluctuations on millennial time scales produce locally complex deposits in coasts and on shelves associated with short term influence on species development this article reviews the sedimentary and biogeographic history of the tropical siliciclastic sunda shelf as an end member of continental shelves regarding extreme width an enormous sediment supply and highest biodiversity in response to rapid sea level fluctuations we describe particular depositional segments as part of a genetic succession of zones from land to the deep sea based on literature data field observations and calculation of hydro isostatic adjustment effects on changing relative sea level these segments are characterized by individual sedimentary processes and deposits and by a specific potential for material storage and re mobilization long term regressive intervals led to overall sigmoidal promoting extremely thick and wide succeeding units in contrast rapid lateral shifts of defined depocentres over long distances took place in response to short term sea level fluctuations fully isolated small scale sediment bodies formed when sea level changed at exceptionally high rates as a result of the high availability of organic rich sediments mangrove and freshwater peats formed frequently over late quaternary times the appearance of thick massive and widespread peats is mainly linked to time intervals of a sea level rise at slow rates whilst organic matter appears much more dispersely in the sediments during episodes of rapidly changing sea level the preservation potential of the regressive units is generally high due to highest initial sediment supply stabilizing soil formation during exposure and rapid subsidence preservation of depositional elements from other periods is more exceptional and either restricted to local morphological depressions or to episodes of rapid sea level change besides complex channel incision an overall lowering of the sediment surface related to erosion as deep as 20 m or more over wide areas took place mainly during sea level lowering the final export of shelf material is documented by enormous mass wasting packages on the associated continental slope from a palaeogeographic perspective the rapid formation or disappearance of special habitat zones such as mangrove fringes and extended mud flats led to species establishment or truncation in distribution in addition the opening or closure of ocean passages as narrow bridges allowing limited species crossing or as fully colonized corridors had severe impact on eco fragmentation and the expansion or contraction of species independent of such particular conditions sea level changes have been too rapid over the past climatic cycle to allow full regeneration and mature development of coast related ecosystems

bottom sediments affect sonneratia mangrove forests in the prograding mekong delta vietnam abstract mangrove forests exert a strong influence on tropical deltas by trapping sediments discharged by rivers and by stabilizing the substrate with roots understanding the dynamics of sediments and morphology in and around mangrove forests is critical in order to assess the resilience of coastlines in a period of accelerated sea level rise in this research sediment samples mangrove forest characteristics and remote sensing data are used to investigate the relationship between mangroves and sediment substrate in the mekong delta vietnam our data show a significant correlation between percent of sand in bottom sediments and density of sonneratia caseolaris forest we ascribe this result to higher sediment disturbance in muddy areas that prevents seedling establishment this correlation potentially allows the determination of substrate characteristics from vegetation attributes detected by remote sensing despite the impenetrability of the forest canopy the results presented herein suggest that a supply of sand from the river and hydrodynamic processes moving the sand ashore control the density of the sonneratia mangrove forests at this location promoting tidal flat colonization and canopy expansion

a submerged late pleistocene delta and other features related to sea level changes in the malacca strait abstract bathymetry sea floor echo characteristics and shallow subbottom reflectors in the malacca strait and the slope of the mergui north sumatra basin of the southern andaman sea show a varied and locally complex character which we interpret as a pleistocene lowered sea level alluvial delta fan system sediment waves and elongate banks in the narrow and shoal southern part of the strait indicate strong bottom currents and a rugged subbottom topography demonstrates erosion during low sea level of the late quaternary shallow subbottom reflectors at the northwest approaches to the strait show progradation and cut and fill valleys and the presence of a compound late pleistocene delta of the confluent sumatran and malayan rivers the younger prograding layers are probably of mid wisconsin wurm age ca 40 20 10 3 yr b p and the deeper foresets may be early illinoian glacial stage riss ca 150 10 3 yr b p farther seaward a small fan progrades from the slope a short distance into the shallow northeastern part of the mergui north sumatra basin

the mekong from satellite imagery a quick look at a large river abstract physical datasets on rivers thousands of kilometres long can be efficiently and rapidly acquired through satellite imagery as large rivers commonly cross several tectonic and climatic zones their channels are composed of a number of units each with characteristic morphology and behaviour separating a large river into such units provides a framework for understanding the geomorphology of the river concerned allowing acquisition of information for environmental impact analysis and river management about 2500 km of the mekong river from the china border to the sea has been examined with spot satellite images with a selection of modis and ikonos scenes in support hydrological data of the mekong river commission hydrographic maps of the commission and field observations have been used to verify and extend the findings from the satellite imagery the technique provides a rapid and holistic conceptualisation of forms and functions along the derived eight unit classification of the mekong river such a framework is valuable for 1 determining selected aspects of the geomorphological behaviour of a large river system 2 rapidly analysing project related environmental impacts and 3 examining the geological evolution of the river investigating large rivers is difficult and resource consuming but satellite imagery provides an easy and rapid tool the cost of the images however may be high as a number of them are required for this type of analysis

an evolutionary model for the holocene formation of the pearl river delta china this paper reconstructs the evolutionary history of the pearl river delta over the last 9000 years and investigates land sea interaction in a large deltaic complex which formed under the influence

asymmetry in holocene river deltas patterns controls and stratigraphic effects abstract understanding controls on sediment distribution in river deltas is paramount to predicting facies relationships and stratal architecture most classifications of deltas have focused on subaerial plan morphology as a simplistic function of river tide and wave energy new work however is revealing the importance of subaqueous deposits and in particular shore parallel sediment redistribution in the shaping of delta planform delta asymmetry has emerged as an important characteristic reflective of patterns of alongshore sediment dispersal asymmetry has been described from onshore and offshore environments from several different types of modern deltas but aspects of asymmetry have not been fully documented and the degree to which these patterns are recorded in deltaic strata is not yet known this study is a comprehensive literature review of sediment distributions in modern deltas focusing on studies with high resolution geomorphic geophysical and geochronological datasets we reviewed 27 deltas using over 100 papers primarily from the past 15 20 years morphological facies and stratigraphic aspects were analyzed across the entire spectrum of deposits from the delta apex to the most distal muds of the prodelta we define quantifiable indices of asymmetry describing upcurrent vs downcurrent distribution of sediment volume sediment area sediment caliber and distributary channels most deltas are asymmetrical to some degree with respect to one or more of these parameters many deltas are increasingly skewed toward the downcurrent side from proximal to distal sub environments i e delta plain delta front prodelta some deltas are skewed toward the upcurrent side in one sub environment and toward the downcurrent side in another sub environment sand is preferentially deposited on the downcurrent side of most deltas but distributary channels tend to develop toward the upcurrent side the highest sand mud ratios are often on the upcurrent side in the lower delta plain but in the delta front these ratios are highest on the downcurrent side these complex patterns of asymmetry reflect different combinations of controls including discharge partitioning lobe abandonment and localized transgression plume deflection by coastal currents dominance of longshore current direction variable subsidence and other factors these processes may result in upcurrent downcurrent variations in clinoform geometry rates of progradation and stratal lapping relationships asymmetry has multiple aspects manifestations and controls even within a single delta but the long term preservation potential of these patterns likely varies depending on depositional setting and shoreline trajectory studies of ancient deltas will be better informed by recognizing the wide variety of controls on sediment distribution and avoiding the tendency toward a single model of delta asymmetry

afterslip and only afterslip following the 2004 parkfield california earthquake 1 an analysis of the first two years of postseismic surface deformations from gps reveals that afterslip is the only mechanism significantly contributing to postseismic deformation following the 2004 m6 parkfield california earthquake finite element modeling shows this event to have been too small to significantly stress the lower crust and upper mantle thus viscoelastic relaxation did not lead to detectable surface displacements similarly coseismically induced pressure changes in the upper crust were not sufficient to induce a measurable poroelastic response from 10 days to two years after the parkfield earthquake postseismic displacements at all gps stations experience the same characteristic decay time 2 weeks this suggests that only afterslip was activated and the distribution of slip remained unchanged in this time period afterslip was found to be broadly distributed in the upper 15 km of the crust and associated with a moment release of much greater magnitude than the coseismic rupture

late holocene depositional environments and coastal evolution of the mekong river delta southern vietnam abstract the mekong river delta in southern vietnam is a tide dominated delta and has been formed by the mekong river system the sediments are dominantly fine grained and were deposited in the receiving basin with slight inclination of pre existing deposits in the south china sea and gulf of thailand around 6000 to 5000 yr bp maximum holocene transgression occurred and seawaves surrounded uplands of the late pleistocene terrace basement rock and weathered land in the northern parts of the delta double notches were undercut by seawaves on the limestone hill in hatien at about 4 5 and 2 5 m above present sea level marine deposits prevailed throughout the delta during the highstand and regressions of relative sea level over the last 4550 yr bp delta progradation has produced a great flat plain of 62 520 km 2 this extremely fast progradation could be due to a very high sediment supply depositing in the slight inclination of the receiving basin b neotectonic movements and relative sea level changes c widespread mangrove forests playing an important role to enhance sediment accumulation former mangrove marshy deposits were significantly developed in the plain of reeds and longxuyen quadrangle mangrove marshes have also been developing well in the camau peninsula and present coastlines moreover regressions over the last 4550 yr bp have produced many distinct sets of relict beach ridges which are useful in distinguishing the former coastlines and evolutionary history of the delta

evidence of sudden rupture of a large asperity during the 2008 mw7 9 wenchuan earthquake based on strong motion analysis 1 we investigate the rupture process of the 12 may 2008 wenchuan earthquake using the records of 26 strong motion stations located 20 120 km from the seismic fault and with a good azimuthal coverage the wave velocity model required to conduct this analysis has been validated against aftershocks for which the point source hypothesis is a very good approximation the inversion of the main shock rupture process confirms the slip distribution and the average rupture velocity 3 km s previously determined however a very peculiar behavior is clearly resolved by the extensive data set used in this study the major slip area of the wenchuan earthquake located at 20 50 km north east of the epicenter is shown to break almost simultaneously 25 s after earthquake initiation this implies that slip in this part of the fault cannot be understood by simple stress release at the rupture front a more likely interpretation is the presence of a strong asperity which could break only when it was completely surrounded by stress increase resulting in a delayed but brutal rupture

oceanographic currents differential subsidence and physiography control three dimensional clinothem growth in the gulf of papua papua new guinea abstract a regional high resolution chirp seismic survey in the gulf of papua gop extends the geographic scope of previous studies by 125 km and reveals that the holocene clinothem along margin geometries surfaces of lap and steepness of foreset slopes appear less dependent on eustatic sea level changes and sediment supply than previously suggested clinothems imaged by chirp profiles and sampled by sediment cores include two older relict clinothems and a younger holocene clinothem divided into three units by two surfaces of lap new radiocarbon ages from this study establish that depositional timing is more recent than previously proposed for the transgressive deposits the holocene clinothem units and surfaces of lap while previous ages suggested that observed stratal relationships could coincide with meltwater pulses or sea level stillstands the younger ages from this study question these results and suggest the dominant control on holocene clinothem architecture may be inherited physiography sw ne differential subsidence along margin currents and seasonal variations in wave energy inherited physiography underlying the holocene clinothem consists of valleys incised during relative lowstands of marine isotope stages mis 4 and 2 valleys in the relict clinothem potentially served as across margin conduits for an older holocene clinothem unit after the older holocene unit filled valleys and accommodation younger holocene units preferentially grew along rather than across margin as bottom shear stresses increased relict clinothems have undergone differential subsidence since deposition due to foreland basin loading which has engendered more accommodation in the northeast gop that systematically decreases toward the southwest areas with less accommodation are more likely to experience toplap as they are exposed to increased shear stresses therefore the surfaces of lap are caused by interactions between accommodation and oceanographic currents examining stratal relationships along the holocene clinothem has broad implications for sequence stratigraphy as many clinothems exhibit a three dimensional stacking pattern measurements of seafloor slopes yield complicated results illustrating that seafloor steepness within lobes is influenced by a complex interplay of accommodation sediment supply underlying physiography and oceanographic currents this suggests that clinothem architecture in the gop cannot simply be interpreted as a rate related problem between rates of sediment supply and relative sea level

holocene delta evolution and sediment discharge of the mekong river southern vietnam abstract evolutionary changes delta progradation and sediment discharge of the mekong river delta southern vietnam during the late holocene are presented based on detailed analyses of samples from six boreholes on the lower delta plain sedimentological and chronostratigraphic analyses indicate clearly that the last 3 kyr were characterized by delta progradation under increasing wave influence southeastward sediment dispersal decreasing progradation rates beach ridge formation and steepening of the face of the delta front estimated sediment discharge of the mekong river for the last 3 kyr based on sediment volume analysis was 144 36 million t yr 1 on average or almost the same as the present level the constant rate of delta front migration and stable sediment discharge during the last 3 kyr indicate that a dramatic increase in sediment discharge owing to human activities as has been suggested for the yellow river watershed did not occur although southeast asian rivers have been considered candidates for such dramatic increases in discharge during the last 2 kyr the mekong river example although it is a typical large river of this region does not support this hypothesis therefore estimates of the millennial scale global pristine sediment flux to the oceans must be revised

far field triggering of foreshocks near the nucleation zone of the 5 september 2012 m w 7 6 nicoya peninsula costa rica earthquake abstract on 5 september 2012 a moment magnitude mw 7 6 earthquake occurred directly beneath the nicoya peninsula an area with dense seismic and geodetic network coverage the mainshock ruptured a portion of a previously identified locked patch that was recognized due to a decade long effort to delineate the megathrust seismic and aseismic processes in this area here we conduct a comprehensive study of the seismicity prior to this event utilizing a matched filter analysis that allows us to decrease the magnitude of catalog completeness by 1 unit we observe a statistically significant increase in seismicity rate below the nicoya peninsula following the 27 august 2012 mw 7 3 el salvador earthquake about 450 km to the northwest and 9 days prior to the nicoya earthquake additionally we identify a cluster of small magnitude

modeling afterslip and aftershocks following the 1992 landers earthquake one way to probe the rheology of the lithosphere and fault zones is to analyze the temporal evolution of deformation following a large earthquake in such a case the lithosphere responds to a known stress change that can be assessed from earthquake slip models constrained from seismology and geodesy here we model the postseismic response of a fault zone that is assumed to obey a rate strengthening rheology where the frictional stress varies as a ln e e being the deformation rate and a 0 a rheological parameter the model is simple enough that these parameters can be estimated by inversion of postseismic geodetic data we apply this approach to the analysis of geodetic displacements following the m w 7 3 1992 landers earthquake the model adjusts well the measured displacements and implies a 0 47 0 53 mpa in addition we show that aftershocks and afterslip follow the same temporal evolution and that the spatiotemporal distribution of aftershocks is consistent with the idea that they are driven by reloading of the seismogenic zone resulting from frictional afterslip

mid holocene mangrove succession and its response to sea level change in the upper mekong river delta cambodia abstract middle holocene vegetation and mangrove successions are clearly evident in the palynological records of two cores from the upper mekong river delta in cambodia spanning from 9 4 to 6 3 cal ka bp the cores mainly record a transgressive sequence from floodplain freshwater marsh to tidal flat which was overlain by mangrove corresponding to the decelerated sea level rise at 8 3 cal ka bp pioneer mangrove species sonneratia alba and sonneratia caseolaris appeared in the sediments and then was replaced by regressive mangrove succession containing upward increasing abundances of rhizophora apiculata and bruguiera spp high salinity and flooding tolerant community s alba was developed at the western core site psg at 8 2 cal ka bp and the eastern core site pk at 7 5 cal ka bp the time difference of s alba appearance between the two sites might be resulted from the complexity of sedimentary environment where a higher sediment supply was provided to the western floodplain than to the eastern floodplain after 7 5 cal ka bp aggradational stacking of intertidal sediments of which the thickness is larger than the present maximum tidal range may have resulted from continuous sea level rise during 7 5 7 0 cal ka bp

cycling of trace metals and rare earth elements ree in acid sulfate soils in the plain of reeds vietnam abstract trace metals and rare earth elements ree are subject to intensive chemical cycling in acid sulfate soils the oxidation of pyrite bearing deposits and the subsequent acidification mobilizes a variety of trace metals and ree in this study solid phase and pore water data of three cores from the plain of reeds in the mekong river delta vietnam were analyzed distinct authigenic enrichments of trace metals and ree were observed in the solid phase of these cores thermodynamic modeling was employed to elucidate the controls on trace metal and ree mobility and the governing factors for the formation of these authigenic enrichments the formation of ree enrichments in these settings is clearly controlled by ph and the availability of potent sorbents under suitable conditions trace metals such as e g zinc nickel and cobalt are likely bound as trace metal monosulfides in the enrichment zones the solid phase and pore water data and the results of a thermodynamic model suggest a possible sequential precipitation of trace metal sulfides however the incorporation of trace metals into iron sulfides cannot be ruled out in the case of lead and arsenic the formation of pb as jarosite proved to be a further major sink these insights provide a better understanding of the controls and governing factors for trace metal and ree mobility in acid sulfate soils and will therefore help to assess impacts on soil and water

late holocene development of a tide induced elongate delta the solo delta east java abstract the solo river east java is a typical monsoonal river the discharge and transport of appreciable amounts of sediment is mainly restricted to the wet season december march and has resulted in the rapid outgrowth of an extensive delta system the solo delta is characterized as a highly mud dominated and rapidly prograding single finger delta a specimen of a high constructive elongate delta type the presence of firmly built natural levees more or less channelizing river outflow the role of flow inertia and the rapid deposition of sediment in front of the main river mouth explain the very strong longitudinal growth of the delta and determine its long rather straight and narrow shape the presence of these pronounced natural levees is clearly a natural response to river outflow conditions since maximum river output is observed at high tide this phenomenon results from the interference of tidal components in the waters surrounding the solo delta the tidal model a conceptual model assumes that the tidal motion is capable of introducing a temporary increase in deltaic river slope during high tide by creating a local difference in the high water levels north and east of the delta the lateral dispersal of sediment is limited and mainly depends on the transport in distributary channels natural crevasses are rather scarce and do not have a stable character

eastward expansion of the tibetan plateau by crustal flow and strain partitioning across faults the tibetan plateau is expanding eastwards but the modes of deformation are poorly understood high resolution seismic images from the region identify localized zones of weak crustal rocks as well as deep faults implying that deformation occurs through a combination of crustal flow and movement of rigid blocks of crust